U.S. patent application number 12/192706 was filed with the patent office on 2008-12-04 for lightweight structural component in particular for aircraft and method for its production.
This patent application is currently assigned to Fraunhofer-Gesellschaft zuer Foerderung der angewandten Forschung e.V.. Invention is credited to Hartmut Brenneis, Berndt Brenner, Jorg Schumacher, Jens Standfub, Bernd Winderlich, Walter Zink.
Application Number | 20080296433 12/192706 |
Document ID | / |
Family ID | 32519995 |
Filed Date | 2008-12-04 |
United States Patent
Application |
20080296433 |
Kind Code |
A1 |
Brenner; Berndt ; et
al. |
December 4, 2008 |
LIGHTWEIGHT STRUCTURAL COMPONENT IN PARTICULAR FOR AIRCRAFT AND
METHOD FOR ITS PRODUCTION
Abstract
Lightweight structural component including at least one panel
and at least one stiffening element oriented one of lengthwise and
crosswise. The at least one stiffening element includes two side
pieces. Each of the two side pieces is at least partially connected
to the panel in a material-locking manner. The two side pieces are
connected to the panel at two separate joint zones. A method of
producing the lightweight structural component includes milling the
at least one panel to form at least one thickened region and
joining the two side pieces to the at least one panel at the two
separate joint zones. This Abstract is not intended to define the
invention disclosed in the specification, nor intended to limit the
scope of the invention in any way.
Inventors: |
Brenner; Berndt; (Dresden,
DE) ; Winderlich; Bernd; (Dresden, DE) ;
Standfub; Jens; (Pirna, DE) ; Schumacher; Jorg;
(Kirchlinteln, DE) ; Brenneis; Hartmut;
(Dudenbuttel, DE) ; Zink; Walter; (Bremen,
DE) |
Correspondence
Address: |
GREENBLUM & BERNSTEIN, P.L.C.
1950 ROLAND CLARKE PLACE
RESTON
VA
20191
US
|
Assignee: |
Fraunhofer-Gesellschaft zuer
Foerderung der angewandten Forschung e.V.
Muenchen
DE
Airbus Deutschland GmbH
Hamburg
DE
|
Family ID: |
32519995 |
Appl. No.: |
12/192706 |
Filed: |
August 15, 2008 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10757419 |
Jan 15, 2004 |
|
|
|
12192706 |
|
|
|
|
Current U.S.
Class: |
244/129.1 ;
29/448; 52/846 |
Current CPC
Class: |
Y10T 29/49867 20150115;
Y02T 50/40 20130101; B64C 1/12 20130101; B64C 2001/0081 20130101;
Y02T 50/42 20130101 |
Class at
Publication: |
244/129.1 ;
52/846; 29/448 |
International
Class: |
B64C 1/00 20060101
B64C001/00; F16S 5/00 20060101 F16S005/00; B23P 11/00 20060101
B23P011/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 16, 2003 |
DE |
103 01 445.4-22 |
Claims
1. A lightweight structural component comprising: at least one
metal panel; at least one metal stiffening element; the at least
one metal stiffening element comprising two side pieces; each side
piece comprising an outer surface, an inner surface, and an end
surface extending between the outer and inner surfaces; and each
end surface of the two side pieces being at least partially
connected to the at least one metal panel in a material-locking
manner, wherein the two side pieces are connected to the at least
one metal panel at two separate weld joint zones, and wherein an
axis passing through each side piece passes through one of the two
separate weld joint zones.
2. The component of claim 1, wherein the component is utilized in
an aircraft and the at least one stiffening element is oriented at
least one of a lengthwise and a crosswise direction relative to the
at least one panel.
3. The component of claim 1, wherein the at least one panel
comprises a skin sheet.
4. The component of claim 1, wherein the at least one panel
comprises a thickened region in an area of the two separate weld
joint zones.
5. The component of claim 1, wherein the at least one stiffening
element comprises a stringer which is oriented in a lengthwise
manner.
6. The component of claim 1, wherein the at least one stiffening
element comprises a rib that runs in a circumferential
direction.
7. The component of claim 1, wherein the two separate weld joint
zones comprise laser beam weld zones.
8. The component of claim 1, wherein the two separate weld joint
zones comprise friction stir weld zones.
9. The component of claim 1, wherein the two side pieces are bent
or oriented away from each other by a total angle .alpha., whereby
inner surfaces of the two side pieces and a surface of the at least
one panel form a generally isosceles triangle.
10. The component of claim 9, wherein the angle .alpha. lies in a
range of between approximately 7.degree. and approximately
50.degree..
11. The component of claim 1, wherein the two side pieces are bent
or oriented at a total angle .alpha. of approximately 180.degree.,
whereby inner surfaces of the two side pieces rest on a surface of
at least one panel.
12. The component of claim 11, wherein the two side pieces are
integrally formed with the at least one stiffening element, whereby
the at least one stiffening element and the two side pieces
comprise a one-piece member.
13. The component of claim 1, wherein the two side pieces are
integrally formed with the at least one stiffening element, whereby
the at least one stiffening element and the two side pieces
comprise a one-piece member.
14. The component of claim 1, wherein the at least one stiffening
element comprises a generally U-shaped profile, whereby the two
side pieces are arranged on opposite ends of a head of the at least
one stiffening element.
15. The component of claim 14, wherein the two side pieces of the
generally U-shaped profile are parallel to each other.
16. The component of claim 1, wherein the at least one stiffening
element comprises an edge area which is oriented in a generally
parallel manner relative to the at least one panel.
17. The component of claim 1, wherein the at least one panel
comprises a panel reinforcing base portion which comprises a first
base portion and a second base portion separated from the first
base portion, wherein lateral outer surfaces of the first and
second base portions rest against or adjacent to inner surfaces of
the two side pieces.
18. The component of claim 1, wherein an area of the at least one
panel comprising the two weld joint zones comprises a surface
formed by metal cutting.
19. The component of claim 1, wherein an area of the at least one
panel comprising the two weld joint zones comprises a surface
formed by metal removal.
20. The component of claim 1, wherein at least one of the two side
pieces comprises cut-outs.
21. The component of claim 1, wherein at least one of the two side
pieces comprises a plurality of through openings.
22. The component of claim 1, wherein each of the two side pieces
comprises cut-outs and wherein the cut-outs are arranged at
generally regular intervals "a".
23. The component of claim 1, wherein each of the two side pieces
comprises through openings arranged at generally regular intervals
"a".
24. The component of claim 23, wherein a distance between an edge
of the through openings and joint surfaces of the two joint zones
is greater than approximately one and a half times a side piece
thickness t.sub.S measured in a plane of each joint zone.
25. The component of claim 23, wherein the through openings in one
of the two side pieces are spaced from each other by a distance "a"
and wherein the through opening of the other of the two side pieces
are spaced from the through openings of the one of the two side
pieces by a distance of approximately a/2.
26. The component of claim 23, wherein the through openings
comprise circular openings.
27. The component of claim 23, wherein one of the through openings
comprise polygonal openings and the through openings comprise
non-circular openings.
28. The component of claim 23, wherein the through openings
comprise triangular openings.
29. The component of claim 28, wherein the triangular openings
comprise approximately equal-sided triangular openings with rounded
corners, and wherein vertices of adjacent triangular openings point
in opposite directions.
30. The component of claim 1, further comprising a doubler plate
made of a damage-tolerant fiber-reinforced laminate attached to
outer surfaces of the two side pieces.
31. The component of claim 1, further comprising at least one
stress relief element located inside the at least one panel.
32. The component of claim 31, wherein the at least one panel
comprises a thickened panel base arranged in an area of the two
separate joint zones and wherein the at least one stress relief
element is arranged within the thickened panel base.
33. The component of claim 32, wherein the at least one stress
relief element is arranged beneath a bar portion of the at least
one stiffening element and between the two separate joint
zones.
34. The component of claim 31, wherein the at least one stress
relief element comprises a material having a higher modulus of
elasticity and a higher fatigue strength than a material of the at
least one panel.
35. The component of claim 31, wherein the at least one stress
relief element comprises a plurality of stress relief elements.
36. The component of claim 31, wherein the at least one stress
relief element comprises a plurality of spaced apart stress relief
elements.
37. The component of claim 31, wherein the at least one stress
relief element comprises a high-strength wire cable.
38. The component of claim 31, wherein the at least one stress
relief element is located directly beneath a panel stiffening base
of the at least one panel and is centrally disposed between the two
separate joint zones.
39. The component of claim 37, wherein the panel stiffening base is
integrally formed with the at least one panel, whereby the panel
stiffening base and the at least one panel comprise a one-piece
member.
40. The component of claim 1, wherein the at least one panel
comprises a panel stiffening base made of material that is deformed
during a rolling-in of a stress relief element into the at least
one panel.
41. The component of claim 1, wherein the at least one panel
comprises a panel stiffening base made of material that is deformed
during a rolling of the at least one panel.
42. The component of claim 1, wherein the at least one panel
comprises a plurality of panel bars arranged generally parallel to
one another and generally parallel to the at least one stiffening
element.
43. The component of claim 1, wherein the at least one panel
comprises a plurality of panel bars arranged generally parallel to
one another and generally perpendicular to the at least one
stiffening element.
44. The component of claim 1, wherein the at least one panel
comprises a plurality of panel bars, some of which are arranged
generally parallel to one another and some of which are arranged
generally perpendicular to one another.
45. The component of claim 1, wherein the at least one panel
comprises a plurality of panel stiffening bases and a plurality of
panel bars, wherein a height of the panel bars generally
corresponds to a height of the panel stiffening bases, wherein the
at least one stiffening element comprises a plurality of stiffening
elements, and wherein a spacing between the stiffening elements is
generally equal to an integral multiple of a spacing "C" between
the panel bars.
46. The component of claim 1, wherein the at least one stiffening
element comprises a head portion that is coupled to a bar
portion.
47. The component of claim 46, wherein the head portion projects
from both sides of the bar portion.
48. The component of claim 47, wherein the head portion projects by
generally equal amounts from both sides of the bar portion.
49. A lightweight structural component comprising: at least one
metal panel comprising at least one thickened region; at least one
metal stiffening element welded to the at least one panel; the at
least one metal stiffening element comprising a bar portion and two
side pieces; each side piece comprising an outer surface, an inner
surface, and an end surface extending between the outer and inner
surfaces and having a width that is narrower than a length of the
outer surface; and each end surface of the two side pieces being at
least partially connected in a material-locking manner to the at
least one thickened region by two separate weld joint zones,
whereby the at least one metal stiffening element is oriented in at
least one of a lengthwise and a crosswise direction.
50. The component of claim 49, further comprising a reinforcing
element located in a cavity formed by the two side pieces and a
surface of the at least one thickened region.
51. The component of claim 50, wherein the at least one thickened
region comprises a panel stiffening base and wherein the
reinforcing element comprises a high-strength material having a
modulus of elasticity that is generally greater than a modulus of
elasticity of a material of at least one of the at least one panel
and the at least one stiffening element.
52. The component of claim 51, wherein the reinforcing element is
connected to at least one of the two side pieces and to the at
least one panel stiffening base in one of a force-locking manner
and a form-locking manner.
53. The component of claim 49, wherein the component is arranged on
an aircraft.
54. The component of claim 49, wherein the at least one stiffening
element comprises a stringer oriented in a lengthwise
direction.
55. The component of claim 49, wherein the at least one stiffening
element comprises a rib oriented in a circumferential
direction.
56. The component of claim 49, wherein the two separate weld joint
zones comprise laser beam weld zones.
57. The component of claim 49, wherein the two separate weld joint
zones comprise friction stir weld zones.
58. The component of claim 49, wherein the two weld joint zones
comprise panel surfaces and surfaces of the two side pieces, and
wherein each of the panel and two side piece surfaces comprises a
machined surface.
59. The component of claim 49, further comprising a reinforcing
element having surfaces which are both force-locked and form-locked
to at least one of inner surfaces of the two side pieces and a
surface of the thickened region.
60. The component of claim 59, wherein the surfaces comprise at
least one of a rough profile and surface profiling.
61. The component of claim 49, further comprising a reinforcing
element which comprises surfaces which are fixed to at least one of
inner surfaces of the two side pieces and a surface of the
thickened region.
62. The component of claim 49, further comprising a cavity formed
by the two side pieces and the at least one thickened region and a
reinforcing element arranged within the cavity.
63. The component of claim 62, wherein a cross-sectional shape of
the cavity generally corresponds to a cross-sectional shaped of the
reinforcing element.
64. The component of claim 63, wherein the cavity comprises a
cross-sectional shape having a form of a generally equal isosceles
triangle with a rounded-off apex.
65. The component of claim 63, wherein the reinforcing element
comprises a cross-sectional shape having a form of a generally
equal isosceles triangle with a rounded-off apex.
66. The component of claim 49, further comprising at least one
reinforcing element arranged within the at least one thickened
region.
67. The component of claim 49, further comprising at least one
reinforcing element arranged between the two side pieces, wherein
the at least one reinforcing element comprises one of a wire, a
rod, a wire rope, a pipe and a tube.
68. The component of claim 67, wherein the at least one thickened
region comprises a curved surface and wherein the two side pieces
comprises curved inner surfaces, whereby the curved surfaces
enclose the at least one reinforcing element.
69. The component of claim 68, wherein the two side pieces contact
at least approximately 180.degree. of a circumferential surface of
the at least one reinforcing element.
70. The component of claim 68, wherein the two side pieces comprise
portions which are arranged parallel to one another, whereby a
spacing between inner surfaces of the two side pieces generally
corresponds to a diameter of the at least one reinforcing
element.
71. The component of claim 49, wherein the at least one thickened
region comprises a panel stiffening base which contains a recess
adapted to receive a reinforcing element.
72. The component of claim 49, further comprising a plurality of
cut-outs arranged in at least one of the bar portion and the two
side pieces, wherein the cut-outs are arranged at regular intervals
"a".
73. The component of claim 49, further comprising a plurality of
through openings arranged in at least one of the bar portion and
the two side pieces, wherein the through openings are arranged at
regular intervals "a".
74. The component of claim 49, further comprising a plurality of
through openings arranged in at least one of the bar portion and
the two side pieces.
75. The component of claim 74, wherein the through openings
comprise a circular through openings.
76. The component of claim 74, wherein the through openings
comprise non-circular through openings.
77. The component of claim 74, wherein the through openings
comprise polygonal through openings.
78. The component of claim 74, wherein the through openings
comprise generally approximately equilateral triangular through
openings with rounded-off corners.
79. The component of claim 78, wherein adjacent triangular through
openings are oriented in opposite directions.
80. The component of claim 74, wherein the through openings of one
of the two side pieces are arranged offset from the through
openings of another of the two side pieces, whereby a distance
between the through openings of each of the two side pieces
comprises a value "a", and whereby a distance between each of the
through openings of one of the two side pieces and each of the
through openings of another of the two side pieces comprises a
value of approximately a/2.
81. The component of claim 49, further comprising a plurality of
stress relief elements arranged within the at least one thickened
region.
82. The component of claim 81, wherein at least one of the
plurality of stress relief elements is arranged on one side of the
bar portion and wherein at least another of the plurality of stress
relief elements is arranged on another side of the bar portion.
83. The component of claim 81, wherein at least one of the
plurality of stress relief elements is arranged near each of the
two separate weld joint zones.
84. The component of claim 81, wherein at least one of the
plurality of stress relief elements comprises a material having a
higher modulus of elasticity and a higher fatigue strength than a
material of the at least one panel.
85. The component of claim 81, wherein at least one of the stress
relief elements comprises a high-strength wire cable.
86. The component of claim 49, wherein the at least one panel
comprises a sheet skin for one of an aircraft, a boat and a
ship.
87. The component of claim 49, wherein the at least one panel
comprises a plurality of integrally formed panel bars.
88. The component of claim 87, wherein the plurality of panel bars
are arranged generally parallel to the at least one stiffening
element.
89. The component of claim 87, wherein the plurality of panel bars
are arranged generally perpendicular to the at least one stiffening
element.
90. The component of claim 87, wherein the plurality of panel bars
are arranged generally parallel to one another and generally
parallel to the at least one stiffening element.
91. The component of claim 87, wherein a height of the panel bars
corresponds to a height of the at least one thickened region.
92. The component of claim 87, wherein the at least one stiffening
element comprises a plurality of stiffening elements which are
spaced apart from one another by an amount equal to an integral
multiple of a spacing "C" of the panel bars.
93. The component of claim 49, wherein the at least one stiffening
element comprises a head which is centrally disposed on the bar
portion.
94. A method of producing the lightweight structural component of
claim 1, the method comprising: milling the at least one metal
panel to form at least one thickened region; and extruding the at
least one metal stiffening element; subjecting the at least one
metal panel to tension; subjecting the at least one metal
stiffening element to tension; and joining the two side pieces to
the at least one thickened region at the two separate weld joint
zones.
95. A method of producing the lightweight structural component of
claim 1, the method comprising: milling the at least one metal
panel to form at least one thickened region; and joining the two
side pieces of the at least one metal stiffening element to the at
least one panel at the two separate weld joint zones.
96. The method of claim 95, further comprising extruding the at
least one stiffening element.
97. The method of claim 95, further comprising subjecting the at
least one panel to tension.
98. The method of claim 95, further comprising subjecting the at
least one stiffening element to tension.
99. The method of claim 95, wherein the joining comprises joining
the two side pieces to the at least one thickened region by laser
beam welding.
100. The method of claim 95, wherein the joining comprises joining
the two side pieces to the at least one thickened region by laser
beam welding, and wherein a laser beam focus is formed such that it
is one of extended in a feed direction and divided into two partial
beams.
101. The method of claim 95, wherein the joining comprises joining
the two side pieces to the at least one thickened region by
friction stir welding.
102. The method of claim 95, wherein the joining comprises
simultaneously joining the two side pieces to the at least one
thickened region.
103. The method of claim 95, wherein the joining comprises
unilaterally joining the two side pieces to the at least one
thickened region.
104. The method of claim 95, wherein the joining comprises joining
the two side pieces one at a time to the at least one thickened
region.
105. The method of claim 95, further comprising, before the
joining, forming the two side pieces by extrusion.
106. The method of claim 95, further comprising extruding the at
least one stiffening element and the two side pieces to form a
one-piece extruded member.
107. The method of claim 95, further comprising forming the at
least one stiffening element as an extruded rib, wherein the two
side pieces comprise inner curved surfaces, and wherein the
thickened region comprises a curved surface.
108. The method of claim 95, wherein the milling comprises chemical
milling.
109. The method of claim 95, wherein the milling comprises
mechanical milling.
110. The method of claim 95, wherein the milling comprises HSC
milling.
111. The method of claim 95, further comprising extruding the at
least one stiffening element and thereafter forming the two side
pieces by splitting, whereby the splitting utilizes press
rollers.
112. The method of claim 95, further comprising extruding the at
least one stiffening element and thereafter forming the two side
pieces by rolling.
113. The method of claim 95, further comprising positioning a
reinforcing element between the two side pieces of the at least one
stiffening element and a surface of the at least one thickened
region.
114. The method of claim 95, further comprising connecting a
reinforcing element to at least one of the two side pieces of the
at least one stiffening element and a surface of the at least one
thickened region.
115. The method of claim 95, further comprising connecting by
mechanical deformation a reinforcing element to at least one of the
two side pieces of the at least one stiffening element and a
surface of the at least one thickened region.
116. The method of claim 115, wherein the mechanical deformation
comprises rolling-in.
117. The method of claim 114, wherein the connecting comprises at
least one of force-locking and form-locking connecting.
118. The method of claim 95, further comprising forming by
co-extrusion the at least one stiffening element and a reinforcing
element.
119. The method of claim 95, further comprising, before the
joining, tensioning at least one of the at least one stiffening
element and the at least one panel.
120. The method of claim 95, further comprising, during the
joining, tensioning at least one of the at least one stiffening
element and the at least one panel.
121. A method of producing the lightweight structural component of
claim 49, the method comprising: milling the at least one metal
panel to form the at least one thickened region; and joining the
two side pieces to the at least one thickened region at the two
separate weld joint zones.
122. A method of producing the lightweight structural component of
claim 49, the method comprising: milling the at least one metal
panel to form the at least one thickened region; forming as a
one-piece member the at least one metal stiffening element and the
two side pieces; and joining the two side pieces to the at least
one thickened region at the two separate weld joint zones.
123. A lightweight structural component comprising: a metal panel
comprising at least one thickened region; at least one stiffening
element coupled to a surface of the at least one thickened region;
the at least one stiffening element being a one-piece metal member
and comprising a head portion, a bar portion and two side pieces
extending from the bar portion; the bar portion comprising a first
thickness; each of the two side pieces comprising a second
thickness; the first thickness being greater than the second
thickness; and end surfaces of the two side pieces being at least
partially connected to the at least one thickened region by two
separate weld joint zones, wherein each of the two side pieces
comprises an outer surface and an inner surface and wherein each
end surface has a width that is narrower than a length of the outer
surface when measured between the bar portion and the end
surface.
124. The component of claim 123, wherein the bar portion and two
side pieces of the at least one stiffening element form a generally
Y-shaped cross-section.
125. The component of claim 123, wherein the bar portion and two
side pieces of the at least one stiffening element form a generally
T-shaped cross-section.
126. The component of claim 123, wherein the at least one
stiffening element has a generally I-shaped cross-section.
127. The component of claim 123, wherein a distance between the two
separate weld joint zones is greater than the first thickness.
128. The component of claim 123, wherein a distance between the two
separate weld joint zones is greater than the second thickness.
129. The component of claim 123, wherein a distance between inner
edges of the two separate weld joint zones is greater than the
first thickness.
130. The component of claim 123, wherein a distance between inner
edges of the two separate weld joint zones is greater than the
second thickness.
131. The component of claim 1, wherein the two separate weld joint
zones are arranged between a thickened region of the panel and the
two side pieces.
132. The component of claim 131, wherein the at least one metal
stiffening element comprises a one-piece metal member.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a continuation application of
parent U.S. patent application Ser. No. 10/757,419 filed on Jan.
15, 2004, the disclosure of which is expressly incorporated by
reference herein in its entirety. The present application also
claims priority under 35 U.S.C. .sctn.119 of German Patent
Application No. 103 01 445.4, filed on Jan. 16, 2003, the
disclosure of which is expressly incorporated by reference herein
in its entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The invention relates to the design and production of
lightweight structural components. Objects in which its application
is expedient and possible are all large-volume lightweight
structures in which an essential part of the bearing pressure
occurs as area load via skin sheets and which are provided with
stiffening elements for load distribution, load diversion,
reduction of deflection or prevention of denting or buckling.
Typically such cases of stressing are particularly marked in many
lightweight structures that are acted on by a pressure difference
between the outside and the inside of the skin sheet in addition to
the structural load. The invention can be used particularly
advantageously for aircraft structures, in particular for fuselage
structures, but also for wing structures, engine intakes, pressure
bulkheads, landing gear shaft covers, etc. Other fields of
application lie in liquid tanks or gas tanks, pressure tanks or
vacuum tanks, components of rockets and rocket engines and fuselage
structures of lightweight watercraft.
[0004] 2. Discussion of Background Information
[0005] Without restricting the generality, the prior art and the
background of the invention will be explained by way of example
based on the construction of aircraft fuselage structures. Usually
aircraft fuselages are produced from riveted panels that are
reinforced by riveted stiffening elements-respectively stringers
running lengthwise along the fuselage pipes and ribs running in the
circumferential direction. Typically a stringer comprises a
suitably formed stringer head, a stringer bar and a stringer base
resting on a panel base at an angle of 90.degree. relative to the
stringer bar, which stringer base is riveted to the panel base.
[0006] The stress on the panel/stiffening element structure is very
complex due to the different load originations and the static and
cyclical loads dependent on many parameters. In the design of
aircraft fuselages constructed in such a way, preset static
strength demands must be met, fatigue strengths taken into account
and safety guaranteed with regard to different preset critical
failure scenarios. In the embodiment of the aircraft fuselage as a
riveted structure, these demands are taken into account through the
location-dependent and loading-dependent selection of the thickness
of panel, stringers, ribs, the shape and the spacing of stringers
or ribs, the dimensions of panel base and rivets and rivet spacing,
etc. The fact that weight-saving potentials in terms of
construction methods have been largely exhausted and that the
production of this type of differential structure is too expensive
because of the limited riveting speed and, in addition, can hardly
be improved on in qualitative terms, have a negative impact on the
design of the conventionally riveted structure.
[0007] It is known, for example, from P. Heider: Lasergerechte
Konstruktion und lasergerechte Fertigungsmittel zum Schwei.beta.en
gro.beta.formatiger Aluminium-Strukturbauteile in:
VDI-Fortschrittsberichte, series 2: Fertigungstechnik, no. 326,
VDI-Verlag Dusseldorf (1994) to replace riveting by welding the
stringer foot to the panel base from both sides simultaneously by
means of two lasers. In order to realize this connection with a
sufficiently well-developed root of the weld seam and in a manner
low in pores, it is necessary for both laser beams to produce a
common melting bath. This is achieved in that the two laser beams,
placed opposite one another, are focused on identical positions
with respect to the joint. Hot cracks are thereby avoided through
the use of suitable wire-shaped weld fillers, such as, e.g., wire
of the alloy AlSi12. Through the very low linear energy of the
process and the energy input symmetrical to the stringer, the
deformation is limited.
[0008] In another embodiment of this principle the construction of
completely welded shell components is possible, including
stringers, ribs, clips, distribution belts and rib heads. See, for
example, P. Brinck et al.: Schalenbauteil fur ein Flugzeug und
Verfahren zur Herstellung, PS DE 198 44 035 C1.
[0009] Despite better static strength and higher rigidity compared
with a riveted connection, the disadvantage of a connection
produced in this way lies in its lower damage tolerance which is
manifested by, e.g., a higher rate of crack growth of a
circumferential crack after crossing the stringer and a lower
residual strength. The reason for this is that, on reaching a
welded-on stiffening element, a crack spreads out into the latter.
Whereas with a conventional differential construction, the crack
growth in the fuselage planking is delayed through the riveted or
adhered reinforcements, such as stringers or ribs, since the crack
tip does not spread into the stiffening elements for a certain
number of load cycles and, moreover, is held together through the
intact reinforcement, in the welded-on stiffening elements the
crack grows in the planking and the stiffening element
simultaneously, without a noticeable crack-delaying effect
occurring. The weight-saving use of laser beam-welded fuselage
shells is thus only possible for fuselage regions for which the
design criteria for damage tolerance do not need to be met, i.e.,
only for the lower shells of the fuselage.
[0010] The reason for this defect or disadvantage is that the known
integral embodiments of the connection of the stiffening elements
does not provide any adequate geometric, stress-related or
microstructural possibilities for stopping a crack, a less damaging
crack branching or an energy dissipation near the crack opening.
The crack can thus spread unhindered into the stiffening
elements.
[0011] Another defect or disadvantage is that the direct tensile
strength of a stringer/panel connection laser beam-welded from both
sides simultaneously decreases with increasing weld seam depth,
i.e., with increasing stringer thickness.
[0012] The reason for this is, i.a., to produce greater weld seam
depths the weld parameters have to be changed such that a greater
linear energy and more unfavorable ratios of wire conveying speed
of the weld filler to welding speed have to be selected. Together
with solid state mechanical influences, both of these lead to a
greater under-matching in the welding zone, a broader overaged
region in the heat affected zone and to an increased risk of
formation of micro-hot cracks.
[0013] To improve the crack growth behavior of shell components
with welded-on stiffening elements, it has become known from, for
example, F. Palm: Metallisches Schalenbauteil, PS DE 199 24 909 C1
to increase the thickness of the bar of the stiffening element near
the welding zone without increasing the connection depth of the
laser beam weld seam made from both sides simultaneously. In other
embodiments of the invention a reduced weld seam depth is made or
notches placed between the weld seam and the increased thickness.
The object of all three measures is to make it more difficult for a
crack to spread in the direction of the stringer head. The crack
can possibly be deflected and can run for a certain distance in the
weld seam or along the weld seam.
[0014] The disadvantage of this solution is that this embodiment is
only geared to the two bay crack type of stress, i.e., the bearing
of a longitudinal or circumferential crack over two rib sections or
stringer divisions. Both for the "tension in the direction of the
head of the stiffening element" type of stress, such as occurs in
the lower fuselage region and for ribs, and for the combined
"bending with bending deflection crosswise to the stiffening
element" and "tension in the direction of the head of the
stiffening element" types of stress, as occurs in the areas of the
fuselage loaded by transverse stress, the proposed solution leads
to a reduction of the bearable loads or to a premature stringer or
rib rupture.
[0015] The reason for the defect is that the two disadvantages of
an integrally welded structure--the lack of an effective mechanism
for delaying cracks and the locally increased crack growth rate in
the weld seam--are combated only with disadvantageous consequences
regarding loading capacity for other types of stress, or cannot be
combated at all.
[0016] An embodiment of a welded arrangement of panel and
reinforcing elements that achieves an increase in residual strength
and thus is also intended to render possible the use of welded
fuselage shells for the side and upper shell area of the fuselage
is known from H. J. Schmidt (PS DE 100 31 510 A1). To this end
reinforcements are applied to the stiffening elements before the
laser beam-welding. The reinforcements can thereby be arranged as
doubler plates or as tension bands.
[0017] The doubler plates comprise high-strength Al alloys or
fiber-reinforced metal laminates and are attached by riveting or an
adhesive bond. The doubler plates must thereby be an adequate
distance from the weld seam, which distance is determined by the
temperature field of the welding process. The tension bands
comprise high-strength steel alloys or titanium alloys or fiber
composites and are inserted and twisted into through bores that are
to be made beforehand. Cross section reinforcements are provided in
the lower bar area of the stiffening element to contain the through
bores. Another variant provides embodying the lower bar area in a
slotted manner, inserting the tension band through a mounting
opening in the slotted lower bar area and connecting the inserted
tension band to the tension band in a form-locking manner by
compressing the slotted bar area and subsequently connecting it to
the skin sheet by laser beam welding in a known manner.
[0018] An increase in residual strength is achieved through the
crack-delaying effect of the reinforcements. This occurs in that
the number of the load cycles necessary for the complete severance
of the stiffening element is increased and the reinforcing element
does not fail until after the stiffening element. Through the
latter effect the reinforcing element can reduce the crack opening
angle and reduce the tensions at the crack tip for the period
between the failure of the stiffening element and the failure of
the reinforcing element.
[0019] The embodiment of the weld seam itself is not changed with
respect to the previously known prior art. This means that the
stringer or rib foot is attached to the skin sheet across its
entire width by a single weld seam, whereby the weld seam is
produced by laser beam welding from both sides simultaneously.
[0020] One defect of the arrangement is that it is not suitable for
improving the prior art with regard to the two critical stress
types "tension in the direction of the head of the stiffening
element" and "bending in the direction crosswise to the stiffening
element." The danger of static failure as a result of the
separation of the stiffening element from the panel, in particular
during transverse stress in the side shells, therefore remains.
[0021] The reason for this is that the unchanged weld seam
arrangement and weld seam vicinity cannot bear any greater direct
tensile stress or bending stress crosswise to the longitudinal
directions of the stiffening elements.
[0022] Moreover, it has a disadvantageous effect that the
reinforcements of the stiffening elements do not reduce the local
crack growth rate in the weld seam and its direct vicinity. This
applies in particular to stress types such as, e.g., transverse
stress in which there is a danger of a crack spreading along the
weld seam.
[0023] The reason for this is that, for reasons determined by the
process and the arrangement, they have to be installed at a
distance from the weld seam at which their effect in terms of
stress relief for the weld seam is very slight.
[0024] Another defect is that the reinforcing elements cannot be
applied to or inserted in the stiffening elements in an economic
manner.
[0025] The reason for this is that additional production steps,
such as, e.g., riveting the doubler plates, adhering the doubler
plates or drilling very long through bores are necessary to apply
the reinforcements, which steps are in themselves very expensive or
in part even more expensive than the riveting of the stiffening
elements to the panel which is to be replaced.
[0026] Furthermore, the fact that the variant with inserted tension
bands is not suitable for ribs has a disadvantageous effect. The
reason for this lies in the impossibility of drilling a curved slot
or of bending together the two side pieces of the slotted lower bar
area after inserting the tension band in a plastic manner without
damage to the materials or a permanent deformation of the entire
stiffening element.
SUMMARY OF THE INVENTION
[0027] The invention is directed to a new kind of lightweight
structural component in particular for aircraft and a method for
its effective and lower-cost production that is also suitable for
complex stress types. It is applicable for both straight and curved
stiffening elements, features an improved damage tolerance, direct
tensile strength, transverse stress loading capacity and bending
resistance, and can be used even with thicker stiffening elements.
Moreover, it does not require expensive additional separate
production steps.
[0028] The invention is also directed to a lightweight structural
component that, despite integral embodiment, features a
differential failure behavior, that leads to reduced load stresses
and strains in the joint zone and its immediate vicinity and that
can be produced simply with modern manufacturing methods.
[0029] The invention also takes account of the following: [0030]
The embodiments of integral lightweight structures, and here in
particular aircraft fuselage structures, known according to the
prior art, do not adequately exploit the possibilities of jointing
technology due to a constructional design not suitable for welding;
[0031] It is also possible to execute integral structures with
locally effective elements that avoid the weld seam weak point and
produce a differential failure scenario; and [0032] A sufficiently
faultless welding is possible even without a laser beam-welding
from both sides simultaneously with a common melting bath, if new
laser beam sources with the highest beam quality and suitable,
process-adapted beam formation (twin spot or elliptical beam) and
suitable weld fillers are used.
[0033] According to one aspect of the invention, in contrast to all
previous solutions known according to the prior art, the bar of the
stiffening element on its side facing the skin sheet comprises not
one foot but two spatially separate side pieces, both of which are
connected to the panel in a material-locking manner by way of two
separate joint zones. This arrangement has the advantages that with
the same weight a clearly stiffer arrangement is realized which,
compared with the previous solution, reduces the mechanical stress
due to direct tension and bending in the weld seams, reacts in a
less sensitive manner to welding defects and is much less demanding
in terms of the requirements of precision control of the two laser
beams relative to one another.
[0034] For higher stresses, such as those that normally prevail,
e.g., in aircraft construction, the panel is embodied so that it
features a thickening in the region of the junction points of the
joint zones.
[0035] The stiffening elements can be embodied as stringers or as
ribs in the embodiment of the structural component according to the
invention.
[0036] The inventive concept is not limited to the joint zone
necessarily being a laser weld seam. The joint zones can just as
well comprise friction stir welded zones or adhered zones.
[0037] Through a local thickening of the skin sheet-here embodied
as a skin stiffening base--the crack growth rate in the
"circumferential crack with broken stringer" stress type can be
clearly reduced while crossing the stringer. Through the supporting
effect of the locally thickened skin sheet in particular the
loading of the weld seam is reduced and the crack growth rate in
the first side piece is reduced. Moreover, the now divided crack
tip has to travel greater distances on several paths until the
second stringer side piece and the entire stringer bar and stringer
head are severed.
[0038] The invention also provides for an advantageous design of
the geometric dimensions of the stiffening element and skin sheet
arrangement.
[0039] The invention can also prove useful for all types of stress
environments in which the stiffening elements can be arranged
outside. In this case the weld seam is not stressed by radial
tensile forces. Through the branching of the crack after it crosses
the weld seam, the crack growth rate is locally reduced compared
with the prior art.
[0040] The inventive concept does not relate only to the heads of
the stiffening elements being embodied in the classic L-shape.
Without violation of the inventive concept, the stiffening profile
can also be embodied, e.g., as a U-profile or as a profile similar
to a U-profile. In this form, the head side of the U-profile can be
connected particularly well to potentially necessary attachment
parts. Embodiments disclosed herein further develop the geometric
shape of the stiffening element for this case.
[0041] The invention also introduces a new additional variant for
improving damage tolerance. They provide the arrangement of
cut-outs within the side pieces of the stiffening element. If a
running crack runs into one of these openings, it can be stopped.
The reason for this is that the very high stress intensity factor
at the crack tip is replaced by the lower notched form factor of
the cut-out after the crack leads into the cut-out. In fact, this
is equivalent to the necessity of a new start of the crack in a
stress field with a lower stress concentration.
[0042] It is known from experiments that once a primary crack has
formed under the stress conditions of the two bay crack criterion,
it is very hard to deflect it from its general crack growth
direction. It can therefore happen that with a conventional
embodiment of the stringer, the crack runs between the two cut-outs
and is even accelerated for a short time due to the locally lower
supporting effect. This is taken into account by the staggered
arrangement of the cut-outs in the two side pieces. For the lower
fuselage region these cut-outs can advantageously be used as
drainage openings at the same time.
[0043] The fact that the load stresses are not distributed
homogenously in an aircraft fuselage is allowed for in an
advantageous manner. Thus in particularly highly stressed areas the
crack growth rate in the stringer foot can be further reduced
through the local application of a doubler plate made of a
damage-tolerant, fiber-reinforced laminate. In particular in the
case that the stiffening element is a rib, the reduction of the
structural static load capacity can be compensated for by the
cut-outs during tensile stress on the rib bar.
[0044] The invention also permits a new approach to reducing the
load stress in the joint zones. In the region of the upper shells
of the fuselage, the joint zones are subjected to a high tensile
stress that can be clearly reduced by stress relief elements
arranged in their direct vicinity.
[0045] Disclosed embodiments also advantageously utilize the
finding that material accumulations can reduce the crack growth in
the panel in a particularly effective manner if they are located in
the direct vicinity of the panel. They contribute less to avoiding
buckling because of their lower moment of resistance, but they are
sufficient to be able to somewhat enlarge the spacing of the
stiffening elements and thus to save weight generally. Moreover,
they are able to improve the acoustic behavior of the fuselage.
[0046] The invention also provides that if heads are arranged
asymmetrically to the longitudinal axis of the stiffening elements,
the stiffening elements deform crosswise to their longitudinal
direction under tensile stress, compressive stress or transverse
stress and thus generate high bending stresses in the weld
seam.
[0047] The invention also provides an arrangement for a lightweight
structural component in which a reinforcing element is located in
the cavity that is formed by the two side pieces of the stiffening
element and the skin sheet, which reinforcing element comprises a
material with a much higher modulus of elasticity than the skin
sheet and the stiffening elements and which is connected to at
least one of the partners stiffening element or skin sheet in a
form-locking and/or force-locking manner. During an elongation of
the panel in the direction of the longitudinal axis of the
stiffening elements, the force-locking and/or form-locking
connection between the reinforcing element and the stiffening
element or the skin sheet reduces the elongation in the foot area
of the stiffening element and thus in the joint zones. Thus stress
on the two weld seams is relieved such that despite a
microstructure in the weld zone that promotes the crack growth, a
locally reduced crack growth rate results. Since the fatigue
strength of the reinforcing elements that have not begun to crack
is much greater than the crack-spreading stress in the skin sheet
and stiffening element, the reinforcing element still remains
intact even when the crack has crossed both side pieces of the
stiffening elements and the panel stiffening base. In addition to a
reduction of the crack growth rate, the residual strength is also
increased. In comparison with the prior art it has a positive
impact in that the reinforcing element is located in the direct
vicinity of the weld seam and thus can effectively reduce the
stress concentration during the approach of the crack to the joint
location between the panel and stiffening element. Thus the
arrangement of the reinforcing elements according to the invention
is suitable for avoiding or reducing the disadvantages of an
integral structure with respect to damage tolerance.
[0048] The invention also provides process steps for producing the
lightweight structural component according to the invention.
Embodiments relate to the use of laser beam welding as the most
favorable process variant. In one embodiment the experience is
utilized that the development of welding defects (pores, discharge)
can be reduced with laser beam welding of aluminum by a suitable
beam formation.
[0049] The inventive concept further contemplates the joining to
take place by friction stir welding or adhesion.
[0050] The invention also provides a process that saves cycle time
by way of joining from both sides simultaneously. However, in
contrast to previous assumptions it is also possible to produce
sufficiently faultless weld seams with weld seams welded
unilaterally in succession and located separately from one another.
The fact that it is thus possible to omit the simultaneous welding
on both sides while forming a common melting bath renders possible
the constructional free spaces for the embodiment according to the
invention of the lightweight structural component as well as a
simplified process cycle.
[0051] The invention also provides for methods with which the
stiffening elements according to the invention can be mechanically
produced in a non-cutting manner in a particularly favorable way.
According to one embodiment extrusion is used as a very
cost-effective method of producing the stiffening elements
including their bars. If the stiffening element is embodied as a
rib, this results in the difficulty that in the case of, e.g., an
aircraft fuselage, it has to be embodied in a curved manner. Such
curved semi-finished products can also be produced in a very
favorable manner, if during extrusion a transverse force is exerted
on the semi-finished product immediately after the extrusion die.
If the height of the stiffening element is too great in relation to
its thickness, it is more favorable to produce the two side pieces
by splitting by way of pressure rollers.
[0052] One process step is the production of the force-locking
and/or form-locking connection between the stiffening element
and/or skin sheet with the reinforcing element. The invention also
provides for favorable variants for this.
[0053] The invention also provides for a lightweight structural
component in particular for aircraft comprising at least one skin
sheet and stiffening elements arranged lengthwise or crosswise or
lengthwise and crosswise thereon, which stiffening elements are
connected completely or at least partially to the skin sheet
respectively by their foot in a material-locking manner, wherein
the bar of the stiffening element on its side facing the skin sheet
is composed of two side pieces that are both connected to the panel
in a material-locking manner by way of two separate joint
zones.
[0054] The panel may feature a thickening in the region of the
connection points of the joint zones. The stiffening elements may
be embodied as stringers running lengthwise. The stiffening
elements may be embodied as ribs running in the circumferential
direction. The separate joint zones may be laser beam weld zones.
The separate joint zones may be friction stir weld zones. The
separate joint zones may be adhered joint zones.
[0055] A panel stiffening base may be located between the inner
surfaces of the side pieces, the thickness of which panel
stiffening base d.sub.Hv is greater than the thickness d.sub.Hs of
the panel base and whose side surfaces are designed such that they
rest on the inner surfaces of the side pieces and the two joint
zones are embodied such that they extend up to the side surfaces of
the panel stiffening base. The two side pieces may be bent by a
total angle .alpha. so that the inner surfaces of the two side
pieces and the surface of the skin stiffening base form an
isosceles triangle and the total angle .alpha. lies in the range
7.degree..ltoreq..alpha..ltoreq.50.degree.. The following ratios
apply for the dimensions of the stiffening element: the ratio
between the side piece thickness in the plane of the joint zone
t.sub.s and the thickness of the stiffening element d.sub.s is
0.5.ltoreq.t.sub.s/d.sub.s.ltoreq.1.8; the ratio between side piece
length s.sub.s and the height of the stiffening element h.sub.s is
0.15.ltoreq.s.sub.s/h.sub.s.ltoreq.0.7; the ratio of the side piece
thickness near the branching of the two side pieces of the
stiffening element b.sub.s0 and the side piece thickness in the
plane of the joint zone t.sub.s is
0.28.ltoreq.b.sub.s0/t.sub.s.ltoreq.1; the angle .beta. between the
panel and the joint surface of the joint zone is
0.degree..ltoreq..beta..ltoreq.25.degree..
[0056] The two side pieces may be bent at a total angle
.alpha.=180.degree. so that the inner surfaces of the two side
pieces rest on the surface of the panel base. The stiffening
element may be formed from a generally U-profile, whereby the two
side pieces extend directly up to the head of the stiffening
element. The head of the stiffening element may extend on both
sides over the side pieces of the U-profile which run parallel. The
two areas of the head of the stiffening element extending over the
side pieces may each feature an edge area pointing in the direction
of the skin sheet. The panel reinforcing base may be embodied in a
divided manner and that the two lateral outer surfaces of the panel
reinforcing base rest on the inner surfaces of the side pieces. The
two joint surfaces and the outer sides of the panel stiffening base
or feature a surface may be produced by metal cutting.
[0057] Cut-outs may be located in the two side pieces, which
cut-outs are arranged at intervals a along the side pieces. The
distance between the edging of the cut-outs and the joint surfaces
may be greater than one and a half times the side piece thickness
t.sub.s in the plane of the joint zone. The cut-outs may be
arranged displaced respectively by the distance a/2. The cut-outs
may feature a cylindrical form. The cut-outs may feature the form
of equal-sided or virtually equal-sided triangles with rounded off
corners, whereby the cut-outs are arranged along the side pieces so
that the vertices of the triangles point alternately in the
direction of the panel and in the direction of the head of the
stiffening element.
[0058] A doubler plate may be made of a damage-tolerant,
fiber-reinforced laminate is attached on both outer surfaces of the
two side pieces of the stiffening element. One to five stress
relief elements may be located inside the panel base symmetrical to
the bar of the stiffening element and near the joint zones, which
stress relief elements comprise a material with a much higher
modulus of elasticity and higher fatigue strength than the material
of the skin sheet. The stress relief elements may be made of
high-strength wire cables. One stress relief element may be located
directly beneath the panel stiffening base. The panel stiffening
base may be made of the material expediently deformed during the
rolling-in of the stress relief element. The panel bars may be
located on the panel parallel or perpendicular or perpendicular and
parallel to the reinforcing elements. The height of the panel bars
may correspond to the height of the panel stiffening base and the
spacing of the stiffening elements on the skin base may be an
integral multiple of the spacing of the panel bars. The head of the
stiffening element may be embodied symmetrically and may be
arranged centrally on the bar of the stiffening element.
[0059] The invention also provides for a lightweight structural
component in particular for aircraft, comprising at least one skin
sheet and stiffening elements arranged thereon lengthwise or
crosswise or lengthwise and crosswise. The stiffening elements are
connected completely or at least partially to the skin sheet
respectively by their foot in a material-locking manner. Each bar
of the stiffening elements on its side facing the skin sheet is
made of two side pieces, both of which are connected in a
material-locking manner to the panel by way of two separate joint
zones. The panel features a thickening in the region of the
connection points of the joint zones. A reinforcing element is
located in the cavity formed by the two side pieces and the panel
stiffening base, which reinforcing element comprises a
high-strength material with a modulus of elasticity that is greater
than the modulus of elasticity of the materials of the skin sheet
or of the stiffening elements. The reinforcing element is connected
to the two side pieces and/or the panel stiffening base in a
force-locking and or form-locking manner.
[0060] The stiffening elements may be embodied as stringers running
lengthwise. The stiffening elements may be embodied as ribs running
in the circumferential direction. The separate joint zones may be
laser beam weld zones. The separate joint zones may be friction
stir weld zones. The separate joint zones may be adhered joint
zones.
[0061] The two joint surfaces and the outer sides of the panel
stiffening base may feature a machined surface.
[0062] A combined force-lock and form-lock may be realized in that
the surface of the reinforcing element features a roughening or a
surface profiling, the impression of which is on the two inner
surfaces of the two side pieces and/or the surface of the panel
reinforcing base.
[0063] The cavity formed by the two side pieces and the panel
stiffening base, and the cross section of the reinforcing element,
may form an equal isosceles triangle with a rounded-off apex.
[0064] The reinforcing element may be embodied as a wire or a pipe,
the panel stiffening base is embodied as a circle segment with the
wire or pipe diameter, and the branching of the two side pieces at
the foot of the stiffening element is embodied such that it
encloses the wire or the pipe at a looping angle of approx.
180.degree. and the two side pieces lie parallel to one another,
whereby the spacing of their two inner surfaces corresponds to the
diameter of the wire or pipe.
[0065] The panel stiffening base may contain a recess to accept the
reinforcing element. The bar or the side pieces of the stiffening
element may feature cut-outs that are arranged along the bar or
along the side pieces at intervals "a". The cut-outs may be
embodied in a circular manner. The cut-outs may feature the shape
of equilateral or almost equilateral triangles with rounded-off
corners, whereby the triangles are arranged along the bar or the
two side pieces such that one apex of the triangles points
alternately in the direction of the panel and in the direction of
the head of the stiffening element. The cut-outs may be arranged
and/or displaced by the distance a/2 respectively.
[0066] Two or four stress relief elements may be located inside the
panel base symmetrical to the bar of the stiffening element and
near the joint zones, which stress relief elements are composed of
a material with a much higher modulus of elasticity and higher
fatigue strength than the material of the skin sheet. The stress
relief elements may be composed of high-strength wire cables.
[0067] Panel bars may be located on the panel parallel or
perpendicular or parallel and perpendicular to the reinforcing
elements. The height of the panel bars may correspond to the height
of the panel stiffening bases and the spacing of the stiffening
elements on the skin sheet is an integral multiple of the spacing c
of the panel bars. The head of the stiffening element may be
embodied symmetrically and may be arranged centrally on the bar of
the stiffening element.
[0068] The invention also provides for a method for producing a
lightweight structural component, in particular for aircraft, as
described above, and made by the following stages: chemical or
mechanical milling to make the thickening of the skin sheet,
extrusion of the stiffening elements, tensioning of the panel;
symmetrical positioning of the stiffening element on the thickening
of the skin sheet; tensioning of the stiffening element to realize
a flat configuration of the joint surfaces; and joining of the
stiffening element to the skin sheet by way of two separate joint
zones with at least local mechanical tension.
[0069] The joining may be carried out by way of laser beam welding.
The laser beam focus may be formed such that it is extended in the
feed direction or divided into two partial beams.
[0070] The joining may be carried out by way of friction stir
welding. The joining may be carried out by adhesion. The joining of
the two side pieces or of the stiffening element to the skin sheet
may be carried out from both sides simultaneously.
[0071] The two side pieces of the stiffening element may be joined
to the skin sheet unilaterally in succession. The two side pieces
may be embodied with the aid of and during extrusion. The
stiffening element may be a rib, the rib is extruded with such a
radius that the radius that is featured by the two undersides of
the side pieces corresponds to the radius of the inside of the
panel base. The stiffening element may be conventionally extruded
and the two side pieces may be produced by a subsequent splitting
by way of press rollers.
[0072] Before the positioning of the stiffening element on the skin
sheet, the reinforcing element may be inserted between the side
pieces of the stiffening element or in the recess of the panel
stiffening base and is connected to the stiffening element or the
panel stiffening base in a form-locking and/or force-locking manner
by way of a mechanical deformation. The mechanical deformation may
be carried out by rolling-in. The force-locking and/or form-locking
connection between the stiffening element and the reinforcing
element may be produced by coextrusion. The mechanical deformation
to produce the force-locking and/or form-locking connection between
the stiffening element and reinforcing element may be effected by
tensioning technology directly before the joining process or in the
course of the joining process.
[0073] The invention also provides for a lightweight structural
component comprising at least one panel, at least one stiffening
element oriented one of lengthwise and crosswise, the at least one
stiffening element comprising two side pieces, and each of the two
side pieces being at least partially connected to the panel in a
material-locking manner, wherein the two side pieces are connected
to the panel at two separate joint zones.
[0074] The component may be utilized in an aircraft. The at least
one panel may comprise a skin sheet. The at least one panel may
comprise a thickened region in an area of the two separate joint
zones. The at least one stiffening element may comprise a stringer
which is oriented in a lengthwise manner. The at least one
stiffening element may comprise a rib running in a circumferential
direction. The two separate joint zones may comprise laser beam
weld zones. The two separate joint zones may comprise friction stir
weld zones. The two separate joint zones may comprise adhered to
joint zones. The two separate joint zones may comprise adhesive
bonded joint zones. The at least one panel may comprise a panel
stiffening base having an outer portion and an inner portion
arranged between inner surfaces of the two side pieces.
[0075] The panel stiffening base may comprise a thickness d.sub.Hv
of the inner portion is greater than a thickness d.sub.Hs of the
outer portion and wherein side surfaces of the inner portion rest
against inner surfaces of the two side pieces. The two separate
joint zones may respectively extend at least partially up to the
side surfaces of the inner portion. The two side pieces may be bent
away from each other by a total angle .alpha., whereby inner
surfaces of the two side pieces and a surface of the at least one
panel form a generally isosceles triangle. The angle .alpha. may
lie in a range of between approximately 7.degree. and approximately
50.degree..
[0076] The at least one stiffening element may comprise the
following: a ratio between a side piece thickness t.sub.s in a
plane of each joint zone and a thickness d.sub.s of the at least
one stiffening element comprises approximately
0.5.ltoreq.t.sub.s/d.sub.s.ltoreq.approximately 1.8; a ratio
between each side piece length s.sub.s and a height h.sub.s of the
at least one stiffening element comprises approximately
0.15.ltoreq.s.sub.s/h.sub.s.ltoreq.approximately 0.7; and an angle
.beta. between the panel and each joint surface of each joint zone
comprises approximately
0.degree..ltoreq..beta..ltoreq.approximately 25.degree..
[0077] The at least one stiffening element further comprises the
following: a ratio of each side piece thickness b.sub.s0 near a
branching of the two side pieces and a side piece thickness t.sub.s
in a plane of each joint zone comprises approximately
0.28.ltoreq.b.sub.s0/t.sub.s.ltoreq.approximately 1.
[0078] The two side pieces may be bent or oriented at a total angle
.alpha. of approximately 180.degree., whereby inner surfaces of the
two side pieces rest on a surface of at least one panel. The two
side pieces may be integrally formed with the at least one
stiffening element, whereby the at least one stiffening element and
the two side pieces comprise a one-piece member. The two side
pieces may be integrally formed with the at least one stiffening
element, whereby the at least one stiffening element and the two
side pieces comprise a one-piece member. The at least one
stiffening element may comprise a generally U-shaped profile,
whereby the two side pieces are arranged on opposite ends of a head
of the at least one stiffening element. The two side pieces may be
of the generally U-shaped profile and may be parallel to each
other. The at least one stiffening element may comprise an edge
area which is oriented in a generally parallel manner relative to
the at least one panel. The at least one panel may comprise a panel
reinforcing base portion which comprises a first base portion and a
second base portion separated from the first base portion, wherein
lateral outer surfaces of the first and second base portions rest
against inner surfaces of the two side pieces.
[0079] An area of the at least one panel may comprise the two joint
zones wherein each joint zone comprises surfaces formed by metal
cutting. An area of the at least one panel may comprise the two
joint zones which each comprise surfaces formed by metal removal.
At least one of the two side pieces may comprise cut-outs. At least
one of the two side pieces may comprise a plurality of through
openings. Each of the two side pieces may comprise cut-outs and the
cut-outs may be arranged at generally regular intervals "a". Each
of the two side pieces may comprise through openings arranged at
generally regular intervals "a". A distance between an edge of the
through openings and joint surfaces of the two joint zones may be
greater than approximately one and a half times a side piece
thickness t.sub.s measured in a plane of each joint zone. The
through openings in one of the two side pieces may be spaced from
each other by a distance "a" and wherein the through opening of the
other of the two side pieces are spaced from the through opening of
the one of the two side pieces by a distance of approximately a/2.
The through openings may comprise circular openings. The through
openings may comprise polygonal openings. The through openings may
comprise non-circular openings. The through openings may comprise
triangular openings. The triangular openings may comprise
approximately equal-sided triangular openings with rounded corners,
whereby vertices of adjacent triangular openings point in opposite
directions.
[0080] The component may further comprise a doubler plate made of a
damage-tolerant fiber-reinforced laminate attached on outer
surfaces of each of the two side pieces.
[0081] The component may further comprise at least one stress
relief element located inside the at least one panel. The at least
one panel may comprise a thickened panel base arranged in an area
of the two separate joint zones and the at least one stress relief
element may be arranged within the thickened panel base. The at
least one stress relief element may be arranged directly beneath a
bar portion of the at least one stiffening element and between the
two separate joint zones. The at least one stress relief element
may comprise a material with a higher modulus of elasticity and a
higher fatigue strength than a material of the at least one panel.
The at least one stress relief element may comprise a plurality of
stress relief elements. The at least one stress relief element may
comprise a plurality of spaced apart stress relief elements. The at
least one stress relief element may comprise a high-strength wire
cable. The at least one stress relief element may be located
directly beneath a panel stiffening base of the at least one panel
and is centrally disposed between the two separate joint zones. The
panel stiffening base may be integrally formed with the at least
one panel, whereby the panel stiffening base and the at least one
panel comprise a one-piece member.
[0082] The at least one panel may comprise a panel stiffening base
made of material that is deformed during a rolling-in of a stress
relief element into the panel. The at least one panel may comprise
a panel stiffening base made of material that is deformed during a
rolling of the panel. The at least one panel may comprise a
plurality of panel bars arranged generally parallel to one another
and perpendicular to the at least one stiffening element. The at
least one panel may comprise a plurality of panel bars arranged
generally parallel to one another and generally perpendicular to
the at least one stiffening element. The at least one panel may
comprise a plurality of panel bars, some of which are arranged
generally parallel to one another and some of which are arranged
generally perpendicular to one another. The at least one panel may
comprise a plurality of panel stiffening bases and a plurality of
panel bars, wherein a height of the panel bars corresponds to a
height of the panel stiffening bases, wherein the at least one
stiffening element comprises a plurality of stiffening elements,
and wherein a spacing between the stiffening elements is generally
equal to an integral multiple of a spacing "C" between the panel
bars. The at least one stiffening element may comprise a head
portion that is coupled to a bar portion. The head portion may
project from both sides of the bar portion. The head portion may
project by generally equal amounts from both sides of the bar
portion.
[0083] The invention also provides for a lightweight structural
component comprising at least one panel comprising at least one
thickened region, at least one stiffening element arranged on the
at least one panel in at least one of a lengthwise and a crosswise
direction, the at least one stiffening element comprising a bar
portion and two side pieces, each of the two side pieces being at
least partially connected in a material-locking manner to the at
least one thickened region by two separate joint zones.
[0084] Each of the two side pieces may instead be non-removably
and/or fixedly secured to the at least one thickened region by two
separate joint zones. The component may further comprise a
reinforcing element located in a cavity formed by the two side
pieces and a surface of the thickened region. The at least one
thickened region may comprise a panel stiffening base and the
reinforcing element may comprise a high-strength material having a
modulus of elasticity that is generally greater than a modulus of
elasticity of a material of at least one of the at least one panel
and the at least one stiffening element.
[0085] The reinforcing element may be connected to at least one of
the two side pieces and the at least one panel stiffening base in
one of a force-locking manner and a form-locking manner. The
component may be arranged on an aircraft. The at least one
stiffening element may comprise a stringer running in a lengthwise
direction. The at least one stiffening element may comprise a rib
running in a circumferential direction. The two separate joint
zones may comprise laser beam weld zones. The two separate joint
zones may comprise friction stir weld zones. The two separate joint
zones may comprise adhered or adhesion joint zones. The two
separate joint zones may comprise adhesive bonded joint zones. The
two joint zones may comprise panel surfaces and surfaces of the two
side pieces, and wherein each of the panel and two side piece
surfaces comprises a machined surface.
[0086] The reinforcing element may comprise surfaces which are both
force-locked and form-locked to at least one of inner surfaces of
the two side pieces and a surface of the thickened region. The
surfaces may comprise one of a rough profile and surface profiling.
The reinforcing element may comprise surfaces which are fixed to at
least one of inner surfaces of the two side pieces and a surface of
the thickened region.
[0087] The component may further comprise a cavity formed by the
two side pieces and the thickened region and a reinforcing element
arranged within the cavity. The cross-sectional shape of the cavity
may generally correspond to the cross-sectional shaped of the
reinforcing element. The cavity may comprise a cross-sectional
shape having a form of a generally equal isosceles triangle with a
rounded-off apex. The reinforcing element may comprise a
cross-sectional shape having a form of a generally equal isosceles
triangle with a rounded-off apex.
[0088] The component may further comprise at least one reinforcing
element arranged within the thickened region. The component may
further comprise at least one reinforcing element arranged between
the two side pieces, wherein the at least one reinforcing element
comprises one of a wire, a wire rope, a pipe and a tube. The at
least one thickened region comprises a curved surface and wherein
the two side pieces comprises curved surfaces, whereby the curved
surfaces enclose the at least one reinforcing element. The two side
pieces may contact at least approximately 180.degree. and/or half
of the outer surface of the at least one reinforcing element. The
two side pieces may comprise portions which are arranged parallel
to one another, whereby a spacing between inner surfaces of the two
side pieces generally corresponds to a diameter of the at least one
reinforcing element. The at least one thickened region may comprise
a panel stiffening base which contains a recess adapted to receive
a reinforcing element.
[0089] The component may further comprise a plurality of cut-outs
arranged in at least one of the bar portion and the two side
pieces, wherein the cut-outs are arranged at regular intervals
"a".
[0090] The component may further comprise a plurality of through
openings arranged in at least one of the bar portion and the two
side pieces, wherein the through openings are arranged at regular
intervals "a". The component may further comprise a plurality of
through openings arranged in at least one of the bar portion and
the two side pieces. The through openings may comprise a circular
through openings. The through openings may comprise non-circular
through openings. The through openings may comprise polygonal
through openings. The through openings may comprise generally
approximately equilateral triangular through openings with
rounded-off corners. Adjacent triangular through openings may be
oriented in opposite directions. The through openings of one of the
two side pieces may be arranged offset from the through openings of
another of the two side pieces, whereby a distance between the
through openings of each of the two side pieces comprises a value
"a", and whereby a distance between each of the through openings of
one of the two side pieces and each of the through openings of
another of the two side pieces comprises a/2.
[0091] The component may further comprise a plurality of stress
relief elements arranged within the thickened region. At least one
of the plurality of stress relief elements may be arranged on one
side of the bar portion and at least another of the plurality of
stress relief elements may be arranged on another side of the bar
portion. At least one of the plurality of stress relief elements
may be arranged near each of the two separate joint zones. At least
one of the plurality of stress relief elements may comprise a
material having a higher modulus of elasticity and a higher fatigue
strength than a material of the at least one panel. At least one of
the stress relief elements may comprise a high-strength wire cable.
The at least one panel may comprise a sheet skin for one of an
aircraft, a boat and a ship. The at least one panel may comprise a
plurality of panel bars. The plurality of panel bars may be
arranged generally parallel to the at least one stiffening element.
The plurality of panel bars may be arranged generally perpendicular
to the at least one stiffening element. The plurality of panel bars
may be arranged generally parallel and generally perpendicular to
the at least one stiffening element. A height of the panel bars may
correspond to a height of the thickened region. The at least one
stiffening element may comprise a plurality of stiffening elements
which are spaced apart from one another by an amount equal to an
integral multiple of a spacing "C" of the panel bars.
[0092] The at least one stiffening element may comprise a head
which is centrally arranged on the bar portion.
[0093] The invention also provides for a method of producing the
lightweight structural component of the type described above,
wherein the method comprises milling the at least one panel to form
at least one thickened region and joining the two side pieces to
the at least one panel at the two separate joint zones.
[0094] The method may further comprise extruding the at least one
stiffening element. The method may further comprise subjecting the
at least one panel to tension. The method may further comprise
subjecting the at least one stiffening element to tension.
[0095] The invention also provides for a method of producing the
lightweight structural component of type described above, wherein
the method comprises milling the at least one panel to form at
least one thickened region, extruding the at least one stiffening
element, subjecting the at least one panel to tension, subjecting
the at least one stiffening element to tension, and joining the two
side pieces to the thickened region at the two separate joint
zones.
[0096] The joining may comprise joining the two side pieces to the
at least one thickened region by laser beam welding. The joining
may comprise joining the two side pieces to the at least one
thickened region by laser beam welding, and wherein a laser beam
focus is formed such that it is one of extended in a feed direction
and divided into two partial beams. The joining may comprise
joining the two side pieces to the at least one thickened region by
friction stir welding. The joining may comprise joining the two
side pieces to the at least one thickened region by adhesion. The
joining may comprise joining the two side pieces to the at least
one thickened region by adhesive bonding. The joining may comprise
simultaneously joining the two side pieces to the at least one
thickened region. The joining may comprise unilaterally joining the
two side pieces to the at least one thickened region. The joining
may comprise joining the two side pieces one at a time to the at
least one thickened region. The two side pieces may be formed by
extrusion.
[0097] The method may further comprise extruding the at least one
stiffening element and the two side pieces to form a one-piece
extruded member. The method may further comprise forming the at
least one stiffening element as an extruded rib, wherein the two
side pieces comprise inner curved surfaces, and wherein the
thickened region comprises a curved surface.
[0098] The milling may comprise chemical milling. The milling may
comprise mechanical milling. The milling may comprise HSC
milling.
[0099] The method may further comprise extruding the at least one
stiffening element and thereafter splitting the two side pieces by
splitting using press rollers. The method may further comprise
extruding the at least one stiffening element and thereafter
forming the two side pieces by rolling. The method may further
comprise positioning a stiffening element between the two side
pieces of the at least one stiffening element and a surface of the
at least one thickened region.
[0100] The method may further comprise connecting a stiffening
element to at least one of the two side pieces of the at least one
stiffening element and a surface of the at least one thickened
region. The method may further comprise connecting by mechanical
deformation a stiffening element to at least one of the two side
pieces of the at least one stiffening element and a surface of the
at least one thickened region. The mechanical deformation may
comprise rolling-in. The connecting may comprise at least one of
force-locking and form-locking connecting.
[0101] The method may further comprise forming by co-extrusion the
at least one stiffening element and a reinforcing element. The
method may further comprise, before the joining, tensioning at
least one of the at least one stiffening element and the at least
one panel. The method may further comprise, during the joining,
tensioning at least one of the at least one stiffening element and
the at least one panel.
[0102] The invention also provides for a method of producing the
lightweight structural component of the type described above,
wherein the method comprises milling the at least one panel to form
the at least one thickened region and joining the two side pieces
to the thickened region at the two separate joint zones.
[0103] The invention also provides for a method of producing the
lightweight structural component of the type described above,
wherein the method comprises milling the at least one panel to form
the at least one thickened region, forming as a one-piece member
the at least one stiffening element and the two side pieces, and
joining the two side pieces to the thickened region at the two
separate joint zones.
[0104] The invention also provides for a lightweight structural
component comprising a metal panel comprising at least one
thickened region, at least one stiffening element coupled to a
surface of the at least one thickened region, the at least one
stiffening element being a one-piece metal member and comprising at
least a bar portion and two side pieces extending from the bar
portion, the bar portion comprising a first thickness, each of the
two side pieces comprising a second thickness, the first thickness
being greater than the second thickness, and ends of the two side
pieces being at least partially connected to the at least one
thickened region by two separate weld joint zones.
[0105] The bar portion and two side pieces of the at least one
stiffening element may form a generally Y-shaped cross-section. The
bar portion and two side pieces of the at least one stiffening
element may form a generally T-shaped cross-section. The at least
one stiffening element may have a generally I-shaped
cross-section.
[0106] Other exemplary embodiments and advantages of the present
invention may be ascertained by reviewing the present disclosure
and the accompanying drawing.
BRIEF DESCRIPTION OF THE DRAWINGS
[0107] The present invention is further described in the detailed
description which follows, in reference to the noted plurality of
drawings by way of non-limiting examples of exemplary embodiments
of the present invention, in which like reference numerals
represent similar parts throughout the several views of the
drawings, and wherein:
[0108] FIG. 1 shows a cross section of a lightweight structural
component in a first embodiment that is particularly simple to
produce;
[0109] FIG. 2 shows a cross section of a structural component
according to the invention for higher demands with regard to damage
tolerance;
[0110] FIGS. 3a-c show a comparison of the bending stresses in a
laser beam-welded stringer/skin connection according to the prior
art to the stringer/skin connection according to the invention;
[0111] FIGS. 4a and 4b show an alternative embodiment for the
design of stringers or ribs with integrated crack stoppers;
[0112] FIG. 5 shows a cross section through a stringer/skin
connection that is embodied by an inserted reinforcing element for
highest demands with regard to transverse stress capacity, damage
tolerance and residual strength;
[0113] FIG. 6 shows a cross section of the most highly
stress-resistant stringer/skin connection in still another
embodiment;
[0114] FIG. 7 shows a cross section of a highly stress-resistant
stringer/skin connection in still another embodiment;
[0115] FIG. 8 shows a cross section through an embodiment with
additional elements for stiffening the panel;
[0116] FIG. 9 shows a cross section of an embodiment in which the
stiffening element is embodied as a U-profile;
[0117] FIG. 10 shows a cross section of an embodiment in which
wire-shaped stress-relief elements are located inside the skin
sheet in the direct vicinity of the two weld zones;
[0118] FIG. 11 shows a cross section with a wire cable-shaped
stress-relief element directly beneath the panel stiffening
base;
[0119] FIG. 12 shows a cross section of a structural component
according to another embodiment of the invention;
[0120] FIG. 13 shows a cross section of a structural component
according to still another embodiment of the invention; and
[0121] FIG. 14 shows a cross section of a structural component
according to still another embodiment of the invention.
DETAILED DESCRIPTION OF THE PRESENT INVENTION
[0122] The particulars shown herein are by way of example and for
purposes of illustrative discussion of the embodiments of the
present invention only and are presented in the cause of providing
what is believed to be the most useful and readily understood
description of the principles and conceptual aspects of the present
invention. In this regard, no attempt is made to show structural
details of the present invention in more detail than is necessary
for the fundamental understanding of the present invention, the
description taken with the drawings making apparent to those
skilled in the art how the several forms of the present invention
may be embodied in practice.
[0123] The following are examples of non-limiting embodiments:
Example 1
[0124] The lower fuselage of an aircraft is to be embodied with a
higher dent resistance and an improved stiffness. At the same time,
production costs and weight are to be reduced. To this end, a
riveted construction is replaced by a laser beam-welded
construction in a configuration according to the invention, as
shown in FIG. 1 for the stringer/panel variant.
[0125] The panel 1 includes a panel base 11. A stringer 2 normally
embodied with a stringer head 12 and stringer bar 4 features on its
side facing the skin sheet 1 two side pieces 5, 6. The lower sides
of the two side pieces 5, 6 extend from the stringer foot 3 and run
horizontally. In this way, they rest on a level panel base 11. Both
side pieces 5, 6 are connected in a material-locking and/or fixedly
secured manner to the panel base 11 with two separate joint zones
7, 8. The center-lines of the joints 7, 8 can form an angle of
respectively .gamma.=approximately 20.degree. to the surface of the
panel base 11 (see FIG. 2). The joint zones 7, 8 are produced by
laser beam welding. The sum of the depths of the two joint zones
can correspond to the stringer thickness d.sub.S. See FIG. 2 for
illustration of this dimension.
[0126] The following exemplary dimensions are selected for the
embodiment of the lightweight element shown in FIG. 1 (the
dimensions are measured in the same way as the corresponding
dimensions in FIG. 2):
[0127] Panel thickness: d.sub.H=approx. 1.6 mm, stringer height:
h.sub.s=approx. 31.5 mm, thickness of the panel base:
d.sub.Hs=approx. 2.4 mm, width of the panel base: b.sub.Hs=approx.
23 mm, side piece length s.sub.s=approx. 8.5 mm, stringer thickness
d.sub.s=approx. 3.3 mm, stringer head width: b.sub.s=approx. 17.5
mm, side piece thickness in the plane of the joint zone and
connection depth of the weld seams t.sub.s=approx. 1.65 mm, angle
between the two side pieces 5, 6 .alpha.=approx. 40.degree.,
spacing of the inside edges of the two side pieces on the panel
base b.sub.rs=approx. 3.0 mm. Both stringer and panel are produced
from a weldable Al (aluminum alloy) material, in this case from
alloy 6013 T4. The thickened panel base 11 of the skin sheet 1 is
preferably formed from the skin sheet by chemical milling, but may
be formed by other acceptable material removing or sheet forming
techniques. The stringers 2 which include the side pieces 5, 6 are
preferably formed by extrusion, but can be formed by other
acceptable techniques. Subsequently, the panel 1 is tensioned by
way of a vacuum tensioning device (not shown). By way of a
following tensioning unit integrated in the welding head, the
stringer 2 is positioned symmetrically on the panel base 11 and
pressed on the panel 1 by the application of a force of approx. 20
kg. Through lateral rolling, the position of the two side pieces 5,
6 relative to one another is prevented from changing due to the
compressive force or due to lateral forces caused by the alignment
of the stringers 2 on the panel 1.
[0128] Joining is carried out with two CO.sub.2 lasers using a
power of approx. 2,800 W (watts) each. Work is performed with
wire-shaped weld filler of alloy AlSi12 to avoid hot cracks. The
welding wire has a diameter of approx. 0.8 mm and is fed at a wire
feed speed of approx. 4,500 mm/min. The welding speed is 4 400
mm/min. To minimize distortion, welding takes place from both sides
simultaneously. The angle .gamma.=approx. 20.degree. and is set as
the angle .gamma. between the laser beam axis and surface of the
panel 1.
[0129] According to the experts, e.g., Heider Lasergerechte
Konstruktion und lasergerechte Fertigungsmittel zum Schwei.beta.fen
gro.beta.formatiger Aluminium-Strukturbauteile in
VDI--Fortschrittsberichte, series 2: Fertigungstechnik, no. 326,
VDI-Verlag Dusseldorf (1994), the disclosure of which is hereby
expressly incorporated by reference in its entirety, a
stringer/skin connection that is low in welding defects (hot
cracking, porosity, discharge) can only be achieved through welding
from both sides simultaneously while guaranteeing a common melting
bath of the two laser beams.
[0130] However, recent tests have shown that crack-free weld seams
that are low in pores and free of discharge can also be achieved by
way of a suitable beam formation of the normally circular laser
beam focus. The separation of the laser beam into two beams lying
one behind the other in the feed direction has proved to be
particularly favorable. A twin spot mirror is therefore used to
avoid discharge and to reduce porosity. Approx. 0.3 mm is selected
as the spacing of the foci with a division of the two power
portions in a ratio of approx. 60:40 with a laser power of approx.
3,500 W.
[0131] After adjusting all the parameters, the welding process is
started and the joint line is traced in a CNC-controlled manner
with the stringer tensioning unit following. After welding, the
panel 1 is conditioned, i.e., artificially aged to condition
T6.
[0132] The ribs can also be embodied with identical geometric
dimensions apart from the height and head of the stiffening element
2. In addition it should be ensured hereby that the panel 1 is
curved with a radius R=approx. 2,820 mm. This means that the ribs
have to be embodied in a curved manner such that the lower side of
the two side pieces 5, 6 describes a cylinder shell with the same
radius. This is realized in that during extrusion a transverse
force is exerted in the direction of the rib head after the
extrusion die. In process terms, the joining of the ribs takes
place analogously to that of the stringers.
[0133] As a result of this, a stiffer aircraft fuselage lower
shell, that is more resistant to buckling and cyclical compressive
loading, is advantageously obtained in a manner that is quicker,
more cost-effective, and without increased weight compared with
riveting.
[0134] In this simplest embodiment the lightweight element
according to the invention is also very suitable for the production
of stiff aluminum bodies of watercraft, in particular sport and
racing boats. Other very advantageous applications include pressure
tanks and vacuum tanks.
Example 2
[0135] Further advantages of the invention are to be explained on
the basis of a further developed embodiment that leads to a clearly
improved damage tolerance behavior. It is particularly suitable for
the side fuselage area but also for the upper fuselage area, i.e.,
for transverse stress and/or tensile stress.
[0136] One preferred geometric embodiment is shown in FIG. 2. In
addition to the features specified in exemplary embodiment 1, the
panel 1 also features a panel stiffening base 13. The two outer
sides 16 of the panel stiffening base 13 are inclined at an angle
of approx. a/2 and are thus adapted to the inner sides of the side
pieces 5, 6 that are also inclined at an angle of approx. a/2 from
the symmetric line (i.e., the center line running through stringer
bar 4). The joint surfaces 9, 10 of the base 13 and of the two side
pieces 5, 6 are inclined at an angle .beta..gtoreq.approx.
.alpha./2 with respect to the surface of the panel 1. With a weld
seam angle .gamma..apprxeq..beta., the weld seam thus lies
generally perpendicular or almost perpendicular to the outer
surface of the side pieces 5, 6. In contrast to exemplary
embodiment shown in FIG. 1, here the side pieces 5, 6 are embodied
with varying thickness and/or are tapered towards the side piece
foot, i.e., the side piece thickness in the plane of the joint
zones t.sub.S is greater than the side piece thickness b.sub.S0
near the branching point 14 of the two side pieces 5, 6.
[0137] This stringer/skin connection is, e.g., designed with the
following dimensions: stringer height h.sub.s=approx. 37 mm,
stringer head width b.sub.s=approx. 21 mm, stringer thickness
d.sub.s=approx. 4.4 mm, thickness of the panel d.sub.H=approx. 2.4
mm, thickness of the panel base d.sub.Hs=approx. 3.4 mm, thickness
of the panel stiffening element 13 d.sub.Hv=approx. 5.0 mm, width
of the panel base b.sub.Hs=approx. 15.2 mm, width of the panel
stiffening base b.sub.rs=approx. 9.2 mm, side piece thickness near
the branching point of the two side pieces b.sub.s0=approx. 2.2 mm,
side piece height s.sub.s=approx. 11.0 mm, side piece thickness in
the plane of the joint zone and simultaneously connection depth of
the weld seam t.sub.s=approx. 2.7 mm, angle between the two side
pieces .alpha.=approx. 40.degree., angle between the joint surface
7, 8 and the panel .beta.=approx. 22.0.degree., radius in the
branching point 14 is approx. 0.6 mm.
[0138] Apart from the manufacture of the panel stiffening base 13,
the production steps run analogously to those in example 1. To
guarantee a matching configuration between the inner sides of the
two side pieces 5, 6 and the side surfaces of the panel stiffening
base 13 and the joint surfaces 9, 10, this section is worked by HSC
milling on the panel 1 previously produced by chemical stripping.
However, it is also possible to omit the chemical stripping
completely and to produce the entire thickness profile of the skin
sheet 1 by HSC milling.
[0139] The following values have been selected as welding
parameters: laser power approx. 2,800 W, welding speed approx.
4,000 mm/min, wire feed speed approx. 4,000 mm/min. The inclination
of the laser beam axis to the surface of the panel is adjusted
according to the inclination of the joint surfaces to
.gamma.=approx. 22.2.degree.. All the other welding parameters are
selected analogously to those of example 1 as described above.
[0140] The lightweight fuselage shell thus produced features
particularly high values with regard to direct tensile strength,
dent stability and buckling stability as well as damage
tolerance.
[0141] Compared with the normal variant that is laser beam-welded
from both sides simultaneously and that has the same stringer
thickness d.sub.s=approx. 4.4 mm, the direct tensile strength is
increased from approx. 230 MPa to approx. 325 MPa. Moreover, the
scatter width of the determined cross tension values is
significantly reduced. Regardless of the higher direct tension
values, with comparable loads the material strain in the weld seam
plane in the variant according to the invention is lower because
the connection width is increased from approx. 4.4 mm to approx.
2.times.2.7 mm=approx. 5.4 mm.
[0142] In this case the following ratios are realized according to
one embodiment: t.sub.s/d.sub.s=approx. 0.61;
b.sub.s0/t.sub.s=approx. 0.81; s.sub.s/h.sub.s=approx. 0.30;
.beta.=approx. 22.degree.. In the event that a subsequent
artificial aging is to be omitted, the side piece thickness in the
plane of the joint zone and connection depth of the weld seam
t.sub.s can be increased to t.sub.s=approx. 4.9 mm. The following
would thus apply with the other geometric parameters kept constant:
t.sub.s/d.sub.s=approx. 1.11; b.sub.s0/t.sub.s=approx. 0.45.
[0143] The improvement regarding the use in fuselage shells loaded
by transverse stress is explained in FIGS. 3a-c. Through the
asymmetrical embodiment of the stringers, a bending moment M in the
direction of the bent end of the stringer head 12 acts on the
stringer (see FIG. 3a) both during a tensile stress and during a
compressive stress and in a particularly marked manner during a
transverse stress. The cross section stressed the most thereby lies
in the weld seam. The force F associated with the bending moment
leads to an effective bending stress at the weld seam surface
of
.sigma. Beff a = 6 * .alpha. KB * F * h S L * d S 2 ( I )
##EQU00001##
with .alpha..sub.KB as a notched form factor for bending in the
position of the weld seam and L as panel or stringer length.
[0144] In the solution according to the invention, however, the
cross section stressed most in terms of bending no longer lies in
the weld seam itself, but in the plane of the side piece branching
(see FIG. 3c). Due to the changed leverage conditions and the much
larger notch radius compared with the weld seam, the effective
bending stress in the most stressed area is reduced to
.sigma. Beff c 0 = 6 * .alpha. KB r * F * ( h S - s S ) L * d S 2 (
II ) ##EQU00002##
with .alpha..sup.r.sub.KB as a notched form factor for bending at
the site of the side piece branching with the radius r. If the
tensions in the respectively most highly stressed cross sections in
terms of bending are compared, the result is the following
diminution factor R.sub.1:
R 1 = .sigma. Beff c 0 .sigma. Beff a = .alpha. KB r * ( h S - s S
) .alpha. KB * h S ( III ) ##EQU00003##
[0145] With the above values of the geometric dimensions and
.alpha..sub.KB.apprxeq.3, .alpha..sup.r.sub.KB.apprxeq.1.1, a
stress diminution factor of R.sub.1.apprxeq.0.26 results. This
means that the highest effective bending stress is reduced to
approx. 26% through the embodiment according to the invention and
in addition is displaced from the weld seam to a region that is not
microstructurally damaged.
[0146] The bending stress in the weld seam due to the supporting
effect of the lower side piece in FIG. 3c is approximately replaced
by a tensile stress .sigma..sup.c.sub.Zeff at
.sigma. Zeff c = .alpha. KZ * h S * F s S * t S * L * sin [ 2 * arc
tan ( b rs / 2 * s S ) ] ( IV ) ##EQU00004##
(with .alpha..sub.KZ as notched form factor for tensile stress at
the site of the weld seam).
[0147] Analogously to the diminution factor R.sub.1, a diminution
factor can also be defined for the stress in the weld seam R.sub.2
due to the effect of the bending moment M:
R 2 = .sigma. Zeff c .sigma. Beff a ( V ) R 2 = .alpha. KZ * d S 2
6 * .alpha. KB * s S * t S * sin [ 2 * arctan ( b rs / 2 * s S ) ]
( VI ) ##EQU00005##
[0148] With the values given in example 2 and
.alpha..sub.KZ.apprxeq..alpha..sub.KB, in a rough calculation the
result is R.sub.2=approx. 0.15. This means that with the solution
according to the invention the bending moments on the weld seam
resulting from the asymmetrical design of the stringers are very
slight and, in contrast to the previous solution, a deterioration
of the properties can be ruled out.
[0149] To sum up this means that the material strain as a result of
compressive stress, transverse stress or tensile stress in the
critical weld seam area is reduced and that laser beam welded
integral structures can thus also be used for side and upper
shells.
[0150] Compared with the solution according to the invention, other
solutions for reducing material strain caused by bending, such as,
e.g., a thickening in the stringer bar or the thickening of the
stringer foot according to FIG. 3b are much less effective. If, for
example, only the stringer foot is thickened, the direct tensile
strength is reduced and the bending stress in the weld seam,
reduced but still present, reduces loading capacity in particular
during transverse stress or tensile stress. Moreover, the fact that
much greater weld seam depths are necessary to compensate for the
reduction in direct tensile strengths, which seam depths lead to an
increase in deformation, has a negative effect.
[0151] With the stress type "crack growth with broken stringer,"
the fact that the panel stiffening base 13 reduces the stress
concentration near the crack tip in the direct vicinity of the weld
seam has a positive effect. The weld seam is thus subject to a
reduced elongation amplitude, which leads to a locally lower crack
growth rate. Even after the failure of the weld seam in the first
side piece, the crack growth rate is reduced by the crack branching
and the material thickening in the skin sheet. Moreover it is
important that the stringer still does not lose its stabilizing
effect even after the failure of the first weld seam and of the
first stringer side piece. Overall, an improved damage tolerance
and residual strength are thus achieved.
[0152] Further advantages exist regarding the avoidance of
unintentional damage to the weld seam during assembly of the
fuselage panel, during disassembly of the fuselage interior or in
the event of repair. Whereas, for example, with a stiffening
element/skin connection carried out according to the prior art, an
unintentional mechanical stress (e.g., through bumping during
assembly) crosswise to the stiffening element (rib or stringer)
cracks can occur in the weld seam at an early stage even before the
development of any signs on the stiffening element (e.g., visibly
permanent deformation), with suitable dimensions of the stiffening
element according to the invention, the plane of the side piece
branching can serve as desired deformation point that reacts before
damaging stress or deformation conditions are reached in the weld
seam.
[0153] From the point of view of quality assurance, the fact
that--due to the separate position of the weld seams--even maximum
cross section weaknesses, such as those that can occur due to
discharge or very long communicating pores, can cover no more than
approx. 50% of the total cross section of both weld seams, has a
positive effect. The fact that the requirements for guaranteeing
high quality, faultless weld seams have been noticeably reduced has
a very advantageous impact in welding technology terms for the
following reasons: [0154] The foci of the two laser beams no longer
have to meet exactly. This reduces the requirements for precision
control of the welding equipment, in particular during the welding
of parts that are not flat. [0155] Bond faults are easier to avoid
because the angle between the laser beam axis and the joint
surface, which currently cannot be reduced much under 20.degree.
due to the dimensions of the laser beam weld heads, can be reduced
to 0.degree. because of the inclination of the joint surfaces now
possible. The requirements for precision control of the laser beam
perpendicular to the feed direction and the risk of bond faults
forming are thus also reduced. [0156] Through the elimination of
the previous demand for an angle between joint surface and laser
beam axis and the resulting predetermined minimal weld seam width,
a lower linear energy can be used in welding, which reduces
deformation. [0157] From the point of view of welding technology,
welding safety and deformation, the requirement for welding from
both sides simultaneously can be eliminated.
[0158] For even more exacting demands on damage tolerance, the two
side pieces 5, 6 can be provided with cut-outs, as shown in a side
view in FIGS. 4a and 4b. These cut-outs act as crack stoppers,
since a crack penetrating into them in order to spread further
first has to initiate a new crack. Shape and size of the cut-outs
15', 15'' are selected thereby such that they entail the lowest
possible loss of stiffness in the longitudinal direction of the
stiffening element, while on the other hand acting as an effective
crack stopper for a crack that has crossed the weld seam. The shape
of the cut-outs can thereby be selected to be circular, oval, a
slit or a rounded triangle. The cut-outs 15', 15'' of the two side
pieces 5, 6 are thereby made in a manner displaced and/or offset
from or relative to one another. With only a small reduction in the
stiffness of the stiffening element 2 it is thereby ensured that
the crack leads out into a cut-out, thus increasing the damage
tolerance.
Example 3
[0159] Another variant for improving the damage tolerance of welded
panel/stringer connections is explained in FIGS. 10 and 11.
[0160] High tensile stresses prevail in the upper shell area along
the weld seams in a stringer/skin connection. They can be reduced
by stress relief elements embedded in the panel base parallel to
the stringer. In one preferred embodiment they comprise wires of
high-strength steel, titanium or Ni materials. Their positive
effect regarding damage tolerance is due to two effects: firstly,
due to their higher modulus of elasticity they put up a higher
resistance to an elongation along the wire axis than the skin
material surrounding them or the weld seam, so they relieve the
stress on their surroundings. The crack growth rate is thus reduced
when the crack approaches the stress relief element and thus the
weld seam. Secondly, the residual strength is improved, because the
stress relief element still remains intact after the crack has
crossed the surroundings of the stress relief element.
[0161] According to the invention, several embodiments are
possible. According to FIG. 10, in a preferred embodiment, e.g.,
two multicore, high-strength wires 22 made of an Inconel alloy can
be rolled (or otherwise placed) into the panel base directly to the
right and left of the two weld seams. The effect of the relief of
stress on the weld seam is particularly marked in this arrangement.
A mechanically sufficiently load-bearing connection of the stress
relief elements to the panel is achieved through the rolling in and
the structured surface of the wire cable.
[0162] In another arrangement shown in FIG. 11, the stress relief
element 22 is rolled in (or otherwise placed) directly beneath the
panel stiffening base. In this embodiment the insertion of the
stress relief element 22 can be coupled in a particularly favorable
manner with the production of the panel stiffening base by way of
metal forming.
Example 4
[0163] The exemplary embodiments shown in more detail in FIGS. 5
through 7 are designed for particularly exacting demands with
regard to damage tolerance.
[0164] These embodiments utilize a reinforcing element 17 of a
higher modulus of elasticity compared to the skin 1. The stiffening
element material 17 is located in the cavity formed by the two side
pieces 5, 6 and the panel stiffening base 13. In the exemplary
embodiment, the reinforcing element 17 is made of the titanium
alloy Ti6Al4V. The modulus of elasticity is approx. 110 GPa
compared to the Al alloy used at approx. 71 GPa. As shown in FIG.
5, the reinforcing element has the cross section of an isosceles
triangle with a rounded-off tip. An arrangement of intersecting
grooves (not shown) can also be impressed in one or all of the
surfaces of the reinforcing element 17 by way of roller
burnishing.
[0165] To realize the arrangement shown in FIG. 5, the same
dimensions for the panel 1 and stiffener 2 can be selected as in
exemplary embodiment shown in, e.g., FIGS. 1, 2 and 10. The
dimensions of the reinforcing element 17 can accordingly be as
follows: Base width: b.sub.V=approx. 9.2 mm, height:
h.sub.V=approx. 10.4 mm.
[0166] The process steps are likewise selected analogously to
exemplary embodiments described above such as, e.g., the embodiment
shown in FIGS. 1 and 2. In addition, the reinforcing element 17 can
be rolled into (or otherwise placed into) the stringer 2 after the
extrusion of the stringer 2.
[0167] The particular advantage of this solution variant with a
reinforcing element 17 in the direct proximity of both the weld
seams is that during a stressing of the panel in the direction of
the stringer longitudinal axis, the elongation in the direct
proximity of the weld seam is reduced by the lower elongation of
the reinforcing element due to the greater modulus of elasticity
and the form-locking or force-locking connection to the stringer.
This results in a reduction of the longitudinal tensile stresses in
the weld seam. An estimate with the above-mentioned geometric
parameters gives a stress diminution in the proximity of the weld
seam of approx. 6%. Due to the strong dependence of the crack
growth rate on the main stress in the critical stress area of 95
MPa, the result is a clear extension of the service life.
[0168] Furthermore, despite the placing of cut-outs 15', 15'' in
the side pieces 5, 6 of the stringers 2, which can be used in the
embodiments shown in FIGS. 5-7, the stiffness of the stringers is
not reduced compared with the prior art, so the crack stop function
is possible without other disadvantageous consequences, such as
reduction of the stiffness of the panel or reduced support function
of the stringers.
[0169] Due to its higher cracking fatigue strength, the reinforcing
element 17 is still intact even when the crack has crossed both
weld seams. This temporarily reduces the crack growth rate even
after the stringer has subsequently cracked. With increasing crack
growth, a load rearrangement occurs on the reinforcing element 17
of the cracked stringer until finally, progressing along the
stringer, the force-locking or form-locking connection to the
stringer is detached and the reinforcing element is pulled out of
the cavity with a consumption of energy. Through this differential
failure the residual strength is increased and the damage
development before the break is less catastrophic.
[0170] The reinforcing element 17 is embodied as a pipe or tube
having a circular cross-section in the variant shown in FIG. 6.
FIG. 7 presents a variant in which the reinforcing element 17 is
rolled into (or otherwise placed in) the skin sheet 1 before
welding of the stiffener 2 to the panel 1. In this variant, the
prevention of elongation in the weld seam by the reinforcing
element 17 is particularly marked.
[0171] Because the weld seam is relieved from bending stress by the
embodiment of the stringers 2 with two side pieces and the
additional local stress-reducing effect of the reinforcing element
17, the stringer head 12 can also be embodied with a greater
thickness without a damaging effect (see FIG. 8). This leads to a
particularly stiff fuselage shell. For this case, alternatively the
spacing of the stringers can also be increased. To obtain larger
spacing between stringers, it can be favorable to provide
additional panel bars 19 in the panel at distances "C" from the
stringer 2 or rib 2. These thickened panel regions or
reinforcements 19 can be produced in a particularly simple manner
by chemical milling.
Example 5
[0172] In some cases the stiffening elements 2 have to contain
additional attachment parts, load bearing elements or an inner skin
in their embodiment as ribs or as stringers. Without violating the
inventive concept it can be advantageous for these applications to
embody the stiffening element 2 as an upside-down U-profile. FIG. 9
shows a suitable embodiment. The two side pieces 5', 6' are thereby
extended up to the head 12' of the stiffening element 2. Another
special feature is that the panel stiffening base 13', 13'' can be
embodied in a divided manner, i.e., first and second panel base
sections 13' and 13'' to further save weight.
[0173] The embodiment shown in FIG. 12 is similar to the embodiment
shown in FIG. 1 but utilizes a head 12 which extends generally
parallel to the panel 1 and which projects generally equally from
both sides of the bar 4. The head 12 may be integrally formed with
the bar portion 4 and pieces 5, 6 or may be formed separately there
from and then fixedly attached to the bar portion 4 using, e.g.,
welding, rivets, fasteners, adhesive bonding, etc.
[0174] The embodiment shown in FIG. 13 is similar to the embodiment
shown in FIG. 1 but utilizes a doubler plate DP made of
damage-tolerant, fiber reinforced laminate pieces which are
attached to the outer surfaces of the side pieces 5, 6. The doubler
plates DP may be fixedly attached to the pieces 5, 6 using, e.g.,
adhesive bonding, etc.
[0175] The stiffener embodiment shown in FIG. 14 utilizes a
T-shaped end whose side pieces 5, 6 are arranged generally parallel
to one another, which extends generally parallel to the panel 1,
and which projects generally equally from both sides of the bar
portion 4. The head 12 may be integrally formed with the bar
portion 4 or may be formed separately there from and then fixedly
attached to the bar portion 4 using, e.g., welding, rivets,
fasteners, adhesive bonding, etc. In this embodiments, the inner
surfaces of the side pieces 5, 6 are fixedly attached to the panel
base 11 at two separate joint zones 7, 8.
[0176] It is noted that the invention contemplates any features
shown in one embodiment may be used in another embodiment. Thus, by
way of example, the embodiment shown in FIG. 2 may use the T-shaped
head 12 shown in FIG. 12 in place of the L-shaped head. The
embodiments shown in FIGS. 1, 2, and 5-14 may utilize the cut-outs
shown in FIGS. 4a-b or the cut-outs 20 of FIG. 7 in any desired
arrangement. The head 12 in each embodiment may be oriented in any
desired angle relative to the bar portion 4 instead of being
generally perpendicular thereto and may be omitted altogether. Each
embodiment may utilize any desired arrangement of reinforcements 17
and/or stress relief elements 22, including the disclosed
arrangements. Additionally, each embodiment may utilize the doubler
plates DP.
[0177] It is noted that the foregoing examples have been provided
merely for the purpose of explanation and are in no way to be
construed as limiting of the present invention. While the present
invention has been described with reference to an exemplary
embodiment, it is understood that the words which have been used
herein are words of description and illustration, rather than words
of limitation. Changes may be made, within the purview of the
appended claims, as presently stated and as amended, without
departing from the scope and spirit of the present invention in its
aspects. Although the present invention has been described herein
with reference to particular means, materials and embodiments, the
present invention is not intended to be limited to the particulars
disclosed herein; rather, the present invention extends to all
functionally equivalent structures, methods and uses, such as are
within the scope of the appended claims.
LIST OF REFERENCE NUMBERS
[0178] 1 Skin sheet, panel [0179] 2 Stiffening elements, stringers,
ribs [0180] 3 Foot of the stiffening element, stringer foot, rib
foot [0181] 4 Bar of the stiffening element, stringer bar, rib bar
[0182] 5 Side piece 1 [0183] 5' Side piece 1 if the stiffening
element is embodied as a U profile [0184] 6 Side piece 2 [0185] 6'
Side piece 2 if the stiffening element is embodied as a U-profile
[0186] 7 Joint zone 1 [0187] 8 Joint zone 2 [0188] 9 Connection
point of joint zone 1, joint surface 1 [0189] 10 Connection point
of joint zone 2, joint surface 2 [0190] 11 Thickening, panel base
[0191] 12 Head of the stiffening element, stringer head, rib head
[0192] 12' Head of the stiffening element if the stiffening element
is embodied as a U-profile [0193] 13 Panel stiffening base [0194]
13' Panel stiffening base if the stiffening element is [0195] 13''
embodied as a U-profile [0196] 14 Branching point of the two side
pieces 5 and 6 [0197] 15' Cut-outs in side piece 1 of the
stiffening element [0198] 15'' Cut-outs in side piece 2 of the
stiffening element [0199] 16 [0200] 16' Outer sides of the panel
stiffening bases 13, 13', 13'' [0201] 16'' [0202] 17 Reinforcing
element [0203] 18 Recess in the panel stiffening base to accept the
reinforcing element [0204] 19 Panel bar [0205] 20 Cut-out in the
bar of the stiffening element [0206] 21 Laser beam axis [0207] 22
Stress relief element [0208] DP Doubler plate [0209] a Spacing of
cut-outs 15' and 15'' in the side pieces 1 and 2 [0210] a' Spacing
of the cut-outs 20 in the bar 4 [0211] b.sub.Hs Width of the panel
base 11 [0212] b.sub.rs Width of the panel stiffening base, spacing
of the two side pieces 5 and 6 on the panel base [0213] b.sub.s
Width of the head of the stiffening element, stringer head width
[0214] b.sub.s0 Side piece thickness near the branching of the foot
of the stiffening element [0215] b.sub.v Base width of the
reinforcing element 17 [0216] C Distance of the panel bar from the
center line of the stiffening element [0217] d.sub.H Panel
thickness [0218] d.sub.Hs Thickness of the panel including panel
base [0219] d.sub.Hv Thickness of the panel including panel
stiffening base [0220] d.sub.s Thickness of the stiffening element,
stringer thickness, rib thickness [0221] F Force on stringer due to
bending moment [0222] F.sub.v Force at branching point due to
bending moment [0223] h.sub.s Height of stiffening element,
stringer height, rib height [0224] h.sub.v Height of reinforcing
element 17 [0225] M Bending moment on stringer [0226] R Curvature
radius of the panel [0227] R.sub.1; R.sub.2 Stress diminution
factors in the weld seam [0228] R Curvature radius of the
transition between the bar of the stiffening element 4 and the two
side pieces 5, 6 [0229] s.sub.s Side piece height [0230] t.sub.s
Side piece thickness in the plane of the joint zone, connection
depth of the weld seam [0231] A Angle between the two side pieces 5
and 6 [0232] .alpha..sub.KB Notched form factor for bending stress
[0233] .alpha..sub.KZ Notched form factor for tensile stress [0234]
.alpha..sup.r.sub.KB Notched form factor for bending stress at the
site of the side piece branching [0235] B Angle between the joint
surface of the joint zone 7 or 8 and panel 1 [0236] .LAMBDA. Angle
between laser beam axis 21 and skin sheet [0237] .sigma..sup.(a, b,
c).sub.Beff Effective bending stress acting on the weld seam in
embodiment (a, b, c) [0238] .sigma..sup.(a, b, c).sub.Zeff
Effective tensile stress acting on the weld seam in embodiment (a,
b, c) [0239] .sigma..sup.(a,b,c).sub.Beff Effective bending stress
acting on the transition between the bar 4 of the stiffening
element and the two side pieces 5, 6 in embodiment (a, b, c)
* * * * *