U.S. patent application number 09/270428 was filed with the patent office on 2002-04-04 for method and apparatus for producing beneficial stresses around apertures by use of focused stress waves.
Invention is credited to EASTERBROOK, ERIC T..
Application Number | 20020038565 09/270428 |
Document ID | / |
Family ID | 22143518 |
Filed Date | 2002-04-04 |
United States Patent
Application |
20020038565 |
Kind Code |
A1 |
EASTERBROOK, ERIC T. |
April 4, 2002 |
METHOD AND APPARATUS FOR PRODUCING BENEFICIAL STRESSES AROUND
APERTURES BY USE OF FOCUSED STRESS WAVES
Abstract
Tooling and a method of employing tooling to produce beneficial
stress waves in a substrate to provide high fatigue life
structures. Stress waves are provided to work a substrate, causing
dimples in the workpiece, along a uniform pressure profile in the
workpiece. By use of the method, uniform beneficial residual stress
is provided at surface and midplane apertures in a workpiece, so as
to improve overall fatigue life. An improved tool shape is
described, having a smooth curve, rather than a flat punch. Also,
the use of a consumable wafer provides additional uniform stress
profile benefits.
Inventors: |
EASTERBROOK, ERIC T.; (KENT,
WA) |
Correspondence
Address: |
R REAMS GOODLOE JR
10725 SE 256TH STREET
SUITE 3
KENT
WA
980316426
|
Family ID: |
22143518 |
Appl. No.: |
09/270428 |
Filed: |
March 16, 1999 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60078356 |
Mar 17, 1998 |
|
|
|
Current U.S.
Class: |
72/334 |
Current CPC
Class: |
B21C 23/001 20130101;
B21J 15/00 20130101; B21D 28/24 20130101; C21D 7/04 20130101; B21J
1/025 20130101; B23P 9/04 20130101; B21J 5/00 20130101; B23P 9/025
20130101; B24B 39/06 20130101; C21D 2221/00 20130101; C21D 7/02
20130101; B21J 15/38 20130101 |
Class at
Publication: |
72/334 |
International
Class: |
B21D 028/00 |
Claims
1. Tooling for working a structure to improve the fatigue strength
at a selected location in said structure, said structure comprising
a first surface, a second surface, and a body therebetween, said
body suitable for forming therein an aperture having at least one
sidewall extending between said first surface and said second
surface, said tooling comprising: a first indenter, said first
indenter comprising a contacting end for engagement with and
deformation of said first surface of said structure to impart a
residual stress profile in said body of said structure, and wherein
said contacting end of said first indenter comprises a surface
shape substantially conforming to the shape which would be created
by inducing a dimple in said first surface of said structure by
application of a uniform pressure profile on said first surface of
said structure throughout a contact area between said contacting
end and said first surface.
2. Tooling as set forth in claim 1, further comprising a second
indenter, said second indenter comprising a contacting end for
engagement with and deformation of said second surface of said
structure to impart a residual stress profile in said body of said
structure, and wherein said contacting end of said second indenter
comprises a surface shape substantially conforming to the shape
which would be created by inducing a dimple in said second surface
of said structure by application of a uniform pressure profile on
said second surface of said structure throughout a contact surface
between said contacting end and said second surface.
3. The tooling as set forth in claim 1, wherein said first indenter
comprises a so that said residual stress profile along said at
least one sidewall of said aperture in said structure is
substantially uniform.
4. Tooling as set forth in claim 1, wherein said first indenter is
hollow.
5. Tooling as set forth in claim 1, wherein said first indenter is
solid.
6. Tooling as set forth in claim 1, wherein said contacting end is
smoothly curved.
7. Tooling as set forth in claim 1, wherein said contacting end
comprised at least one beveled edge.
8. Tooling as set forth in claim 7, wherein said contacting end of
said indenter comprises at least two beleled edges.
9. Tooling as set forth in claim 1 or claim 2, further comprises at
least one consumable workpiece, said consumable workpiece sized and
shaped for insertion between said indenter and said first
workpiece.
10. Tooling as set forth in claim 2, wherein said first indenter
and said second indenter are of unequal cross-section.
11. A method for working a structure to improve the fatigue
strength at a selected location in said structure, said structure
comprising a first surface, a second surface, and a body
therebetween, said body suitable for forming therein an aperture
having at least one sidewall extending between said first surface
and said second surface, said method comprising: providing a first
indenter, said first indenter comprising a contacting end for
engagement with and deformation of said first surface of said
structure to impart a residual stress profile in said body of said
structure, and wherein said contacting end of said first indenter
comprises a surface shape substantially conforming to the shape
which would be created by inducing a dimple in said first surface
of said structure by application of a uniform pressure profile on
said first surface of said structure throughout a contact area
between said contacting end and said first surface, engaging said
first indenter with said workpiece with sufficient force to impart
a stress wave in said workpiece, so as to cause formation of a
dimple in said workpiece, disengaging said first indenter from said
workpiece, machining said workpiece to form an aperture of
pre-selected size and shape, so as to provide substantially uniform
residual stress along the side walls of said aperture, so as to
improve fatigue life of said workpiece.
12. The method as set forth in claim 11, further comprising:
providing a second indenter, said second indenter comprising a
contacting end for engagement with and deformation of said second
surface of said structure to impart a residual stress profile in
said body of said structure, and wherein said contacting end of
said second indenter comprises a surface shape substantially
conforming to the shape which would be created by inducing a dimple
in said second surface of said structure by application of a
uniform pressure profile on said second surface of said structure
throughout a contact surface between said contacting end and said
second surface engaging said second indenter with said workpiece to
cause formation of a dimple in said workpiece; disengaging said
second indenter from said workpiece.
13. The method as set forth in claim 11, wherein said first
indenter comprises a shaped profile, so that said residual stress
profile along said at least one sidewall of said aperture in said
structure is substantially uniform.
14. The method as set forth in claim 11, wherein said first
indenter is hollow.
15. The method as set forth in claim 11, wherein said first
indenter is solid.
16. The method as set forth in claim 11 or in claim 12, wherein
said first indenter contacting end is smoothly curved.
17. The method as set forth in claim 11, wherein said contacting
end comprises at least one beveled edge.
18. The method as set forth in claim 11, wherein said contacting
end of said indenter comprises at least two beleled edges.
19. The method as set forth in claim 11 or in claim 12, further
comprising at least one consumable workpiece, said consumable
workpiece sized and shaped for insertion between said indenter and
said first workpiece.
20. The methos as set forth in claim 12, wherein said first
indenter and said second indenter are of unequal cross-section.
Description
[0001] A portion of the disclosure of this patent document contains
material that is subject to copyright protection. The owner has no
objection to the facsimile reproduction by anyone of the patent
document or the patent disclosure, as it appears in the Patent and
Trademark Office patent file or records, but otherwise reserves all
copyright rights whatsoever.
FIELD OF THE INVENTION
[0002] This invention is related to novel methods and tools for use
in manufacturing parts with improved fatigue life, particularly for
parts having fastener apertures therein, or cutouts therein, and
which parts are subject to repeated or prolonged stress. More
specifically, this invention relates to improved techniques for
utilizing the stress wave method for working parts, to improved
tools for utilizing the stress wave method for working parts, and
to finished parts made thereby, which parts have improved stress
fatigue resistance characteristics.
BACKGROUND
[0003] Metal fatigue is a problem common to just about everything
that experiences cyclic stresses. Such problems are especially
important in transportation equipment, such as aircraft, ships,
trains, cars, and the like. Metal fatigue can be defined as the
progressive damage, usually evidenced in the form of cracks, that
occurs to structures as a result of cyclic loading. This failure
mode is not to be confused with a failure due to overload. The
lower surface of an aircraft wing is a classical example of the
type of loading that produces fatigue. The wing is subjected to
various cyclic stresses resulting from gust, maneuver, taxi and
take-off loads, which over the lifetime of a particular part
eventually produces fatigue damage. Similarly, the pressurized
envelope of an aircraft, including the fuselage skin and rear
pressure bulkhead, are subject to a stress cycle on each flight
where the aircraft interior is pressurized.
[0004] One problem inherent in fatigue damage is that it can be
hidden since it generally occurs under loads that do not result in
yielding of the structure. Fatigue damage is most often observed as
the initiation and growth of small cracks from areas of highly
concentrated stress. Undetected, a crack can grow until it reaches
a critical size. At that point, the individual structural member
can suddenly fail. Catastrophic failure of an entire structure can
also occur when other members of the adjacent portions of the
overall structure can not carry the additional load that is not
being carried by the failed structural member.
[0005] Even stationary objects, such as railroad track or pressure
vessels, may fail in fatigue because of cyclic stresses. Cyclic
loads for railroad track are caused by repeated loading from the
wheels running over an unsupported span of track. In fact, some of
the earliest examples of fatigue failures were in the railroad
industry and in the bridge building industry. Sudden pressure
vessel failures can be caused by fatigue damage that has resulted
from repeated pressurization cycles. Importantly, government
studies report that fatigue damage is a significant economic factor
in the U.S. economy.
[0006] Fatigue can be defined as the progressive damage, generally
in the form of cracks, that occur in structures due to cyclic
loads. Cracks typically occur at apertures (holes), fillets, radii
and other changes in structural cross-section, as at such points,
stress is concentrated. Additionally, such points often are found
to contain small defects from which cracks initiate. Moreover, the
simple fact that the discontinuity in a structural member such as a
fuselage or wing skin from a hole or cutout forces the load to be
carried around the periphery of such hole or cutout. Because of
this phenomenon, it is typically found that stress levels in the
structure adjacent to fastener holes or cutouts experience stress
levels at least three times greater than the nominal stress which
would be experienced at such location, absent the hole or
cutout.
[0007] It is generally recognized in the art that the fatigue life
in a structure at the location of a through aperture or cutout can
be significantly improved by imparting beneficial residual stresses
around such aperture or cutout. Various methods have been
heretofore employed to impart beneficial residual stress at such
holes or cutouts. Previously known or used methods include roller
burnishing, ballizing, and split sleeve cold expansion, split
mandrel cold working, shot peening, and pad coining. Generally, the
compressive stresses imparted by the just mentioned processes
improve fatigue life by reducing the maximum stresses of the
applied cyclic loads at the edge of the hole. Collectively, these
processes have been generically referred to as cold working. The
term cold working is associated with metal forming processes where
the process temperature is lower than the recrystallization
temperature of the metal. A similar term, "cold expansion", as used
by Fatigue Technology Inc., of Tukwila, Washington, is often used
interchangeably with cold working, but as applied specifically to
their split sleeve cold expansion process. However, of all the
methods used to cold work holes, presently the most widely used
processes are the split sleeve process and split mandrel process.
Together, these processes are referred to as mandrel cold working
processes
[0008] Historically, mandrel cold working was accomplished through
strictly manual means. As an example, split sleeve cold expansion
of holes is still done using hand-held hydraulic tools attached to
air-actuated hydraulic power units. The variables involved in tool
selection, implementation, and control of the cold expansion
process requires skilled operators to reliably produce properly
treated holes. Unfortunately, the requirement of having a skilled
operator to perform the task is a disadvantage in that it
continuously presents the risk of improper or inaccurate
processing. Also, such labor-intensive techniques effectively
preclude automated feedback necessary for statistical process
control. Although development of that process continues, the
complexity of the split sleeve processes and the apparatus utilized
presently precludes the widespread adoption of the process for
automated fastening environments. The split mandrel process it at a
similar stage of development; manually performed, but with some
minor automation.
[0009] The mandrel cold working processes have a particular
disadvantage in that they require precision in the size of the
starting holes, usually in the range of from about 0.002 inch to
about 0.003 inch in diametric tolerance, in order to achieve
uniform expansion. Also, an undersize starting hole is required in
that process, in order to account for the permanent expansion of
the hole and the subsequent final ream that is necessary to remove
both the localized surface upset around the periphery of the hole,
as well as the axial ridge(s) left behind by the edges of the
sleeve split or mandrel splits at their working location within the
aperture, and of course, to size the holes. Moreover, treatment
requires the use of two reamers; one that is undersized, for the
starting hole diameter, and one which is provided at the larger,
final hole diameter.
[0010] Another undesirable limitation of mandrel cold working
processes is the requirement for, presence of, and residual effect
of lubricants. For the split sleeve cold expansion process the
starting hole must be free of residual lubricants (used for
drilling) to prevent sleeve collapse during processing. A collapsed
sleeve can be very difficult to remove and necessitates increasing
the hole diameter beyond the nominal size, to remove the subsequent
damage. The split mandrel process uses a liquid cetyl alcohol
lubricant that must be cleaned from the hole after cold working, in
order to ensure proper paint adhesion. In either case, the cold
worked hole must be cleaned with solvents, in order to remove
lubricants. Such chemical solvents are costly, require additional
man-hours for handling and disposal, and if not effectively
controlled during use or disposal, can have a deleterious effect on
operators and/or the environment.
[0011] Still another limitation of the prior art mandrel cold
working processes is their effect on the surface of the aperture
being treated, i.e. the metal wall which defines the hole. The
"split" in the split sleeve or the multiple splits in a split
mandrel can cause troublesome shear tears in type 7050 aluminum,
and in some other alloys. Shear tears, which are small cracks in
the structural material near the split(s), are caused by the
relative movement of the material near the split. Significantly,
the increasing use of type 7050 aluminum in aircraft structures has
created a large increase in the number of shear tears reported.
Although such tears are generally dismissed as cosmetic flaws, they
nevertheless produce false positives in non-destructive inspections
for cracks.
[0012] Also, in the mandrel cold working processes, the sliding
action of a mandrel produces a large amount of surface upsetting
around the periphery of the hole, especially on the side of the
structure where the mandrel exits the hole. In the split mandrel
process, this effect is clearly seen, because of the direct contact
of the mandrel with the aperture sidewall. The undesirable surface
upset can increase the susceptibility to fretting, which may lead
to a reduction in life for fastened joints. Additionally, surface
upset in a stackup of structural layers can cause disruption of the
sealant in the faying surface. To some extent the undesirable
surface upset can be reamed out when sizing the final hole
diameter, but at least some portion (and normally a substantial
portion) remains.
[0013] Pad coining is another process that has been used to improve
the fatigue life of holes and other cutouts. This process is
described in U.S. Pat. No. 3,796,086 issued Mar. 12, 1974 to
Phillips for Ring Pad Stress Coining, and the related, commonly
owned U.S. Pat. No. 3,434,327, issued Apr. 16, 1974 to Speakman for
Ring Pad Stress Coining Tooling. This method uses opposing dies to
cold work an existing hole or aperture. The pad coin process leaves
a characteristic concentric impression around the periphery of the
cutout. The reduced thickness impression is a major drawback of the
process, since the reduced section thickness reduces the bearing
area of the hole. Further, the impression makes attaching thin
structure at treated fastener holes problematic, since a panel may
buckle when the fastener is tightened. Moreover, the process does
not attempt to perform ring pad stress coining on a structure prior
to machining the hole.
[0014] As described in U.S. Pat. No. 3,824,824 issued to Leftheris
on Jul. 23, 1974, and entitled Method and Apparatus for Deforming
Metal, the stress wave phenomenon has previously been used to
deform a metal workpiece by passing stress waves through the
workpiece to momentarily render the metal plastic. Such methods and
related devices have been employed for metal forming, riveting and
spot welding operations.
[0015] Another invention by Leftheris, U.S. Pat. No. 4,129,028
issued on Dec. 12, 1978 for a Method and Apparatus for Working a
Hole, couples mandrel cold working to the aforementioned stress
wave process. The object of this latter mentioned invention was to
simultaneously cold work and control the finish and dimensional
characteristics of a hole. The process treats both straight and
tapered starting holes by driving tapered mandrels through or into
an existing hole, using a stress wave generator. The invention
teaches production of close tolerance holes to a surface finish of
30 micro-inch RMS. However, as with the other mandrel cold working
methods, this process requires a close tolerance starting hole, and
is subject to the same surface upset problem as the other mandrel
cold working methods. Thus, while this variation of Leftheris's
work realized that it would be advantageous to utilize stress waves
to impart residual stresses in structures in an amount sufficient
to provide improved fatigue life, the process still suffers from
the same starting hole methodology that is used with the mandrel
cold working processes.
[0016] Another attempt to provide a method for cold working holes
was developed by Wong and Rajic, as taught in WIPO International
Publication Number WO 93/09890, published May 27, 1993, entitled
Improving Fatigue Life of Holes. The method was an improvement over
the pad coining methods, because the impression made in the
structure being treated is smaller than the hole diameter, thus
eliminating the undesirable concentric ring provided in coining
methods. Also, although such teaching was advantageous in that it
eliminated the need for preparing the starting hole that is
required with the mandrel and coining processes, a significant
drawback to the Wong process was that it required relatively high
loads to indent or cold work the structure being treated. This can
be understood from considering minimum quasi-static mandrel load
necessary to initially indent a sheet. The initial mean contact
pressure, P.sub.M, for initial yield (indentation) is estimated by
the following equation:
[0017] P.sub.M=1.10 .times.(compressive yield stress)
[0018] The load P for initial yielding or indenting is calculated
by multiplying P.sub.M, by the cross sectional area of the mandrel.
Therefore:
[0019] Mandrel Load (P)=1.10 (compressive yield stress) (mandrel
cross sectional area)
[0020] In practicality, the load necessary to impart fatigue
improvement is far greater. For example, the 0.063 inch (1.6 mm)
thick 2024-T3 aluminum specimens used in the Wong/Rajic disclosure
were cold worked with a (0.158 inch diameter) 4.0 mm diameter
cylindrical mandrel. The initial mandrel indentation load using
these parameters is calculated at 835 pounds (3714 Newtons).
Because the indentation process must go well beyond the initial
indentation load to achieve fatigue life improvement, the force
used in the Wong/Rajic test ranged from 3595 pounds (15991 N) to
4045 pounds (17994 N) for the (0.158 in.) 4.0 mm diameter mandrel.
As a comparison, the forces necessary to cold work (indent) a
common 1/4 inch (6.35 mm) diameter fastener can be as high as
10,000 pounds (44484 N). Unfortunately, loads of such magnitude
generally require large and bulky machinery such as power presses,
hydraulic presses, drop hammers, etc., and as a result, their use
is precluded from widespread use in automated fastening
systems.
[0021] The impracticality of such just mentioned heavy, large
equipment for automated fastening are identified by Zieve in U.S.
Pat. No. 4,862,043. Commenting on the prior art apparatus, Zieve
states, ". . . a C-yoke squeezer is a large, expensive device which
extends around the workpiece to provide an integral backing member.
However, such devices are impractical for many applications, since
the throat depth requirements, i.e., the distance of the rivet from
the edge of the workpiece, result in an apparatus which is
impractically large and expensive because of the corresponding
stiffness demanded for the required throat depth." It is clear that
the Wong/Rajic invention dos not teach the propagation of stress
waves into the metal for deformation and subsequent residual stress
development. Therefore, they do not anticipate the use of stress
wave technology to significantly lower the strength and size
requirements of the processing device or its supporting
structure.
[0022] The mandrels in the Wong/Rajic disclosure are designed for
the purposes of both indenting and hole punching. While their
invention allows for mandrel end shapes to be flat or conical, they
do not use the shape of the mandrel end to optimize the extent of
the residual stresses. A large and uniform zone of residual
stresses is required to produce the highest fatigue life. A mandrel
that has a flat end is well suited for forming or punching the
hole, but induces a low amount of residual stress at the surface of
the sheet. On the other hand, mandrels that have a conical end
increase surface residual stresses but tend to "plow" the material
radially and produce substantial surface upsetting. It is clear
then that the prior art, in regards to the configuration of the
mandrel ends, does not optimize the extent and depth of the
residual stresses.
[0023] The Wong/Rajic process can also be used to treat
non-circular cutouts using either of two methods. The first method
uses a solid mandrel with the same cross sectional shape of the
hole. The second method treats selected areas of the cutout using
solid circular mandrels prior to machining the cutout. The second
method is similar to the invention of Landy, U.S. Pat. No.
4,885,829 which uses the split sleeve cold expansion process to
treat selected radii of the cutout. After machining the cutout
sufficient residual stresses remain in the radii to improve fatigue
life. Another invention by Easterbrook and Landy, U.S. Pat. No.
4,934,170, treats existing non-circular holes and cutouts using
tools that conform to the shape of the hole. A common weakness of
each of these methods are that only selected areas (radii) of the
cutout are cold expanded. The non-uniformity of the residual
stresses caused by treating only the radii of the cutout allows for
tensile stresses to be present at the hole edge. This has the
potential to reduce fatigue life.
[0024] The aforementioned invention by Zieve, and others similar to
it, are used to drive rivets and fasteners using electromagnetic
drivers. Such techniques and apparatuses, however, are not used for
cold working a metal structure prior to machining the hole. Hence,
in summary, present methods of cold working holes and other cutouts
using tapered mandrel methods, coining, punching, and such are not
adaptable to automated fastening systems and other automated
environments because of their complexity and bulkiness of
equipment. Also, presently known methods used by others do not
treat the entire periphery of non-circular cutouts leading to
potential fatigue life degradation. Finally, prior art countersink
cold working methods require re-machining of the formed countersink
to achieve the desired fastener flushness.
[0025] Thus, the following items summarize the shortcomings of the
current methods and will be used as a basis for comparing with my
novel, improved stress wave fabrication method. Heretofore known
processes are not entirely satisfactory because:
[0026] They often require mandrels, split or solid, and disposable
split sleeves, which demand precision dimensions, which make them
costly;
[0027] Mandrels and sleeves are an inventory and handling item that
increases actual manufacturing costs when they are employed;
[0028] "Mandrel only" methods require a different mandrel for
roughly each 0.003 to 0.005 inch change in hole diameter, since
each sleeve is matched to a particular mandrel diameter, and
consequently, the mandrel system does not have the flexibility to
do a wide range of hole existing hole diameters;
[0029] Each hole diameter processed with "mandrel only" methods
requires two sets of reamers to finish the hole, one for the
starting dimension and another for the final dimension;
[0030] Mandrel methods rely on tooling and hole dimensions to
control the amount of residual stress in the part, and therefore
the applied expansion can be varied only with a change of
tooling;
[0031] Mandrel methods require some sort of lubricant; such
lubricants, and especially the liquids, require solvent clean
up;
[0032] Splits in a sleeve or splits in a mandrel can cause
troublesome shear tears in certain 7000 series aluminum alloys;
[0033] The pulling action against mandrels, coupled with the
aperture expansion achieved in the process, produces large surface
marring and upsets around the periphery of the aperture;
[0034] Split sleeve methods are not easily adapted to the
requirements of automation, since the cycle time is rather long
when compared with the currently employed automated riveting
equipment;
[0035] Mandrel methods are generally too expensive to be applied to
many critical structures such as to aircraft fuselage joints, and
to large non-circular cutouts;
[0036] Mandrel methods have limited quality control/quality
assurance process control, as usually inspections are limited to
physical measurements by a trained operator.
OBJECTS, ADVANTAGES, AND NOVEL FEATURES
[0037] My novel stress wave manufacturing process can be
advantageously applied to apertures for fasteners, to large holes,
to non-round cutouts from a workpiece, and to other structural
configurations. Treating a workpiece structure for fatigue life
improvement, prior to fabricating the aperture itself, has
significant technical and cost advantages. The method is simple,
easily applied to robotic manufacturing methods, and is otherwise
superior to those manufacturing methods heretofore used or
proposed.
[0038] From the foregoing, it will be apparent to the reader that
one important and primary object of the present invention resides
in the use of a novel method for treating a workpiece to reduce
fatigue stress degradation of the part while in service, and to
novel tool shapes for achieving such results. The method and novel
tools simplify the manufacturing costs, and importantly, simplifies
and improves quality control in the manufacture of parts having an
improved fatigue life.
[0039] Other important but more specific objects of the invention
reside in the provision of an improved manufacturing process for
enhances service life metal parts subject to fatigue stress, as
described herein, which:
[0040] eliminates the requirement for purchase, storage, and
maintenance of mandrels;
[0041] eliminates the requirement for purchase, storage, and
maintenance of split sleeves;
[0042] eliminates the need for disposal of split sleeves;
[0043] eliminates the need for lubrication and subsequent clean-up
during manufacture of structures containing apertures
therethrough;
[0044] enables the manufacture, in a single manufacturing
apparatus, of a wide range of aperture diameters, in which a wide
range of fastener diameters can be employed;
[0045] allows the magnitude and depth of the residual stresses to
be carefully controlled, by way of the amount of energy input into
the stress wave;
[0046] enables process control to be established using stastical
feedback into the manufacturing system, thus enhancing quality
assurance;
[0047] eliminates shear tears in a workpiece that are commonly
encountered in mandrel manufacturing methods;
[0048] significantly reduces or effectively eliminates surface
marring and upset associated with mandrel methods, thus
significantly increasing fatigue life;
[0049] is readily adaptable to automated manufacturing equipment,
since manufacturing cycle times are roughly equivalent to, or less
than, cycle times for automated riveting operations;
[0050] eliminates bulky hydraulic manufacturing equipment typically
used in mandrel methods, and substitutes simple electromagnetic
equipment;
[0051] enables aperture creation after fatigue treatment, by a
single reaming operation, rather than with two reaming operations
as has been commonly practiced heretofore;
[0052] is sufficiently low in cost that it can be cost effectively
applied to a number of critical structures, including fuselage
structures.
[0053] Other important objects, features, and additional advantages
of my invention will become apparent to the reader from the
foregoing and from the appended claims and the ensuing detailed
description, as the discussion below proceeds in conjunction with
examination of the accompanying drawing.
SUMMARY
[0054] I have now invented, and disclose herein, an improved cold
metal working process that uses stress waves to impart beneficial
residual stresses to holes and other features in parts subject to
strength degradation through stress fatigue. This improved stress
wave process does not have the above-discussed drawbacks common to
heretofore utilized cold working methods of which I am aware. The
process overcomes the heretofore-encountered shortcomings of the
stress wave cold working process. Also, it eliminates undesirable
equipment necessary for the more commonly utilized alternative
processes, such as the need for bulky hydraulic equipment,
precision mandrels, disposable split sleeves and messy lubricants.
Thus, it is believed that my novel method will substantially reduce
manufacturing costs. In addition, my stress wave process is readily
adaptable to use in automated manufacturing equipment. As a result,
the unique process described herein is a major improvement over
other processes in common use today, including mandrel
processes.
[0055] My improved stress wave method imparts beneficial stresses
using a dynamic probe that impinges the surface of the metal,
preferably in a normal direction to the surface. The action of the
probe causes waves of elastic and plastic stress to develop and
propagate through the metal. In some cases a stationary indenter is
used to support thin workpiece materials. Such "backing" indenters
also assist in the reflection of the plastic wave off of the other
side of the workpiece.
[0056] After a properly applied and focused plastic stress wave has
imparted a large zone of residual stress, the area is now ready for
the hole. A drill, reamer or other cutting device is positioned
concentric to the impact zone from the probe and anvil. When the
hole is machined a small rebound of the stresses surrounding the
hole occurs. Such rebound manifests itself as shrinking of the
machined hole. For this reason, the cutting tools used in my stress
wave method may require the use of a feature that takes into
account the inward metal movement in a hole. Otherwise, the
workpiece material has the possibility of binding on the cutting
tool. This could lead to short tool life or poor hole finish. For a
drill or reamer, a simple solution to this requirement is to
provide a back-taper feature. As a result, beneficial residual
compressive stresses are allowed to remain in the finished
structure. These residual compressive stresses counteract
potentially damaging stresses focused at the aperture edge.
BRIEF DESCRIPTION OF THE DRAWING
[0057] FIG. 1 shows a summary of normalized contact pressure
distribution, .sigma..sub.z/P.sub.m, at radial distance from a
punch centerline, for a structure undergoing deformation by (a) a
cone, (b) a sphere, and (c) a circular, flat bottomed punch.
[0058] FIG. 2 is a vertical cross-section of a fiat bottom circular
punch, of the type which displays the curve depicted in FIG. 1 when
cold working metal.
[0059] FIG. 3 is a vertical cross-section of a spherical bottom
bunch, of the type which displays the curve depicted in FIG. 1 when
cold working metal.
[0060] FIG. 4 is a vertical cross section of a cone shaped punch,
of the type which displays the curve depicted in FIG. 1 when cold
working metal.
[0061] FIG. 5 is graphical illustration of the deflection of a
workpiece structure, at radial distance from a punch centerline,
for a workpiece structure undergoing deformation by (a) a flat
bottom cylindrical indenter, with a configuration such as is
illustrated in FIG. 2 above, (b) a spherical bottom indenter such
as is illustrated in FIG. 3 above, and (c) an optimized, uniform
pressure type indenter as taught herein, one embodiment of which is
illustrated in FIG. 6 below.
[0062] FIG. 6 is a partial cross-sectional view, showing an
optimized tooling indenter as taught herein, and indicating key
parameters for shaping such a tool.
[0063] FIG. 7 illustrates the set-up step for utilizing my stress
wave method in a small aperture on a relatively thin workpiece.
[0064] FIG. 8 illustrates the step of impacting one-side of a
workpiece with an indenter to create a stress wave and form a
dimple in the workpiece at a pre-selected location for creation of
an aperture.
[0065] FIG. 9 represents a workpiece in which a dimple has been
formed at a pre-selected location wherein it is desired to create
an aperture such as a "rivet-hole".
[0066] FIG. 10 illustrates the set-up step for utilizing my
improved stress wave method in creation of dimple on a workpiece
with an indenter, while utilizing a backing anvil for support of
the workpiece, in preparation for forming an aperture in the
workpiece at a pre-selected location.
[0067] FIG. 11 illustrates the step for utilizing my stress wave
method by impacting one-side of a workpiece with an indenter to
create a stress wave and form a dimple in the workpiece at a
pre-selected location where it is desired to create an aperture
such as a "rivet-hole".
[0068] FIG. 12 illustrates the set-up step for utilizing my
improved stress wave method for impacting both the obverse and the
reverse sides of a workpiece with an indenter to create a stress
wave and form dimples in both sides of the workpiece at a
pre-selected location where it is desired to create an aperture
such as a "rivet-hole".
[0069] FIG. 13 illustrates the step of impacting both the obverse
and the reverse sides of a workpiece with an indenter to create a
stress wave and form dimples in both sides of the workpiece at a
pre-selected location where it is desired to create an aperture
such as a "rivet-hole".
[0070] FIG. 14 illustrates another embodiment of my optimized
indenter for use in stress wave cold working to create desirable
residual stress patterns in a workpiece; in this embodiment, one or
more chamfers are used at the edges to approximate a desirable
curvature near and at the edge of the indenter.
[0071] FIG. 15 illustrates yet another embodiment of my optimized
indenter for use in stress wave cold working to create desirable
residual stress patterns in a workpiece; in this embodiment, the
indenter utilizes a pre-selected radius of curvature.
[0072] FIG. 16 is a vertical cross-sectional view of a first and of
a second optimized shaped indenter in the set-up step for utilizing
my improved stress wave method for forming a dimple in both the
obverse and reverse sides of a workpiece at a preselected location
where it is desired to create an aperture.
[0073] FIG. 17 is a vertical cross-sectional view of a first and of
a second optimized shaped indenter during the step of impacting a
workpiece to form a dimple in both the obverse and reverse sides of
the workpiece at a preselected location where it is desired to
create an aperture.
[0074] FIG. 18 is a cross-sectional view of a workpiece after
formation of dimples in both the obverse and reverse sides.
[0075] FIG. 19 is a perspective view of a workpiece in which a
dimple has been formed a pre-selected location for formation of an
aperture in the workpiece.
[0076] FIG. 20 is a cross-sectional view of a workpiece after
formation of dimples in the obverse side.
[0077] FIG. 21 is a perspective view of the step of drilling an
aperture in a workpiece in which a dimple has been formed, in order
to create an aperture at a pre-selected location.
[0078] FIG. 22 illustrates the set-up step for utilizing my
improved stress wave method for impacting both the obverse and the
reverse sides of a workpiece with indenters of a selected, unequal
diameter, to create a stress wave and form dimples in both sides of
the workpiece at a pre-selected location where it is desired to
create a shaped aperture such as a "rivet-hole", particularly for a
flush rivet hole.
[0079] FIG. 23 illustrates the step of impacting both the obverse
and the reverse sides of a workpiece with indenters of a selected,
unequal diameter, to create a stress wave and form dimples in both
sides of the workpiece at a pre-selected location where it is
desired to create an apertures such as a "rivet-hole", particularly
for a flush rivet hole.
[0080] FIG. 24 is a side elevation view of a set-up step for
utilizing my improved stress wave method for impacting both the
obverse and the reverse sides of a workpiece with indenters of both
different shape and of unequal size, to create a stress wave and
form dimples in both sides of the workpiece at a pre-selected
location where it is desired to create an aperture such as a
chamfered "flush-rivet hole".
[0081] FIG. 25 is a partial cross-sectional view, illustrating the
set-up step for utilizing my improved stress wave method for
impacting both the obverse and the reverse sides of a workpiece
with indenters which are both of different shape and of unequal
size, and where the indenters are centered by use of pilot guide
tool received in a centering receiving slot in each indenter.
[0082] FIG. 26 is a partial cross-sectional view, illustrating the
set-up step for utilizing my improved stress wave method for
impacting both the obverse and the reverse sides of a workpiece
with indenters which are of identical shape and size, and where the
indenters are centered by use of pilot guide tool received in a
centering receiving slot in each indenter.
[0083] FIG. 27 is a cross-sectional view which shows the set-up
step for utilizing yet another embodiment of my improved stress
wave method for impacting both the obverse and the reverse sides of
a workpiece, utilizing indenters which are both of identical shape,
and utilizing a consumable wafer between the indenters and the
workpiece.
[0084] FIG. 28 is a cross-sectional view which illustrates the step
of disengaging opposing indenters from their respective consumable
wafers on opposing sides of a workpiece, while utilizing my
improved stress wave method for impacting both the obverse and the
reverse sides of a workpiece to create dimples at pre-selected
locations.
[0085] FIG. 29 is a side elevation view which illustrates the
set-up step for drilling a pair of workpieces at an aligned,
pre-selected location, and using workpieces wherein both have been
prepared with dimples in the obverse and reverse side, and wherein
the dimples have been centered in order to create beneficial
residual stresses at a desired location for lap joint between the
first and second workpieces.
[0086] FIG. 30 is a side elevation view which illustrates the
set-up step for drilling a pair of workpieces at an aligned,
pre-selected location, and using workpieces wherein only workpiece
one has been prepared by my improved stress wave method for
improved fatigue life, and wherein the dimples in the first
workpiece have been centered over a desired location in the second
workpiece in order to secure the first and second workpieces at a
desired location for the beneficial residual stresses in a
joint.
[0087] FIG. 31 is a top plan view of a structure with one
non-circular hole therein which can advantageously be treated with
my improved stress wave process to provide beneficial residual
stress levels at desired locations adjacent the interior edge wall
of the hole.
[0088] FIG. 32 is a top plan view of yet another structure having a
non-circular hole there with can advantageously be treated with my
improved stress wave process to provide beneficial residual stress
levels at desired locations in the structure.
[0089] FIG. 33 is a perspective view of a opposing, hollow,
oversize indenters which can be shaped, at least along the outer
periphery of the indenter, in an optimized manner for treatment of
over-sized or non-round holes in a workpiece,
[0090] FIG. 34 is a vertical cross-sectional view of a pair of
hollow, oversize indenters at the step of being positioned adjacent
the obverse and the reverse side of a workpiece, prior to imparting
a stress wave on a structure in order to create beneficial residual
stress at desired locations in an oversize or non-circular
cutout.
[0091] FIG. 35 is a vertical cross-sectional view of a pair of
hollow, oversize indenters at the step of impacting the obverse and
the reverse side of a workpiece, while imparting a stress wave on a
structure in order to create beneficial residual stress at desired
locations in an oversize or non-circular cutout.
[0092] FIG. 36 is a vertical cross-sectional view of the structure
just shown in FIGS. 34 and above, but now shown with dimples in the
obverse and reverse sides at the locations where my optimized
indenters have been utilized to impart beneficial residual stresses
in the structure.
[0093] FIG. 37 is the oversize structure first shown in FIGS. 34,
35, and 36, but now showing the structure after cutout of material
necessary to form an aperture of a desired size, which structure
retains beneficial residual stress substantially uniformly from the
obverse side to the reverse side, along pre-selected portions of
the interior sidewall that defines the aperture through the
structure.
[0094] FIG. 38 is modularly exploded cross-sectional view of three
workpieces being acted on by a flat cylindrical punch type
indenter, showing in detailed color the stress field created in the
workpieces by such an indenter.
[0095] FIG. 39 is a modularly exploded cross sectional view of
three workpiece sheets after being cold worked by a flat
cylindrical punch, as shown in FIG. 38, now showing in detailed
color the stress field present in the workpieces after drilling to
create an aperture through the workpieces.
[0096] FIG. 40 is modularly exploded cross-sectional view of three
workpieces being acted on at the obverse and reverse sides by flat
cylindrical punch type indenters, showing in detailed color the
stress field created in the workpieces by such indenters.
[0097] FIG. 41 is a modularly exploded cross-sectional view of
three workpieces after being cold worked by a flat cylindrical
punch, as shown in FIG. 38, now showing in detailed color the
stress field present in the workpieces after drilling to create an
aperture in the workpiece.
[0098] FIG. 42 is a modularly exploded cross-sectional view of
three workpieces after the step of stress wave input to the
workpieces by utilizing indenters and a consumable wafer body
between the workpieces and each of the indenters, as the desirable
deformation is imparted into both the obverse and reverse sides of
the three workpiece package.
[0099] FIG. 43 is a modularly exploded cross-sectional view of
three workpieces after the step of drilling an aperture at a
pre-selected location in a workpiece, after the step shown in FIG.
42 of utilizing indenters and a consumable wafer body between the
workpiece and each of the indenters, and now clearly showing the
desirable and beneficial residual stress pattern provided
substantially uniformly along the interior edge wall of the
aperture in the workpiece.
[0100] FIG. 44 is a graphical plot of the tangential stress (in
thousand pounds per square inch) versus radial distance from a hole
(aperture) edge in a structure which has been cold worked by prior
art flat bottomed cylindrical mandrel methods.
[0101] FIG. 45 is a graphical plot of the residual tangential
stress versus radial distance from a hole (aperture) edge, in a
structure which has been cold worked utilizing one embodiment of my
optimized cylindrical indenter in my improved stress wave
method.
[0102] FIG. 46 is a graphical plot of the residual tangential
stress versus radial distance from a hole (aperture) edge, in a
structure which has utilized the split sleeve method for improving
fatigue life, as compared to both surface and aperture mid-plane
residual tangential stress results for a structure fabricated
utilizing in Type 2024-T3 Aluminum.
[0103] FIG. 47 is semi-log graphical plot of the maximum net stress
versus cycles to failure for (a) untreated sample results reported
by The Boeing Company, (b) samples results reported by Boeing for
parts treated by cold expansion; (c) untreated control samples of
the type treated and tested herein, and (d) results for sample
structures treated by the improved stress wave method taught
herein.
[0104] FIG. 48 illustrates a partial cross-sectional view of a
structure which utilizes a fastener to join first and second
structural parts, and wherein each of the first and second
structural parts have had the fastener apertures prepared by my
improved stress wave method for imparting beneficial residual
stress in a structural part.
[0105] FIG. 49 provides a partial cross-sectional view of a second
structure which utilizes a countersunk type fastener to join first
and second structural parts, and wherein each of the first and
second structural parts have had the fastener apertures prepared by
my improved stress wave method, to provide improved fatigue life in
the finished structural part.
[0106] FIG. 50 is a cross-sectional view which shows the set-up
step for utilizing yet another embodiment of my improved stress
wave method for impacting a single side of a workpiece, here
showing work on the obverse side of a workpiece, although the
reverse side could similarly be worked, by utilizing one indenter
and a consumable wafer between the indenter and the workpiece, in
order to provide beneficial residual stress near the fastener
apertures in the finished structure.
DESCRIPTION
[0107] In my novel, revolutionary cold metal working process,
stress waves are effectively utilized to impart beneficial residual
stresses to holes and other features in parts that are subject to
strength degradation resulting from fatigue damage. The stress wave
method imparts beneficial stresses using a dynamic indenter that
impinges the surface of a workpiece, normal to the surface of the
workpiece. The action of the dynamic indenter on the workpiece at a
selected velocity and with sufficient force causes waves of elastic
and plastic stress to develop and to propagate through the
workpiece.
[0108] Unlike the various prior art methods, I have discovered that
by carefully controlling the amount and distribution of pressure
applied to a workpiece, the resulting residual stress in the
workpiece can be substantially stratified, in cross-section, so
that a substantially uniform residual tangential stress is
maintained along the sidewall profile of an aperture through a
structure. To understand this phenomenon, it is helpful to look at
FIG. 1, which shows a summary of normalized contact pressure
distribution, .sigma..sub.z /P.sub.m, at radial distance from a
punch centerline, for a structure undergoing deformation by (a) a
cone, (b) a sphere, and (c) a circular, flat bottomed punch. When
using a punch 60 having a circular, flat bottom 62 as as depicted
in FIG. 2, the pressure is relatively uniform only near the center
of the area of engagement, and increases exponentially near the
edge of the contact area, where shearing tends to occur, as
indicated by broken line 64 in FIG. 1. Thus, flat cylindrical
indenters are efficient at imparting large zones of residual
stresses in thick sheets as they displace material uniformly over
their entire surface area. Unfortunately, the edge at the periphery
of the end of the flat cylindrical indenter shears the edge of the
surface of the structure being worked, leading to a much small zone
of residual stress at the surface of a workpiece. As a result, the
reduced amount of residual stress (as more particularly seen in
FIGS. 38 and 39, discussed hereinbelow) leads to a reduced fatigue
life of a structure fabricated using such method.
[0109] When using a punch 66 with a spherical bottom shape 68, as
shown in cross-section in FIG. 3, increased pressure is seen at the
center of the contact area (reference line 0.00 in FIG. 1), and the
contact pressure drops off exponentially toward the edge of the
contact area, as indicated by solid line 70 in FIG. 1. Resultingly,
indenters with spherical end shapes provide large zones of
beneficial residual stresses at the sheet surface, but lack the
ability to treat thick structures because of the reduced contact
area associated with the spherical shape. Also, such spherical
indenter shapes tend to induce large amounts of upset at the
surface of a workpiece, which may require further attention in
order to provide a suitable final product.
[0110] If a cone shaped punch 72 is utilized, as depicted in cross
section in FIG. 4, the pressure exerted by the punch 72 is very
high toward the center of the contact area, but drops off sharply
toward the edge, as indicated by dotted line 74 in FIG. 1. Such
cone shaped indenters are not very effective at producing desirable
residual stress profiles, since they tend to tear the surface and
move it radially outward.
[0111] In my method of producing beneficial stresses at desired
locations in structures, it is instructive to examine the amount of
deflection achieved on a workpiece by a properly optimized indenter
tool shape. This phenomenon is graphically represented in FIG. 5,
which compares the amount of deflection achieved at radial distance
from the centerline of a contact surface area being acted upon by
spherical, flat, and optimized indenters. The deflection achieved
by a spherical indenter of shape such as is depicted in FIG. 3, is
shown in line 80 of FIG. 5. The deflection achieved by a flat
bottomed cylindrical indenter, such as is depicted in cross-section
in FIG. 2, is shown in line 82 of FIG. 5. Importantly, the
generalized pressure profile of my improved, optimized indenter
which provides uniform pressure across the contact surface areas
results in deflection in a workpiece that, at the center of the
contact area, results in more deflection than that achieved by a
traditional prior art flat bottomed punch, but at the edge of the
contact area, results in less deflection (and hence, less shearing)
than that achieved by a traditional flat bottomed punch. See FIG.
5, broken line 84 which depicts the variable deflection achieved by
my optimized indenter when acting on a workpiece.
[0112] FIG. 6 is a partial cross-sectional view, showing an
optimized tooling indenter as taught herein, and indicating key
parameters for shaping such a tool. For purposes of this
discussion, FIG. 6 is best envisioned as depicting in one-half
cross-section a circular indenter 90 or punch with a contact face
92 with shaped profile, in the radially outward direction, as
further discussed 92 hereinbelow. The overall radius is a, and the
instantaneous radius is r at any pre-selected location in the
radially outward direction along the contact face 92. The initial
mean contact pressure, PM, for initial yield (indentation) is in
the normal direction z, and is indicated by the downward arrow in
FIG. 6, and such mean contact pressure is provided by the
instantaneous contact pressure .sigma..sub.z at any point in the
along the profile of contact face 92, or more generally shown
below, in the contacting end 94 of the indenter 90.
[0113] In one embodiment, a preferred indenter 90 contact face 92
profile shape is determined from the deflected shape of a dimple
induced by a uniform pressure acting on a selected workpiece. By
selecting the deflected shape for a pre-selected depth of
indentation in a workpiece resulting from uniform pressure as the
shape for the contact face profile 92 of the contacting end 94, the
optimum shape for my improved indenter can be determined. Such
deflected shape, or, more precisely as shown, the shape for the
contract face profile 92 in tool 90, is given by the elliptical
integral as follows: 1 u z = C ( 4 ( 1 - v 2 ) P M a E 0 / 2 1 - r
2 a 2 sin
[0114] Where:
[0115] a=a pre-selected radius of uniform pressure
[0116] C=a constant related to optimum depth in selected material
to create pre-selected stress levels in a selected workpiece,
ranging from about 1 to about 4
[0117] E=Elastic modulus
[0118] P.sub.M=contract pressure distribution
[0119] r=radial distance
[0120] u.sub.z=normal displacement of the workpiece
[0121] v=Poisson's Ratio
[0122] Since beneath the indenter 90 at the center of contact face
profile 92, i.e., the center of contact where r=0, the normal
displacement measured with respect to the first surface of a
workpiece is given by the following expression: 2 u z = 2 ( 1 - v 2
) P M a E
[0123] At the outer edge of the dimple (where r=the total radius a
of the indenter), the normal displacement measured with respect to
the first surface of a workpiece is given by the following
expression: which expression: 3 u z = 4 ( 1 - v 2 ) P M a E
[0124] Overall, the resultant shape of my improved indenter 90 is
somewhat similar to a flat punch but with a gradually sloping face
104 along the low radial distance (r/a) points (up to about r/a
0.66 or slightly more), and a somewhat radiused edge face 106 along
the high radial distance (r/a) points (where r/a is over about 0.66
or is in excess of about 0.75 or so). Another embodiment of an
indenter 90', slightly easier to manufacture, is shown in FIG. 14.
In this embodiment, the contact face profile 92 discussed above is
instead provided by a relatively flat central portion 110, and one
or more preferably flat, chamfered edges. Here, one edge 112 is
shown on a first side 114 of indenter 90', and two edges 116 and
118 are depicted a second side 120 of indenter 90'. As depicted,
edge 112 is offset from flat central portion 110 by an angle alpha
({acute over (.alpha.)}), which is preferably in the 30 degree to
60 degree range. Also, edge 116 is offset from the flat central
portion 110 by an angle beta (.beta.), which is preferably provided
in the range from about zero degrees up to about fifteen degrees.
In those tools in which a second chamfer is desired, the second
chamber angle tau (.tau.) is generally about the same as alpha, or
from about 30 up to about 60 degrees.
[0125] In FIG. 15, yet another embodiment of my indenter 90" is
illustrated. Depending upon the materials of construction of a
particular structure, and on the amount of beneficial residual
stress necessary to provide in the workpiece to be used in such
structure, in some cases it may be possible to achieve the results
taught herein, at least to some significant degree, via use of a
simple indenter 90" with a rounded contacting end 120. In such
cases, I have found that the shape of the contacting end 120 can be
described as having a curvature of radius R, where radius R is
equal to or between 2D and 8D. When restated in terms of radius as
shown above, R is in the range from (2a) to 8(2a), inclusive, i.e,
R is between 4a and 16 a. More preferably, R is between about 3Dand
5 D,and most preferably, R is about 3.2 D.
[0126] Turning now to FIGS. 7, 8, and 9, the use of a single
dynamic indenter 130 is illustrated for application of the stress
wave process via contacting end 131 to a workpiece structure 132.
FIG. 7 shows the single, (here, upper) pre-selected indenter 30 of
diameter 2a being positioned and aligned over the centerline 134 of
a workpiece structure 132 in which an aperture 136 of a preselected
diameter A.sub.D is desired. In FIG. 8, the step of dynamically
driving indenter 130 in the direction of reference arrow 138 and
into the obverse surface 140 of workpiece structure 132 to create a
stress wave in workpiece structure 132 is depicted. A stylized view
of the stress waves passing through workpiece 132 in response to
impact from the dynamic indenter 130 is shown in FIG. 9. With a
sudden compressive impact, such as by a dynamic indenter 130 on
workpiece 132, an elastic wave of compression 150 hits the
workpiece, and moves through the thickness T of the workpiece. This
is followed by a plastic wave 152 which travels more slowly. The
elastic wave moves through the metal at a velocity of
[0127] c=(E/.rho.).sup.1/2
[0128] where
[0129] c=the speed of sound in the material, and
[0130] .rho.=the material density
[0131] The velocity of the plastic wave front is a little more
complex, and is generally represented as follows:
[0132] C=[S(.epsilon.)/.rho.]1/2
[0133] where
[0134] S(.epsilon.)=the modulus of deformation (elastic or
plastic), which is equal to the change in stress divided by the
change in strain speed, (d .sigma./ d.epsilon.).
[0135] In my improved stress wave process, it is important to note
that by shaping contacting end 131 of the indenter 130, the stress
wave is focused normally along the direction z of the impact of the
indenter 130 on workpiece 132, and between the contacting end 131
of the indenter and the first or obverse surface of the workpiece
132.
[0136] Turning now to FIGS. 10 and 11, a slightly different method
is illustrated, wherein the workpiece 132 is provided with a
backing anvil 160 to support the workpiece 132 from the second or
reverse side 162 of workpiece 132. In FIG. 10, the set-up step for
utilizing my improved stress wave method in creation of dimple on a
workpiece with an indenter 130 is shown, in preparation for forming
an aperture in the workpiece at a pre-selected location. FIG. 11
illustrates the step of impacting one side of workpiece 132 with
indenter 130 to create a stress wave and form a dimple in the
workpiece 132 at a pre-selected location where it is desired to
create an aperture such as a "rivet-hole".
[0137] In FIGS. 12 and 13, the two-sided method of using my
improved stress wave method for impacting both the obverse 140 and
the reverse 162 sides of a workpiece 132 with a first indenter
130(1) and a second indenter 130(2) to create a stress wave and
form dimples 170 (see FIG. 18) in both sides of the workpiece at a
pre-selected location where it is desired to create an aperture A
such as a "rivet-hole" of diameter A.sub.D. The step of both the
obverse 140 and the reverse 162 sides of a workpiece 132 with
indenters 130(1) and 130(2), respectively, to create a stress wave
150 and form dimples 170 in both sides of the workpiece 132 at a
pre-selected location where it is desired to create an aperture
such as a "rivet-hole". This technique is shown in additional
detail in FIGS. 16, 17, 18, and 19, wherein the indenters 130(1)
and 130(2) are shown in partial cross-sectional view. Another aid
to understanding the improvement offered in the art by my process
is to look at FIG. 17, wherein workpiece 132' is being impacted by
the indenters 130(1) and 130(2). Reference arrows 200, 202, 204,
and 204 depict equal impact pressure lines experienced by workpiece
132'. It is important to note that workpiece 132' is not indented
by an equal amount in the z direction (normal direction) when
examined from side to side along the radial distance r/ of the
indenter, as earlier presented in FIG. 5. In other words, the
length of reference arrow 200, from indenter 130(1) to indenter
130(2), is longer than radially inward reference arrow 202, which
also extends from indenter 130(1) to indenter 130(2). y direction
(side-to-side). Similarly, the length of reference arrow 206, from
indenter 130(1) to indenter 130(2), is longer than radially inward
reference arrow 204 also extending from indenter 130(1) to indenter
130(2). Thus, equal pressure at the radial positions indicated by
reference arrows 200, and 202 results in unequal deformation of the
workpiece 132'. The idealized slope of this curve, for one
embodiment of my invention, was earlier described in FIG. 6.
[0138] Also depicted in FIGS. 16 and 17 are some reference marks
for analytical tools further revealed in FIGS. 38 through 43. It
can be observed that workpiece 132' is arbitrarily divided into
three parts, 132(A), 132(B), and 132(C), as indicated by separation
lines U and L. In FIGS. 38 through 43, further discussed below, the
separation lines are used to modularly explode stress analysis
diagrams of workpiece 132' into three layers, representing parts
132(A), 132(B) and 132(C).
[0139] FIGS. 18 and 20 represent a workpieces 132' and 132,
respectively, wherein a dimple 170 has been created in both side of
workpiece 132' , or in a single side of a workpiece 132, as
desired. It is important to note that the actual dimple depth DP
when under pressure, as indicated in 17, may be slightly more than
the dimple depth H after the indenter 130(1) and/or 130(2) has been
withdrawn due to the slight elastic sprinback of the workpiece once
the deforming pressure of the indenter(s) has been released.
[0140] As indicated in FIG. 21, a back-taper type drill having a
backtaper angle lambda (X) can be used to advantageously remove
unwanted metal in workpiece 132 or 132'. Preferably, the working
face diameter of the drill DD used to achieve an aperture of
diameter A.sub.D is larger than the dimple diameter D(170) by a
distance 2Q, where Q representes the radial distance from the edge
220 of a dimple to the edge 222 of the hole being drilled. In any
event, Q is small, but in some cases, may range down to zero.
[0141] FIGS. 22 through 26 represent variations in my method which
may be utilized to achieve desired results in unique situations.
First, in FIG. 22, a first indenter 300 with contacting end 302 and
a second indenter 304 with a contacting end 306 are provided
wherein the contacting end 302 area and the contacting end 306 are
of differing surface area. FIG. 22 shows the set-up step for using
such differential area indenters, and FIG. 23 illustrates the step
of providing a dynamic pulse on the first 300 and second 304
indenters to provide a stress wave on workpiece 332, to create a
dimple therein. So, as shown in FIG. 22, the set-up step for
utilizing my improved stress wave method allows for the impacting
both the obverse and the reverse sides of a workpiece with
indenters of a selected, unequal diameter or non-standard shape, to
create a stress wave and form dimples in both sides of the
workpiece at a pre-selected location where it is desired to create
a shaped aperture such as a "rivet-hole", or preferably, a flush
rivet hole. FIG. 23 depicts the actual step of impacting both the
obverse and the reverse sides of a workpiece with the indenters 300
and 306 of a selected, unequal diameter, to create a stress wave
and form dimples in both sides of the workpiece 332 at a
pre-selected location where it is desired to create an apertures
such as a "rivet-hole" or preferably a flush rivet hole of diameter
A.sub.D.
[0142] When chamfered or countersink type rivets are utilized, the
set up illustrated in FIG. 24 can be advantageously employed for
utlizing my improved stress wave method, to impacting both the
obverse 340 and the reverse 362 sides of a workpiece 362 with
indenter 370 (with contacting end 371) and indenter 372 of both
different shape and of unequal size, to create a stress wave and
form dimples in both sides of the workpiece 362 at a pre-selected
location where it is desired to create an aperture such as a
countersink 374 edge"flush-rivet hole". In this case, indenter 372
is provided with a beveled edge 376 to impart stresses in the
desired direction, as well as in the normal face end 378 of the
indenter 372.
[0143] FIGS. 25 and 26 show the use of a pilot hole defined by wall
380 in workpiece 378. In FIGS. 25, an elongated alignment shaft 382
protrudes through workpiece 278 and is received by companion,
complementary sized and shaped receiving chamber 384 in indenter
372'. Similarly, in FIG. 26, an elongated alignment shaft 382,
preferably affixed to the distal end 371"of indenter 370",
protrudes through workpiece 378 and is received in a companion,
complementary sized and shaped receiving chamber 384".
[0144] Both FIGS. 25 and 26 are partial cross-sectional views which
illustrate the set-up step for utilizing my improved stress wave
method for impacting both the obverse side 390 and the reverse side
392 workpiece 378 with indenters which are both of different shape
and of unequal size, and where the indenters are centered by use of
pilot guide tool received in a centering receiving slot in at least
one of the indenter.
[0145] Turning now to FIG. 27 the set-up step for utilizing yet
another embodiment of my improved stress wave method for impacting
both the obverse and the reverse sides of a workpiece is depicted.
Here, the use of indenters 430(1) and 420(2), which are of
identical shape and size, are provided. This variation in my method
utilizes a consumable wafer W(1) between the indenters 430(1) and
the obverse surface 431 of the workpiece 432. Likewise, a
consumable waver W(2) is utilized between the indenter 430(2) and
the reverse surface 434 of the workpiece 432. Each of the
consumable wafers has a workpiece side WW and an indenter side WI.
The workpiece side WW impacts the workpiece, and when the wafer is
squeezed between the workpiece 432 and an indenter, the waver W(1)
and/or W(2) deforms slightly, spreading the force to the workpiece
outward radially a small distance QW from the radius of the
indenter utilized, which force decreases rapidly beyond the end of
the contacting end 462 or 464 of the respective indenter. After the
step of dynamic impacting of the wafers W(1) and W(2), in the
manner discussed hereinabove, the step of disengaging the opposing
indenters 430(1) and 430(2) is illustrated in FIG. 28. Here, a
cross-sectional view shows the disengagement of the opposing
indenters from their respective consumable wafers W(1) and W(2) on
opposing sides of a workpiece 432, while utilizing my improved
stress wave method for impacting both the obverse 431 and the
reverse 434 sides of a workpiece 432 to create dimples 450 and 454
at pre-selected locations. It is preferred that the consumable
wafers W(1) and W(2) be of the same or a similar material to the
material comprising workpiece 432, although it should be understood
that the method is not limited thereto, and any material which
provides the desired uniformity in resultant residual stress
profile, as further discussed herein below, can be utilized.
However contact end 462 of indenter 430(1), and contacting end 464
of indenter 430(2), can be selected from a variety of shapes, so
long as the pressure distribution to the workpiece 432 is
substantially uniform, assuming that straight sides are desired on
the apertures being created in the workpiece. Importantly, it
should be noted that the actual aperature hole edge location 480
may be located radially inward, of or radially outward of, the
peripheral edge 482 of the indenter 430(1) or 430(2). Similarly,
the anticipated edge wall location which defines the aperture
through workpiece 432 may be radially inward of (wall 480) or
within the zone QW earlier discussed (wall 480"), or radially
outward of zone QW (wall 480"'). The choice of wall location is
dependent on various factors, most importantly of course the amount
of beneficial residual stress present, after treatment, at the
pre-selected wall location, and also whether or not a slight
indentation ID would be advantageous at the outer peripheral edge
of a fastener. In this regard, see FIG. 49, which shows the
peripheral edge 800 of a fastener 802, with a small indentation ID
adjacent thereto. FIG. 49 is particularly interesting since it
provides an indication that a countersunk type outer edge wall 804
can be prepared according to the methods described herein to
provide a desirable beneficial residual stress pattern in the body
806 of structure 808. Likewise, the body 810 of structure 812
adjacent to the more conventional perpendicular edge wall 814 can
be treated to provide a desirable beneficial stress pattern in the
body 810. More conventionally, as shown in FIG. 49, a fastener 840
having an externally protruding head 842 is provided to join
structural members 844 and 846. In such structures, apertures
defined by sidewalls 848 and 850, respectively, accomodate the
fastener. The beneficial residual stress is advantageously provided
in both structural member 844 and 846.
[0146] Although it is generally expected that most structures would
substantially benefit from the use of increased fatigue resistance
being imparted from both the obverse and the reverse sides, in some
applications, there may arise useful results when only a single
side is treated by my improved stress wave method. Such one-sided
treatment of a structure is depicted in FIG. 50. That
cross-sectional view shows the set-up step for utilizing yet
another embodiment of my improved stress wave method for impacting
a single side of a workpiece 432', by showing work on only the
obverse side 431' of that workpiece. The reverse side 434' could
similarly be worked, by utilizing one indenter and a consumable
wafer between the indenter and the workpiece 432', in order to
provide beneficial residual stress near the fastener apertures in
the finished structure fabricated from the workpiece 432'. FIGS. 29
and 30 illustrate the set-up step for drilling a pair of workpieces
at an aligned, pre-selected location. In FIG. 29, a workpiece 500
having dimples 502 and 504 therein, and workpiece 510, having
dimples 512 and 514 therein, are aligned along a centerline so that
an concentric apertures can be created through workpieces 500 and
510 by drilling therethrough with drill 516 or preselected diameter
DX. Similarly, in FIG. 30, workpieces 500 and 520 (in which no
beneficial stress relief pattern has been creatred) are aligned
along a centerline, so that concentric apertures can be created
through both workpieces by drill 516. In both FIGS. 29 and 30, lap
type joints can be created using at least one workpiece wherein a
fastener receiving aperture has been provided with an improved
fatigue life, by preparing dimples in the obverse and reverse side,
and wherein the wherein the dimples have been centered in order to
create beneficial residual stresses at a desired location with
respect to the final fastener receiving apertures.
[0147] It is also important to understand that unusual
configuration, non-circular type apertures can be treated with my
improved stress wave process to provide beneficial residual stress
levels at desired locations adjacent the interior edge wall of the
hole. In FIGS. 31 and 32, top plan views of two such structures,
600 and 602, are illustrated. In structure 600, an interior
side-wall 604 is provided of generally elliptical shape. In
structure 602, a parallelogram 604 shaped aperture wall having
radiused corners 606, 608, 610, and 612 are provided. Thus,
structures having non-circular holes therein can advantageously be
treated with my improved stress wave process to provide beneficial
residual stress levels at desired locations in the structure.
[0148] In order to provide the beneficial residual stress levels in
oversized circular apertures, or in unusual shaped structures, I
have found it useful to provide opposing, hollow, oversize
indenters as depicted in FIGS. 33, 34, and 35. Such indenters
630(1) and 630(2) can be shaped, at least along the outer periphery
P of the contacting end 662 and 664 of the indenters 630(1) and
630(2), respectively, in an optimized manner for treatment of
over-sized or non-round holes in a workpiece 632. In FIG. 33, a
perspective view of such indenters 630(1) and 630(2) is shown. In
FIG. 34, a vertical cross-sectional view of a pair of hollow,
oversize indenters 630(1) and 630(2) are shown at the step of being
positioned adjacent the obverse 690 and the reverse 692 side of a
workpiece 632, prior to imparting a stress wave on a structure in
order to create beneficial residual stress at desired locations in
an oversize or non-circular cutout. Particularly in FIG. 34, and
also throughout following FIGS. 35, 36, and 37, note that the
broken lines 700 and 702 indicating the intended dimensions of an
interior aperture through the workpiece 632, with that sidewalls
704 and 706 are ultimately created along the lines indicated at 700
and 702. In FIG. 37, an end wall 708 is also shown, which would be
at another cross-sectional location at the rear of the indenters
shown in the perspective view of FIG. 33. Note that the dimples 694
and 695 on the obverse side 690, and the dimples 696 and 697 on the
reverse side 692 are located inwardly (here, radially, with
circular indenters) from the broken sidewall indicating lines 700
and 702, so that the beneficial residual stress is at desired
locations in an oversize or non-circular cutout.
[0149] The treated workpiece 632, before machining for removal of
unwanted material, is shown in FIG. 37. The oversize or non-round
cutout treatment is finally represented in FIG. 27, showing how
interior sidewalls 704, 706, and 708 extend between the obverse and
reverse sides of workpiece 632, with beneficial residual stresses
in the structure.
[0150] Attention is now directed to the series of drawings FIG. 38
through FIG. 43, where the results of my optimized stress wave
process can be clearly compared to and its improvement shown over
the prior art. A modularly exploded cross sectional view of a stack
of three workpiece sheets is shown in FIG. 38 after being cold
worked by a flat cylindrical punch. The detailed color code, with a
legend on the side indicating beneficial residual stress in pounds
per square inch, clearly shows in FIG. 39 that the residual stress
field present in the workpieces after drilling to create an
aperture in the workpieces provides workpieces only modest
beneficial stress increase near the obverse surface, and similarly
on modest beneficial stress increase near the reverse surface.
[0151] However, such prior art techniques create significant
benefical residual stress at the mid-plane of the aperture, as is
seen in FIG. 39. Unfortunately, this allows fatigue cracks to
initiate at the surface (either obverse or reverse sides) and
grow.
[0152] In contrast, with my optimized indenter, and using the
method taught herein, by examining closely the results shown in
FIG. 40 and FIG. 41, it can be clearly seen in this modularly
exploded cross-sectional view of three workpiece sheets being acted
on at the obverse and reverse sides by an optimized indenter, that
beneficial residual stress is imparted substantially uniformly
throughout the workpiece structure. Particularly in FIG. 41, it can
be seen that workpiece after drilling to create an aperture in the
workpiece, provides a uniform beneficial residual stress
profile.
[0153] In one particularly advantageous embodiment, depicted in
FIG. 42 and 43, an exploded cross-sectional view is shown of three
workpiece sheets after the step of stress wave input to the
workpiece by utilizing opposing indenters and a consumable wafer
body between the workpiece and each of the indenters. Desirable
deformation is imparted into both the obverse and reverse sides of
the workpiece.
[0154] Importantly, FIG. 43 depicts a is a modularly exploded
cross-sectional view of three workpieces after the step of drilling
an aperture at a pre-selected location in a workpiece, after the
step shown in FIG. 42 of utilizing indenters and a consumable wafer
body between the workpiece and each of the indenters, and now
clearly showing the desirable and beneficial residual stress
pattern provided substantially uniformly along the interior edge
wall of the aperture in the workpiece. Thus, it can clearly be seen
that uniformity is achieved in the sidewall beneficial residual
stress.
[0155] The above finding can be further confirmed by comparison of
FIGS. 44, and 45. In FIG. 44, the graphical plot of the tangential
stress (in thousand pounds per square inch) versus radial distance
from a hole (aperture) edge in a structure which has been cold
worked by prior art flat bottomed cylindrical mandrel methods is
shown. Clearly, there is a large departure between stress at some
distance from the obverse or reverse surface edge of the hole.
However, with my improved stress wave method, and using the my
optimized cylindrical indenter, the beneficial residual stress
becomes fairly closely matched, between the surface and the
mid-plane areas.
[0156] In fact, my improved method provides beneficial residual
stress at least as good as the split speeve process, as revealed in
FIG. 46. In that graph, a plot of the residual tangential stress
versus radial distance from a hole (aperture) edge, in a structure
which has utilized the split sleeve method for improving fatigue
life, as compared to both surface and aperture mid-plane residual
tangential stress results for a structure fabricated utilizing in
Type 2024-T3 Aluminum.
[0157] Most importantly, when direct comparisons of fatigue life
are conducted, as is illustrated in FIG. 47, structures prepared by
my process clearly show improved fatigue life. FIG. 47 is a
semi-log graphical plot of the maximum net stress versus cycles to
failure for (a) untreated sample results reported by The Boeing
Company, (b) samples results reported by Boeing for parts treated
by cold expansion; (c) untreated control samples of the type
treated and tested herein, and (d) results for sample structures
treated by the improved stress wave method taught herein. My
samples showed fatigue life in the 400,000 to 4,000,000 cycles,
more or less, depending upon applied loading, and slightly exceeded
the predicted life for one of the best commercially employed
methods, cold expansion.
[0158] It should also be noted that the present invention can be
used with any convenient apparatus which results accelerates the
indenter with sufficient force to effect the necessary stress wave
in the workpiece. In this regard, electromechanical impact type
apparatuses such as that revealed the above mentioned Zieve patent,
or in the other patents mentioned or otherwise known in the prior
art, may be advantageously applied by those of skill in the art and
to whom this disclosure is directed, in order to achieve the
results and to practice the improved stress wave method taught
herein.
[0159] It is to be appreciated that my novel stress wave process
for cold working parts to reduce fatigue stress degradation of the
part, is an appreciable improvement in the state of the art of cold
working metal parts subject to fatigue concerns. The stress wave
process treats the process of cold working from a new perspective,
preferably by entirely treating the hole before it is machined. The
stress wave method improves on currently used treatment methods by
eliminating expansion mandrels, sleeves, and lubricants.
[0160] In my improved stress wave method, control of the magnitude
and depth of residual stress is determined by the nature of the
stress wave, and not by tooling tolerances. Importantly, the stress
wave process is readily automated and can be put into any automated
fastening environment. The stress wave process also eliminates
distortions and tears around the holes caused during expansion by
mandrel methods. Although only a few exemplary embodiments of this
invention have been described in detail, it will be readily
apparent to those skilled in the art that my novel stress wave cold
working process, and the apparatus for implementing the process,
may be modified from those embodiments provided herein, without
materially departing from the novel teachings and advantages
provided by this invention, and may be embodied in other specific
forms without departing from the spirit or essential
characteristics thereof. Therefore, the embodiments presented
herein are to be considered in all respects as illustrative and not
restrictive. As such, the claims are intended to cover the
structures described herein, and not only structural equivalents
thereof, but also equivalent structures. Thus, the scope of the
invention is intended to included all variations described herein,
whether in the specification or in the drawing, including the broad
meaning and range properly afforded to the language and description
set forth herein to describe such variations.
[0161] It will thus be seen that the objects set forth above,
including those made apparent from the preceding description, are
efficiently attained. Since certain changes may be made in carrying
out the construction of a method for metals processing according to
the teachings herein, it is to be understood that my invention may
be embodied in other specific forms without departing from the
spirit or essential characteristics thereof. Many other embodiments
are also feasible to attain advantageous results utilizing the
principles disclosed herein. Therefore, it will be understood that
the foregoing description of representative embodiments of the
invention have been presented only for purposes of illustration and
for providing an understanding of the invention, and it is not
intended to be exhaustive or restrictive, or to limit the invention
only to the precise forms disclosed.
[0162] All of the features disclosed in this specification
(including any accompanying claims, the drawing, and the abstract)
may be combined in any combination, except combinations where at
least some of the features are mutually exclusive. Each feature
disclosed in this specification (including any accompanying claims,
the drawing, and the abstract), may be replaced by alternative
features serving the same or similar purpose, unless expressly
stated otherwise. Thus, unless expressly stated otherwise, each
feature disclosed is one example only of a generic series of
equivalent or similar features. Further, while certain materials
are described for the purpose of enabling the reader to make and
use certain embodiments shown, such suggestions shall not serve in
any way to limit the claims to the materials disclosed, and it is
to be understood that other materials, including other metals and
various compositions, may be utilized in the practice of my
methods, and in the manufacture of my novel structures.
[0163] The intention is to cover all modifications, equivalents,
and alternatives falling within the scope and spirit of the
invention, as expressed herein above and in the appended claims. As
such, the claims are intended to cover the structures, apparatus,
and methods described herein, and not only the equivalents or
structural equivalents thereof, but also equivalent structures or
methods. The scope of the invention, as described herein and as
indicated by the appended claims, is thus intended to include
variations from the embodiments provided which are nevertheless
described by the broad meaning and range properly afforded to the
language of the claims, as explained by and in light of the terms
included herein, or the equivalents thereof.
* * * * *