U.S. patent application number 11/180398 was filed with the patent office on 2006-02-02 for fatigue-resistance sheet slitting method and resulting sheet.
Invention is credited to Max W. Durney, Justin Koch, Alan D. Pendley.
Application Number | 20060021413 11/180398 |
Document ID | / |
Family ID | 35839788 |
Filed Date | 2006-02-02 |
United States Patent
Application |
20060021413 |
Kind Code |
A1 |
Durney; Max W. ; et
al. |
February 2, 2006 |
Fatigue-resistance sheet slitting method and resulting sheet
Abstract
A sheet of material (111) having a plurality of bend-inducing
structures (113) configured and positioned to produce bending along
a bend line (115). The bend-inducing structures (113) have arcuate
return portions (122) extending from opposite ends (121) back along
the bend-inducing structures (113) toward the other return portion
(122) and each return portion (122) has a length dimension and a
radius of curvature reducing stress concentrations. Preferably, the
length dimension of the arcuate return portion (122) is in excess
of twice the thickness. The lateral distance, LD, to which the
bend-inducing structures (113) is formed in the sheet away from the
bend line (115) is preferably minimized by small radius arcs (125)
which connect the return portions (122) to the remainder of the
bend-inducing structures (113). A method of forming a structure
(131) from a sheet of material (111) to resist cyclical loading is
also disclosed, as is a method to increase the fatigue resistance
of a structure (131) formed by bending a sheet of material (111)
along a bend line (115) having a plurality of bend-inducing
structures (113).
Inventors: |
Durney; Max W.; (San
Francisco, CA) ; Pendley; Alan D.; (Peteluma, CA)
; Koch; Justin; (Deer Creek, IL) |
Correspondence
Address: |
David J. Brezner;Dorsey & Whitney LLP
Intellectual Property Department
555 California Street, 10th Floor
San Francisco
CA
94104
US
|
Family ID: |
35839788 |
Appl. No.: |
11/180398 |
Filed: |
July 12, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10672766 |
Sep 26, 2003 |
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11180398 |
Jul 12, 2005 |
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10256870 |
Sep 26, 2002 |
6877349 |
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10672766 |
Sep 26, 2003 |
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09640267 |
Aug 17, 2000 |
6481259 |
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10256870 |
Sep 26, 2002 |
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60587470 |
Jul 12, 2004 |
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Current U.S.
Class: |
72/324 |
Current CPC
Class: |
B31F 1/0012 20130101;
B21D 31/00 20130101; B29C 53/06 20130101; B21D 11/20 20130101; Y10T
83/0341 20150401; B21D 5/00 20130101; Y10T 428/12354 20150115 |
Class at
Publication: |
072/324 |
International
Class: |
B21D 5/00 20060101
B21D005/00 |
Claims
1. A sheet of material formed for bending along a bend line
comprising: a sheet of material having a plurality of bend-inducing
structures configured and positioned to produce bending along the
bend line, at least one bend-inducing structure having arcuate
return portions extending from opposite ends of the bend-inducing
structure and returning along the bend-inducing structure toward
the other return portion, the return portions each being configured
to significantly reduce stress concentrations resulting from loads
oriented in a direction transverse to the bend line.
2. The sheet of material as defined in claim 1 wherein, the
bend-inducing structures are one of slits, grooves and steps.
3. The sheet of material as defined in claim 2 wherein, the arcuate
return portions curve away from the bend line proximate the
opposite ends and curve back toward the bend line at distal ends of
the return portions.
4. The sheet of material as defined in claim 3 wherein, the arcuate
return portions have a length dimension along the bend line equal
to at least about 2 times the thickness dimension of the sheet of
material.
5. The sheet of material as defined in claim 3 wherein, the arcuate
return portions each have a length dimension along the bend line
equal to at least about 20 percent of the overall length of the
bend-inducing structure along the bend line.
6. The sheet of material as defined in claim 4 wherein, the arcuate
return portions have a radius of curvature section over a majority
of their length of at least 2 times the thickness dimension of the
sheet of material.
7. The sheet of material as defined in claim 4 wherein, the arcuate
return portions have a radius of curvature section over a majority
of their length of at least 3 times the thickness dimension of the
sheet of material.
8. The sheet of material as defined in claim 2 wherein, the
bend-inducing structure is an arc having a convex side facing and
extending along the bend line, and the opposite ends of the arc
transition to the return portions along arcs having a radius of
curvature of between about 0.1 to about 1.0 times the thickness
dimension of the sheet of material.
9. The sheet of material as defined in claim 2 wherein, the
transverse dimension of the bend-inducing structure from the bend
line is less than about 20 percent of the overall length dimension
of the bend-inducing structure.
10. The sheet of material as defined in claim 2 wherein, the
bend-inducing structures are formed by a plurality of connected
arcuate sections that are symmetrical about a transverse center
line of the bend-inducing structure perpendicular to the bend line,
and the opposite ends of the bend-inducing structure have radii of
curvatures less than the radii of curvature of the return
portions.
11. The sheet of material as defined in claim 10 wherein, the radii
of curvature of the return portions are at least about 5 times the
radii of curvature of the opposite ends of the bend-inducing
structure.
12. The sheet of material as defined in claim 2 wherein, each
return portion has a length equal to between about 2 to about 4
times the thickness dimension of the sheet of material and a radius
of curvature between about 2 and about 4 times the thickness
dimension of the sheet of material.
13. The sheet of material as defined in claim 12 wherein, each end
portion has a radius of curvature not greater than approximately
the thickness dimension of the sheet of material.
14. The sheet of material as defined in claim 4 wherein, the
bend-inducing structures are configured and positioned to produce
edge-to-face engagement of the sheet of material on opposite sides
of the bend-inducing structures during bending.
15. A sheet of material formed for bending along a bend line
comprising: a sheet of material having a plurality of bend-inducing
slits positioned in longitudinally staggered relation on
alternating sides of the bend line and configured to produce
edge-to-face contact of the sheet of material on opposite sides of
the slits during bending, the slits each being arcuate with convex
sides closest to the bend line and having arcuate return portions
at opposite ends of the slits extending back along the slits toward
the other return portion, and the arcuate return portions having a
length dimension and radius of curvature reducing stress
concentrations.
16. The sheet of material as defined in claim 15 wherein, the
arcuate return portions have chords oriented substantially parallel
to the bend line.
17. The sheet of material as defined in claim 16 wherein, the radii
of curvature of the arcuate return portions are at least about 2
times a thickness dimension of the sheet of material.
18. The sheet of material as defined in claim 17 wherein, the radii
of curvature of the return portions are between about 2 and about 4
times the thickness dimension of the sheet of material.
19. The sheet of material as defined in claim 15 wherein, the sheet
of material is bent along the bend line.
20. The sheet of material as defined in claim 19 wherein, the sheet
of material is bent along the bend line into a three-dimension
structure suitable for loading transversely to the bend line.
21. A method of increasing the fatigue resistance of a structure
formed by bending a sheet of material along a bend line having by a
plurality of bend-inducing structures positioned and configured to
cause bending along the bend line, the method comprising the step
of: forming the bend-inducing structures to extend along the bend
line and have arcuate return portions extending away from opposite
ends of the bend-inducing structures and back along the
bend-inducing structures toward the other return portion, the
return portions having a length dimension along the bend line and a
radius of curvature selected to be sufficiently large to
significantly reduce stress concentration and significantly
increase resistance to fatigue upon cyclical loading of the bent
sheet of material the transverse to the bend line.
22. The method as defined in claim 21 wherein, the step of forming
the bend-inducing structures with arcuate return portions is
accomplished by forming the arcuate return portions to have a
radius of curvature between about 2 and about 4 times the thickness
dimensions of the sheet of material.
23. The method as defined in claim 22 wherein, during the forming
steps, forming the bend-inducing structures as one of slits,
grooves and steps in the sheet of material.
24. The method as defined in claim 23 wherein, during the forming
steps, forming the bend-inducing structures with a configuration
producing edge-to-face engagement of the sheet of material on
opposite sides of the bend-inducing structures during bending.
25. A method of preparing a sheet of material for bending along a
bend line into a three-dimensional structure and subsequent loading
of the structure transverse to the bend line, comprising the steps
of: forming a plurality of bend-inducing structures along the bend
line, the bend-inducing structures being at least one of slits,
grooves and steps in the sheet of material positioned proximate the
bend line; and during the forming step, forming the bend-inducing
structures with return portions extending from opposite ends of the
bend-inducing structures away from the bend line and back along the
bend-inducing structures toward the other return portion with each
return portion having a length dimension along the bend line
sufficient to substantially increase the resistance to fatigue
failure when the structure undergoes transverse loading.
26. The method as defined in claim 25 wherein, during the forming
step, forming the arcuate return portions with chords oriented
substantially parallel to the bend line.
27. The method as defined in claim 26 wherein, during the forming
step, forming the arcuate return portions with a radius of
curvature at least equal to about 2 times the thickness dimension
of the sheet of material.
28. The method as defined in claim 27, and the step of: after the
forming step, bending the sheet of material into a
three-dimensional structure.
29. The method as defined in claim 28, and the step of: after the
bending step, loading the three-dimensional structure transversely
to the bend line.
30. A method of forming a structure from a sheet of material to
resist cyclical loading comprising the steps of: forming a sheet of
material with a plurality of bend-inducing structures configured
and positioned along a bend line to produce bending of the sheet of
material along the bend line; bending the sheet of material along
the bend line to produce a three-dimensional bent structure; and
during the forming step forming each bend-inducing structure as a
continuous arcuate slit having opposite ends curving away from the
bend line and having arcuate return portions extending from the
opposite ends and curving back along the slit toward the other
return portion with distal ends of the return portions curving back
toward the bend line.
31. The method as defined in claim 30, and the step of: after the
bending step, cyclically loading the bent structure transversely of
the bend line.
32. The method as defined in claim 31 wherein, the step of forming
each bend-inducing structure is accomplished by forming the arcuate
return portions with a chord length extending along the bend line
by at least about 20 percent of the length of the bend-inducing
structure along the bend line.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This application claims priority to U.S. Provisional Patent
Application No. 60/587,470, filed Jul. 12, 2004, entitled METHOD
FOR INCREASING THE FATIGUE RESISTANCE OF STRUCTURES FORMED BY
BENDING SLIT SHEET MATERIAL AND PRODUCTS RESULTING THEREFROM, the
entire contents of which is incorporated herein by this
reference.
[0002] This application is also a Continuation-in-Part Application
of U.S. patent application Ser. No. 10/672,766, filed Sep. 26,
2003, and entitled TECHNIQUES FOR DESIGNING AND MANUFACTURING
PRECISION-FOLDED, HIGH STRENGTH, FATIGUE-RESISTANT STRUCTURES AND
SHEET THEREFOR, which is a Continuation-in-Part Application of U.S.
patent application Ser. No. 10/256,870, filed Sep. 26, 2002, and
entitled METHOD FOR PRECISION BENDING OF SHEET OF MATERIALS, SLIT
SHEETS FABRICATION PROCESS, now U.S. Pat. No. 6,877,349, which is a
Continuation-in-Part Application of U.S. patent application Ser.
No. 09/640,267, filed Aug. 17, 2000, and entitled METHOD FOR
PRECISION BENDING OF A SHEET OF MATERIAL AND SLIT SHEET THEREFOR,
now U.S. Pat. No. 6,481,259, the entire contents of which is
incorporated herein by this reference.
BACKGROUND OF THE INVENTION
[0003] 1. Field of the Invention
[0004] The present invention relates, in general, to the bending of
sheets of material having bend-inducing structures formed therein,
such as slits, grooves, perforations or steps, and more
particularly, relates to improving the resistance of structures
formed by bending such sheets to fatigue failure during cyclical
loading.
[0005] 2. Description of Related Art
[0006] A commonly encountered problem in connection with bending
sheet material using conventional sheet bending equipment, such as
a press brake, is that the locations of the bends are difficult to
control because of bending tolerance variations and the
accumulation of tolerance errors. For example, sheet metal may be
bent along a first bend line within certain tolerances. A second
bend, however, often is located based upon the first bend, and
accordingly, the tolerance errors can accumulate. Since there can
be three or more bends which are involved to create an enclosure or
closed structure, the effect of cumulative tolerance errors in
conventional prior art bending techniques can be significant.
[0007] One approach to this problem has been to try to control the
location of bends in sheet material through the use of
bend-inducing or bend-controlling structures, such as slits,
grooves, perforations or the like. Bend-inducing structures can be
formed in sheet stock at very precise locations, for example, by
the use of computer numerically controlled (CNC) devices to
manipulate lasers, water jets, punch presses, knives or even single
point tools.
[0008] Slits, grooves, perforations, dimples and score lines have
been used in various patented systems as bend-inducing or producing
structures for bending sheet material. U.S. Pat. No. 6,640,605 to
Gitlin et al. employs parallel offset slits to create bendable
sheets in which connecting twisted straps or "stitches" span across
the bend line. The Gitlin et al. slitting technique was developed
to achieve decorative affects, and the resulting bends were
reinforced in most applications to provide the necessary structural
strength. U. S. Pat. No. 5,225,799 to West et al uses a
grooving-based technique to fold up a sheet of material to form a
microwave wave guide or filter. In U.S. Pat. No. 4,628,661 to St.
Louis, score lines and dimples are used to fold metal sheets. In
U.S. Pat. No. 6,210,037 to Brandon, slots and perforations are used
to bend plastics. The bending of corrugated cardboard using slits
or die cuts is shown in U.S. Pat. No. 6,132,349 and PCT Publication
WO 97/24221 to Yokoyama, and U.S. Pat. No. 3,756,499 to Grebel et
al. and U.S. Pat. No. 3,258,380 to Fischer, et al. Bending of
paperboard sheets also has been facilitated by slitting, as is
shown in U.S. Pat. No. 5,692,672 to Hunt, U.S. Pat. No. 3,963,170
to Wood and U.S. Pat. No. 975,121 to Carter.
[0009] In most of these prior art sheet bending systems, however,
the bend-inducing structures greatly weaken the resulting
structure, or the bend-inducing structures do not produce the
desired precision in the location of the bends, or both.
[0010] The problems of precision bending and retention of strength
are much more substantial when bending metal sheets, and
particularly metal sheets of substantial thickness. In many
applications it is highly desirable to be able to bend metal sheets
with low force, for example by hand, with using only hand tools or
with only moderately powered tools.
[0011] Well known conventional fabrication techniques for producing
rigid three-dimension structures include the joining together of
sheet material by jigging and welding, or clamping and adhesive
bonding, or machining and using fasteners. In the case of welding,
problems arise in the accurate cutting and positioning of the
individual pieces during welding, and the labor required to
manipulate a large number of parts is significant, as are the
quality control and certification burden. Additionally, welding has
potential problems in connection with dimensional stability caused
by the heat affected zone of the weld.
[0012] Welding of metal sheets or plates having significant
material thickness is often achieved using parts having beveled
edges made by grinding or single point tools. This adds
significantly to the fabrication time and cost. Moreover, fatigue
failure of heat affected metals under cyclical loading is a problem
for joints whose load bearing geometries are based upon welding,
brazing or soldering.
[0013] A new system for precise bending of sheet material,
including thick sheets, has been devised in which improved
bend-inducing or bend-controlling structures are employed. The
bend-inducing structures are configured and positioned in a manner
such that the three-dimensional structure resulting upon bending of
the sheet has substantially improved strength and dimensional
precision as compared to prior art slitting techniques, such as,
for example, are disclosed in the Gitlin et al. U.S. Pat. No.
6,640,605. The position and configuration of these new and improved
bend-inducing structures facilitate bending of the sheet precisely
along the bend line, most preferably by causing edge-to-face
engagement of the sheet material on opposite sides of the
bend-inducing structures during the entire bend for control of the
bend location.
[0014] The configurations and positioning of these new and improved
bend-inducing slits, grooves and steps are described in much more
detail in the above set forth Related Applications, which are
hereby incorporated by reference in their entireties into this
application.
[0015] Using the improved bend-inducing structures for bending
sheet material has many advantages, not the least of which is the
ability to use a series of precisely located bends to close the
sheet of material back upon itself during bending, for example, in
order to fabricate a box beam. Press brake bending, by contrast, is
not well suited to form closed structures such as box beams. Box
beams are exemplary of structures that have many applications and
have heretofore been formed more traditionally by welding together
of metal sheets or plates, rather than by bending of a single sheet
or plate into a closed hollow beam structure.
[0016] Bending sheet material to form a box beam has substantial
cost-saving advantages over fabrication of the beam by welding, if
the resultant beam has substantially the same strength, and if it
does not fail prematurely due to fatigue during the cyclical
loading. When a box beam is loaded during use, it typically will be
loaded transversely to its length, that is, transversely to the
longitudinally extending corners of the beam along which the sheets
or plates are welded together, or in the case of a folded single
sheet, along the longitudinally extending bend lines. Such loading
is often cyclical and results in fatiguing of the beam at its
corners. For welded box beams, therefore, fatigue failure typically
occurs along the welded corners, and if a bent sheet is to be used,
the corner bend lines will also be the area most likely to
fail.
[0017] Accordingly, it is an object of the present invention to
provide a method for increasing the fatigue resistance of
structures formed by bending slit sheet material.
[0018] It is another object of the present invention to provide an
improved configuration of a bend-inducing structure for sheet
material that will substantially improve the fatigue resistance of
three-dimensional object formed by bending the sheet material.
[0019] A further object of the present invention is to provide
increased fatigue resistance in bent sheet material and improve
strength at the bend line of the sheet material.
[0020] Still a further object of the present invention is to
provide a method and apparatus for enhancing the fatigue resistance
of bent, slit sheet material which does not undesirably increase
the fabrication costs, can be applied to a wide range of
structures, and is adaptable for use with sheets of various
thicknesses and types of materials.
[0021] The method and apparatus of the present invention have other
objects and features of advantage which will become apparent from,
or are set forth in more detail in, the accompanying drawing and
the following description of the Best Mode of Carrying Out the
Invention.
SUMMARY OF THE INVENTION
[0022] In one aspect, the present invention is comprised of a sheet
of material formed for bending along a bend line and having a
plurality of bend-inducing structures configured and positioned to
produce bending along the bend line. At least one of the
bend-inducing structures, and preferably all of them, have arcuate
return portions extending from opposite ends of the bend-inducing
structure and returning along the bend-inducing structure toward
the other return portion. The return portions each are configured
to significantly increase resistance to fatigue resulting from
cyclical loads oriented in a direction transverse to the bend line
by having arcuate lengths and radii reducing stress concentrations.
The bend-inducing structures preferably are slits, grooves or steps
which are configured to produce edge-to-face engagement on opposite
sides of the bend-inducing structures during bending. Stress
concentrations can be reduced by forming the arcuate return
portions with a cord length at least approximately twice the
thickness dimension of the sheet of material. The arcuate return
portions further preferably have chords oriented substantially
parallel to the bend line, and a radii of curvature of the return
portions which are at least approximately three times the thickness
dimension of the sheet of material.
[0023] In another aspect of the present invention a method of
increasing the fatigue resistance of a structure formed by bending
a sheet of material along a bend line having a plurality of
bend-inducing structures is provided. The method comprises,
briefly, the step of forming the bend-inducing structures to extend
along the bend line and have arcuate return portions extending from
opposite ends of the bend-inducing structures back along the
bend-inducing structures toward the other return portion. The
return portions have a length dimension along the bend line and a
radius of curvature selected to be sufficiently large to
significantly increase resistance to fatigue upon cyclical loading
of the structures transverse to the bend line.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] FIG. 1 is a top plan view of a sheet of material having
bend-inducing structures formed therein as shown in the Related
Applications.
[0025] FIG. 2 is a top plan, schematic representation of the slits
of FIG. 1, and FIG. 2A is an enlarged, top plan view of the ends of
the slits of FIG. 2.
[0026] FIG. 3 is a top plan, schematic representation corresponding
to FIG. 2 of an alternative embodiment of the slits showing arcuate
return portions.
[0027] FIG. 3A is an enlarged, top plan view of an end of the slit
of FIG. 3.
[0028] FIG. 4 is a top plan, schematic representation corresponding
to FIG. 2 of a further alternative embodiment of the slits showing
an extended arcuate return portions.
[0029] FIGS. 4A and 4B are enlarged, top plan views of the end of
the slit of FIG. 4.
[0030] FIG. 5 is a top plan, schematic representation corresponding
to FIG. 2 of a further alternative embodiment of slits having a
configuration and constructed in accordance with the present
invention.
[0031] FIGS. 5A and 5B are enlarged, top plan views of the end of
the slit of FIG. 5.
[0032] FIG. 6 is a schematic, side elevation view of a fatigue test
stand with a box beam constructed using the slit configurations of
FIG. 4 in position for testing.
[0033] FIG. 6A is an end view of the beam of FIG. 6.
[0034] FIG. 7 is a graph of stress versus cycles-to-failure for
beams tested using the fatigue test stand of FIG. 6 and showing
welding curves for class B to class G welds.
[0035] FIG. 8 is a table showing the test results for the beams
tested using the test stand of FIG. 6.
DETAILED DESCRIPTION OF THE INVENTION
[0036] Reference will be made in detail to the preferred embodiment
of the invention, an example of which is illustrated in the
accompanying drawings. While the invention will be described in
conjunction with the preferred embodiment, it will be understood
that it is not intended to limit the invention to that embodiment.
On the contrary, the invention is intended to cover alternatives,
modifications and equivalents, which may be included within the
spirit and scope of the invention as defined by the appended
claims.
[0037] The present method and apparatus for precision bending of
sheet material is based upon the bend-inducing slits, grooves or
steps disclosed in the above-identified Related Applications, and
particularly, as disclosed in U.S. patent application Ser. No.
10/672,766, filed Sep. 26, 2003 and entitled TECHNIQUES FOR
DESIGNING AND MANUFACTURING PRECISION-FOLDED, HIGH STRENGTH,
FATIGUE-RESISTANT STRUCTURES AND SHEETS THEREFOR. FIG. 6 of U.S.
patent application Ser. No. 10/672,766 has been incorporated in
this application as FIG. 1 to illustrate the changes made by the
present invention to the slit groove or step configurations in
order to increase fatigue resistance.
[0038] Referring specifically to FIG. 1, a sheet of material 41 to
be bent or folded along a bend line 45 is formed with a plurality
of longitudinally extending bend-inducing structures. These
bend-inducing structures may be any one of slits, grooves or steps
43 positioned along bend line 45, but for brevity they will be
referred to herein as "slits" or "bend-inducing structures." Each
bend-inducing structure 43 is shown as having a kerf in FIG. 1 and
essentially no kerf in FIGS. 2 through 5B. The presence or absence
of a kerf does not form a part of the present invention. Slits 43
also have enlarged stress-relieving end openings 49, or a curved
end section 49a (the slit on the right-hand end of FIG. 1). In
addition, the slits may have a curved end. Curved end 49a
terminates the slits in a relatively low stress zone, thereby
decreasing the likelihood that cracking will initiate at a terminus
of the curved end. Slits 43 are configured in a manner producing
bending and twisting of obliquely oriented bending straps 47 about
a virtual fulcrum superimposed on bend line 45. The configuration
and positioning of the bend-inducing structures, including
selection of the jog distance and kerf width, causes the sheet
material on opposite sides of the bend-inducing structures to tuck
or to move into an edge-to-face interengaged relationship during
bending, as is set forth in detail in the Related Applications and
will not be repeated herein. Most preferably, edge-to-face
interengagement occurs throughout the bend to its completion; but,
the jog distance and kerf also can be selected to produce
edge-to-face interengagement only at the start of the bend, which
also will tend to ensure precise bending. Thus, as used herein, the
expression "during bending" is meant to include edge-to-face
interengagement at any stage of the bend that will produce precise
bending. Interengagement only at the end of the bend will not
control the location of the bend with the same degree of
precision.
[0039] As shown in FIG. 1, pairs of elongated slits 43 are
preferably positioned on opposite sides, of and proximate to, bend
line 45 so that pairs of longitudinally adjacent slit end 51 on
opposite sides of the bend line define a bending web, spline or
strap 47, which can be seen to extend obliquely across bend line
45. "Oblique" and "obliquely," shall mean that the longitudinal
central axis of straps 47 cross the bend line, and cross at an
angle other than 90 degrees. Thus, each slit, groove or step end
portion 51 diverges away from bend line 45 so that the center line
of the strap is skewed or oblique to the bend line. This produces
bending as well as twisting of the strap.
[0040] Unlike the slits or grooves of the prior art Gitlin, et al.
U.S. Pat. No. 6,640,605, which are parallel to the bend line in the
area defining the bending straps, the divergence of the
bend-inducing structures 43 from bend line 45 results in oblique
bending straps that do not require the extreme twisting present in
the straps of the Gitlin, et al. patent. Moreover, the divergence
of bend-inducing structures 43 from bend line 45 results in the
width dimension of the straps increasing as the straps connect with
the remainder of sheet 41. This increasing width enhances the
transfer of loading across the bend so as to reduce stress
concentrations and to increase fatigue resistance of the
straps.
[0041] As above noted, the width or kerf of slits and the
transverse jog distance across the bend line between slits, are
preferably dimensioned to produce interengagement of sheet material
on opposite sides of the slits during bending. If the kerf width
and jog distance are so large that contact does not occur, the bent
or folded sheet will still have some of the improved strength and
fatigue-resistance advantages of oblique bending straps. In such
instances, however, there are no actual fulcrums for controlled
bending to occur so that bending along bend line 45 becomes less
predictable and precise. Similarly, if the strap defining
structures are grooves 43 which do not penetrate through the sheet
of material, the grooves will define oblique, high-strength bending
straps, but edge-to-face sliding will not occur during bending
unless the groove is so deep as to break-through during bending and
become a slit.
[0042] It is also possible for the slits 43 to actually be on the
bend line or even across the bend line (a negative jog distance)
and still produce precise bending from the balanced positioning of
the actual fulcrum faces 55 and the edges of lips 53 sliding
therealong. A potential disadvantage of bend-inducing structures 43
being formed to cross the bend line 45 is that an air-gap would
remain between the opposed edges and faces. An air-gap, however,
may be acceptable in order to facilitate subsequent welding,
brazing, soldering, adhesive filling or if an air-gap is desired
for venting.
[0043] In the slit sheet of FIG. 1, both oblique bending straps 47
and stress-reducing opening or enlargements 49 have been employed
in an attempt to increase the resistance to fatigue failure of the
structure formed by bending sheet 41. Additionally, the right-hand
slit or groove 43 has been formed with an arcuate return portion or
extension 49a in order to terminate slits 43 in a zone of
relatively low stress. While effective to some extent, these
strategies for increasing the fatigue resistance of bend-inducing
slits, grooves or steps sill have not achieved the fatigue
resistance that is desirable for structures which are subjected to
repeated heavy cyclical loading.
[0044] More particularly, box beams which are formed using the
sheet slitting, grooving or step-forming techniques as taught by
the above-identified Related Applications are often subjected to
cyclical loading in bending. Such loading can cause premature
fatigue failure of the beams, with disastrous effects.
[0045] FIGS. 2, 2A, 3, 3A, 4, 4A and 4B schematically illustrate
the evolution of the configuration of the bend-inducing structures
which have resulted in the greatly improved, fatigue-resistant
geometry shown in FIGS. 5, 5A and 5B.
[0046] FIGS. 2 and 2A correspond to FIG. 1 except that the
bend-inducing structures 43 are shown with ends 51 which do not
have stress-relieving openings or enlargements 49, as shown in FIG.
1. Similarly, ends 51 in FIGS. 2 and 2A do not have a return
portion 49a which curves back along the slits.
[0047] In FIGS. 2 and 2A, diverging slit ends 51 again define
oblique bending straps 47, which will produce precise bending of
sheet 41 along bend line 45. When the sheet of FIGS. 2 and 2A is
bent and then loaded transversely to bend line 45, failure of the
resulting structure under cyclical loading will most likely occur
at the ends of slits 43, as schematically shown in broken lines at
39 in FIG. 2A. Crack 39 will propagate transversely away from bend
line 45 and can cause failure of the three-dimensional structure
formed by bending sheet 41.
[0048] In FIGS. 3 and 3A, sheet 71 is formed with a plurality of
bend-inducing structures, such as slits 73, which are positioned
relative to bend line 75 in a manner taught by the Related
Applications. In the embodiment shown in FIGS. 3 and 3A, end
portions 81 of the slits are formed with relatively large diameter
arcuate return portions 82. Thus, the return portions 82 are
similar in concept to that shown in FIG. 1 by arcuate end 49a, but
the radius of curvature of end return portions 82 is much greater
than was the case for return portion 49a. Again, the concept is to
bring any stress-increasing crack tips to a low stress zone so that
cracks do not initiate from the tips.
[0049] It was discovered, however, that when a three-dimensional
structure was formed by bending sheet 71 along bend line 75, and
thereafter the structure was loaded transversely to bend line 75,
fatigue failure did not occur at end 83 of return portion 82, but
instead, occurred, as shown by broken line 69, proximate point 84
of return portion 82 which is farthest away from bend line 75.
[0050] In an effort to attempt to avoid the stress concentration
resulting from the configuration of arcuate return portion 82,
sheet 91 in FIGS. 4, 4A and 4B was formed with bend-inducing slits
93 along bend line 95 As best may be seen in FIGS. 4A and 4B, the
bend-inducing structures are formed with return portions 102 which
flatten out or have relatively larger radii of curvature in the
area which failure might occur. The return portions then hook back
in at 103, again to attempt to avoid stress concentration at the
end of the bend-inducing structures. When bent along bend line 95
and then transversely loaded, however, cracking again occurred upon
failure of the structure at crack 89, shown by a broken line in
FIGS. 4A and 4B. This cracking occurred at 104, which is
approximately the position which is furthest from bend line 95.
[0051] FIGS. 5, 5A and 5B illustrate the configuration
bend-inducing slits, grooves or steps which have been found to have
substantially increased resistance to fatigue failure. This
configuration is also shown in prior U.S. patent application Ser.
No. 10/672,766 as FIG. 11.
[0052] In FIG. 5, a sheet of material 111 has been slit, grooved or
stepped with bend-inducing structures 113 along bend line 115 in a
manner as set forth in the above-identified Related Applications.
The bend-inducing structures 113 are generally continuous compound
arcuate shapes and have end portions 121 which define bending
straps 117 that extend obliquely across bend line 115 in a manner
also described above and in the Related Applications. Arcuate
return portions 122 are provided on opposite ends 121 of
bend-inducing slits 113, with ends 121 being connected to return
portions 122 by relatively smaller diameter arcs 125. Each return
portion 122 returns along bend line toward the other return
portion. Finally, the return portions most preferably include ends
123 which hook or extend back toward bend line 115.
[0053] As will be seen from the Examples set forth hereinafter, a
dramatic improvement in the fatigue resistance of the bent
structures formed using the slit configuration of FIG. 5 over that
of FIG. 4, and over that of commercially available welding, has
been experienced.
[0054] Comparing the slits of FIGS. 4 and 5, the dramatic increase
in resistance to fatigue is believed to reside in one or more of
the following factors. First, the length of the arcuate return
portion 102 in FIG. 4A can be seen to be substantially shorter than
the length of the arcuate return portion 122 in FIG. 5A. The ends
of the slits in FIG. 4 are continuous curves which transition from
end radius 105 to return radius 102 and then to the terminal radius
103. The arc angle of the return 102 for the FIG. 4 slits was only
3.7 degrees. The arc angle for the slits of FIG. 5, by contrast,
was 26.7 degrees. Thus, the chord subtended by arc 122 in FIG. 5A
is much longer than the chord in FIG. 4A. This is believed to be
very important in avoiding stress risers which will produce fatigue
failure.
[0055] Another way of expressing this increased return length is
that return portions 122 extend along the slit by a much greater
percentage of the slit length than is the case for return portions
102. Thus, the chord lengths of return portions 122 are on the
order of about 20% of the overall slit length in the FIG. 5
configuration, while they are only about 4% of the slit length in
the FIG. 4 configuration. Most preferably, and as is the case in
both configurations, the return portion chords are substantially
parallel to the bend lines 95 and 115, respectively.
[0056] The radius of return portion 102 in FIG. 4B, however, is
actually longer than the radius of curvature of return portion 122
in FIG. 5B. The radius of curvature of return 102 in FIG. 4B is
4.32 times the thickness of the sheet of material, which was 0.125
inches in this case. In FIG. 5B, the radius of curvature of return
portion 122 can be seen to be only 3.161 times the thickness
dimension of the sheet of material, also 0.125 inches. While it is
believed that the radius of curvature of the return portion should
not be too small so as to arc away from the bend line 115 in a
manner which provides a site for stress risers, over a level, which
is not yet known, there is believed to be a reasonable amount of
latitude with respect to the radius of curvature of the return
portion.
[0057] As will also be seen from FIGS. 4B and 5B, the radius of
curvature of end portion 125 is less than the radius of curvature
of end portion 105. Thus, a radius of 0.124 times the thickness
dimension of the sheet of material is employed in the slits of FIG.
5B, while a radius of 0.468 times the thickness dimension of the
sheet of material is employed in the slits of FIG. 4B. The lateral
distance, LD, to position 104 in FIG. 4 from bend line 95 is
significantly greater than the lateral distance, LD, of the
equivalent position in the geometry of FIG. 5B.
[0058] Minimizing the lateral distance to which the slits extend
away from the bend line is thought to be important because the
slits cut into the native material on either side of the bend line.
When the beam is loaded as shown in FIG. 6, the bottom side 143 of
the beam will be under tension so that a band of native material
just above the slits will be called upon to resist the tension
forces along the length of the beam. As the arcuate slits have an
end radius 105 which increases, the band of unbroken native
material moves away from the bend line by the lateral distance, LD
(see FIG. 5B), subjecting it to more stress in resisting the
tension loading forces.
[0059] At this point, sufficient testing has not been conducted in
order to generate complete curves as to the effects of return
portion arc angles, return portion radii, or end arc radii (lateral
distances into the native material) so as to demonstrate where the
substantially enhanced fatigue resistance begins to be significant.
It is believed that these are likely to be continuous curves with
the arc angle of the return portion being the most critical factor.
It is also believed that the configuration of FIG. 5 will scale off
of the thickness dimension of the sheet of material. Since the
improvement in fatigue resistance allows a beam to be folded from
sheet material and have a fatigue resistance many times that which
can be achieved in welded equivalent structures, the exact point at
which the performance exceeds a welded structure's performance may
tend to be somewhat academic. Suffice it to say that the
configuration of FIGS. 5, 5A and 5B will substantially out perform
box beams which are welded together from plate material in fatigue
resistance.
EXAMPLES
[0060] FIG. 6 schematically illustrates a box beam as positioned on
a fatigue test stand. The box beams tested each had a square
cross-section with a dimension of 4 inches on each side and
included a flange 132 which was folded inside one of the sidewalls
and secured thereto by fastener assemblies 133, in this case a bolt
and nut. The fasteners were placed every 4 inches along the length
of the beam, and beam 131 had an overall length of 48 inches. A
support assembly 135 was provided proximate each end of beam 131,
and forced distributing plates 137 used to avoid local
concentrations of stress at support stands 135.
[0061] Beam 131 was loaded at two locations 139 on either side of
the center of the beam. The loads were spaced from each other by a
distance of approximately 6 inches. Again, load distributing plates
were employed at 139, and arrows 141 schematically illustrate that
the beam was loaded from a minimum load up to a maximum load.
Loading was cycled between minimum and maximum load until beam
failure occurred. As will be seen from FIGS. 6 and 6A, therefore, a
bottom side 143 of the beam was cycling in tension, while a top
side 145 was compressed under the transverse bending load of the
beam. In each case, failures occurred along bottom side 143 of the
beam with cracks propagating upwardly from side 143 towards side
145.
[0062] FIG. 7 shows the test results for various beams which were
tested using the test stand of FIG. 6. The stress was measured in
Mega-Pascals, (MPa), and has been plotted versus Cycles to Failure.
Also, shown on FIG. 7 are the Cycles to Failure curves for welded
box beams, as a function of the class of the weld. Thus, a class B
weld is shown as the top curve, while a class G weld is the bottom
curve. The data represented by the "class B weld" to "class G weld"
curves was generated testing "class B weld" through "class C weld"
steel box beams, which beams are welded at the corners using the
various welding class standards, which are known in the industry.
Typically, commercially available box beams will be welded at the
level of a class F weld.
[0063] The data points on FIG. 7 were for two types of box beams,
namely one series using the slits of FIG. 4 and the other series
using the slits of FIG. 5. When the initial tests were run, the
trial load range was relatively low, namely 17.5 (e.g., Stress
Range of approximately 90-100 MPa). Data points 161, 162, 163 and
164 were all run using the lower magnitude of cyclical loading as a
trial. The data points 161, 162 and 163 are all for box beams
formed using the slit of FIG. 4. The data point 164 is for a box
beam having FIG. 5 slits and having a trial load of 17.5 (e.g.,
Stress Range of approximately 100 MPa), but the beam did not fail
at data point 164.
[0064] It was decided that the load range should be increased for
final testing and data points 171, 172, 173, 174 and 175 are for
beams which were loaded with a load range of 26 (e.g., Stress Range
of approximately 150 MPa). Data points 172, 173 and 174 are for box
beams folded from sheet material formed with slits having the
configurations of FIG. 4 while data points 171 and 175 are for box
beams which were folded from sheets slit in accordance with FIG.
5.
[0065] Data point 171 is a relatively early failure which occurred
in a FIG. 5 box beam, not because the beam failed at any of the
slits, but because the beam went out of square into a rhombus mode
during cycling. This rhombus mode cycling resulted in a premature
failure. Data points 164 and 175 are for the same type of beam,
namely a beam with FIG. 5 slits. The beam was cycled up to
2,100,000 cycles at the low trial load range of 17.5 (e.g., Stress
Range of 100 MPa) and, since no failure occurred, the loading was
then increased to 26 (e.g., Stress Range of 150 MPa). The beam
loading was then continued up to 3,827,753 cycles, at which point
the test could not be completed because the failure occurred at one
of the load points 139, indicating that failure was not purely a
function of the beam's characteristics but instead a function of
the beam/test configuration. Thus, the test essentially was not
completed to find the ultimate real limit of beams having FIG. 5
slits.
[0066] As will be seen, data point 175 is above the curve for a
class C weld, much less that of a class F weld, the commercially
available welds. A class F weld would fail, on average, at about
600,000 cycles at the load range of 26 (e.g., Stress Range of
approximately 150 MPa). Thus, a bent or folded box beam using the
slit configuration of FIG. 5 has more than six times the cycling
capacity of a commercially welded, class F, box beam, and the upper
limit of the box beam of the present invention is still not
known.
[0067] FIG. 8 shows a table of the test results used to generate
the data of FIG. 7.
[0068] The foregoing descriptions of a specific embodiment of the
present invention has been presented for the purpose of
illustration and description. It is not intended to be exhaustive
or to limit the invention to the precise form disclosed, and
obviously many modifications and variations are possible in light
of the above teaching. The embodiment was chosen and described in
order to best explain the principles of the invention and its
practical application to thereby enable others skilled in the art
to best utilize the invention and the embodiment with various
modifications as are suited to the particular use contemplated. It
is intended that the scope of the invention be defined by the
claims appended hereto and their equivalents.
* * * * *