U.S. patent application number 10/286937 was filed with the patent office on 2003-12-11 for structural members having improved resistance to fatigue crack growth.
Invention is credited to Bray, Gary H., Denzer, Diana K., Garratt, Matthew D., Ulysse, Patrick.
Application Number | 20030226935 10/286937 |
Document ID | / |
Family ID | 29714952 |
Filed Date | 2003-12-11 |
United States Patent
Application |
20030226935 |
Kind Code |
A1 |
Garratt, Matthew D. ; et
al. |
December 11, 2003 |
Structural members having improved resistance to fatigue crack
growth
Abstract
An extruded structural member having improved damage tolerance.
The structural member comprising: a base section; a rib stiffening
section having at least one pair of structural ribs, the structural
ribs integral with the base section and projecting outwardly
thereof; and at least one intra-rib area positioned between the
pair of structural ribs, the intra-rib area having a microstructure
with intentionally increased amounts of fiber texture to reduce the
rate of fatigue crack growth in the extruded structural member.
Inventors: |
Garratt, Matthew D.;
(Pittsburgh, PA) ; Bray, Gary H.; (Murrsyville,
PA) ; Denzer, Diana K.; (Lower Burrell, PA) ;
Ulysse, Patrick; (Pittsburgh, PA) |
Correspondence
Address: |
ALCOA INC
ALCOA TECHNICAL CENTER
100 TECHNICAL DRIVE
ALCOA CENTER
PA
15069-0001
US
|
Family ID: |
29714952 |
Appl. No.: |
10/286937 |
Filed: |
November 2, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60339715 |
Nov 2, 2001 |
|
|
|
Current U.S.
Class: |
244/123.7 |
Current CPC
Class: |
B21C 23/14 20130101;
C22F 1/057 20130101; B21C 23/001 20130101; B21B 2003/001 20130101;
Y10T 428/12389 20150115; B21B 1/08 20130101; Y10T 428/12375
20150115; B21B 2001/022 20130101; B21B 1/026 20130101; B64C 3/24
20130101; C22C 21/16 20130101; Y10T 428/1241 20150115; C22F 1/053
20130101; Y10S 148/902 20130101 |
Class at
Publication: |
244/123 |
International
Class: |
B64C 001/00; B64C
003/00; B64C 005/00 |
Claims
What is claimed is:
1. An extruded structural member having improved damage tolerance,
said structural member comprising: at least one area having a
substantially unrecrystallized microstructure with intentionally
increased amounts of fiber texture to reduce the rate of fatigue
crack growth in said extruded structural member.
2. The extruded structural member of claim 1 wherein said least one
area has an intentionally increased amount of <100> and
<111> fiber components.
3. The extruded structural member of claim 1 wherein said
intentionally increased fiber texture of said least one area is
formed by intentionally extruded local geometries which promote
primarily axisymmetric metal flow and then removing excess metal in
said local geometries.
4. The extruded structural member of claim 1 wherein said
structural member has at least one pair of structural ribs and said
least one area is an intra-rib area formed between said pair of
structural ribs.
5. The extruded structural member of claim 4 wherein said
intentionally increased fiber texture of said intra-rib area is
created by intentionally extruded local geometries which promote
primarily axisymmetric metal flow and then machining said local
geometries which promote primarily axisymmetric metal flow after
extrusion.
6. The extruded structural member of claim 4 wherein said
intentionally increased fiber texture of said intra-rib area is
created by intentionally extruded local geometries which promote
primarily axisymmetric metal flow and then milling said local
geometries which promote primarily axisymmetric metal flow after
extrusion.
7. The extruded structural member of claim 4 wherein said
intentionally increased fiber texture of said intra-rib area is
created during extrusion by the use of spreaders in the die used to
form the extrusion.
8. The extruded structural member of claim 4 wherein said
intentionally increased fiber texture of said intra-rib area is
created during extrusion by the use of feeder plates or double
extrusion.
9. The extruded structural member of claim 4 wherein said
structural ribs have a T-shaped cross-sectional area.
10. The extruded structural member of claim 4 wherein said
structural ribs have a J-shaped cross-sectional area.
11. The extruded structural member of claim 4 wherein said
structural ribs have a L-shaped cross-sectional area.
12. The extruded structural member of claim 4 wherein said
structural ribs have a hat-shaped cross-sectional area.
13. The extruded structural member of claim 4 wherein said
structural ribs have a Z-shaped cross-sectional area.
14. The extruded structural member of claim 4 wherein said
structural ribs are substantially parallel.
15. The extruded structural member of claim 1 wherein said extruded
structural member is fabricated from a single aluminum alloy.
16. The extruded structural member of claim 1 wherein said extruded
structural member is fabricated from at least two aluminum alloys
which are co-extruded.
17. The extruded structural member of claim 1 wherein said extruded
structural member is fabricated from a substantially
unrecrystallized extrusion selected from the group consisting of
2xxx, 5xxx, 6xxx, 7xxx and 8xxx alloys.
18. The extruded structural member of claim 1 wherein said extruded
structural member is fabricated from a substantially
unrecrystallized extrusion selected from the group consisting of
2x24, 2x26, 2x27 and 2x2x alloys.
19. The extruded structural member of claim 1 wherein said local
geometries which promote primarily axisymmetric metal flow are
selected from the group consisting of circles, squares, polygons
and irregular shapes with aspect ratio within the range of about
0.5 to about 2.0.
20. The extruded structural member of claim 1 wherein said
structural member has increased fatigue crack growth
resistance.
21. The extruded structural member of claim 1 wherein said
structural member has increased resistance to fatigue
initiation.
22. The extruded structural member of claim 1 wherein said
structural member has increased toughness.
23. The extruded structural member of claim 1 wherein said
structural member is a monolithic structure.
24. The extruded structural member of claim 1 wherein said
structural member is an integrally stiffened panel.
25. The extruded structural member of claim 1 wherein said extruded
structural member is fabricated from a substantially
unrecrystallized extrusion, said extrusion comprising: about 3.6 to
about 4.9 wt. % copper, about 1.0 to about 1.8 wt. % magnesium,
about 0.15 to about 0.9 wt. % manganese, about 0.05 to about 0.25%
zirconium, less than about 0.25% zinc, less than about 0.8 silver,
less than about 0.3% iron, less than about 0.25% silicon, the
balance substantially aluminum, incidental elements and
impurities.
26. The extruded structural member of claim 1 wherein said extruded
structural member is fabricated from a substantially
unrecrystallized extrusion selected from the group consisting of
7xxx, 7x50, 7x55 and 7085 alloys.
27. The extruded structural member of claim 1 wherein said extruded
structural member is fabricated from a substantially
unrecrystallized extrusion selected from the group consisting of
2x9x and 8x9x alloys.
28. The extruded structural member of claim 1 wherein said extruded
structural member is fabricated from a substantially
unrecrystallized extrusion selected from the group consisting of
aluminum 2xxx, 5xxx, 6xxx, 7xxx and 8xxx alloys containing
lithium.
29. The extruded structural member of claim 1 wherein said extruded
structural member is fabricated from a substantially
unrecrystallized extrusion, said extrusion comprising: about 0.5 to
about 2.7 wt. % lithium, about 1.0 to about 4.5 wt. % copper, less
than about 1.3 wt. % magnesium, less than about 0.10 wt. %
manganese, about 0.04 to about 0.16% zirconium, less than about
0.25% zinc, less than about 0.8 silver, less than about 0.3% iron,
less than about 0.20% silicon, the balance substantially aluminum,
incidental elements and impurities.
30. The extruded structural member of claim 1 wherein said extruded
structural member is fabricated from a substantially
unrecrystallized extrusion selected from the group consisting of
aluminum alloys containing less than about 3.0% lithium.
31. The extruded structural member of claim 1 wherein said extruded
structural member is fabricated from a substantially
unrecrystallized extrusion selected from the group consisting of
Al--Cu--Mg--Ag system.
32. The extruded structural member of claim 1 wherein said extruded
structural member is useful in aerospace structures of the group
consisting of rib stiffening sections for wing box, empennage and
fuselage.
33. The extruded structural member of claim 1 wherein said extruded
structural member is useful in aerospace structures of the group
consisting of ribs, spars, stringers, stiffeners, monolithic spar
caps and built-up spar caps.
34. An extruded structural member having improved damage tolerance,
said structural member comprising: a base section; a rib stiffening
section having at least one pair of structural ribs, said
structural ribs are integral with said base section and projecting
outwardly thereof; and at least one intra-rib area positioned
between said pair of structural ribs, said intra-rib area having a
microstructure with intentionally increased amounts of fiber
texture to reduce the rate of fatigue crack growth in said extruded
structural member.
35. A wrought aluminum alloy structural member having improved
fatigue crack gowth resistance, said structural member comprising
areas with intentionally increased fiber texture.
36. The wrought aluminum alloy structural member of claim 34
fabricated by rolling.
36. The wrought aluminum alloy structural member of claim 34
fabricated by a method comprising: rolling an ingot section having
a height is 2-4 times its width, and reducing the ingot by rolling
so that the ratio of half the entry width to the contact length of
the plate and roll is less than 1.75 to produce an unrecystallized
product.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a perfection of U.S. Provisional Patent
Serial No. 06/339,715, filed Nov. 2, 2001. The text thereof is
incorporated herein by reference.
FIELD OF THE INVENTION
[0002] This invention pertains to damage tolerant aluminum
structures made from aluminum alloys and the methods of making such
structures.
BACKGROUND OF THE INVENTION
[0003] As the size of new jet aircraft get larger, or as current
jetliner models grow to accommodate heavier payloads and/or longer
flight ranges to improve performance and economy, the demand for
weight savings of structural components, such as fuselage, wing and
spar parts continues to increase. The aircraft industry is meeting
this demand by specifying higher strength, metal parts to enable
reduced section thickness as a weight savings expedient. In
addition to strength, the durability and damage tolerance of
materials are also critical to an aircraft's fail-safe structural
design. Such consideration of multiple material attributes for
aircraft applications eventually led to today's damage tolerant
designs, which combine the principles of fail-safe design with
periodic inspection techniques.
[0004] A traditional aircraft wing structure comprises a wing box
generally designated by numeral 2 in accompanying FIG. 1. It
extends outwardly from the fuselage as the main strength component
of the wing and runs generally perpendicular to the plane of FIG.
1. That wing box 2 comprises upper and lower wing skins 4 and 6
spaced by vertical structural members or spars 12 and 20 extending
between or bridging upper and lower wing skins. The wing box also
includes ribs, which can extend generally from one spar to the
other. These ribs lie parallel to the plane of FIG. 1 whereas the
wing skins and spars run perpendicular to said FIG. 1 plane. During
flight, the upper wing structures of a commercial aircraft wing are
compressively loaded, calling for high compressive strengths with
an acceptable fracture toughness attribute. The upper wing skins of
today's most large aircraft are typically made from 7XXX series
aluminum alloys such as 7150 (U.S. Reissue Pat. No. 34,008) or 7055
aluminum (U.S. Pat. No. 5,221,377). Because the lower wing
structures of these same aircraft wings are under tension during
flight, they will require a higher damage tolerance than their
upper wing counterparts. Although one might desire to design lower
wings using a higher strength alloy to maximize weight efficiency,
the damage tolerance characteristics of such alloys often fall
short of design expectations. As such, most commercial jetliner
manufacturers today specify a more damage-tolerant 2XXX series
alloy, such as 2024 or 2324 aluminum (U.S. Pat. No. 4,294,625), for
their lower wing applications, both of said 2XXX alloys being lower
in strength than their upper wing, 7XXX series counterparts. The
alloy members and temper designations used throughout are in
accordance with the well-known product standards of the Aluminum
Association.
[0005] Upper and lower wing skins, 4 and 6 respectively, from
accompanying FIG. 1 are typically stiffened by longitudinally
extending stringer members 8 and 10. Such stringer members may
assume a variety of shapes, including "J", "I", "L", "T" and/or "Z"
cross sectional configurations. These stringer members are
typically fastened to a wing skin inner surface as shown in FIG. 1,
the fasteners typically being rivets. Upper wing stringer member 8
and upper spar caps 14 and 22 are presently manufactured from a
7XXX series alloy, with lower wing stringer 10 and lower spar caps
16 and 24 being made from a 2XXX series alloy for the same
structural reasons discussed above regarding relative strength and
damage-tolerance. Vertical spar web members 18 and 26, also made
from 7xxx alloys, fasten to both upper and lower spar caps while
running in the longitudinal direction of the wing constituted by
member spars 12 and 20. This traditional design with stiffening
elements such as stringers attached to skin or web elements by
fasteners is also known as "built-up" construction. Obviously, the
fasteners and fastener holes along the stringers are structural
weak links, for example as preferred locations for the initiation
of fatigue cracks. An alternative construction method is to make
the stiffening elements integral with the web or skin. For example,
in a lower or upper wing skin panels this can be accomplished by
machining both the wing skin and stringers from a single thick
plate or by extruding the wing skin and stringers as a single
extrusion to produce an integrally stiffened panel.
[0006] This method of "integral" construction has several
advantages over traditional or "built-up" construction. For one,
integrally stiffened panels are less costly to make and assemble by
eliminating the need for fasteners to attach the stiffening members
to the web or skin. In addition, the spacing or distance between
the stiffeners (also referred to in the art as ribs) can be more
readily optimized in an integral panel. For example, typical
stringer spacing in a "built-up" structure is 5 to 6 inches in
medium to large commercial aircraft. A reduction in the stringer
spacing to 3.5 inches can provide a significant increase in the
compressive buckling strength of an upper wing panel, for example,
or the stiffness of a lower wing skin panel. Narrower spacings are
not typically used in "built-up" stiffened panels because there is
a trade-off between the increased performance and the additional
assembly and fastener costs associated with attaching additional
stringers. Finally, the addition of more fastener holes also
increases the number of fatigue initiation sites increasing the
likelihood that a fatigue crack will form.
[0007] Integral wing panels have been utilized in smaller aircraft
such as regional jets but much less so in larger commercial
aircraft due to increased damage tolerance requirements in the
latter. While integral panels have fewer sites for fatigue
initiation, if a crack does initiate in the web or skin for example
it can continue to propagate without interruption through the
stiffening members. Likewise, a fatigue crack initiating in the
stiffening member can propagate into the web or skin. In a built-up
structure the web and stiffeners are separate members so a crack
propagating in one member need not cause the other to crack. In
fact, a fatigue crack propagating in the web can be bridged by an
uncracked stiffener transferring part of the skin load to the
stiffener and slowing the crack. The reduced damage tolerance of an
integral structure relative to a built-up structure has limited
their used on larger aircraft particularly in structure
predominantly loaded in tension such as the lower wing panel.
[0008] While lower and upper wing panels are specifically mentioned
in the above discussion, skins or webs stiffened by stiffened by
stiffening members are used throughout the aircraft, for example in
the fuselage and empennage. Other aircraft components such as spars
and ribs can also be of built-up design and stiffening caps
separated by a web. The advantages of an integral design with
respect to improvements in structural performance and reduced cost
and weight and its disadvantage with respect to damage tolerance is
also applicable to these structures. The invention described herein
provides a method for retaining the advantages of integral
structure while significantly improving the damage tolerance of
same.
[0009] The important desired properties for a structural components
in high capacity aircraft include higher strength, better fatigue
life and improved fracture toughness. Current alloys for lower wing
skin members in commercial jet aircraft all lack in the property
needs required for tomorrow's high capacity aircraft.
[0010] An object of the invention is to provide a method of
increasing the damage tolerance of existing aluminum alloys.
[0011] A principal object of the invention is to provide an
aluminum alloy and extrusion product formed therefrom, the
extrusion product having improved fracture toughness and resistance
to fatigue crack growth (FCG) while maintaining high strength
properties, good formability and corrosion resistance.
[0012] A further object of the present invention is to provide
aluminum alloy extrusion products having improved fracture
toughness and resistance to fatigue crack growth for aircraft
panels.
[0013] Yet a further object of the present invention is to provide
aluminum alloy extrusion products and a process for producing the
sheet products so as to provide improved fracture toughness and
increased resistance to fatigue crack growth while still
maintaining high levels of strength. Other properties that are
important are bearing strength, compression strength and tensile
strength.
[0014] These and other objects will become apparent from a reading
of the specification and claims and an inspection of the claims
appended hereto.
SUMMARY OF THE INVENTION
[0015] The present invention is directed to an extruded structural
member having improved damage tolerance. The structural member
comprising: at least one area having a microstructure with
intentionally increased amounts of fiber texture to reduce the rate
of fatigue crack growth in the extruded structural member. In a
preferred embodiment, the extruded structural member is formed from
a substantially unrecrystallized extrusion selected from the group
consisting of 2xxx, 5xxx, 6xxx, 7xxx and 8xxx aluminum alloys. In a
most preferred embodiment, the aluminum alloys are 2x24, 2x26 and
2x27 alloys.
[0016] Another embodiment of the invention is an extruded
structural member having improved fatigue crack growth resistance.
The structural member comprising: a base section; a rib stiffening
section having at least one pair of structural ribs, the structural
ribs being integrally formed with the base section and projecting
outwardly thereof; and at least one intra-rib area positioned
between the pair of structural ribs, the intra-rib area having a
microstructure with intentionally increased amounts of fiber
texture to reduce the rate of fatigue crack growth in the extruded
monolithic structural member.
[0017] A third embodiment of the invention is a structural member
made from rolled plate having improved fatigue crack growth
resistance and damage tolerance. The structural member comprising:
at least one area having a microstructure with intentionally
increased amounts of fiber texture to reduce the rate of fatigue
crack growth in the structural member.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] Further features, objectives and advantages of the present
invention will be made clearer from the following detailed
description of preferred embodiments made with reference to the
accompanying drawings in which:
[0019] FIG. A is a traverse cross-sectional view of a typical wing
box construction of an aircraft including front and rear spars of
conventional three-piece built-up design
[0020] FIG. 1A is Fatigue crack propagation rates for 2026-T3511
extrusions of varying thickness;
[0021] FIG. 2 illustrates cross sectional area of the test
extrusion used in the examples. The extrusion contains two square
areas imposed on a rectangular extrusion.
[0022] FIG. 3 illustrates the aspect ratio of a control extrusion
used in the extrusion used in the examples;
[0023] FIG. 4 is a macro-etched cross section of the test extrusion
illustrated in FIG. 2;
[0024] FIG. 5 is a macro-etched cross section of the test extrusion
illustrated in FIG. 3;
[0025] FIG. 6 is an illustration of the areas examined by light
optical microscopy in the extrusions of FIGS. 2 and 3;
[0026] FIG. 7 is a Three dimensional optical micrographs of
extrusion of FIG. 2;
[0027] FIG. 8 is a light optical micrographs of all three planes
taken from areas as indicated from extrusion of FIG. 3,
Anodized;
[0028] FIGS. 9(a) & 9(b) are texture measurements represented
through (111) pole figures showing: (a) fiber texture in extrusion
of FIG. 2 and (b) rolling-type texture of extrusion of FIG. 3;
[0029] FIG. 10 graphs showing tensile properties of extrusion with
corresponding location for the extrusion of FIG. 2
[0030] FIG. 11 is a bar graph showing tensile properties for
extrusion with corresponding location for the extrusion of FIG.
3;
[0031] FIG. 12 shows the location of FCG M(T) specimen for constant
load tests in extrusion of FIG. 2;
[0032] FIG. 13 is a graph showing constant load FCG tests of
textured extrusion of FIG. 2 and rectangular control shape;
[0033] FIG. 14 is a perspective view showing the location of FCG
M(T) specimen for constant DK=15 tests in extrusion of FIG. 2;
[0034] FIG. 15 is a chart showing the FCG results of for constant
DK=15 tests in extrusion of FIG. 2;
[0035] FIG. 16 is a perspective view of an integrally stiffened
panel for Center wing box with 6" spacing;
[0036] FIG. 17 is schematic view of a crosss section of an
integrally stiffened panel for Center wing box with 6" spacing
showing the placement of an intentionally extruded local square
which promote primarily axisymmetric metal flow;
[0037] FIG. 18 is a graph of predicted life of integrally stiffened
panel fabricated with both conventional extrusion techniques and
with fiber texture for improved FCG reistance of the present
invention;
[0038] FIG. 19 is a perspective view Schematic of how a fiber
texture may be inserted into welded stiffened panels for improved
damage tolerance.
[0039] FIG. 20 is a schematic view of how a fiber texture may be
inserted into welded stiffened panels for improved damage
tolerance.
[0040] FIG. 21 illustrates the placement of three different sets of
local geometries which promote primarily axisymmetric metal
flow.
[0041] FIGS. 22-28 are graphs showing FCG data for square and
rectangular sections of 2026-T3511.
DEFINITIONS
[0042] For the description of preferred alloy compositions that
follows, all percentage references are to weight percents (wt. %)
unless otherwise indicated.
[0043] The term "ksi" means kilopounds per square inch.
[0044] The term "minimum strength" or a minimum for another
property or a maximum for a property refers to a level that can be
guaranteed and can mean the level at which 99% of the product is
expected to conform with 95% confidence using standard statistical
methods. And while typical strengths may tend to run a little
higher than the minimum guaranteed levels associated with plant
production, they at least serve to illustrate an invention's
improvement in strength properties when compared to other typical
values in the prior art.
[0045] The term "damage tolerance" is used herein to refer to the
design criteria employed for engineered structures often assume
that preexisting intrinsic flaws (i.e., cracks) are present, and
that these flaws may exist safely in the structure provided they
remain well below a critical size. This approach to design,
referred to as a damage tolerant design methodology, is usually
applied to structures that are susceptible to time-dependent flaw
growth. However, the presence of flaws necessitates the
establishment of quality inspection intervals to ensure that a
critical flaw size is not reached through crack growth processes.
Materials that fulfill the damage tolerant design requirements are
often referred to as "damage tolerant materials". Damage tolerance
design philosophy is particularly important when considering that
most engineered alloys contain intrinsic flaws. For structures
subject to cyclic stresses, a common convention for quantifying
damage tolerance is to monitor the progression of a propagating
crack under cyclic loading. This approach evaluates the ability of
a material to resist fatigue crack growth (FCG), and quality
inspection intervals are established on this basis.
[0046] The term "substantially unrecrystallized", it is meant that
the plate products of this invention are preferably 85 to 100%
unrecrystallized, or at least 60% of the entire thickness of said
plate products are unrecrystallized.
[0047] The term "substantially-free" means having no significant
amount of that component purposefully added to the composition to
import a certain characteristic to that alloy, it being understood
that trace amounts of incidental elements and/or impurities may
sometimes find their way into a desired end product. All preferred
first embodiments of this invention are substantially
vanadium-free. On a preferred basis, these same alloy products are
also substantially free of, bismuth, lead and cadmium.
[0048] The expression "consisting essentially of" is meant to allow
for adding further elements that may even enhance the performance
of the invention so long as such additions do not cause the
resultant alloy to materially depart from the invention and its
minimum properties as described herein and so long as such
additions do not embrace prior art.
[0049] The term "2XXX" or "2000 Series" when referring to alloys
means those structural aluminum alloys with copper as the alloying
element present in the greatest weight percent as defined by the
Aluminum Association.
[0050] The term "2X2X" when referring to alloys means those
structural aluminum alloys with copper and magnesium as the
alloying element present in the greatest weight percent as defined
by the Aluminum Association.
[0051] The term "ingot-derived" means solidified from liquid metal
by a known or subsequently developed casting processes and
includes, but is not limited to, direct chill (DC) continuous
casting, electromagnetic continuous (EMC) casting and variations
thereof, as well as truly continuous cast slab and other ingot
casting techniques.
[0052] When referring to any numerical range of values herein, such
ranges are understood to include each and every number, decimal
and/or fraction between the stated range minimum and maximum. A
range of about 3.6 to 4.2 wt. % copper, for example, would
expressly include all intermediate values of about 3.61, 3.62, . .
. 3.65, . . . 3.7 wt. % and so on all the way up to and including
4.1, 4.15 and 4.199 wt. % Cu. The same applies to all other
elemental ranges, property values (including strength levels)
and/or processing conditions (including aging temperatures) set
forth herein.
[0053] It is believed that various aspects of this invention would
also apply to military and commercial aircraft.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0054] In FIG. 1, there is shown a rough schematic illustrating the
wing box 2. As described above, upper and lower wing skins, 4 and 6
respectively, are typically stiffened by longitudinally extending
stringer members 8 and 10. Such stringer members may assume a
variety of shapes, including "J", "I", "L", "T" and/or "Z" cross
sectional configurations. These stringer members are typically
fastened to a wing skin inner surface as shown in FIG. 1, the
fasteners typically being rivets. It should be remembered at all
times that FIG. 1 is merely a schematic representation of a wing
and not a scale or detailed drawing of any commercial jet aircraft
component part.
[0055] Integrally stiffened members illustrated in FIG. 16 can be
fabricated by extruding to near net shape. Surprisingly, the damage
tolerance of these structural components can be increased if these
stiffening shapes are fabricated by a.) intentionally extruding
oversized local geometries which promote primarily axisymmetric
metal flow during extrusion and then b.) removing excess metal in
these local geometries. The oversized local geometries which
promote primarily axisymmetric metal flow are geometries selected
from the group consisting of circles, squares, polygons and
irregular shapes with aspect ratio within the range of about 0.5 to
about 2.0. These local geometries having a substantially
unrecrystallized microstructure with intentionally increased
amounts of fiber texture.
[0056] After extrusion excess metal (in excess of the metal needed
to form the desired cross section) is removed to form the desired
cross-section. The excess metal is then removed by means of
machining, chemical milling, laser ablation or other techniques
known in the art. After the metal is removed, the extrusion has a
shape that is normally obtained by extruding to near net shape. It
is counter intutitive to add extra metal that is intended to be
disgarded or machined and thereby increase scrap.
[0057] Unexpectedly, the increased amount of amount of <100>
and <111> fiber components in these local geometries, locally
reduce the rate of fatigue crack growth. The improvement in damage
tolerance has been demonstrated in 2x2x alloys and it is expected
that similarly extruded and machined structural member fabricated
from a substantially unrecrystallized extrusion selected from the
group consisting of 2xxx, 5xxx, 6xxx, 7xxx and 8xxx alloys will
also exhibit an increase in damage tolerance.
[0058] The damage tolerance of AA alloys 2026 and 2027 are believed
to be especially sensitive to the the amount of <100> and
<111> fiber components. Without wishing to be bound by any
theory, it is believed that the tighter control on the levels of
iron and silicon in 2026 as compared to 2024 is in part responsible
for the increased damage tolerance of 2026 with increased amount of
<100> and <111> fiber components.
1TABLE 1 Designation Si Fe Cu Mn Mg Cr Zn Ti Zr 2024* 0.50 0.50
3.8-4.9 0.30-0.9 1.2-1.8 0.10 0.25 0.15 -- 2124* 0.20 0.30 3.8-4.9
0.30-0.9 1.2-1.8 0.10 0.25 0.15 -- 2224* 0.12 0.15 3.8-4.4 0.30-0.9
1.2-1.8 0.10 0.25 0.15 -- 2324* 0.10 0.12 3.8-4.4 0.30-0.9 1.2-1.8
0.10 0.25 0.15 -- 2026* 0.06 0.07 3.6-4.3 0.3-0.8 1.0-1.6 -- 0.10
0.06 0.05-0.25 2027* 0.12 0.15 3.9-4.9 0.5-1.2 1.0-1.5 -- 0.20 0.08
0.05-0.15 *Others: each 0.05, total 0.15 Note: Where a range is
indicated, the range indicates a min and max. Where a single number
is indicated, the single number indicates a max.
[0059] In addition, it is believed that the presense of zirconium
in 2026 is in part responsible for the unrecrystalized
mircrostructure which in term promotes better damage tolerance. In
conjunction with the invention, the damage tolerance of 2026
increases unexpectedly. Therefore, 2027 is very likely to have a
pronounced increase in damage tolerance when used as the alloy in
fabricating structural parts of the present invention.
[0060] The invention is particularly useful in the aerospace
industry for designing structural components for which damage
tolerance is a primary consideration. The improved resistance to
fatigue crack initiation and fatigue crack growth, FCG, allow for
longer periods between inspections or higher operating stresses,
which reduce aircraft weight. In addition, the invention allows for
aircraft designers to incorporate monolithic (unitized) design,
thereby reducing the number of aircraft parts and subsequent costs
associated with aircraft manufacture and maintenance, without
increasing the weight of the aircraft.
[0061] Representative structural component parts include extruded
stiffening members, extruded spar members, integrally stiffened
panels and monolithic spar members the like that are extruded or
machined from thick wrought sections, including rolled plate. Such
stiffening and spar members can be used in the wingbox, fuselage
and empennage structures of high capacity aircraft, or in any
application of the aircraft where damage tolerance is critical to
design and certification.
[0062] The present invention is particularly suitable for
manufacturing high strength extrusions and forged aircraft
components, such as, for example, main landing gear beams. Such
aircraft include commercial passenger jetliners, cargo planes (as
used by overnight mail service providers) and certain military
planes. To a lesser degree, the alloys of this invention are
suitable for use in other aircraft including but not limited to
turbo prop planes. In addition, this invention is applicable to the
production of any structural member or application in which damage
tolerance is a critical design property.
[0063] Conventional melting and casting procedures are employed to
formulate the alloy. Care must be taken to maintain high purity in
the aluminum and the alloying constituents so that the trace and
impurity elements, especially iron and silicon, are at or below the
requisite maximums. Ingots are produced from the alloy using
conventional procedures such as semi-continuous direct chill
casting. Once the ingot is formed, the alloy must not undergo a
conventional homogenization, for example, by subjecting the ingot
to elevated temperature of about 915.degree. F. The conventional
homogenization treatment, while entirely adequate for providing an
essentially uniform distribution of alloying elements, results in a
coarse distribution of dispersoids.
[0064] In similar 2XXX alloys, the alloying element Mn is mostly in
supersaturation after ingot casting. During homogenization, Mn
undergoes a solid state reaction and forms dispersoid particles of
approximate stoichiometry Al.sub.20Cu.sub.2Mn.sub.3. These
dispersoids can inhibit recrystallization when present in high
number density by pinning recrystallization nuclei in early stages
of recrystallization via a metallurgical process called Zener drag.
However, the conventional homogenization results in dispersoid
distributions which are in general too coarse to inhibit
recrystallization.
[0065] The alloy product is aimed at being substantially
unrecrystallized. It addresses the above-described shortcomings of
similar 2XXX alloys by the introduction of an additional
dispersoid-forming element Zr, coupled with a well controlled
homogenization treatment that balances the elemental redistribution
while at the same time, provides a dense distribution of Mn-bearing
dispersoids as well as an additional distribution of Zr-bearing
dispersoids. The specially controlled homogenization is carried out
by slowly heating the ingot over a course of about 9 hours (or
longer) to a temperature between about 855.degree. and 880.degree.
F. and is maintained therein for about 18 hours, followed by air
cooling to room temperature.
[0066] Following the homogenization step, the alloys are cooled to
room temperature at any desired rate. This cooling to room
temperature is preferably air cooling. The alloys may be optionally
cooled following homogenization to at least 800.degree. F.
(426.7.degree. C.) at a rate of less than 100.degree. F.
(37.8.degree. C.) per hour, preferably at a rate of less than
70.degree. F. (21.1.degree. C.) per hour. This optional slow
cooling is followed by cooling of the alloys to room temperature at
any desired rate. This cooling to room temperature is preferably
air cooling.
[0067] After cooling the homogenized alloy to substantially room
temperature, the ingot is sawed into billets of appropriate lengths
and scalped. Then the billets may be reheated to an elevated
temperature for extrusion. The reheat process can be carried out
either by induction heating or in an air furnace. In the case of
induction heating, the billet is rapidly heated to the desired
extrusion temperature and extruded. The reheat temperature
represents the optimum starting point for extruding the billet into
the desired configuration based on producing commercially
acceptable product and available press tonnage. The selection of a
reheat temperature can have a major impact on the productivity and
thus the profitability or an extrusion press. Reheating the billet
to too low a temperature results in recrystallization during
subsequent solution heat treatment and hence, lower or failing
strength (depending on the specification). Reheating the billet to
too high a temperature results in low extrusion speeds in order to
produce acceptable product.
[0068] After the material has reached the reheat temperature, it is
ready to be placed in the extrusion press and extruded. In an
effort to avoid unnecessary cooling of the billet, care is taken to
minimize the time it takes to transport the material from the
reheat furnace to the extrusion press. The billet is placed into a
heated compartment or container in the extrusion press. All of the
foregoing steps relate to practices that are well known to those
skilled in the art of casting and extruding. Each of the foregoing
steps is related to metallurgical control of the metal to be
extruded.
[0069] The billets can then be extruded. As will be described in
more detail below, when hot working the alloy to produce
extrusions, extreme care must be taken to prevent any substantial
recrystallization or tearing of the extrusion surface. As stated
above, the term "substantially unrecrystallized" means that less
than about 20 vol. % of the alloy microstructure in a given product
is in a recrystallized form, excepting surface layers of extrusions
which often show complete recrystallization. In any event, the
surface layers of extrusion products are often removed during
fabrication into final part configurations. As will be described in
more detail below, recrystallization (including the surface layer)
can be minimized by maintaining the temperature of the alloy during
hot working at levels that cause annealing out of internal strains
produced by the working operation such that recrystallization will
be minimized during the working operation itself, or during
subsequent solution treatment.
[0070] After the alloy is extruded into a product, the product is
typically solution heat treated at a temperature on the order of
920.degree. F. for a time sufficient for solution effects to
approach equilibrium. Once the solution effects have approached
equilibrium, the product is quenched using conventional procedures,
normally by spraying the product with or immersing the product in
room temperature water. After quenching, extruded products may be
stretched or stress relieved to develop adequate strength, relieve
internal stresses and straighten the product.
[0071] Large intermetallic compounds formed during solidification,
fabrication and heat treatment will lower the fracture toughness of
the alloy. It is therefore most important to maintain the level of
the elements which form intermetallic compounds at or below the
allowable maximum set forth above. Intermetallic compounds may be
formed from the major alloying elements copper, magnesium and
manganese, as well as from impurity elements, such as iron and
silicon. The amount of the major alloying element copper is
constrained so that the maximum amount of this element will be
taken into solid solution during the solution heat treatment
procedure, while assuring that excess copper will not be present in
sufficient quantities to cause the formation of any substantial
volume of large, unwanted intermetallic particles containing this
element. The amounts of the impurity elements iron and silicon are
also restricted to the very low levels as previously indicated in
order to prevent formation of substantial amounts of iron and
silicon containing particles.
[0072] If the total of large intermetallic compounds formed by
copper, magnesium, manganese, iron and silicon, such as CuAl.sub.2,
CuMgAl.sub.2, Al.sub.12(Fe,Mn).sub.3Si, Al.sub.7Cu.sub.2Fe and
Mg.sub.2Si in an alloy otherwise made in accordance with the
present invention exceeds about 1.5 vol. % of the total alloy, the
fracture toughness of the alloy will fall below the desired levels,
and in fact may fall below the fracture toughness levels of similar
prior art alloys of the 2024 type. The fracture toughness
properties will be enhanced even further if the total volume
fraction of such intermetallic compounds is within the range of
from about 0.5 to about 1.0 volume percent of the total alloy. If
the foregoing preferred range of intermetallic particles is
maintained, the fracture toughness of the alloy will substantially
exceed that of prior art alloys of similar strength.
[0073] The extrusion process involves a considerable amount of
deformation energy. Most of this energy transforms into heat, but
part of the deformation energy is stored in the material. The lower
the extrusion temperature and/or the higher the extrusion speed,
the higher the stored energy of deformation. The 2XXX alloys, as is
the case with most aerospace aluminum alloys, require a solution
heat treatment subsequent to extrusion, during which the stored
energy of deformation is dissipated. For materials with a high
stored energy, the stored energy dissipation manifests in the
undesirable recrystallization.
[0074] Recrystallization causes the loss of the strengthening
deformation texture built up during extrusion. It also changes the
grain structure by replacing the low angle grain boundaries in the
deformed or unrecrystallized state with high angle grain
boundaries. The high angle grain boundaries are susceptible to
heterogeneous precipitation during the quenching operation of the
subsequent solution heat treatment. The high angle grain boundaries
with heterogeneous precipitates are weak links in fracture
processes and preferred sites for anodic corrosion attack. A
recrystallized 2XXX product, therefore, may fail to meet certain
property specifications such as strength, toughness and corrosion
resistance. It is important to have an unrecrystallized structure
in the product after thermal and mechanical processing so that the
fiber texture is retained in the final product.
[0075] The extrusion procedure itself is controlled to minimize
recrystallization in the final product and to thus maintain the
strength and toughness of the product at the desired improved
levels. U.S. Pat. No. 4,294,625 discloses that desired properties
can be achieved if the alloy is extruded at temperatures at or
above about 770.degree. F. while holding the extrusion speed such
that the degree of recrystallization in the final wrought product
is minimized.
[0076] The extrusion conditions (speed and temperature) of hard
aluminum alloys are determined empirically and kept below safe
speed and temperature limits by experience to reduce the risk of
impairing the quality and properties of the extruded product.
[0077] Exact extrusion speeds and temperatures are of course
dependent upon such factors as starting billet size, extrusion size
and shape, number of die openings, press tonnage and method of
extrusion (direct or indirect). It is necessary to achieve a
substantially unrecrystallized structure in the extruded product in
order to obtain the desired mix of properties. The unrecrystallized
structure thus produced is very beneficial to strength. An 8.8
(18%) ksi or greater differential has been noted between
unrecrystallized and recrystallized structures of extrusions of the
alloy. Likewise, the unrecrystallized structure is usually superior
to its recrystallized counterpart in fracture toughness, as it is
more difficult to propagate cracks in the finer unrecrystallized
structure of the alloy in which the heterogeneously nucleated grain
boundary precipitates are much finer.
[0078] Extruded products may be stretched as a final working
procedure in order to straighten and strengthen the product and to
remove residual quenching stresses from the product. It should be
noted that the stress patterns in the cold worked alloy are
reversed from those of normal solution treated and quenched
material; i.e., the surface layers of the alloy are in tension and
the center is in compression. Stretching a product beyond 2% to 3%
up to about 8% provides a continual increase in strength. Where
such increased strength is not needed, extrusions are stretched 1%
to 3%, as is normally required for all commercial alloys for
aerospace applications.
[0079] The benefit of the present invention is illustrated in the
following examples.
EXAMPLES
[0080] Integrally stiffened members represented in shapes used in
aerospace structures are fabricated by extruding to near net shape.
As described in priority document of U.S. Provisional Patent Serial
No. 06/339,715, the variability observed in fatigue crack growth
(FCG) tests of damage-tolerant extrusion alloy 2026 in the -T3511
condition was a result of the crystallographic texture that
developed during processing. Two extrusion processing parameters
were identified as having the most significant influence on texture
evolution and subsequent FCG behavior: extrusion aspect ratio (AR)
and extrusion ratio.
[0081] The observed effect of aspect ratio, extrusion width divided
by the thickness, is clearly illustrated in FIG. 1. As the aspect
ratio increases from unity, i.e. an extrusion with a square cross
section, to values from 15 to 20, i.e. extrusions with rectangular
cross-sections, the resistance to FCG decreases. Therefore, the
square extrusion areas were shown to have improved FCG resistance
compared to the rectangular extrusion areas. It was further shown
that the crystallographic texture that developed in the square
extrusions with an AR=1 was similar to that of extruded aluminum
rod and bar, whereas the texture measured in the extrusions with
high aspect ratios (>15) was similar to that observed in rolled
aluminum plate. The results from the thesis work concluded that
texture was the principal microstructural feature that contributed
to the difference in FCG behavior, and the fiber texture
characterized by the <111> and <100> crystallographic
directions provided the best resistance to FCG in 2026-T3511
extrusions. Therefore, the best FCG performance for 2026-T3511
extrusions occurs when a fatigue crack grows through an area with a
fiber texture, and a plane strain, rolling-type, texture provides
the least resistance comparatively.
[0082] Extrusion ratio, defined as the billet cross-section area
divided by the extrusion cross-section area, was identified as the
second processing parameter that governed texture development and
the subsequent FCG behavior. However, the effect was found to be
much less significant than extrusion aspect ratio. If the extrusion
ratio was significantly high (ER>15-20), the intensity of the
texture that developed was adequate for the aspect ratio to govern
FCG behavior. It should also be noted that the formation of the
proper fiber texture in extrusions is also dependent upon the
inhibition of recrystallization during extrusion processing.
Therefore, the appropriate diserpsoids must be present, and the
extrusion processing parameters, namely extrusion speed and
temperature, must be properly controlled.
[0083] It has been found that square extrusions with a fiber
texture provided the best resistance to FCG in 2026-T3511
extrusions. Initially, this discovery was merely incidental of the
extrusion geometry, and therefore of no practical value since it
could be controlled. We have found that it is possible to
selectively insert a fiber texture into a region of an extrusion
where a fiber texture would not naturally develop. It was believed
that the FCG resistance of such an extrusion would be enhanced
significantly from the presence of a fiber texture. In addition to
improvements in FCG, 2026-T3511 extrusions with a fiber texture
exhibited improved S--N fatigue, higher strength and presumably
higher toughness as well, and therefore, it was believed that by
selectively tailoring the texture these properties would be
enhanced too.
[0084] Targeted applications for this invention are focused on
aerospace structure, both conventional and newly developing
monolithic concepts. Conventional, "built-up" aluminum aerospace
structure would benefit from this invention by extending the period
of time required for initial damage inspection, which is based on
fatigue performance. Frequently, fatigue cracks initiate around
mechanically fastened joints, which increase the local stress
concentration. If a fiber texture were developed in this region,
the time of crack initiation would be extended, effectively
increasing the period for initial inspection. Furthermore, the
inspection intervals, which are established by the FCG performance,
proceeding the initial inspection could be set further apart as the
fiber texture would improve the characteristic da/dN performance of
the material. Therefore, the improved fatigue and FCG performance
granted by selectively tailoring the texture in aerospace structure
would lead to less frequent repairs and longer service life of the
aircraft, both which lead to a reduction in costs associated with
inspection and repairs.
[0085] Similarly, if it can be shown that the FCG resistance
offered by an improved material or product is significantly better
than the incumbent such that an aircraft component will experience
an equivalent service life at elevated stresses, then cost savings
can be realized through more efficient fuel economy by reducing
parts thickness and aircraft weight. Furthermore, increased
fracture toughness characteristics of a material allow for
formation of larger cracks and fatigue damage before failure
occurs. Therefore, safety inspectors have additional opportunities
to find and repair this damage, which improves the safety of the
aircraft and enables extended inspection intervals. Therefore, the
improved fracture toughness, resistance to fatigue initiation and
FCG provided by aluminum products with fiber texture translate to
potential cost savings in aircraft design by longer and less
frequent inspection intervals and improved aircraft
performance.
[0086] The impact this invention could have on new aerospace design
concepts is also quite promising. The improved FCG resistance that
the fiber texture provides would enable increased damage tolerance,
which would enable original equipment manufacturers (OEMs) to
introduce monolithic/unitized structure. The use of monolithic
design concepts are driven by the desire for OEMs to save weight
and lower manufacturing costs through parts consolidation. However,
these new design concepts require improvements in material
properties and new design approach to carry additional loads and
arrest crack growth.
[0087] In conventional, "built-up", structure attaching additional
structural member with rivets creates crack arrest features. These
structural members reduce the local load in a cracked or damaged
component and reduce the driving force for crack growth. The
results of the present invention suggest that aluminum components
with improved damage tolerance from controlled texture development
may provide the improved properties required to complement these
new design concepts.
[0088] Furthermore, it is also believed that other alloy systems
and alloy families will likely benefit from this invention. FCG
behavior in many Al--Li alloys to be affected by crystallographic
texture, and therefore, it is likely that textured extrusions with
improved properties could be developed for these alloys. In
addition to extrusion processing, it is also possible to control
the evolution of texture in other wrought processing techniques,
such as rolling. By rolling aluminum plate and then rotating it
90.degree., it has been shown that a fiber texture can be created,
which is not expected.
Examples
[0089] The purpose of the current examples is to create an
extrusion with a fiber texture selectively inserted in a region of
the extrusion where a fiber texture would not naturally develop. A
fiber texture can be created in an extrusion through the use of
channel dies, feeder plates, spreader dies, two-step extrusions (or
sometimes referred to as double-extrusion). In addition, a fiber
texture fiber texture will naturally develop in areas where the
metal flow is axisymmetric, such as in thick square sections or
round section. Axisymmetric metal flow was promoted in local areas
of an extrusion by inserting two square areas in the cross-section
of the extrusion die that would otherwise be rectangular.
[0090] An experimental extrusion die, was designed, and finite
element analysis was used to predict the metal and verify that the
proper texture would develop. A drawing of the extrusion profile is
provided in FIG. 2, which shows two square areas imposed on a
rectangular extrusion. The two square areas were inserted to
produce a fiber texture. A second extrusion, illustrated in FIG. 3,
was produced as baseline for comparison, which had a simple
rectangular cross section.
[0091] The extrusions were both fabricated from extruded AA2026 on
an indirect press at temperatures ranging from 500-800.degree. F.,
and a speed of 5-15 feet/minute. The extrusions are then solution
heat treated at 920-930.degree. F. for up to 30 minutes. The parts
were then stretched 1-3% and allowed to naturally age for 4
days.
[0092] Both extrusion were fully characterized and compared to one
another. The microstructure was characterized through optical
microscopy and x-ray diffraction. Mechanical properties were
characterized through static, fatigue, toughness and FCG tests. In
addition, the corrosion resistance was characterized through
exfoliation corrosion tests.
[0093] The samples were macro-etched in a caustic solution to
reveal the grain structure and to identify if a large amount of
recrystallization had occurred. FIGS. 4 and 5 shows both test
extrusion and the control extrusion were primarily unrecrystallized
with only minimal areas of local recrystallization around the
corners and edges.
[0094] Light optical microscopy was performed on both the test
extrusion and rectangular control extrusion, and three-dimensional
micrographs were constructed. The areas of the extrusion that were
evaluated are provided in FIG. 6. Optical micrographs for extrusion
taken at locations L3 & L5 are provided in FIG. 7. The grains
at location L3 are axisymmetric in the transverse plane and highly
elongated along the extrusion axis, typical of extruded rod or bar.
In contrast, the grains at location L5 are flat and pancake like
and highly elongated along the extrusion axis, which is typical of
rolled aluminum plate. Both the "rod" or "fiber"--like grains and
the flat pancake grains are indicative of the local extrusion
geometry due to the constraint these geometries place on metal flow
through the extrusion die. It is therefore not surprising that
resultant grain dimensions would be indicative of the local
extrusion geometry.
[0095] The grain shapes observed in FIG. 8 of the 6.times.0.375 in.
extrusion at location L5 are similar to those at L5 in extrusion of
FIG. 2, and the grain structure is consistent throughout.
Considering that the local geometry at location L5 in extrusion of
FIG. 2 is flat and rectangular, it is, therefore expected that the
rectangular extrusion would have flat pancake-like grains in all
locations.
[0096] Texture measurements were performed on both extrusion of
FIG. 2 and the rectangular extrusion. Texture measurements were
taken from the locations indicated in FIGS. 9(a) and 9(b). The data
from the texture measurements from the extrusions are provided as
(111) pole figures, and the data show that extrusion of FIG. 2 has
a fiber texture within the square region and the 6.0.times.0.375
in. rectangular extrusion has a rolling-type texture. The texture
results are quantified in Table 2 with the volume fractions of the
individual components for each texture provided, and a square and
rectangular shape for which the textures are known to be fiber and
rolling-type respectively are provided for comparison.
2TABLE 2 Texture Components present in the various extruded shapes
represented as volume fractions (%) of total crystallographic
orientations Fiber Component Rolling Component Excess Excess
.alpha.- Alloy Shape <111> <100> brass Cu S 2026-T3511
4.85 .times. 4.85 in. 28.77 0.5 -- -- -- 2026-T3511 18.5 .times.
1.125 in. -- -- 6.11 2.82 5.93 2026-T3511 Z-1817 15.16 7.56
2026-T3511 6.0 .times. 0.375 in. 7.18 1.37 5.5
[0097] The data in Table 2 conclusively show that there is a strong
intensity of <111> and <100> fiber components in the
4.85.times.4.85 in. square extrusion and in the square area of
extrusion of FIG. 2. The data also show that the beta, copper and S
components, indicative of rolled aluminum sheet and plate, are
present in the two rectangular extrusions. These data are
consistent with the optical micrograph analysis, as well.
[0098] Mechanical Properties
[0099] Mechanical tests were performed to compare the mechanical
properties between the experimental extrusion of FIG. 2, and the
rectangular extrusion.
[0100] Tensile Results:
[0101] Tensile tests were performed at various locations along the
cross-section of the extrusions according to FIG. 10. These
locations indicate the change in strength as a function of the type
of texture that developed. Locations 2 and 3 are regions with the
strong fiber texture, and consequently they have the highest values
of strength, UTS 80 ksi. Whereas, the flat thin section of location
with a rolling-type texture has the lowest strengths with a UTS of
only 74 ksi. The increase in strength exhibited in the fiber region
is expected, and the rest of the extrusion maintains strength
properties comparable to other extrusions, i.e. there is no loss in
strength. This is illustrated in FIG. 11, which shows that the
tensile properties of the rectangular shape are roughly constant
throughout the cross-section and are equal to the lower bound
properties of the of FIG. 2 extruded shape. UTS of roughly 76 ksi
was measured across the rectangular shape, and the lowest UTS
measured for shape of FIG. 2 at Location 5 was 74 ksi. Therefore it
can be concluded that the fiber texture in the of FIG. 2 extruded
shape improved the tensile strengths locally with no degradation in
strength in the other areas.
[0102] Fatigue Crack Growth Results:
[0103] Constant load and constant stress intensity range, .DELTA.K,
FCG tests were performed on the 2026-T3511 extrusions. The constant
load tests were performed on 4 in. wide M(T) specimens sectioned
from the extrusion with the starter notch centered within the cross
section of the extrusion as shown in FIG. 12. A constant load of
8330 lbs was applied to both samples, and the results from these
tests are plotted as crack length, a, versus the number of applied
cycles, N in FIG. 13. The rectangular extrusion failed after
500,000 cycles, whereas extrusion of FIG. 2 did not fail until
almost 10,000,000 cycles; an order in order in magnitude of
improvement. Thus, it can be concluded that the FCG resistance of
the textured of FIG. 2 extrusion was significantly improved over
the baseline rectangle. It should also be noted that the same size
FCG sample was employed for both extrusions, therefore the
differences in life are not indicative of the extrusion shape, but
only of the inherent resistance to FCG of the material.
[0104] The constant .DELTA.K FCG tests were performed on M(T)
panels, which were the full width, 5.3 in., of extrusion of FIG. 2
and rectangular extrusion as shown in FIG. 14. The results from
these tests are plotted in FIG. 15 with crack growth rate, da/dN,
as a function of the crack length, a. The FCG rates measured on the
rectangular extrusions were relatively constant at roughly
5*10.sup.-6 and did not change as the crack length increased.
However, the FCG rates measured for the extrusions with the fiber
texture were not constant. The rate of propagation was for the
fatigue crack in extrusion of FIG. 2 was equivalent to the
rectangular extrusion initially. But, as the fatigue crack grew
outwards from the center of the extrusion, the rate of propagation
decreased dramatically as the crack approach the region with a
fiber texture. The FCG rate reached a minimum of 7E-7 in/cycle,
which is about 7.times. slower relative to the rectangular
extrusion. The crack growth rate increases as the crack leaves the
region with the fiber texture and approached the rate initially
measured. It can therefore be concluded that selectively inserting
into an extrusion significantly improves the FCG resistance.
[0105] Based on these findings and the findings referenced in the
previous work on fiber texture and FCG, it can conclusively be
determined that inserting a fiber texture in given areas of an
extrusion significantly improves strength, FCG resistance, fatigue
initiation and toughness. In addition, there is no decrease in
other properties such as corrosion resistance, formability or any
other properties required by structural metals for aerospace or
other applications.
[0106] As this technology has been identified as having benefit for
aerospace applications, particularly in monolithic structure, a
computer analysis was performed to illustrate the benefit FCG
resistance that might be expected for a particular application. The
application selected was an integrally stiffened panel for lower
wing applications. The FCG resistance of alloy 2026-T3511 was
modeled using a FASTRAN analysis, and typical FCG curves were
generated for an extrusion having both a rolling-type texture and a
fiber texture.
[0107] After the typical FCG curves were produced for the different
textures they were applied to the integrally stiffened panel for
which the driving force for crack growth as a function of crack
length is known. The configuration of the panel is provided in FIG.
16, and an illustration of how a fiber texture may be inserted into
this shape to improve the FCG performance is given in FIG. 17. In
FIG. 17 the region where the squares are inserted would be machined
away after extrusion, but the fiber texture would remain. The
predicted life of this panel provided that a crack initiated near
one of the stiffeners is provided in FIG. 18. As the crack grows
for the first two inches, the FCG rates and therefore, the a vs N
curve for the two curves are identical. However, as the crack
approaches the fiber region the crack begins to slow, and therefore
there is a change in the a vs. N curve.
[0108] This improvement in FCG resistance results in a total life
4,000 cycles for the extrusion with the fiber texture, compared to
only about 2,000 cycles for the panel fabricated by conventional
extrusion. These results are not absolute, but the trends that are
provided are valuable. It can be concluded from these results that
an integrally stiffened panel fabricated with a fiber texture
intentionally inserted would gain a significant amount of
resistance to FCG, and therefore the FCG life of the panel would
increase.
[0109] In addition to individual panels, this technology could also
be used to improve the FCG performance of welded panels to further
enable monolithic designs in aerospace applications. In FIG. 19, a
rendering of a friction stir welded integrally stiffened panel is
shown, and FIG. 20 shows how these panels might be fabricated to
stop cracks that might initiate from within the weld zones from
progressing throughout the rest of the structure so rapidly.
Friction stir welding, laser welding and other welding techniques
can be used with the present invention.
[0110] FIG. 21 illustrates the placement of three different sets of
local geometries which promote primarily axisymmetric metal flow.
In `A`, these local geometries are square shaped and they are
located around the stiffening ribs and in the intra rib area. In
`B`, the local geometries are used to create texture in the panel.
A pluarity of geometries are used to create areas in with fatigue
crack growth rates are retarded. In `C`, circular cross-sectional
geometries are used to create fiber texture in the base of the
stiffening ribs and the area of the panel surrounding the ribs. In
each case, the excess metal is removed to produce the same
structure shape.
[0111] 2026-T3511 Extrusions were fabricated with a square cross
section (with a fiber texture) and rectangular cross sections (with
a rolling type texture). The FCG curves are shown n FIGS.
22-28.
[0112] The lower FCG rates shown in FIG. 22 for the square
extrusions provide us with the basis for the patent. These results
are quantified below in Table 3. Therefore the actual improvement
can be calculated by comparing the FCG rates of different samples
at a given value of .DELTA.K.
3 TABLE 3 da/dN (in/cycle) @ .DELTA.K (ksi.check mark.in): Alloy
Shape .DELTA.K = 10 .DELTA.K = 15 .DELTA.K = 20 .DELTA.K = 25
.DELTA.K = 30 .DELTA.K = 35 2026 Square 1.06E-06 2.30E-06 8.55E-06
1.20E-05 3.08E-05 9.48E-05 2026 Square 6.67E-07 1.74E-06 4.41E-06
2.30E-05 3.89E-05 1.04E-04 2026 Rect. 3.48E-06 6.24E-06 1.86E-05
4.61E-05 1.14E-04 2.32E-04 2026 Rect. 3.97E-06 1.34E-05 2.74E-05
7.46E-05 1.58E-04 3.76E-04
[0113] In FIG. 23 there is little difference among the shapes,
therefore texture does not have the same effect on 2024.
[0114] In FIG. 24 there is little difference among the shapes;
therefore texture does not have the same effect on 2224 as it does
for 2026.
[0115] In FIG. 25 shows further evidence of texture influence on
2026-T3511 extrusions: FIG. 25 is a collection of various shapes.
The aspect ratio for these shapes was calculated based on the local
geometry where the specimen was taken. From these FCG rates, it is
evident that the resistance to FCG decreases as the aspect ratio
increases.
[0116] In FIG. 26 shows FCG Spectrum Data. The specimens are from
the various alloys were also subjected to a spectrum FCG test. The
TWIST spectrum simulates the load sequence that a twin aisle
passenger jet would experience during regular operations. The
spectrum FCG data are more meaningful to airframe designers as they
more closely represent service conditions, and often improvements
in FCG resistance exhibited during constant amplitude FCG tests are
absent during spectrum tests. These data are plotted below with the
fatigue crack length shown as a function of flights. These data
show that the 2026 square extrusion did not fail until over 15,000
flights, whereas the 2024 square extrusions and 2026 rectangular
extrusions, which survived about the same number of flights, failed
at just over 8,000 flights. Therefore, it can be concluded that
2026 square extrusions with a fiber texture provided nearly twice
the service life as both 2024 square extrusions with a fiber
texture and 2026 extrusions with a rolling-type texture.
[0117] In FIG. 27 illustraates improvement in fatigue initiation
(S--N fatigue). Additional test results show that the improvement
the fiber texture provides to fatigue crack initiation and growth
through fatigue life tests. FIG. 27 shows that the square
2026-T3511 extrusions provide longer fatigue lives relative to the
rectangular 2026-T3511 extrusions. The average life exhibited by
the square extrusions is nearly 281,000 cycles, whereas the
rectangular extrusions lasted only 157,000 cycles. That is nearly
an 80% improvement, but it should be recognized that variability is
inherent to fatigue data, and this is only illustrative of the
benefits. The actual improvement in service depends greatly on the
loading conditions, environment and application.
[0118] In FIG. 28 show results from Experimental Extrusion of FIG.
2. The key results are shown in the FIG. 2, which shows an order in
magnitude of improvement to resist fatigue crack growth.
[0119] Table 4 below shows property data from testing.
4 TABLE 4 da/dN (in/cycle) @ .DELTA.K (ksi.check mark.in): Alloy
Shape .DELTA.K = 10 .DELTA.K = 15 .DELTA.K = 20 .DELTA.K = 25
.DELTA.K = 30 .DELTA.K = 35 2024 Square 1.06E-05 2.84E-05 4.92E-05
8.35E-05 1.35E-04 2.37E-04 2024 Square 6.87E-06 2.51E-05 4.38E-05
6.46E-05 1.04E-04 1.98E-04 2024 Rect. 3.98E-06 9.39E-06 2.88E-05
1.24E-04 2.88E-04 5.85E-04 2024 Rect. 2.53E-06 1.44E-05 3.83E-05
8.40E-05 1.43E-04 3.42E-04 2026 Square 1.06E-06 2.30E-06 8.55E-06
1.20E-05 3.08E-05 9.48E-05 2026 Square 6.67E-07 1.74E-06 4.41E-06
2.30E-05 3.89E-05 1.04E-04 2026 Rect. 3.48E-06 6.24E-06 1.86E-05
4.61E-05 1.14E-04 2.32E-04 2026 Rect. 3.97E-06 1.34E-05 2.74E-05
7.46E-05 1.58E-04 3.76E-04 2224 Square 6.18E-06 3.06E-05 5.03E-05
7.69E-05 9.38E-05 1.39E-04 2224 Square 1.98E-06 5.55E-06 9.90E-06
3.65E-05 6.14E-05 1.41E-04 2224 Rect. 2.86E-06 7.93E-06 1.85E-05
6.63E-05 1.67E-04 3.42E-04 2224 Rect. 3.93E-06 1.16E-05 2.47E-05
7.33E-05 1.80E-04 3.42E-04
[0120] It is to be appreciated that certain features of the present
invention may be changed without departing from the present
invention. Thus, for example, it is to be appreciated that although
the invention has been described in terms of a preferred embodiment
in which 2xxx alloys are used, aluminum alloys comprehended by the
present invention are 2xxx, 5xxx, 6xxx, 7xxx and 8xxx alloys. A
preferred composition for the 2xxx alloys is about 3.6 to about 4.9
wt. % copper, about 1.0 to about 1.8 wt. % magnesium, about 0.15 to
about 0.9 wt. % manganese, about 0.05 to about 0.25% zirconium,
less than about 0.25% zinc, less than about 0.8 silver, less than
about 0.3% iron, less than about 0.25% silicon, the balance
substantially aluminum, incidental elements and impurities.
[0121] In addition, the invention is intended to be used on
aluminum lithium alloys. A preferred aluminum lithium composition
about 0.5 to about 2.7 wt. % lithium, about 1.0 to about 4.5 wt. %
copper, less than about 1.3 wt. % magnesium, less than about 0.8
silver, less than about 0.10 wt. % manganese, about 0.04 to about
0.16% zirconium, less than about 0.25% zinc, less than about 0.3%
iron, less than about 0.20% silicon,the balance substantially
aluminum, incidental elements and impurities.
[0122] In addition, the invention is not limited to forming the
increased amount of amount of <100> and <111> fiber
components in the local geometries by intentionally extruding
oversized local geometries which promote primarily axisymmetric
metal flow during extrusion and then b.) removing excess metal in
these local geometries. Those skilled in the art will recognize
that die design, spreader plates and/or feeder plates can be used
to form increased amount of amount of <100> and <111>
fiber components in the local geometries.
[0123] Although the invention was described in terms of a single
step extrusion, a double extrusion practice could be used in which
a billet is extruded to a round extrusion which is then used as
billet to extrude to a final near net shape. The first extrusion
step would produce a fiber texture would would be retained in
critical regions of the the near net shape shape.
[0124] In addition, those skilled in the art will recognize that
the invention is not limited to extrusion. Other forming methods
can be employed to form the increased amount of amount of
<100> and <111> fiber components in the local
geometries. Upset forging and rolling may also be used.
[0125] If ingot rolling is to be employed, it is preferred that the
ingot section have a height that is 2-4 times the width. Most
aluminum ingot has a thickness dimension that is much lower than
the width dimension. As rolling proceeds, the thickness is reduced
and the width remains virtually unchanged as the ingot is rolled to
plate or sheet. It is well known, however, that if an ingot is
rolled starting with a thickness greater than the width,
substantial increase in width will occur as rolling proceeds. As a
result of this width increase, a fiber texture develops. The fiber
texture will provide an increase in the damage tolerance as
described earlier. Therefore, the reduction during ingot rolling
are preferably such that the ratio of half the entry width to the
contact length of the plate and roll is less than 1.75.
[0126] Furthermore, it is contemplated that those skilled in the
art will recognize that the the structural members do not
necessarily need to be randomly altered with the increased amount
of amount of <100> and <111> fiber components in the
local geometries. Rather, the local geometries can be positioned
within the structural members in areas in which damage tolerance
and an increased resistance to fatigue crack growth is especially
critical. Thus for example of the amount of <100> and
<111> fiber components can be increased in an area which will
be riveted or welded and a larger area surrounding these areas.
[0127] It is also to be appreciated that although the invention has
been described in terms of metals, the present invention may also
be employed with metal matrix composites, metal laminates,
bimetallics and cermets.
[0128] What is believed to be the best mode of the invention has
been described above. However, it will be apparent to those skilled
in the art that numerous variations of the type described could be
made to the present invention without departing from the spirit of
the invention. The scope of the present invention is defined by the
broad general meaning of the terms in which the claims are
expressed.
[0129] Having described the presently preferred embodiments, it is
to be understood that the invention may be otherwise embodied
within the scope of the appended claims.
* * * * *