Patent Publication Number: US-6711928-B1

Title: Method and apparatus for producing beneficial stresses around apertures, and improved fatigue life products made by the method

Description:
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. This application is a continuation-in part of U.S. patent application Ser. No. 09/401,065, filed Sep. 22, 1999, which is a continuation-in-part of U.S. patent application Ser. No. 09/270,428, filed Mar. 16, 1999, and claims the benefit of provisional application 60/078,356 filed Mar. 17, 1998, the disclosures of each of which are incorporated herein by this reference, including, in particular, color figures provided in the drawing. 
    
    
     TECHNICAL FIELD 
     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 novel manufacturing techniques for providing improved fatigue life in parts, to 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 
     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, helicopters, ships, trains, cars and the like. Fatigue can also be present in other less obvious applications such as pressurized vessels, space vehicles, farm equipment, internal combustion engines, turbine engines, medical implants, industrial equipment, sporting equipment. 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. 
     Fatigue can be a problem for holes and cutouts found in frames and bulkheads of fighter aircraft. Typically these structures have a variety of hole shapes and sizes (some non-round in shape) for the purpose of routing cables, wires, tubing and actuators through the aircraft. They can also serve as a means for allowing fuel flow from one bay to the next. In addition to serving as passageway holes they can also serve as lightening holes for reducing the weight of the structure. Lightening holes can also be found on bridges, trusses, construction equipment, semi-trailers and the like. Regardless of the function or purpose of the hole if they experience cyclic stresses they are subject to fatigue damage. 
     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 cannot carry the additional load that is not being carried by the failed structural member. 
     Automotive vehicles are also subjected to the damaging effects of cyclic stress. Vehicles driven on rough roads or off-road experience far more damaging loads on suspension, steering, wheels and the like than for those driven on smooth pavement. The firings of the pistons create cyclic loads on valves, valve guide holes, piston and connecting rod assembly, holes in and connecting both blocks and heads. Some fatigue is a result of high vibration of small stress. Metal covers surrounding and protecting mechanical assemblies may crack at holes due to vibratory loads. Holes created for the purpose of providing flow of lubricant or fluids are sometimes located in areas of high stress. These too, may experience fatigue damage. 
     Fortunately, failure due to the fatigue of an automotive component has generally less severe consequences than with an aircraft component failure. Even so, fatigue in automotive components has a large economic impact on the manufacturer because of the extent of the problem. Fatigue failures may show up only after the production of hundreds of thousands of units. Warranties work on that many vehicles can be very expensive and create a negative public image. Since fatigue damage usually occurs on highly stressed and typically more expensive parts these are the ones that are generally most costly to fix. 
     Large cylindrical and tubular rollers used in the manufacture of paper are perforated with thousands of holes allowing for the escape of liquids associated with the pulping process. The rollers used in paper production are basically rotating cylinders that are simply supported at both ends. The action of squeezing the pulp or pressing the paper under very high pressures creates bending stresses in the rollers. At the bottom of the roller tensile stresses are created and at the top of the roller compressive stresses are created. As the roller rotates through one complete turn the material experiences one cycle of alternating stresses; negative to positive. These applied cyclic stresses, coupled with the stress risers of many thousands of holes produce many potential fatigue damage sites on the rollers. Even non-perforated rollers experience fatigue because of the need for high speed, vibration free operation. Since the rollers typically rotate at a high velocity, any imbalance in the system can cause severe vibration. Typically, balance weights are attached to the roller through small bolt holes. The holes are subjected to the previously mentioned alternating stress cycle. Because the holes concentrate the stress they are a major source of fatigue cracks. 
     Orthopedic implants are subjected to repetitive cyclic loading from patient movements. Consequently, such implants are designed to resist fatigue. Orthopedic implants frequently include holes through which screws and other fasteners pass to attach the implant to the bone. The holes, while necessary for attachment to the bone, reduce the overall strength of the implant since they provide less cross-sectional area to accommodate the loads being transferred to the implant through the bone and also act as stress risers which reduce the ability of the implant to tolerate cyclic fatigue loading. The problem is particularly acute in trauma implants, such as bone plates, intramedullary nails and compression hip screws since these devices, in effect, stabilize broken bone fragments until healing occurs. Thus the loading imposed on the bone during the normal movements of the patient is immediately translated to the trauma implant which is then placed under greater stresses than a permanent prosthetic implant might be. The situation is aggravated if the bone does not heal as expected. In that case, the implant is required to accommodate not only greater stresses but also a longer cyclic loading period. Under such conditions, fatigue failure of the trauma implant is more likely. 
     Even stationary objects, such as railroad track or pressure vessels, may fail in fatigue because of cyclic stresses. The repeated loading from wheels running over an unsupported span of track causes fatigue loads for railroad 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. 
     Fatigue can be defined as the progressive damage, generally in the form of cracks, which occur in structures due to cyclic loads. Cracks typically occur at apertures (holes), notches, slots, 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, cutout or notch. Because of this phenomenon, it is typically found that stress levels in the structure adjacent to fastener holes, cutouts or changes in section experience stress levels at least three times greater than the nominal stress which would be experienced at such location, absent the hole, cutout or notch. 
     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, Wash., 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. 
     Cold working has shown to be effective on a wide variety of materials including cast iron, ductile iron, carbon steels, low alloy steels, intermediate alloy steels, stainless steels, high alloy steels, aluminum alloys, magnesium, beryllium, titanium alloys, high temperature alloys, bronze and the like. 
     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 require 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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     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&#39;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. 
     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 M , for initial yield (indentation) is estimated by the following equation: 
     
       
         P M ==1.10×(compressive yield stress)  
       
     
     The load P for initial yielding or indenting is calculated by multiplying P M , by the cross sectional area of the mandrel. Therefore: 
     
       
         Mandrel Load (P)=1.10 (compressive yield stress) (mandrel cross sectional area)  
       
     
     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 ¼ inch (6.35 mm) diameter fastener can be as high as 10,000 pounds (44484 N). Unfortunately, loads of such magnitudes 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. 
     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 does 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. 
     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. 
     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. 
     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. 
     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: 
     They often require mandrels, split or solid, and disposable split sleeves, which demand precision dimensions, which make them costly; 
     Mandrels and sleeves are an inventory and handling item that increases actual manufacturing costs when they are employed; 
     “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; 
     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; 
     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; 
     Mandrel methods require some sort of lubricant; such lubricants, and especially the liquids, require solvent clean up; 
     Splits in a sleeve or splits in a mandrel can cause troublesome shear tears in certain 7000 series aluminum alloys; 
     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; 
     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; 
     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; 
     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 
     My novel stress wave manufacturing process can be advantageously applied to apertures for fasteners, to large holes in structures, to countersunk 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 manufacturing cost advantages. The method is simple, easily applied to robotic, automated manufacturing methods, and is otherwise superior to those manufacturing methods heretofore used or proposed. 
     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. 
     Another objective of my method, and my novel tools us for carrying out the method, is to simplify the manufacturing procedures, which importantly, simplifies and improves quality control in the manufacture of parts having an improved fatigue life. 
     Other important but more specific objects of the invention reside in the provision of an improved manufacturing process for enhanced service life metal parts subject to fatigue stress, as described herein, which: 
     eliminates the requirement for creating a starting hole, as well as tooling and labor costs associated therewith; 
     eliminates the requirement for purchase, storage, and maintenance of mandrels; 
     eliminates the requirement for purchase, storage, and maintenance of split sleeves; 
     eliminates the need for disposal of split sleeves; 
     eliminates the need for lubrication and subsequent clean-up during manufacture of structures containing apertures therethrough; 
     enables the manufacture of a wide range of aperture diameters, in which appropriate fastener diameters can be employed; 
     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; 
     enables process control to be established using statistical feedback into the manufacturing system, thus enhancing quality assurance; 
     eliminates shear tears in a workpiece that are commonly encountered in mandrel manufacturing methods; 
     significantly reduces or effectively eliminates surface marring and upset associated with mandrel methods, thus significantly increasing fatigue life; 
     is readily adaptable to automated manufacturing equipment, since manufacturing cycle times are roughly equivalent to, or less than, cycle times for automated riveting operations; 
     eliminates bulky hydraulic manufacturing equipment typically used in mandrel methods, and substitutes simple, preferably electromagnetic equipment; 
     enables aperture creation after fatigue treatment, by a single reaming operation, rather than with two reaming operations as has been commonly practiced heretofore; 
     is sufficiently low in cost that it can be cost effectively applied to a number of critical structures, including fuselage structures. 
     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 
     I have now invented, and disclose herein, an improved metal cold 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 cold working processes. Also, it eliminates undesirable equipment necessary for the more commonly utilized alternative processes, such as the need for starting holes, for bulky hydraulic equipment, for precision mandrels, for disposable split sleeves, and for 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. 
     My improved stress wave method imparts beneficial stresses using a dynamic indenter that impinges the surface of the metal, preferably in a normal direction to the surface. The action of the indenter causes waves of elastic and plastic stress to develop and propagate through the metal. In some cases a stationary indenter or an anvil is used to support thin workpiece materials. Such “backing” indenters or anvils also assist in the reflection and or creation of plastic waves off or from the other side of the workpiece. 
     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 indenter 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, substantially uniform beneficial residual compressive stresses remain in finished structures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The file of this patent contains six (6) frawings executed in color. Copies of this patent with color drawings will be provided by the Patent and Trademark Office upon request and payment of the necessary fee. 
     FIG. 1 shows a summary of normalized contact pressure distribution,  z /P 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. 
     FIG. 2 is a vertical cross-section of a flat bottom circular punch, of the type which displays the curve depicted in FIG. 1 when cold working metal. 
     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. 
     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. 
     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. 
     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. 
     FIG. 7 illustrates one type of set-up step for utilizing my stress wave method in preparing a high fatigue life aperture on a relatively thin workpiece. 
     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 having improved fatigue life properties. 
     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”, having improved fatigue life properties. 
     FIG. 10 illustrates one type of 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 having improved fatigue life properties in the workpiece at a pre-selected location. 
     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 having improved fatigue life properties, such as a “rivet-hole”. 
     FIG. 12 illustrates one type of set-up step for utilizing my improved stress wave method for impacting both the obverse and the reverse sides of a workpiece with indenters 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 with improved fatigue life properties, such as a “rivet-hole”. 
     FIG. 13 illustrates the step of impacting both the obverse and the reverse sides of a workpiece with indenters 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 with improved fatigue life properties, such as a “rivet-hole”. 
     FIG. 14 illustrates another embodiment of an 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 of the indenter to approximate a desirable curvature near, and preferably at, the edge of the indenter. 
     FIG. 15 illustrates yet another embodiment of an 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. 
     FIG. 16 is a vertical cross-sectional view of first and second optimized shape indenters in one type of 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 having improved fatigue life. 
     FIG. 17 is a vertical cross-sectional view of first and second optimized shape indenters 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 having improved fatigue life. 
     FIG. 18 is a cross-sectional view of a workpiece after formation of optimized dimples in both the obverse and reverse sides. 
     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 having improved fatigue life in the workpiece. 
     FIG. 20 is a cross-sectional view of a workpiece after formation of a dimple only in the obverse side at a location where it is desired to place an aperture having enhanced fatigue life. 
     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 having improved fatigue life at a pre-selected location. 
     FIG. 22 illustrates one type of 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 pre-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 having enhanced fatigue life, such as a “rivet-hole”, particularly for a flush rivet hole. 
     FIG. 23 illustrates the step of impacting both the obverse and the reverse sides of a workpiece with indenters of a pre-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 aperture having improved fatigue life, such as a “rivet-hole”, and in one embodiment, for a flush rivet hole. 
     FIG. 24 is a side elevation view of one type of 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 having improved fatigue life, such as a chamfered “flush-rivet hole”. 
     FIG. 25 is a partial cross-sectional view, illustrating one type of 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 receiver in one or the other indenter. 
     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 receiver in one or the other indenter. 
     FIG. 27 is a cross-sectional view which shows one type of 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 further utilizing a consumable wafer between the indenters and the workpiece. 
     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, after 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 where it is desired to create an aperture having improved fatigue life. 
     FIG. 29 is a side elevation view which illustrates one type of 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. 
     FIG. 30 is a side elevation view which illustrates one type of set-up step for drilling a pair of workpieces at an aligned, pre-selected location, and using workpieces wherein only one workpiece 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. 
     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 in a bounding portion adjacent the interior edge wall of the hole. 
     FIG. 32 is a top plan view of yet another structure having a non-circular hole therein which can advantageously be treated with my improved stress wave process to provide beneficial residual stress levels in a bounding portion at desired locations in the structure adjacent an interior edge wall portion of a passageway in the structure. 
     FIG. 33 is a perspective view of a opposing, shaped (and preferably hollow) oversize indenters which can be shaped, at least along the outer periphery of the indenter, in an optimized manner for treatment of a large cut-out, or of a non-round hole in a workpiece. 
     FIG. 34 is a vertical cross-sectional view of a pair of shaped (preferably 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 a large cutout, or of a non-circular cutout. 
     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 cutout or in a non-circular cutout. 
     FIG. 36 is a vertical cross-sectional view of the structure just shown in FIGS. 34 and 35 above, but now shown with dimples in the obverse and reverse sides at the locations where my optimized shaped indenters have been utilized to impart beneficial residual stresses in the structure. 
     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. 
     FIG. 38 is modularly exploded cross-sectional view of three workpiece sheets being acted on by a pair of flat cylindrical punch type indenters, showing in detail the stress field created in the workpieces by such an indenter. 
     FIG. 39 is a modularly exploded cross sectional view of three workpiece sheets after being cold worked by a pair of flat cylindrical punch type indenters, as shown in FIG.  38 , now showing in detail the stress field present in the workpiece sheets after drilling to create an aperture through the workpieces. 
     FIG. 40 is modularly exploded cross-sectional view of three workpiece sheets being acted on at the obverse and reverse sides by a pair of optimized shaped indenters, showing in detail the stress field created in the workpiece sheets by such indenters. 
     FIG. 41 is a modularly exploded cross-sectional view of three workpiece sheets after being cold worked by a pair of optimized shape indenters, as shown in FIG. 40, now showing in detail the stress field present in the workpiece sheets after drilling to create an aperture in the workpiece. 
     FIG. 42 is a modularly exploded cross-sectional view of three workpiece sheets after the step of stress wave input to the workpiece sheets by utilizing indenters and a consumable lamina or wafer body between the workpiece sheets and each of the indenters, as the desirable deformation is imparted into both the obverse and reverse sides of the three workpiece package. 
     FIG. 43 is a modularly exploded cross-sectional view of three workpiece sheets 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 sheets 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 each of the workpiece sheets. 
     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. 
     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. 
     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 results at both the surface and at an aperture mid-plane in a structure fabricated in Type 2024-T3 Aluminum utilizing my new process. 
     FIG. 47 is semi-log graphical plot of the maximum net stress versus cycles to failure for (a) sample results for parts treated by cold expansion; (b) untreated control samples of the material type treated and tested herein, and (c) results for sample structures treated by the improved stress wave method taught herein. 
     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. 
     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. 
     FIG. 50 is a cross-sectional view which shows one type of 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. 
     FIG. 51 includes a series of FIGS. 51A,  51 B,  51 C,  51 D, and  51 E, which shows a variety of ways in which an indenter can be utilized to place a dimple in one or both sides of a workpiece, as taught herein. 
     FIG. 52 shows the use of opposing indenters of equal size which have, on the face thereof, alignment punches. 
     FIG. 53 shows the use of opposing indenters of unequal size which have, on the face thereof, alignment punches. 
     FIG. 54 shows the use of an indenter having an optimized shape contacting edge, but with a small alignment punch located at the center. 
     FIG. 55 shows use of my process for providing a fatigue enhanced hole in a composite structure. 
     FIG. 56 illustrates the step of match drilling a hole through a metal/composite high strength workpiece. 
     FIG. 57 illustrates a finished hole which has been prepared in the workpiece first shown in FIG.  55 . 
     FIG. 58 illustrates the use of my method to treat a tubular structure for improving fatigue life of holes placed in the structure. 
     FIG. 59 shows the tubular structure first shown in FIG. 58 being treated with use of an optional support plug as a backing anvil. 
     FIG. 60 shows the tubular structure first shown in FIGS. 58 and 59, as finished. 
     FIG. 61 shows the first step in a process for fastening wing structures together, showing two workpieces being held together with a tack fastener. 
     FIG. 62 shows the next step in a process for fastening wing structures together, showing the structure after the fastener head has been removed. 
     FIG. 63 illustrates the step of indenting the workpieces by using compression of the fastener shank. 
     FIG. 64 illustrates the next step of intending the wing part workpieces with upper and lower indenters. 
     FIG. 65 illustrates the step of drilling the wing part workpieces and the deformed fastener to form an aperture of desired diameter. 
     FIG. 66 illustrates the finished wing part workpieces as prepared for installation of the final fastener 
     FIG. 67 shows the use of my process to treat a lug structure. 
     FIG. 68 shows a lug type structure where a preselected location has been identified for an aperture. 
     FIG. 69 shows a lug type structure wherein a slot has been added. 
     FIG. 70 shows a completed lug type structure having sidewalls at the aperture with residual compressive stress for enhancement of fatigue life. 
    
    
     DESCRIPTION 
     In my 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 as a result of fatigue damage. The stress wave method imparts beneficial stresses using a dynamic indenter that impinges the surface of a workpiece, preferably normal to the surface of the workpiece. In this new process, the action of the dynamic indenter on the workpiece, at a pre-selected velocity and with sufficient force, causes waves of elastic and plastic stress to develop and to propagate through a workpiece, resulting in instantaneous plastic deformation of the structure. 
     Unlike the techniques employed in 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,  z /P 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 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  63  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 smaller 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 less than optimal fatigue life of a structure fabricated using such method. 
     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 axis at r/a=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 . Resultantly, 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. 
     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. 
     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 various tools, and then to achieve improved results by way of optimization of the deflection achieved, by use of 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 . For ease of understanding, in one basic apparatus and method, the preferred generalized pressure profile provided by use of my optimized indenter tool results in a uniform pressure across the contact surface areas, so as to achieve a varying amount of deflection in a workpiece which, 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. To understand this phenomenon, see FIG. 5, wherein the broken line  84  depicts the variable deflection achieved by my optimized indenter when acting on a workpiece. 
     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, P M , 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  z  at any point along the profile of contact face  92 , or more generally shown below, in the contacting end  94  of the indenter  90 . 
     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:          u   z     =           (     4                   (     1   -     ν   2       )       )                     P   M                   a     E                       ∫   0     π        /        2                1   -         r   2       a   2                       sin   2                   θ                            θ                           
     Where: 
     a=a pre-selected radius of uniform pressure 
     E=Elastic modulus 
     P M =mean contract pressure distribution (above yield strength, but below that which causes structural deformation) 
     r=radial distance (instantaneous) 
     u z =normal displacement of the workpiece 
     ν=Poisson&#39;s Ratio 
     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:          u   z     =         (     2                   (     1   -     ν   2       )       )                     P   M                   a     E                     
     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:          u   z     =         (     4                   (     1   -     ν   2       )       )                     P   M                   a       3.14159                   (   E   )                         
     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). 
     In another embodiment, shown in FIG. 14, an optimized indenter  90 ′ can be provided in a design which is slightly easier to manufacture, yet still substantially achieve many of the benefits of my invention. 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 (), 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 (), 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 () is generally about the same as alpha, or from about 30 up to about 60 degrees. 
     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. 
     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  130  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 D  is desired. It should be understood that in one variation in my method, the indenter may be positioned in touching engagement with the workpiece  132 , and the necessary force to create the desired result in the workpiece may be applied to the indenter while it remains in such touching contact. 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 
     
       
         c=(E/) ½   
       
     
     where 
     c=the speed of sound in the material, and 
     =the material density 
     The velocity of the plastic wave front is a little more complex, and is generally represented as follows: 
     
       
         C=[S()/] ½   
       
     
     where 
     S()=the modulus of deformation (elastic or plastic), which is equal to the change in stress divided by the change in strain speed, or (d/d). 
     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 both normally along the direction z of the impact of the indenter  130  on workpiece  132  and radially along the first or obverse side of the workpiece  132 . 
     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”. 
     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 stress waves 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 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 stress waves  150  and  152  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. Also, as a slight variation in my method, but using the apparatus illustrated in FIGS. 12,  13 , and  16 , for example, one of the indenters can be provided in a stationary position adjacent the workpiece, and the necessary force to create the stress wave indentations can be provided by the action of a single indenter. 
     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  206  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 . 
     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 actually three separate parts (sheets  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 components, representing sheets  132 (A),  132 (B) and  132 (C). 
     FIGS. 18 and 20 represent a workpieces  132 ′ and  132 , respectively, wherein a dimple  170  has been created in both sides 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 FIG. 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 springback or rebound of the workpiece, once the deforming pressure of the indenter(s) has been released. 
     As indicated in FIG. 21, a back-taper type drill having a backtaper angle lambda() 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 D  is larger than the dimple diameter D( 170 ) by a distance 2 Q, where Q represents 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. 
     FIGS. 22 through 26 represent variations in my method that 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 dimples therein. So, as shown in FIG. 22, one type of 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 stress waves 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 stress waves and form dimples in both sides of the workpiece  332  at a pre-selected location where it is desired to create an aperture such as a “rivet-hole” or preferably a flush type rivet hole of shank diameter A D , as seen in FIG.  24 . 
     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 stress waves 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  373  of the indenter  372 . 
     FIGS. 25 and 26 show the use of a pilot hole defined by wall  380  in workpiece  378 . In FIG. 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 ″. 
     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. 
     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  430 ( 2 ), which are of identical shape and size, are provided. This variation in my method utilizes a consumable lamina or wafer W( 1 ) between the indenters  430 ( 1 ) and the obverse surface  431  of the workpiece  432 . Likewise, a consumable lamina or wafer W( 2 ) is utilized between the indenter  430 ( 2 ) and the reverse surface  434  of the workpiece  432 . Each of the consumable lamina has a workpiece side WW and an indenter side WI. The workpiece side WW impacts the workpiece, and when the lamina is squeezed between the workpiece  432  and an indenter, the lamina or wafer 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 lamina or 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 lamina or 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. 
     The method of use of lamina or wafers is further enhanced via application of a lubricant that performs under high stress, preferably a dry film type lubricant, on the indenter side WI of a wafer while leaving the workpiece side WW of the wafer free of lubricant. This method allows the efforts of the combination of the indenter and wafer to impart even greater residual stresses to the workpiece. Importantly, the reduction in friction on the indenter side WI of the wafer allows it to plastically flow more readily away from the indenter during execution of the process. Also, the clean, non-lubricated condition of the workpiece side WW of the wafer increases friction and allows the wafer to “lock” onto the workpiece surface. In this way, the increased radial displacement from the lubricated side is transferred to the non-lubricated side of the wafer, which is, in turn, transferred to the surface of the workpiece This results in greater radial flow of the workpiece surface, which results in increased residual stress at, and improved fatigue life of, an aperture created at such location in the workpiece. 
     Importantly, it should be noted that the anticipated actual aperture 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  802  of a fastener  800 , with a small indentation IF 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. 48, 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, accommodate the fastener shank  852 . The beneficial residual stress is advantageously provided in both structural member  844  and  846 . 
     Although it is generally expected that most structures would substantially benefit from increased fatigue resistance being imparted from both the obverse and the reverse sides of the structure. However, in some applications, there may arise useful results when only a single side is treated. 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. Note that use of a dry film lubricant DF is indicated between the indenter surface  462  and the indenter side WI of workpiece W( 1 ). The reverse side  434  could similarly be worked, by utilizing another indenter  430 ( 2 ) and an additional consumable wafer W( 2 ) between the indenter  430 ( 2 ) 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 order to eventually produce a finished fabricated structure as depicted in FIG. 48 or  49 . 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 concentric apertures can be created through workpieces  500  and  510  by drilling therethrough with drill  516  of preselected diameter DX. Similarly, in FIG. 30, workpieces  500  and  520  (in which no beneficial stress relief pattern has been created) 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. 
     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 sidewall  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. 
     In order to provide the beneficial residual stress levels in relatively large or oversized circular apertures, or in unusual shaped structures, I have found it useful to provide opposing, oversize indenters, preferably hollow (or at least with partial relief between the contacting face and the workpiece, even if not hollow, i.e., so that the footprint of the indenter is not the same as the area bounded by the sidewalls of the indenter) as depicted in FIGS. 33,  34 , and  35 . Such indenters  630 ( 1 ) and  630 ( 2 ) can be shaped, at least along the distal periphery P of the contacting end  662  and  664  of the indenters  630 ( 1 ) and  630 ( 2 ), respectively, in an optimized manner which is especially advantageous for treatment of large or non-round holes in a workpiece  632 ; however, it must be understood that the use of such indenter shapes is not limited to such over-size or non-round structures. 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 type 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. 
     The treated workpiece  632 , before machining for removal of unwanted material, is shown in FIG.  36 . The oversize or non-round cutout treatment is finally represented in FIG. 37, 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. 
     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 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. However, such prior art techniques create significant beneficial 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). 
     In contrast, with my optimized indenter, and using the methods 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 more uniform beneficial residual stress profile. 
     In one particularly advantageous embodiment, depicted in FIGS. 42 and 43, an exploded cross-sectional view is shown of three workpiece sheets after the step of stress wave input to the three workpiece sheets by utilizing opposing indenters and a consumable wafer body between the indenters and the workpiece which the indenter strikes. Desirable deformation is imparted into both the obverse and reverse sides of each of the workpieces. 
     Importantly, FIG. 43 depicts a modularly exploded cross-sectional view of three workpiece sheets after the step of drilling an aperture at a pre-selected location through the stack of three workpieces, which step has been performed after the step shown in FIG. 42 of utilizing opposing indenters and a consumable wafer body between the indenters and the workpiece on which the indenters act. In this figure, it is now clearly shown how the desirable and beneficial residual stress pattern is 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 beneficial residual stress along the aperture sidewall. 
     This important 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 my optimized cylindrical indenter, the beneficial residual stress in an aperture wall, when examined along the through-wall direction, becomes substantially matched between the surface and the mid-plane areas. 
     In fact, my improved method provides beneficial residual stress at least as good as the split sleeve 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. 
     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) sample results reported for parts treated by cold expansion; (b) untreated control samples of materials of the type treated and tested herein, and (c) results for sample structures treated by the improved stress wave method taught herein. My samples showed fatigue life in the range of about 400,000 to about 4,000,000 cycles, more or less, depending upon applied loading, and slightly exceeded the predicted life for one of the best methods of fatigue life enhancement which is currently commercially employed, specifically, split-sleeve cold expansion. 
     It should also be noted that the present invention can be used with any convenient apparatus which utilizes an indenter with a preselected force to effect the necessary deformation of the workpiece to provide a desired residual stress pattern. However, it is preferably practiced with the use of an optimized, shaped indenter acting on a workpiece with sufficient force to effect a stress wave in the workpiece. In the latter 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 methods taught herein. 
     A simplistic review of certain variations in the method of practicing my invention are shown in FIGS. 51A,  51 B,  51 C,  51 D, and  51 E. In FIG. 51A, the process is practiced with a single indenter  800  acting on only an obverse side  802  of workpiece  804 . No support is required for the backside of the workpiece as the strength or inertia of the workpiece  804  structure provides sufficient reaction force for the process. Also, although an anticipated aperture through workpiece  804  is shown via dotted line  806  so as to provide a through passageway defined by an edge wall at line  806 , it should be understood that the process is advantageous for providing improved fatigue life where only a dead end hole is provided (e.g., starting at the obverse side  802  and ending at a bottom indicated by line  808 ), rather than a through passageway. Turning now to FIG. 51B, the method is shown being practiced with a single indenter  800  acting on workpiece  804 , with use of a backing surface  810  on a relatively large backing structure  812 . In this method of operation, a dimple is formed on the processed, obverse side  802  of the workpiece  804 . This method may be advantageously employed when the inherent strength or inertia of the workpiece  804  is not sufficient to utilize the method depicted in FIG.  51 A. In FIG. 51C, another variation is depicted, where a single indenter  800  acts on the obverse side  802  of workpiece  804 , and where an anvil  820  or other suitable backing structure, preferably of complementary size and shape to the indenter  800  and having a forming surface  822  complementary to the surface  824  shape of indenter  800 , is provided to act on the reverse  826  side of workpiece  804 . This variation of my method leaves dimples of substantially equal depth on the obverse  802  and the reverse  826  sides of the workpiece  804 . Yet another variation is depicted in FIG. 51D, where the indenters  800  and  800 ′ are used to act on the obverse  802  and reverse  826  sides of workpiece  804 . This “two-sided” method provides the greatest penetration depth of stress waves, and thus of the resultant residual stresses. Finally, FIG. 51E depicts another two-sided process of a workpiece using unequal diameter and/or shaped indenters  800 ″ and  800 ′″. This variation on my method may be utilized for the processing of a structure that contains flush type fasteners. Also, via comparison of FIGS. 51E and 51C, it should be understood that unequal diameter and/or shaped indenters can be advantageously utilized. Also, the use of a wafer intermediate the indenter  800  and the workpiece  804  may be employed in any of the methods depicted in FIGS. 51A through 51E, in the manner just depicted above in relation to FIG.  50 . Moreover, it should be appreciated that the indenters  800 , etc., depicted in these FIGS. 51A through 51E can employ both prior art tooling, such as flat bottom punches, as well as my optimized tooling which provides a unique indenter end shape, and hollow indenters, and unequal diameter and/or shaped indenters, all as discussed and taught herein, while practicing my simple two-step (indent, drill) manufacturing process for structures having improved fatigue life fastener apertures therein. 
     In addition to the variations in my method as just briefly reviewed, it should be appreciated that with respect to certain workpieces, and various automated drilling equipment, it would be advantageous to add a drill locating feature to the method. Further variations on this theme are suggested in FIGS. 52,  53 , and  54 . In FIG. 52, equal size indenters  900  and  902  are shown acting on workpiece  904 . Indenters  900  and  902  have centering guides  906  and  908 , respectively, which are sized and shaped (preferably, generally cone shaped) for interfitting engagement with depressions  910  and  912  (each V-shaped in this cross-sectional view), respectively, in obverse  916  and reverse  918  sides of workpiece  904 . Similarly, in FIG. 53, unequal size indenters  920  and  922  are shown acting on workpiece  904 . Indenters  920  and  922  have centering guides  926  and  928 , respectively, which are sized and shaped (preferably, cone shaped) for interfitting engagement with depressions  930  and  932  (each V-shaped in this cross-sectional view), respectively, in obverse  916  and reverse  918  sides of workpiece  904 . As depicted in FIG. 54, I prefer the use of an indenter  950  having a lower optimized shaped surface  952 , but having at the center thereof a small downwardly projecting centering punch portion  954 . The downwardly projecting centering punch portion  954  is preferably cone shaped, and, upon impact of the face (here, the obverse side  958  of workpiece  960 ), leaves a small indentation as projected by broken line  962 , so as to provide a centering mechanism for the subsequent hole forming operation. 
     My method of treating workpieces to improve the fatigue life has many unique and innovative applications for the manufacture of parts with improved fatigue life. For example, in FIGS. 55,  56 , and  57 , a method for treating a composite structure is illustrated. As shown, this type of structure is often encountered in high performance aircraft construction, where titanium  1800  or other light metal is layered with a fibrous, high strength composite.  1802  to provide a workpiece stack  1804  with high strength. To prepare a finished aperture defined by sidewall  1806  (see FIG.  57 ), an initial aperture in composite  1802  is provided, defined by sidewall  1808 . The diameter D 55  of the hole in the composite defined by sidewall  1808  is slightly undersized from the diameter D 57  of the finished hole, as shown in FIG.  57 . Importantly, diameter D 55  is slightly larger than indenter I 55 A and I 55 B, to allow passage of the indenter I 55 A through the composite material  1802 . The titanium  1800  is dimpled ( 1810 ,  1812 ) by indenters I 55 A and I 55 B, respectively. Then, the workpiece  1804  is match drilled by drill  1816  to provide final hole diameter D 57  as defined by sidewall  1806 . Note that sidewall  1806 T in the titanium (or other metal) sheet now matches the diameter of sidewall  1806 C in the composite portion, as illustrated in FIG.  57 . Also note, as illustrated in FIG. 57, that the aperture can be provided with a countersunk shoulder  1818  to accommodate an aerodynamic flush fastener (not shown). Further, as noted in the various figures and accompanying discussion above, the titanium or other metal  1800  may be indented on either the obverse  1820  or the reverse  1822  side, or both. 
     Next, as illustrated in FIGS. 58,  59 , and  60 , my unique method may be applied to tubular objects; such objects are found in important applications such as medical implants, pressurized piping, drilling equipment, process equipment, or in aircraft applications such as struts, actuators and the like. Indenters I 58 A and/or I 58 B are provided to act on a tubular workpiece  1830  having inside diameter I.D. and outside diameter O.D. as illustrated in FIG.  58 . Indenters I 58 A and/or I 58 B have a unique concavity in the working surface  1831  which is designed to apply a substantially uniform pressure profile against the outer surface  1832  of workpiece  1830 , as generally described herein above. Typically, the diameter D of indenters I 58 A and/or I 58 B is slightly less than the diameter D 60  of the finished hole as noted in FIG.  60 . Indenters I 59 A and/or I 59 B similarly act on workpiece  1830  when an optional support plug  1840  is utilized as a backing anvil. Note that one or more holes defined by sidewall  1842  (and with diameter D 60 ) can be provided in a workpiece  1830  by use of this method, after the step of machining the desired one or more holes. 
     One important application of my method is in the preparation of aircraft wing components. Typically, wing skin and underlying structures are fastened with temporary fasteners called tack fasteners. Such fasteners are often removed, and then cold worked, with considerable labor and processing time involved. A much-improved method for accomplishing an equivalent or better result, from a fatigue life standpoint (and much improved from a manufacturing cost standpoint), is illustrated in FIGS. 61 through 66. In FIG. 61, a workpiece  1850  and a workpiece  1852  are joined with a temporary fastener  1854 . As shown, a typically (but not necessarily) undersized fastener  1854  has a button type head  1856  (any convenient type head could be utilized) and a swaged collar  1858 . This configuration as illustrated in FIG. 61 is to hold together in proper alignment the workpieces  1852  and  1850 , while the necessary coldwork to improve fatigue life is performed. In FIG. 62, the first step of shaving head  1856  off of the fastener  1854  is shown, leaving flush stub  1860 . The next step, shown in FIG. 63, illustrates using the shank  1862  of fastener  1854  as a tool for radial expansion of the hole defined by sidewall  1864  in workpiece  1850 , and by sidewall  1866  in workpiece  1852 . Sleeve guide  1870 , having swage containing sidewalls  1872  and  1874 , are utilized to preclude radial movement of the swaged collar  1858  during compression by indenters I 63 A and I 63 B. During compression, the diameter of shank  1862  increases, and thus force is transmitted against sidewalls  1864  and  1866 . The radial movement of the shank  1862  is sufficient to provide radial plastic flow of sidewalls  1864  and  1866  of workpieces  1850  and  1852 . This action creates beneficial residual stresses in workpieces  1850  and  1852  adjacent sidewalls  1864  and  1866 , thereby increasing the fatigue life. In FIG. 64, the step of indenting shank  1862  and workpieces  1850  and  1852  is illustrated. FIG. 65 shows the step of using a drill  1870  to drill out the fastener shank  1854  and swage collar  1858  to provide a finished aperture diameter D 66  as shown in FIG.  66 . The aperture in workpieces  1850  and  1852  is defined by sidewalls  1872  and  1874 , respectively. A countersunk sidewall  1876  can be provided, and normally is, for flush fasteners for aerodynamic surfaces. Then, fastener of desired size can be installed for completing the finished joint. 
     Finally, in FIGS. 67 through 70, the use of my process to treat a lug type structure is illustrated. In FIG. 67, a side view is shown of indenters I 67 A and I 67 B as they are used to impact the obverse  1902  and reverse  1904  sides of workpiece  1900  at a preselected location. The workpiece  1900  is compressed at a preselected location for a hole diameter D 68  to be formed later. Sufficient displacement of the indenters provides radial plastic flow throughout the entire thickness of workpiece  1900 , to assure the presence of residual compressive stress at walls  1910  (and  1912 , where a slot  1914  is milled from workpiece  1900 ). Plan views of the workpiece  1900  are provided in FIGS. 68 and 70. 
     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. 
     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 include 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. 
     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 methods for coldworking of metals 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. 
     All of the features disclosed in this specification (including any accompanying claims, and the figures of the drawing) may be combined in any combination, except combinations where at least some of the features are mutually exclusive. Alternative features serving the same or similar purpose may replace each feature disclosed in this specification (including any accompanying claims, the various figures of the drawing), unless expressly stated otherwise. Thus, each feature disclosed is only one example 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. 
     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.