Patent Publication Number: US-6664506-B2

Title: Method using laser shock processing to provide improved residual stress profile characteristics

Description:
CONTINUATION DATA 
     The present application hereby claims the benefit under Title 35, United States Code §119(e) of United States provisional application no. 60/309,447 filed August 1, 2001. 
    
    
     CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is related to co-pending U.S. patent applications entitled “SYSTEM FOR LASER SHOCK PROCESSING OBJECTS TO PRODUCE ENHANCED STRESS DISTRIBUTION PROFILES”and “ARTICLES HAVING IMPROVED RESIDUAL STRESS PROFILE CHARACTERISTICS PRODUCED BY LASER SHOCK PEENING”, filed concurrently herewith by the same inventors and assigned to the same assignee as the present application, the contents thereof being incorporated herein by reference hereto. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to laser shock peening techniques, and, more particularly, to processing methods employing various laser shock peening procedures to enhance the deep compressive residual stress characteristics induced by laser shock peening and to selectively modify in a controlled manner the compressive residual stress distribution profile developed in a processed workpiece, such as an airfoil. 
     2. Description of the Related Art 
     Laser shock processing has found use in applications involving the enhancement of certain structural features such as the leading and trailing edges of turbine engine compressor or other airfoils. Various strategies have focused upon finding adequate laser beam spot patterns to process the airfoil. However, little attention has been given to determining useful techniques that can provide desired shockwave groups and accompanying stress distribution profiles. 
     In a typical application, when a shockwave from a single laser irradiated spot on the surface of a material propagates into the material from the surface, the peak pressure is highest at the surface and then decreases (i.e., attenuates) with increasing depth into the material. If the peak pressure is high enough, namely, above the dynamic yield strength of the workpiece, the shockwave plastically deforms the material below the surface in an amount generally proportionate to the amount that the peak pressure is above the dynamic yield strength. 
     The plastic yielding gives rise to plastic strain in the material, which creates the compressive residual stresses desired by the process. As the peak pressure of the shockwave decreases with increasing depth below the surface, the amount of plastic strain also decreases. This factor limits the depth of the compressive residual stress that can be introduced into the workpiece. 
     SUMMARY OF THE INVENTION 
     Various processing methods are provided that employ laser shock peening procedures to enhance the deep compressive residual stress characteristics induced within a workpiece by laser shock peening, such as with the introduction of an asymmetrical or other selectively configured compressive residual stress distribution profile within the workpiece. One operation may involve processing an airfoil to develop an asymmetrical stress distribution profile through the thickness dimension of a thin section of the airfoil. The asymmetrical stress distribution profile will be selectively tailored to produce compressive residual stress properties within the airfoil that have desired behaviors and objectives, such as retarding crack propagation, inhibiting the growth of incipient flaws, strengthening the material at high fatigue locations, increasing the high cycle fatigue strength at specific location, providing a desired shape or curvature, and other such uses as typically understood in the art. 
     According to one processing method, the workpiece is simultaneously irradiated with a set of laser beams to form a corresponding set of adjacent non-overlapping laser shock peened surfaces. The spaced-apart relationship between the laser beam spots is chosen such that the respective shockwaves induced by laser shock peening will encounter one another as they propagate through the workpiece. The shockwaves will intersect at a location disposed generally between the laser shock peened surfaces. 
     In one form, the encountering shockwaves will interact in a manner generally exhibiting a constructive interference effect. In this manner, the respective deep compressive residual stress regions that extend from each of the adjacent non-overlapping laser shock peened surfaces will overlap and significantly increase the peak pressure experienced by the material in the vicinity of the shockwave intersection plane. Various laser spot beam patterns may be developed to produce selective arrangements of shockwave interaction locations. 
     According to another method, the workpiece is irradiated at opposing sides thereof at different times to form opposing laser shock peened surfaces. In this manner, the opposing time-staggered shockwaves induced by laser shock peening will meet at a location apart from a mid-plane of the workpiece, producing an asymmetrical compressive residual stress profile through a thickness dimension of the workpiece. The relative difference between the arrival times of the laser beams used to laser shock peen the opposing sides of the workpiece is chosen to facilitate control of the profile characteristics by selectively determining the interior location where the opposing shockwaves will encounter one another. 
     According to another processing method, the workpiece is irradiated simultaneously at opposing sides thereof using laser beams having different pulse lengths to form opposing laser shock peened surfaces. The use of such differential laser beam pulse lengths results in the development of opposing shockwaves induced by laser shock peening that attenuate at different rates as they propagate through the workpiece. This disparate attenuation in the shockwaves will produce compressive residual stress regions extending from the respective laser shock peened surfaces having a stress distribution profile that exhibits an asymmetry along a thickness dimension of the workpiece. 
     According to another processing method, the workpiece is irradiated simultaneously at opposing sides thereof to form a set of laterally offset laser shock peened surfaces. This lateral offset has the effect of creating an imbalance in the forces that are developed within the workpiece as the shockwaves induced by laser shock peening propagate through the workpiece. This force imbalance exerts a moment force on the material, tending to rotate it around an axis perpendicular to the displacement vector connecting the laterally offset laser shock peened surfaces, and lying in the nominal mid-thickness phase between the opposing laser-peened surfaces. 
     Additionally, the oppositely-directed shockwaves will interact in a generally asymmetrical manner relative to a mid-plane of the workpiece, producing a shockwave interaction zone generally centered about the mid-plane but exhibiting wing-type portions that extend toward opposite ones of the workpiece surfaces in an oblique manner relative to the mid-plane. A corresponding asymmetrical stress distribution profile will accompany this particular form of shockwave interaction associated with the simultaneous formation of laterally offset laser shock peened surfaces disposed at opposing sides of the workpiece. 
     The invention, in one form thereof, is directed to a method that involves laser shock peening an object to form at least one set of at least two simultaneously formed, non-overlapping adjacent laser shock peened surfaces. 
     In one form, the laser shock peening step further includes the step of forming a selective laser beam spot pattern on the object which is sufficient to enable the formation of at least two overlapping regions each having compressive residual stresses imparted by laser shock peening, wherein each region extends into the object from a respective laser shock peened surface. 
     In another form, the laser shock peening step further includes the step of forming a selective laser beam spot pattern on the object which is sufficient to enable at least two respective shockwaves induced by laser shock peening in connection with the simultaneous formation of at least two respective non-overlapping adjacent laser shock peened surfaces to encounter one another within the object. 
     In another form, the laser shock peening step further includes the step of forming a selective laser beam spot pattern on the object, which is configured to effectuate the formation of at least one row of spaced-apart shockwave intersection sites in the object, wherein each shockwave intersection site is defined by an encounter between shockwaves induced by laser shock peening traveling from neighboring spaced-apart laser beam spots. 
     In another form, the laser shock peening step includes comprises the step of forming a selective laser beam spot pattern on the object including at least one row of laser beam spots arranged in spaced-apart overlapping pairs. The spatial relationship between adjacent pairs is sufficient to enable the formation of a shockwave intersection site disposed at least in part therebetween, wherein each shockwave intersection site is defined by an encounter between shockwaves induced by laser shock peening traveling from nearest neighbor laser beam spots of adjacent laser beam spot pairs. 
     In another form, the laser shock peening step further includes the step of forming a selective laser beam spot pattern on the object including at least one row of non-overlapping laser beam spots configured to define a selective pattern of shockwave intersection sites. Each shockwave intersection site is defined by an encounter between shockwaves induced by laser shock peening traveling from neighboring laser beam spots. 
     In another form, the laser shock peening step further includes the step of forming a selective laser beam spot pattern on the object including at least one row of overlapping laser beam spots, wherein the spot pattern is configured to effectuate the formation of at least one row of spaced-apart shockwave intersection sites in the object. Each row of shockwave intersection sites is generally disposed between respective adjacent ones of the laser beam spot rows, while each shockwave intersection site is defined by an encounter between shockwaves induced by laser shock peening traveling from laser beam spots of adjacent rows. 
     In another form, the laser shock peening step further includes the step of sequentially forming at least one selective laser beam spot pattern on the object, wherein each pattern is configured to effectuate the formation of at least one row of spaced-apart shockwave intersection sites in the object. Each shockwave intersection site is defined by an encounter between shockwaves induced by laser shock peening traveling from neighboring laser beam spots. Preferably, each row of spaced-apart shockwave intersection sites associated with a respective laser beam spot pattern has a respective orientation characteristic defining a directional orientation of the shockwave intersection sites associated therewith. 
     The object preferably includes a gas turbine engine component such as an airfoil. 
     The invention, in another form thereof, is directed to a method that involves laser shock peening an object to form at least one set of at least two non-overlapping adjacent laser shock peened surfaces simultaneously formed with one another. Each laser shock peened surface is associated with a respective shockwave induced by laser shock peening. Moreover, the respective shockwaves which are associated with at least one selective set of at least two simultaneously formed, non-overlapping adjacent laser shock peened surfaces encounter one another within the object. 
     The invention, in another form thereof, is directed to a method that involves simultaneously laser shock peening an object at a plurality of locations to form at least one pair of adjacent, spaced-apart laser shock peened surfaces on the object and to induce the generation of a respective shockwave in association with the formation of each laser shock peened surface. The respective spaced-apart relationship between the respective laser shock peened surfaces of at least one respective laser shock peened surface pair is sufficient to enable the respective shockwaves associated therewith to encounter one another within the object. 
     The invention, in another form thereof, is directed to a method that involves laser shock peening an object to form at least one set of at least two simultaneously formed, spaced-apart adjacent laser shock peened surfaces. Each laser shock peened surface is associated with a region of compressive residual stresses extending into the object therefrom and imparted by laser shock peening. The laser shock peening operation is configured to enable the formation of at least one region overlap location, wherein each region overlap location defines a respective overlap of at least two respective compressive residual stress regions respectively associated with at least two corresponding simultaneously formed, spaced-apart adjacent laser shock peened surfaces. 
     The configuration step further includes the step of selecting a predetermined spaced-apart relationship for use in forming neighboring ones of the laser shock peened surfaces. 
     The invention, in another form thereof, is directed to a method that involves providing a laser shock processor; and operating the laser shock processor to laser shock process the object in a manner sufficient to cause at least one set of at least two shockwaves having mutually non-interfering initial wavefronts to develop simultaneously at a selective side of the object and subsequently interact with one another within the object. 
     In one form, the laser shock processor is operated to simultaneously form two spaced-apart laser shock processed surfaces on the object. 
     The invention, in another form thereof, is directed to a method for use with an object having a first side and a second side disposed generally opposite one another. The method involves laser shock peening the object to form at least one set of an associated first laser shock peened surface and a second laser shock peened surface on the first side and the second side of the object, respectively. The associated first and second laser shock peened surfaces of each respective set of laser shock peened surfaces are formed at different times. Preferably occurring between 1 nanosecond and 2000 nanoseconds apart in time. 
     In a preferred form, the associated first and second laser shock peened surfaces of at least one respective set of laser shock peened surfaces are disposed generally opposite one another. 
     The invention, in another form thereof, is directed to a method for use with an object having a first side and a second side disposed generally opposite one another. The method involves forming at least one pair of associated laser shock peened surfaces each disposed at a different one of the first and second sides of the object. Each pair of associated laser shock peened surfaces is formed by laser shock peening the object at the first and second sides thereof at different times. 
     The invention, in another form thereof, is directed to a method for use with an object having a first side and a second side disposed generally opposite one another. The method involves operating a laser shock processor to laser shock process the object in a manner sufficient to cause at least one set of generally opposing shockwaves to develop in a time-staggered relationship to one another at different ones of the first and second sides of the object. 
     In a preferred form, the laser shock processor is operated to form a first laser shock peened surface on one of the first and second sides of the object; and form a second laser shock peened surface on the other of the first and second sides of the object, at a time later than the formation of the first laser shock peened surface. 
     The invention, in another form thereof, is directed to a method for use with an object having a first side and a second side disposed generally opposite one another. The method involves operating a laser shock processor to laser shock process the object in a manner sufficient to cause at least one set of generally opposing shockwaves to develop at different times at different ones of the first and second sides of the object and to subsequently encounter one another within the object at a location apart from a mid-plane of the object. 
     In a preferred form, the laser shock processor is operated to laser shock peen the object at the first and second sides thereof at different times to form generally opposing laser shock peened surfaces. 
     The invention, in another form thereof, is directed to a method for use with an object having a first side and a second side disposed generally opposite one another. The method involves operating a laser shock processor to laser shock process the object in a manner sufficient to cause the formation of at least one set of generally opposing regions each extending from respective laser shock peened surfaces formed at different times at different ones of the first and second sides of the object. Each region has compressive residual stresses imparted by laser shock processing. Furthermore, each set of opposing regions defines an asymmetrical compressive residual stress distribution profile appearing generally along a respective thickness dimension of the object. 
     The invention, in another form thereof, is directed to a method for use with an object having a first side and a second side disposed generally opposite one another. The method involves simultaneously laser shock peening the object at the first and second sides thereof using laser beams having different pulse lengths to respectively form first and second laser shock peened surfaces on the first and second sides of the object, respectively. 
     In a preferred form, the first and second laser shock peened surfaces are disposed generally opposite one another. 
     The invention, in another form thereof, is directed to a method for use with an object having a first side and a second side disposed generally opposite one another. The method involves laser shock peening the object to form at least one set of simultaneously formed laser shock peened surfaces each disposed at a different one of the first and second sides of the object. The respective laser shock peened surfaces of at least one respective set of simultaneously formed laser shock peened surfaces are respectively formed using laser beams having different pulse lengths. 
     The invention, in another form thereof, is directed to a method for use with an object having a first side and a second side disposed generally opposite one another. The method involves operating a laser shock processor to laser shock process the object in a manner sufficient to cause at least one set of generally opposing shockwaves to develop simultaneously at different ones of the first and second sides of the object and to subsequently experience different rates of attenuation during propagation within the object. 
     In a preferred form, the laser shock processor is operated to simultaneously laser shock peen the object at the first and second sides thereof with laser beams having different pulse lengths. 
     The invention, in another form thereof, is directed to a method for use with an object having a first side and a second side disposed generally opposite one another. The method involves laser shock peening the object to form a plurality of laser shock peened surfaces. The plurality of laser shock peened surfaces includes at least one set of laterally offset, simultaneously formed laser shock peened surfaces each disposed at a different one of the first and second sides of the object. 
     In a preferred form, the lateral offset relationship is sufficient to enable associated shockwaves induced by laser shock peening to encounter one another within the object. 
     The invention, in another form thereof, is directed to a method for use with an object having a first side and a second side disposed generally opposite one another. The method involves laser shock peening the object to form a plurality of laser shock peened surfaces. The plurality of laser shock peened surfaces includes at least one set of laterally offset, simultaneously formed laser shock peened surfaces each disposed at a different one of the first and second sides of the object. Each laser shock peened surface is associated with a respective shockwave induced by laser shock peening. Additionally, the respective shockwaves which are associated with at least one selective set of laterally offset, simultaneously formed laser shock peened surfaces encounter one another within the object. 
     The invention, in another form thereof, is directed to a method for use with an object having a first side and a second side generally opposing one another. The method involves operating a laser shock processor to laser shock process the object in a manner sufficient to cause at least one set of laterally offset shockwaves to develop simultaneously at different ones of the first and second sides of the object and subsequently interact with one another within the object. 
     In one form, the laser shock processor is operated to simultaneously form a set of laterally offset laser shock processed surfaces on different ones of the first and second sides of the object. 
     The invention, in another form thereof, is directed to a method for use with an object having a first side and a second side disposed generally opposite one another. The method involves simultaneously laser shock peening the object at the first and second sides thereof to form first and second laser shock peened surfaces at the first and second sides of the object, respectively, wherein the first and second laser shock peened surfaces have a lateral displacement therebetween. 
     The invention, in another form thereof, is directed to a method for use with an object having a first side and a second side generally opposing one another. The method involves operating a laser shock processing apparatus to laser shock process the object in a manner sufficient to produce at least one zone of compressive residual stress in the object that is characterized by an asymmetrical stress distribution profile relative to a reference plane. 
     In one form, the laser shock processing apparatus is operated to laser shock peen the object at the first and second sides thereof at different times to form opposing first and second laser shock peened surfaces disposed at the first and second sides of the object, respectively. 
     In another form, the laser shock processing apparatus is operated to simultaneously laser shock peening the object at the first and second sides thereof using laser beams having different pulse lengths to form opposing first and second laser shock peened surfaces disposed at the first and second sides of the object, respectively. 
     In another form, the laser shock processing apparatus is operated to simultaneously laser shock peening the object at the first and second sides thereof to form laterally offset first and second laser shock peened surfaces disposed at the first and second sides of the object, respectively. 
     The invention, in another form thereof, is directed to a method for use with an object having a first side and a second side generally opposing one another. The method involves causing the formation in the object of at least one asymmetrical compressive residual stress distribution profile imparted by suitable laser shock processing of the object. Each asymmetrical compressive residual stress distribution profile appears generally along a respective thickness dimension of the object. 
     In one form, the object is laser shock peened at the first and second sides thereof at different times to form opposing first and second laser shock peened surfaces disposed at the first and second sides of the object, respectively. 
     In another form, the object is simultaneously laser shock peened at the first and second sides thereof using laser beams having different pulse lengths to form opposing first and second laser shock peened surfaces disposed at the first and second sides of the object, respectively. 
     In another form, the object is simultaneously laser shock peened at the first and second sides thereof to form laterally offset first and second laser shock peened surfaces disposed at the first and second sides of the object, respectively. 
     One advantage of the invention is that the laser shock peening process can increase the penetration depth of compressive residual stress formed below a laser shock peened surface beyond that available from a single pulse. 
     Another advantage of the invention is that the laser shock peening process can be used to tailor the sub-surface residual stress profile developed through the thickness of a thin section. 
     Another advantage of the invention is that asymmetry can be introduced into the stress distribution profile by simultaneously laser shock peening both sides of the thin section in the manner described herein, thereby allowing modification of the profile. 
     Another advantage of the invention is the availability of better control of the depth of the residual stress below the laser shock peened surface, and the intensity of the interaction of the shockwaves at mid-thickness of the thin section. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is an upper perspective sectional schematic view of a workpiece portion illustrating a configuration of laser shock peened surface areas, according to a first embodiment of the present invention; 
     FIG. 2A is a partial cross-sectional schematic view of the workpiece portion shown in FIG. 1 illustrating the interaction between shockwaves induced by laser shock peening; 
     FIG. 2B is a partial cross-sectional schematic view of the workpiece shown in FIG. 1, illustrating a representative stress contour line; 
     FIG. 3 is an upper planar view of an illustrative laser beam spot pattern, according to a first form of the first embodiment of FIG. 1; 
     FIG. 4 is an upper planar view of an illustrative laser beam spot pattern, according to a second form of the first embodiment of FIG. 1; 
     FIG. 5 is an upper planar view of an illustrative laser beam spot pattern, according to a third form of the first embodiment of FIG. 1; 
     FIG. 6 is an upper planar view of an illustrative laser beam spot pattern, according to a fourth form of the first embodiment of FIG. 1; 
     FIG. 7 is an upper planar sectional view of a workpiece portion showing an illustrative configuration of laser beam spots useful in developing compressive residual stresses around a hole, applying the principles of the first embodiment of the invention depicted in FIG. 1; 
     FIG. 8A is a planar cross-sectional view of a workpiece section to illustrate a typical dual-sided laser shock peening operation; 
     FIG. 8B is a diagrammatic representation residual stress profile pertaining to the processed workpiece of FIG. 8A; 
     FIG. 9A is a planar cross-sectional view of a workpiece section to illustrate a dual-sided laser shock peening operation, according to a second embodiment of the present invention; 
     FIG. 9B is a diagrammatic representation of the compressive residual stress distribution profile pertaining to the processed workpiece of FIG. 9A; 
     FIG. 10A is a planar cross-sectional view of a workpiece section to illustrate a dual-sided laser shock peening operation, according to a third embodiment of the present invention; 
     FIG. 10B is a diagrammatic representation of the compressive residual stress distribution profile pertaining to the processed workpiece of FIG. 10A; 
     FIGS. 11A-C illustrate various cross-sectional views of a workpiece section that is laser shock peened using different amounts of lateral offset between the dual-sided laser shock peened surfaces, according to a fourth embodiment of the present invention; 
     FIG. 12 is a schematic illustration of a dual-sided laser beam peening arrangement to depict the bending moments produced by the laser shock processing conducted in connection with FIGS. 11A-C; 
     FIG. 13 is a cross-sectional schematic view of a workpiece section to illustrate the mid-thickness plane and the positioning of substantially opposite laser beam spots for a workpiece having non-parallel opposing sides; 
     FIG. 14 is a schematic diagram of a laser shock peening apparatus for use in practicing the present invention; 
     FIG. 15 is a schematic perspective view of an engine blade capable of being processed and produced by the present invention; 
     FIG. 16 is a cross-sectional schematic view of the airfoil portion of the engine blade shown in FIG. 15, taken along lines  15 — 15 ; 
     FIG. 17 is a block diagram representation of a laser shock processing system configured to practice the present invention; 
     FIGS. 18A-F show examples of shockwaves passive through a workpiece with no tensile wave interaction; 
     FIGS. 19A-E show examples of shockwaves interacting; and 
     FIG. 20 is a new processing pattern for thin section workpieces. 
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views. The exemplification set out herein illustrates one preferred embodiment of the invention, in one form, and such exemplification is not to be construed as limiting the scope of the invention in any manner. 
     DETAILED DESCRIPTION OF THE INVENTION 
     According to the present invention, various laser shock processing methods are provided to establish selective compressive residual stress distribution profiles within a workpiece. For example, an asymmetrical stress distribution profile may be formed through the thickness of a thin section of a gas turbine engine airfoil. 
     According to one embodiment of the present invention discussed in relation to FIGS. 1-7, a laser shock processing method involves simultaneously irradiating a workpiece with a set of laser beams to form a corresponding set of adjacent non-overlapping laser shock peened surfaces, enabling the shockwaves to encounter one another. Simultaneous as used in the present application means either pulses created from the same initial laser oscillator or shockwaves formed in the workpiece within 5×10 −9  sec, therefore, effectively simultaneous as seen by the workpiece. 
     In another embodiment discussed in relation to FIGS. 8-9, opposite sides of the workpiece may be irradiated at different times to form opposing laser shock peened surfaces, enabling the shockwaves to meet at a location apart from the mid-plane. 
     In another embodiment discussed in relation to FIG. 10, opposite sides of the workpiece may be irradiated simultaneously using laser beams having different pulse lengths to form opposing laser shock peened surfaces. The resulting shockwaves experience different rates of attenuation within the workpiece, producing regions of compressive residual stress having different stress gradient profiles. 
     In another embodiment discussed in relation to FIGS. 11-12, opposite sides of the workpiece may be irradiated simultaneously to form a set of laterally offset laser shock peened surfaces. The resulting shockwaves have a relative displacement therebetween, i.e., the shockwaves are not directly fully opposed to one another. 
     The force imbalance produced by the offset shockwaves exerts a localized moment influence on the workpiece tending to rotate it around an axis perpendicular to the displacement vector defined by the lateral offset, allowing geometric features such as deformations, curvatures, and bends to be introduced into the workpiece interaction zone. Also, the shockwave interaction zone includes portions having an orientation that are not parallel to the workpiece surfaces, i.e., an oblique shockwave interaction area having a directionality that is angled relative to a midplane of the workpiece. 
     Before proceeding with the details of the invention, a discussion is first provided concerning laser shock processing and the workpieces that may be treated by such processing methods. 
     By way of background, laser shock processing or laser shock peening refers to a process for producing a region having deep compressive residual stresses which are introduced by the influence of traveling pressure or shockwaves induced by laser shock peening a surface area of the workpiece. In preferred forms, the laser shock peening operation will produce a plurality of regions having deep compressive residual stresses imparted by laser shock peening and extending into the workpiece from respective laser shock peened surface areas. 
     Laser beams generated by a laser beam source are directed to impact the workpiece and thereby generates an exploding high pressure plasma caused by instantaneous ablation or vaporization of a painted, coated, or un-coated surface, which produces a strong localized compressive force applied to the workpiece. The shockwaves produced in connection with such ablation effectively provide a form of cold work hardening that creates compressive residual stresses extending into the solid body. These residual compressive stresses foster an increase in fatigue properties of the part and alleviate and/or counteract the presence of other weaknesses such as crack fronts and tensile fields. 
     In one typical form, laser peening employs an opaque overlay applied to the surface of the workpiece and a transparent overlay applied to the opaque overlay. The opaque overlay may include materials such as an oil-based or acrylic-based black paint, while the transparent overlay may include materials such as a curtain of flowing water, mineral oil, or glass. 
     A high-energy laser beam pulse is fired through the curtain of flowing water and focused upon the opaque overlay on the solid body. The energy of the laser beam is absorbed by the black paint to create peak power densities having an order of magnitude of a gigawatt/cm 2 , causing a rapid ablation or vaporization of the paint layer into a plasma which produces a rapidly increasing, high-amplitude pressure on the surface of the material. 
     The normally expanding plasma is confined at the workpiece surface by the curtain of flowing water causing the rapidly rising plasma pressure to generate traveling shockwaves (i.e., pressure waves) that propagate through the surface into the interior of the workpiece. The transparent overlay effectively acts to confine or otherwise contain the shockwaves proximate the workpiece surface and to redirect the shockwaves into the body of the workpiece. The shockwaves cold-work the surface of the workpiece and create compressive residual stresses extending from the surface into the interior of the workpiece. The amplitude length and quantity of the shockwaves determine in part the depth and intensity of the resulting residual compressive stresses formed in the material. 
     The paint-based opaque layer serves both to protect the target surface from direct incidence of the laser beam and from the high temperature plasma. Ablated paint material is either washed away by the curtain of flowing water, or removed later. 
     The laser beam(s) may be fired repetitively and in iterative sequence to produce different sets of laser spot patterns on the workpiece. In one form, the workpiece may be treated by developing a matrix of overlapping laser beam spots that cover a critical zone of interest. Additionally, the same or adjacent areas may be repeatedly processed by cyclically directing an energy pulse to the desired target area. 
     Multiple laser beams may be used to produce any suitable laser beam spot pattern on the workpiece. For example, single-sided and double-sided laser peening operations are possible to form laser peened surfaces on one side and opposite sides, respectively, of a workpiece. For this purpose, multiple laser beams may be generated from multiple laser sources or with other suitable beam technology (e.g., a beam splitter). One type of laser adaptable for use with the invention is the Nd:Glass Laser manufactured by LSP Technologies, Inc. of Dublin, Ohio. 
     Further descriptions of laser shock peening technology may be found in U.S. Pat. Nos. 5,131,957, 5,741,559, and 5,911,890, collectively assigned to the same assignee as the present invention and incorporated herein by reference thereto. 
     Various parameters may be controlled by the production manager to tailor the laser shock processing operation. For example, among the operational parameters that the designer can select and adjust, these include (but are not limited to) the location of the incident beam spot, the number of spots, spacing between spots, distance of spots to certain workpiece features (e.g., leading and trailing edge of integrally bladed rotor), angle of incidence, laser firing duration and repetition, and beam intensity. 
     One advantage of laser shock processing is found in its ability to increase the fatigue properties of the part by selectively developing pre-stressed regions within certain critical areas where incipient flaws or cracks typically appear. The technique has been applied with favorable success to the processing of the pressure and suction sides of leading and trailing edges of fan and compressor airfoils and blades in turbine engines. 
     The various effects of laser peening on the fatigue properties of welded samples has been reported in “Shockwaves and High Strained Rate Phenomena in Metals” by A. H. Clauer, J. H. Holbrook and B. P. Fairand, Ed. by M. S. Meyers and L. E. Murr, Plenum Press, New York (1981), pp. 675-702 (incorporated herein by reference thereto). 
     Referring briefly to FIG. 14, there is shown an illustrative laser shock processing (LSP) environment  100  that is representative of the type of configuration capable of being used in connection with the present invention. 
     The illustrated LSP environment  100  includes a target chamber  102  in which the laser shock process takes place. The target chamber  102  includes an opening  104  to receive a laser beam  106  generated by laser  108 , a source of coherent energy. Laser  108 , by way of example, may be a commercially available high power pulse laser system capable of delivering more than approximately 40 joules in 5 to 100 nanoseconds. The laser pulse length and focus of the laser beam may be selectively adjusted. 
     A representative workpiece  110  is held in position within target chamber  102  by means of a suitable positioning mechanism  112 . Positioning mechanism  112  may be of the type that includes a robotically controlled arm or other apparatus to precisely position workpiece  110  relative to the operational elements of laser shock peening system  100 . 
     In one illustrative configuration, LSP environment  100  includes a material applicator  114  for applying an opaque overlay, such as a water-based black paint, onto workpiece  110  to create a coated portion. Material applicator  114  may be provided in any suitable form such as a solenoid-operated painting station or other construction, e.g., a jet spray or aerosol unit to provide a small coated area onto workpiece  110 . 
     The material utilized by material applicator  114  is preferably an energy absorbing material, typically a black, water-based paint such as 1000 F AQUATEMP (TM) from Zynolite Product Company of Carson, Calif. Another opaque coating that may be utilized includes ANTI-BOND, a water soluble gum solution including graphite and glycerol from Metco Company, a Division of Perkin-Elmer of Westbury, N.Y. Alternatively, other types of suitable opaque coatings may be used, such as a latex paint made by Sherwin Williams. 
     LSP environment  100  further includes a transparent overlay applicator  116  that applies a fluid or liquid transparent overlay to workpiece  110  over the portion coated by material applicator  114 . The transparent overlay material should be substantially transparent to the incident radiation, with water being the preferred overlay material. 
     As shown, material applicator  114  and transparent overlay applicator  116  are shown directly located within target chamber  102 . However, this is merely illustrative, since in a production environment, only the necessary operative portions need be accessible to the processing environment of target chamber  102 , such as the portion through which the materials actually flow, e.g., a fluid dispenser head. The supply tanks for the transparent overlay materials and other energy absorbing materials may be located outside of target chamber  102  or any other suitable location. 
     A control unit such as controller  118  is operatively associated with the combination of functional elements including material applicator  114 , transparent overlay material applicator  116 , laser  108 , and positioning mechanism  112 . In particular, controller  118  is connected to laser  108 , positioning mechanism  112 , material applicator  114 , and transparent overlay material applicator  116  via control lines  120 ,  122 ,  124 , and  126 , respectively. Controller  118  controls the operation and timing of each of the applicators  114  and  116 , laser  108 , and selective operation of positioning mechanism  112  to ensure proper sequence and timing of system  100 . In one configuration, controller  118  may be a programmed personal computer or microprocessor. 
     In a typical operation, workpiece  110  is located within targeting chamber  102  by positioning mechanism  112 . Controller  118 , in one illustrative operating sequence, activates material applicator  114  to apply a laser energy absorbing coating such as a water-based black paint onto a particular location of workpiece  110  intended for laser shock processing. Controller  118  next directs transparent overlay material applicator  116  to apply a transparent overlay to the previously coated portion of workpiece  110 . 
     At this point, laser  108  is directed by controller  118  to fire a laser beam  106  that impacts the coated portion. The time between applying the transparent water overlay and the step of directing the laser energy pulse may be on the order of 1.0×10 −3  to 3.0 seconds, for example. By directing this pulse of coherent energy to the coated portion, a shockwave is created at the workpiece surface. As the plasma expands from the impact area, it creates a compressional shockwave passing against and through workpiece  110  that imparts regions of compressive residual stresses within workpiece  110 . 
     The above-described process or portions of the process may be iteratively repeated to shock process the desired surface area of workpiece  110 . Depending upon the energy levels and the amount of laser shock peening desired on workpiece  110 , controller  118  may instruct positioning mechanism  112  to reposition or re-index workpiece  110  to a new location or orientation. This mobility of workpiece  110  and/or laser  108  (by means not shown) enables further laser shock peening operations to be performed that may process the same or different portions of the workpiece, for example, the formation of a matrix of laser beam spots overlapping the previously peened area. Each additional operating sequence typically requires its own set of coatings to be applied to the workpiece and an accompanying sequence of laser firings from laser  108 . Any suitable means may be provided to change the relative spatial relationship (e.g., orientation and distance) between the laser and workpiece. 
     The present invention may be practiced in connection with any suitable workpiece or object. A workpiece may include any solid body, article, or other suitable structure that is amenable to or otherwise capable of being treated by laser shock processing. The workpiece may represent a constituent piece forming part of an in-production assembly, a final production article, or any other desired part. Accordingly, the laser shock processing treatment may be applied at any stage of production, i.e., a pre- or post-manufacturing step or other intervening time. 
     In certain industrial applications, the present invention finds significant use in processing the airfoils of turbine engines, most notably in the region proximate the leading and trailing edges where flaws and other high-cycle failures pose serious problems affecting the performance and durability of the engine. 
     Referring briefly to FIG. 15, there is shown a perspective view of an illustrative aircraft gas turbine engine airfoil  200  with which the present invention can be practiced. FIG. 16 is a planar cross-sectional schematic view of the airfoil section of engine blade  200 , taken along lines  15 — 15  in FIG.  15 . 
     The illustrated aircraft engine blade  200  includes an airfoil  202  extending radially outward from a blade platform  204  to a blade tip  206 . The engine blade  200  includes a root section  208  for attachment to a rotor. Alternately, some blades are machined from a forged or cast integrally with a rotor, to produce blisk or integrated rotor and disk assembly. Airfoil  202  includes a leading edge LE and a trailing edge TE. 
     Referring further to FIG. 16, a chord C of airfoil  202  is the line between the leading edge LE and the trailing edge TE at each cross-section of the engine airfoil. Airfoil  202  extends in a chordwise direction between the leading edge LE and trailing edge TE. A pressure side  210  of airfoil  202  faces in the general direction of rotation, while a suction side  212  is on the other side of airfoil  202 . A mean-line ML is generally disposed midway between the two faces (i.e., pressure and suction sides) in the chordwise direction. 
     The airfoil tip  206  extends along the tip of airfoil  202  from the leading edge LE to the trailing edge TE. The airfoil section depicted by FIG. 16 is of solid body construction. 
     Arrows  218  generally depict the orientation of a potential laser peening operation against airfoil  200 . Of course, other orientations and positions of laser peening may be applied to blade  200 . For example, referring to FIG. 16, pressure side  210  and suction side  212  may be laser shock peened to produce respective laser shock peened surfaces  220  and  222  having respective regions  224  and  226  with deep compressive residual stresses imparted by laser shock peening extending into airfoil  202  from the laser shock peened surfaces. 
     Turning now to the present invention, reference is first made to FIG. 1, which illustrates a representative laser beam spot configuration forming a set of laser shock peened surface areas on workpiece  10 , according to a first embodiment of the present invention. FIG. 2A is a cross-sectional planar view of workpiece  10  taken along lines  1 — 1  of FIG.  1 . FIG. 2B is a similar cross-sectional planar view of workpiece  10 , showing an illustrative stress contour line that is representative of the stress distribution profile which follows from the type of laser shock processing depicted in FIG.  1 . 
     Workpiece  10  is irradiated with a set of laser beam spots  12  and  14  applied simultaneously to the surface  16  of workpiece  10  to form a respective set of adjacent non-overlapping laser shock peened surface areas coextensive with the dimensions of the laser beam spots. This treatment of workpiece  10  is conducted in accordance with a suitably configured laser shock processing operation. As shown, the laser beam spots  12 ,  14  are configured to have a selective spatial separation distance “d”. 
     The spaced-apart relationship between laser beam spots  12 ,  14  (and hence the respective laser shock peened surfaces) is a parameter that is chosen with a view towards enabling the traveling shockwaves induced by laser shock peening to encounter one another within the interior of workpiece  10 , as explained more fully in connection with FIG.  2 A. It should be understood that the adjacent non-overlapping relationship between the laser shock peened surfaces formed in accordance with the first embodiment encompasses any suitable proximate relationship sufficient to achieve the advantages of the first embodiment, namely, to enable the generated shockwaves to meet and interact with one another. In one preferred form, the laser beam spots  12 ,  14  will be in close proximity to one another, with a spatial separation of 5 mm or less. 
     Additionally, while it is preferred that laser beam spots  12 ,  14  be simultaneously applied to workpiece  10 , this temporal condition should be understood as encompassing a nominally simultaneous, substantially simultaneous, or other adequate form of concurrent laser beam application that provides a near-simultaneous result sufficient to achieve the desired purposes. This understanding of simultaneity also applies to other uses of this term herein. Use of the same laser oscillator on the beams or shockwaves starting or having a peak within 5×10 −9  sec or less is simultaneous for the needs of the present application. 
     Moreover, although FIG. 1 shows one set of adjacent non-overlapping laser beam spots  12 ,  14 , this configuration is provided for illustrative purposes only and should not be seen in limitation of the present invention, as any number of such beam spots can be similarly formed. For example, a set of three adjacent non-overlapping laser beam spots configured in a triangular arrangement can be simultaneously formed, according to the first embodiment. Additionally, although the individual laser beam spots preferably have a circular shape, this is provided for illustrative purposes only, as any suitable laser beam spot shape can be employed. 
     Referring now to FIG. 2A, laser beam spots  12 ,  14  from FIG. 1 are depicted as forming respective laser shock peened surface areas  20  and  22  on workpiece  10 . As known, the laser shock peening activity that forms laser shock peened surfaces  20 ,  22  induces a shockwave that radiates or emanates into the body of workpiece  10  from its respective laser shock peened surface. 
     The shockwaves induced by laser shock peening generally define a traveling pressure-type energy vector that traverses a medium in a three-dimensional manner and occupies a volumetric space. For example, in workpiece  10 , it may be considered that the shockwaves have constituent components with mutually orthogonal directions of propagation that include a direction  24  normal to the surface projecting underneath the laser shock peened surface, a radial or lateral direction  26  projecting away from the laser shock peened surface (i.e., towards the adjacent laser shock peened surface), and a transverse direction  28 , such directions may be applied even if surface is curved by using tangent links to the surface. 
     The shockwaves may initially take the form of planar wavefronts that become more spherical during propagation. For example, shockwaves generally illustrated at  30  and  32  emanating from respective laser shock peened surfaces  20  and  22  are shown propagating in the illustrated manner. For purposes of description, each shockwave  30 ,  32  is respectively depicted in the form of a series of representative wavefronts  34 ,  36  with leading edges  42 ,  44  having the indicated propagation directions specified by arrows  38 ,  40 . 
     The shaded area behind the wavefront represents the distance behind the wavefront at which the magnitude of the shock pressure decreases to one half the Hugoniot Elastic Limit (HEL) for the workpiece material. The HEL is the threshold shockwave pressure above which the material yields dynamically and develops plastic strain or cold work. When these portions of the intersecting shockwaves overlap the combined pressures jump about the HEL. The material yields, and increases further plastic strain. Since the amount of plastic strain produced in this region of the material is higher than the at produced by as isolated or single laser beam spot, the higher plastic strains will generate higher compressive residual stresses compared to a single shot. 
     Regarding shockwave  30 , particular interest is drawn to the propagation direction  46  which illustrates the manner in which shockwave  30  moves laterally from the surface towards the vicinity of the adjacent or neighboring laser shock peened surface  22  and its associated shockwave  32 . Likewise, attention is drawn to the propagation direction  48  which illustrates the manner in which shockwave  32  moves laterally from the surface towards the vicinity of the neighboring laser shock peened surface  20  and its associated shockwave  30 . 
     The proximal relationship between laser shock peened surfaces  22  and  20  is such that the simultaneously generated traveling shockwaves  30  and  32  will meet or otherwise encounter one another at a generally planar location disposed nominally midway between the laser shock peened surfaces  22 ,  20 , as specified by plane  50 . This shockwave intersection zone is generally shown at  52  and is generally circumscribed by the illustrated overlap between the shaded areas of the respective shockwaves  30 ,  32 . 
     In various forms, the interaction of the shockwaves may be understood as involving an encounter, reinforcement, collision, meeting, intersection, interface, or engagement between the shockwaves. In a preferred form, it is understood that the encountering shockwaves favorably experience a synergistic interaction that generally exhibits a constructive interference-type effect or wave superposition that essentially yields an additive combination of the pressures provided by each of the shockwaves. 
     Since the shockwaves experience such additive synergism (at least in part), it therefore becomes possible for the shockwave interaction zone  52  to feature an enhanced compressive residual stress region having compressive residual stresses that are higher than that available from a single isolated shockwave. Combining the pressures of the shockwaves significantly increases the peak pressure experienced by the material in the vicinity of the intersection plane  50 . 
     In particular, the higher peak pressures generally along and about the intersection plane  50  result in higher plastic strains along and about plane  50 . Because of the additive nature of this shockwave interaction, peak pressures above the dynamic yield strength of the workpiece (e.g., metal) are also sustained to a much deeper depth in the vicinity of the intersection plane  50  than would otherwise be the case if a single shockwave was proceeding through the material. This feature enables higher plastic strains to be driven deeper into the workpiece, and causes the magnitude of the compressive residual stresses to be higher, located further below the laser shock peened surface. 
     Moreover, the radial emanation of the shockwaves from their point of origin at the laser shock peened surface enables the formation of a shockwave interaction zone  52  that lies at least in part at a subsurface location disposed between the volumetric areas lying immediately beneath the laser shock peened surfaces  20 ,  22 . This provides designers with the opportunity to create strengthened compressive residual stress regions in locations that are beyond the subsurface areas which extend directly immediately below the laser shock peened surface. 
     Referring now to FIGS. 3-6, there are shown various laser beam spot patterns configured to create various arrangements of shockwave interaction locations that drive compressive residual stresses deeper locally, according to various alternative forms of the first embodiment of FIG.  1 . 
     Following upon the discussion above, reference is made to FIG. 2B to show in representative fashion the manner in which the stress distribution profile is modified in connection with the laser shock peening process of FIG.  1 . Line  51  is a representative uniform stress contour line corresponding to a particular level of deep compressive residual stress induced by laser shock peening, namely, the formation of laser shock peened surfaces  20  and  22 . In one form, line  51  can be considered an isopiestic feature, i.e., a feature marked by equal pressure or an isobar. It is of course the case that a complete stress distribution profile will include a plurality of individual spaced-apart stress contour lines each corresponding to a specific stress level. 
     FIG. 2B diagrammatically correlates the pressure level represented by stress contour line  51  to both thickness and lateral or radial dimensions. For example, stress contour line  51  extends fully across the lateral dimension that encompasses the area under both laser shock peened surfaces  20 ,  22  and the separation distance “d” therebetween. Notable, in the vicinity of the shockwave intersection plane  50  that traverses this separation distance, stress contour line  51  exhibits a peak portion generally illustrated at  53 . In one form, the peak portion  53  can be considered to have a generally bell-shaped curvature. 
     A correlation of peak portion  53  to the thickness dimension of the workpiece indicates that the compressive residual stress represented by stress contour  51  extends deeper into workpiece  10  relative to the penetration depth that exists in the regions underlying the laser shock peened surfaces  20 ,  22 . As noted above, this deeper penetration depth occurs because the induced shockwaves that emanate from surfaces  20 ,  22  will synergistically interact with one another within the vicinity of plane  50  in a manner that causes the related pressure fields (i.e., imparted stress) to combine constructively, yielding the peak portion  53  of stress contour  51 . Other stress contour lines will exhibit a similar peak pressure feature. 
     Referring first to FIG. 3, the illustrated laser beam spot pattern  300  includes at least one row of circular laser beam spots  302  arranged in individual pairs  306  of first and second laser shock peened surfaces  308 ,  310  spaced-apart from one another by a sufficient distance (such as distance “d” in FIG. 1) to enable the formation of a shockwave interaction location (such as zone  52  in FIG. 2A) generally illustrated at  312 . Adjacent laser beam spot pairs  304 ,  306  within the same row are overlapped. 
     Shockwave interaction site  312  is formed between laser beam spot  308  and laser beam spot  310  of laser beam spot pair  306  in a manner similar to the formation of shockwave interaction zone  52  formed between laser shock peened surfaces  20  and  22  in FIG.  2 A. The spot pattern  300  may be simultaneously formed all at once or in a sequence of laser shock peening stages, such as pair-wise formation of the laser shock peened surfaces associated with each shockwave interaction site. 
     Within each row  302 , it is seen that each shockwave interaction site  312  alternates with a spot overlap site generally illustrated at  318  defined by the overlap between associated laser beam spot pair  304  (consisting of laser beam spots  316  and  314 ) and laser beam spot pair  306  (consisting of laser beam spots  308  and  310 ). Within each row, then, there is provided a sequence of uniformly spaced deep residual stress points (namely, the compressive residual stress regions produced in conjunction with each shockwave interaction site  312 ) alternating with laser beam spot overlap points  318 . The beam spot overlap region  318  has a mild effect in intensifying the laser peening under the overlapped area. 
     Although FIG. 3 shows shockwave interaction sites  312  being formed in a periodic or regular manner across a row  302 , it should be understood that a different laser beam spot pattern may be used which forms a row sequence of shockwave interaction sites  312  having a selectively aperiodic, irregular, or random spacing. The irregular spacing may be selected to define a configuration of shockwave interaction sites, for example, that match against a known, expected, or unknown defect pattern in the workpiece, thereby serving as a counter-distortion or workpiece life span enhancement measure. 
     The relationship among the rows may take various alternate forms. For example, neighboring rows  302  and  330  may be staggered or offset from one another as shown in FIG.  3 . In this form, the shockwave interaction sites  312  and beam spot overlap sites  318  sequentially alternate from one row to the next to define a column-type alignment, as exemplified along illustrative column line  332 . Alternately, neighboring rows can be identically aligned such that similarly situated shockwave interaction sites from different rows will line up with one another in a column-type linear format. Likewise, similarly positioned beam spot overlap sites from different rows will line up with one another in a column-type format. 
     Additionally, the laser beam spots of neighboring rows may or may not overlap with one another. In a configuration that permits neighboring row overlaps, it is illustratively seen for example that associated laser beam spots  308 ,  310  of pair  306  in row  302  may overlap respectively with laser beam spots  334  and  336  belonging to different (but adjacent) laser beam spot pairs in neighboring row  330 . 
     For all rows of circular laser beam spots discussed herein, it is preferable that the laser beam spots within a row have their center points in linear alignment. 
     Referring next to FIG. 4, the illustrated laser beam spot pattern  400  includes at least one row of circular laser beam spots  402  arranged in non-overlapping individual pairs  404  (consisting of laser beam spots  406  and  408 ). Adjacent laser beam spot pairs  406 ,  408  within the same pair  404  and  402  row are spaced-apart from one another by a sufficient distance (such as distance “d” in FIG. 1) to enable the formation of a shockwave interaction location (such as zone  52  in FIG. 2A) generally illustrated at  412 . 
     Shockwave interaction site  412  is formed between laser beam spot  406  and laser beam spot  408  in a manner similar to the formation f shockwave interaction zone  52  formed between laser shock peened surfaces  20  and  22  in FIG.  2 A. The spot pattern  400  may be simultaneously formed all at once or in a sequence of laser shock peening stages, such as pair-wise formation of the laser shock peened surfaces associated with each shockwave interaction site. 
     Within each row  402 , it is seen that each shockwave interaction site  412  alternates with a bridging site generally illustrated at  418  defined by the non-overlap area between associated laser beam spot pairs. Within each row, then, there is provided a sequence of spaced deep residual stress points (namely, the compressive residual stress regions produced in conjunction with each shockwave interaction site  412 ) that alternate with bridging regions  418 . 
     Although FIG. 4 shows shockwave interaction sites  412  being formed in a periodic or regular manner across a row  402 , it should be understood that a different laser beam spot pattern may be used which forms a row sequence of shockwave interaction sites  412  having a selectively aperiodic, irregular, or random spacing. The irregular spacing may be selected to define a configuration of shockwave interaction sites, for example, that match against a known or expected defect pattern in the workpiece. 
     The relationship among the rows may take various alternate forms. For example, neighboring rows  402  and  420  may be staggered or offset from one another as shown in FIG.  4 . In this form, the shockwave interaction sites  412  positionally alternate in a zig-zag fashion from one row to the next along a general column-type orientation such as direction  422 . Alternately, neighboring rows can be identically aligned such that similarly situated shockwave interaction sites from different rows will line up with one another in a column-type linear format. Likewise, similarly positioned bridging regions from different rows will line up with one another in a column-type format. 
     Referring next to FIG. 5, the illustrated laser beam spot pattern  500  includes at least one row of overlapping circular laser beam spots  502  in which individual laser beam spots  504  overlap with neighboring laser beam spots  506 ,  508  to form spot overlap regions generally illustrated at  510  and  512 , respectively. Within laser beam spot row  502 , for example, the individual spot overlap regions have a generally linear row alignment, as exemplified by illustrative line  514 . 
     Select ones of the adjacent rows such as rows  502  and  520  may be configured relative to one another such that a row-aligned sequence of shockwave intersection sites (generally illustrated at  522 ) may be formed between a respective set of laser beam spots assigned to adjacent rows. For example, sequence  522  includes an illustrative shockwave intersection site  524  formed between proximately spaced laser beam spots  526  and  528  belonging to beam spot rows  502  and  520 , respectively. 
     For this purpose, the individual rows  502 ,  520  would be formed such that illustrative laser beam spots  526 ,  528  would be spaced-apart by a sufficient distance (such as distance “d” in FIG. 1) to enable formation of the shockwave interaction site  524  therebetween, similar to zone  52  in FIG.  2 A. 
     In particular, shockwave interaction site  524  is formed between laser beam spots  526  and  528  in a manner similar to the formation of shockwave interaction zone  52  formed between laser shock peened surfaces  20  and  22  in FIG.  2 A. The spot pattern  500  may be simultaneously formed all at once or in a sequence of laser shock peening stages, such as pair-wise formation of the laser shock peened surfaces associated with each shockwave interaction site. 
     One significant feature of laser beam spot pattern  500  is that each sequence  522  of row-aligned shockwave interaction sites  524  has a common orientation or directionality. Thus, if the rows of shockwave interaction sites are located and oriented properly to known crack propagation directions (e.g., oriented generally perpendicular to the crack propagation direction), the shockwave interaction regions will present a significant enhancement to the crack retarding capability of the laser peened areas and compressive residual stress regions. 
     Moreover, regarding adjacent laser beam spot rows  530  and  532 , it is seen that these rows may be configured to define another sequence  534  of row-aligned shockwave interaction sites  536  disposed therebetween. Notably, the shockwave interaction sites  536  of sequence  534  have a common orientation or directionality different than that relating to sequence  522 . This orientation is possible, for example, by suitably configuring the laser beam spot rows  530 ,  532  to enable the formation of illustrative shockwave interaction site  536  between proximal laser beam spots  538  and  540  of rows  530  and  532 , respectively. A staggering or offset between the rows may produce the desired orientational characteristic, such as shown in FIG.  5 . 
     It is possible to establish any matrix of sequential shockwave interaction sites having selective orientations by suitable configuration of the laser beam spot rows. Thus, as one proceeds in row-like fashion through pattern  500 , it is possible to create any arrangement of orientations among the rows. For example, the orientation may remain the same or alternate among the rows. 
     Additionally, rows of sequential shockwave interaction sites (such as  522  and  534 ) may be interleaved with row(s) of spot overlap regions, such as the spot overlap row defined between adjacent laser beams spot rows  520  and  530 . 
     Referring next to FIG. 6, the illustrated laser beam spot pattern  600  includes plural rows  602  of circular laser beam spots produced by a succession of individual laser beam spot patterns applied to the workpiece to generate a selective arrangement of shockwave interaction sites  604 . Pattern  600  is the composite result of iteratively applying multiple layers of laser shock peening to the workpiece surface, including areas that are repeatedly laser shock peened. Each processing iteration, for example, would produce a configuration of shockwave interaction sites having a desired orientation. Multiple iterations would therefore introduce several such orientations into the workpiece. 
     For example, during a first laser shock processing iteration, a suitable laser beam pattern may be applied that induces the formation of illustrative shockwave interaction sites  604  having the indicated first orientation direction. A second laser shock processing iteration would then commence following the first iteration to induce the formation illustrative shockwave interaction sites  606  having the indicated second orientation direction different from the first orientation. 
     Generally the same workpiece surface area would be laser shock peened to facilitate the creation of both configurations of shockwave interaction sites  604  and  606 . For example, in the first processing iteration, a suitable set of laser shock peened surface areas associated with laser beam spots  608  and  610  would be formed in the manner set forth in connection with FIGS. 1 and 2 to form shockwave interaction site  604 . Similarly, in the second processing iteration, a suitable set of laser shock peened surface areas associated with laser beam spots  610  and  612  would be formed to create shockwave interaction site  606 . The surface area associated with laser beam spot  612  would have been irradiated during the first iteration to form the indicated shockwave interaction site  614  having the same orientation as site  604 . 
     FIG. 6 shows the effect of addressing multiple orientations of the shockwave interaction regions when applying multiple layers of laser peening to the same area. In this manner, the orientation of the interactions region can be changed from layer to layer to address multiple row orientations for the interaction regions. Using this iterative layering approach to form successive laser beam spot patterns each having a corresponding orientation for the shockwave interaction regions, it is possible to surround each circular-type laser peened surface area with six shockwave interaction regions by employing an equal number (6) of laser beam spot pattern layers. 
     This use of successive laser beam spot pattern layers permits an adaptive strategy to be developed that tailors the configuration of shockwave interaction sites to specified features in the workpiece. 
     For example, referring to FIG. 7, there is shown a workpiece surface  700  having a hole  702  where incipient flaws and cracks can easily develop. The processing techniques illustrated by FIGS. 1-6 are used to laser shock peen the circumferential portion about hole  702  to increase the circumferential compressive residual stresses around the hole. 
     As shown, a set of laser beam spots  704 ,  706  and  708  are applied circumferentially about the peripheral edge of hole  702  to form corresponding laser shock peened surface areas. For example, in a first processing iteration, a suitable set of laser beams may be used to form neighboring laser beam spots  704  and  706  to develop a corresponding shockwave interaction region located along and proximate to illustrative intersection plane  710  disposed nominally midway between spots  704  and  706 . 
     Similarly, in a second processing iteration, a suitable set of laser beams may be used to form neighboring laser beam spots  706  and  708  to develop a corresponding shockwave interaction region located along and proximate to illustrative intersection plane  712  disposed nominally midway between spots  706  and  708 . 
     This pair-wise formation of neighboring laser beam spots is continuously repeated until the entire circumference has been covered. As a result, there is developed about the periphery of hole  702  a plurality of regions having deep compressive residual stresses each formed in the vicinity of a shockwave interaction site, as identified by illustrative shockwave intersection planes  710  and  712 . 
     The laser beam spot patterns shown herein are provided for illustrative purposes only and should not be considered in limitation of the present invention. Rather, it should be apparent that any suitable laser beam spot pattern can be used to facilitate the advantages described herein. 
     Reference is now made to FIGS. 8 and 9 to illustrate a laser shock processing operation, according to a second embodiment of the present invention. As discussed further, this embodiment involves a dual-sided laser shock peening operation that forms fully opposing laser shock peened surfaces at opposing sides of a workpiece using an operating sequence that forms the relevant laser beam spots at different times. 
     For comparison purposes, reference is first made to FIG. 8A, which depicts the double-sided laser shock peening of a thin section using laser beams arriving at opposite sides of the workpiece at the same time. In particular, workpiece section  800  having opposite sides  802 ,  804  is simultaneously laser shock peened at sides  802 ,  804  to form laser shock peened surfaces  806  and  808 , respectively. 
     As shown, shockwaves  810 ,  812  induced by laser shock peening propagate toward one another from respective laser shock peened surfaces  806 ,  808 . Since the shockwaves  810 ,  812  were likewise created simultaneously, the shockwaves  810 ,  812  will meet nominally at the mid-plane  814  through the thickness dimension of workpiece  800 , as illustrated by the intimate confronting relationship along mid-plane  814  between the leading wavefront edges  816  and  818  of shockwaves  810  and  812 , respectively. 
     The shockwave interaction depicted in FIG. 8A will produce a high amount of plastic strain at mid-thickness, as represented diagrammatically in the corresponding residual stress profile of FIG.  8 B. For the purpose of this discussion it will be assumed that the residual compressive stress and the tensile strain profiles are similar. The graphic depiction of the profile is shown in juxtaposition to a schematic representation of the workpiece cross-section to facilitate an understanding of how the compressive residual stress (and tensile strain) varies with depth from the laser shock peened surface. 
     As shown in FIG. 8B, the profile curve  820  indicates that the distribution of plastic strain and compressive stress through the thickness is nominally symmetrical about the mid-thickness plane  814 . In particular, the compressive residual stress levels resulting from both shockwaves  810 ,  812  decrease with the same gradient (i.e., curvature) from their respective laser shock peened surfaces to the mid-plane  814 . 
     Referring next to FIG. 9A, there is shown a cross-sectional view which depicts the double-sided laser shock peening of a thin section using laser beams arriving at opposite sides of the workpiece at different times, according to the second embodiment of the present invention. 
     In particular, with regard to the same workpiece section  800  shown in FIG. 8, workpiece  800  is laser shock peened at different times at sides  802 ,  804  to form laser shock peened surfaces  806  and  808 , respectively. The individual laser beams producing laser shock peened surfaces  806 ,  808  are timed to arrive at their respective surface destinations (namely, opposite sides  802 ,  804 ) at a selected time interval apart (i.e., a delay time). In a preferred form, this time interval will nominally be on the order of 1 to 100 ns. 
     It is preferable that the laser shock peened surfaces  806 ,  808  be shaped and dimensioned identically. This requires, for example, the use of identical laser beam spots. Additionally, it is preferable that the laser shock peened surfaces  806 ,  808  be formed so as to be fully opposing one another, namely, that there is no lateral offset therebetween. 
     In the illustration of FIG. 9A, the laser beam incident on lower surface  804  arrives later than the laser beam incident on upper surface  802 , inducing a time-staggered set of shockwaves  850  and  852  that propagate toward one another from respective laser shock peened surfaces  806 ,  808 . Consequently, unlike the shockwaves of FIG. 8 which meet nominally at the mid-thickness plane, the time-staggered shockwaves  850  and  852  induced by laser shock peening meet at a point apart from mid-plane  814 . 
     This shift or displacement in the shockwave meeting point occurs because the shockwaves  850 ,  852  are generated in a time-staggered manner, allowing the first-generated shockwave  850  to pass through mid-plane  814  before it encounters the later-generated shockwave  852 . Their meeting point is indicated by the illustrative intersection plane  854  defining the generally planar junction where the respective leading edges  856  and  858  of shockwaves  850  and  852  encounter one another. 
     The intersection plane  854  is shown spaced-apart and below mid-plane  814  by representative distance d m . This separation distance from the mid-plane  814  will increase with a longer delay time between the incidence of the laser peening beams, and with increasing sound velocity in the material, as discussed below. 
     The following analysis describes the relationship between the delay time, material properties, and distance from the midplane of the thin section at which the time-staggered shockwaves meet. 
     The following parameters are defined: 
     t o =transit time of the shockwave, generated by the first laser beam, from the workpiece surface until it meets the shockwave generated by the second laser beam within the material; 
     t d =delay time between the time of arrival of the first beam pulse and the second beam pulse at the surface; 
     c=sound velocity in the material being laser peened; 
     h=section thickness of the workpiece at the laser peening location; 
     d 1 =distance into the material traveled by the shockwave from the first beam; 
     d 2 =distance into the material traveled by the shockwave from the second beam; 
     d m =distance from the section mid-plane where the shockwaves from opposite surfaces meet. 
     The opposing time-staggered shockwaves meet within the workpiece when the following conditions are satisfied: 
     (1) d 1 +d 2 =h, where 
     (2) d 1 =to * c and 
     (3) d 2 =(t o −t d ) * c. 
     Manipulating these equations yields: 
     (4) t o =(h/2c)+(t d /2). 
     The shockwaves will meet off-center by the distance: 
     (5) d m =(h/2)−d 2 . 
     Substituting equations (3) and (4) into (5) yields: 
     
       
           d   m =( h/ 2)−[( h/ 2 c )+( t   d /2)− t   d )] *  c , which provides  
       
     
     (6) d m =(t d * c)/2, for t d &lt;h/c, independent of the thin section thickness. 
     As an example, in Ti-6Al-4V where c=5.13 mm/ps, for a delay time of 20 ns (0.02 μs), d m  would be 0.05 mm (0.002 inches). If the delay were 100 ns (0.1 μs), d m  would be 0.26 mm (0.010 inches). In thin sections only 1 or 2 mm thick, such as in compressor airfoils of aircraft gas turbine engines, this is a significant effect. 
     Returning to FIG. 9A in conjunction with FIG. 9B, the result which follows from the shockwaves encountering one another at a location apart from the mid-thickness plane is the production of an asymmetrical compressive residual stress distribution through the section thickness relative to the mid-plane, as depicted by the stress distribution profile curve  860  of FIG.  9 B. This graphical depiction of the stress distribution is shown juxtaposed to a schematic representation of the workpiece cross-section to facilitate an understanding of how the compressive residual stress (and tensile strain) varies with depth from the laser shock peened surface. 
     The shockwave interaction depicted in FIG. 9A will produce an overall stress distribution  860  resulting from the combination of stress level distributions  862  and  864  that represent the variation in compressive residual stress levels imparted by shockwaves  850 ,  852  propagating from laser shock peened surfaces  806  and  808 , respectively and their interaction at intersection plane  854 . These stress level distributions generally join at curve portion  866  defined at the shockwave intersection plane  854 , producing an asymmetrical stress distribution profile curve  860  with respect to mid-plane  814 . The larger plastic strain produced about the intersection plane  854  skews the compressive residual stress distribution to be higher towards the surface having the delayed pulse or shown in FIG.  9 B. If the intersection plane occurs just beneath the laser shock peened surface  808 , the compressive residual stress in this surface would be increased relative to the opposing surface  806 . 
     The timing process that defines the time interval between formation of the laser shock peened surfaces on opposite sides of the workpiece is preferably selectively chosen to provide a shockwave intersection plane that coincides with a location where such shockwave synergy is desired. 
     Although the intersection plane is shown below the mid-plane in FIG. 9, this is for illustrative purposes only, as it should be apparent that the intersection plane can be positioned at any level relative to the mid-plane either above or below it by suitable selection of the time interval between irradiation of the opposite sides of the workpiece. For example, an intersection plane above the mid-plane would involve first laser peening lower surface  804  and then laser peening upper surface  806 . 
     Referring next to FIG. 10A, there is shown a cross-sectional view which depicts the double-sided, simultaneous laser shock peening of a thin section using laser beams having different pulse lengths, according to the third embodiment of the present invention. 
     Illustrative workpiece  900  includes sides  902  and  904  disposed generally opposite one another and a mid-thickness plane  914 . The opposing sides  902  and  904  are simultaneously laser shock peened with laser beams having different pulse lengths to form opposing laser shock peened surfaces  906  and  908 , respectively. As shown, the laser shock peened surfaces  906  and  908  are respectively irradiated with representative laser beams  910  and  912 , where the pulse length of laser beam  910  is longer than the pulse length of laser beam  912 . 
     It is preferable that the laser shock peened surfaces  906 ,  908  be shaped and dimensioned identically. This requires, for example, the use of identically sized laser beam spots. Additionally, it is preferable that the laser shock peened surfaces  906 ,  908  be formed so as to be fully opposing one another, namely, that there is no lateral offset therebetween. 
     The residual stress profile through the thickness of workpiece  900  is modified due to the use of different pulse lengths for the laser beams simultaneously irradiating the opposite sides of workpiece  900 . This modification arises because the shockwave generated by the shorter pulse will attenuate faster (i.e., the peak pressure will decrease faster) with distance into the material than the shockwave generated by the longer pulse at the opposite side. The effects of this modification are discussed below in connection with FIG.  10 B. 
     FIG. 10B graphically illustrates the compressive residual stress distribution through the workpiece thickness relative to the mid-plane  914 . This graphical depiction of the stress distribution is shown juxtaposed to a schematic representation of the workpiece cross-section to facilitate an understanding of how the compressive residual stress (and tensile strain) varies with depth from the laser shock peened surface. 
     Referring to the stress distribution profile curve  920 , the gradient in the compressive residual stress (which decreases with increasing distance from the laser shock peened surface) will be steeper on the side affected by the shorter laser pulse, i.e., the compressive stress will not extend as far below the surface of the short pulse side as compared to the long pulse side. This steeper gradient is generally indicated by portion  922  of curve  920  corresponding to the stress distribution that relates to forming laser shock peened surface  908  with a shorter laser beam pulse. The more gradual gradient for the stress distribution associated with the longer laser beam pulse is generally indicated by portion  924  of curve  920 . 
     As curve portion  922  indicates, the compressive residual stresses due to the shorter laser beam pulse may terminate at a location antecedent to mid-plane  914 . This condition ensues from the fact that the shockwaves induced by the shorter laser beam pulse may attenuate sufficiently that the peak pressure is reduced below the HEL before reaching the mid-plane  914 . 
     It is also seen that the interaction of the shockwaves at mid-plane  914  will be decreased by the amount that the shorter pulse has attenuated compared to the longer pulse. It is possible that the shockwave interaction at mid-plane  914  is so weak as to generate little or no plastic strain, and therefore little or no additional compressive residual stress due to the interaction. However, this asymmetry in compressive residual stress could provide a significant advantage in using this processing method to form curved thin sections or for counteracting distortion in thin sections. 
     Referring next to FIGS. 11 and 12, FIGS. 11A-C show a series of cross-sectional views of a workpiece section that is laser shock peened using various amounts of lateral offset between the dual-sided laser shock peened surfaces, according to the fourth embodiment of the present invention. 
     FIG. 11A shows a lateral offset where the laser shock peened surfaces oppose each other in part, i.e., there is a partial overlap of the lateral dimensions. FIG. 11B shows a lateral offset where the neighboring edges of the laser shock peened surfaces line up with one another, but there is no overlap. FIG. 11C shows a lateral offset where the neighboring edges of the laser shock peened surfaces are spaced-apart from one another, i.e., there is no overlap of the lateral dimensions. 
     Referring first to FIG. 11A, workpiece  940  is simultaneously laser shock peened at both of its opposing sides  942  and  944  using respective laser beams  946  and  948  to form respective laser shock peened surfaces  950  and  952 . As shown, the laser shock peened surfaces  950  and  952  are laterally offset or displaced from one another along the lateral direction  954 . Traveling shockwaves  956  and  958  induced by laser shock peening are associated with the formation of laser shock peened surfaces  950  and  952 , respectively. Various effects are produced by such lateral offset relationship. 
     The balance of forces (manifested as metal movement) from the shockwaves moving through the thickness is not directly balanced. This imbalance exerts a moment force on the material that tends to rotate the workpiece around an axis perpendicular to the displacement vector connecting the offset laser shock peened surfaces. The force moments will act directly to deform and bend the material. 
     For example, referring to FIG. 12, there is shown a dual-sided laser beam peening arrangement that depicts the bending moments produced by the laser shock processing conducted in connection with FIGS. 11A-C. An illustrative set of three (3) laser beam pairs  988 ,  989  and  990  is shown having associated laser beams  992  and  993  applied simultaneously in the indicated lateral offset manner to respective opposite sides  942  and  944  of the workpiece. 
     The lateral offset among the respective laser beams of each pair is different for each of the laser beam pairs  988 ,  989  and  990 . In particular, the lateral offset increases through laser beam pairs  988 ,  989  and  990 . Increasing the offset between the opposing laser beams has the effect of increasing the bending moment on the thin section. Accordingly, in ascending order, the bending moments increase through laser beam pairs  988 ,  989  and  990 . The direction of the rotary bending force induced by the bending moments is specified by the indicated arrows. 
     Additionally, the lateral offset relationship between the simultaneously applied incident laser beam spots also has the effect of modifying the shape and location of the shockwave interaction zone, which normally is located at mid-thickness for opposite-positioned laser beam spots. In particular, when the laser beam spots are positioned directly opposite one another across the mid-thickness plane, the shockwave interaction zone remains nominally along the mid-thickness plane, as shown in FIG.  8 A. 
     However, when the laser beams are offset, the shockwave interaction zone has a reduced portion at mid-thickness, and will exhibit newly appearing wing-type portions that extend obliquely from the mid-thickness plane towards the workpiece surface. The portion of the intersection zone of interest for the purpose of this invention is the zone in which the combined pressure of the interacting shockwaves is higher than the HEL, in a region to have the shockwave from only over of the laser beam spots would have a pressure below the HEL. As a consequence of this local increase in pressure above the HEL, this zone will have additional plastic strain creating additional compressive residual stress. This effect will therefore introduce asymmetry into the compressive residual stress distribution through the thickness and thereby extend the field containing residual compressive stress beyond the material volume directly under the laser shock peened spot. This will effectively increase the extent of the compressive residual stresses as compared to directly opposed laser shock peened spots. In addition, this effect will produce localized bending of the thin section. 
     It will also be apparent that progressive increases in the lateral offset will further shorten the shockwave interaction zone at mid-thickness, while the wing-type portions of the zone will curve more obliquely outwards to the opposite surfaces. Eventually, a threshold lateral offset may be reached beyond which the shockwaves will be nominally perpendicular to the workpiece surfaces when they intersect and any bending now will be small and ineffective. However, if the combined pressure of the intersecting shockwaves is higher than HEL, plastic strain will develop and localized compressive residual stress will exist in the interaction zone. 
     Returning again to FIG. 11A, the mid-plane portion of the shockwave interaction zone (specified generally at  957 ) is generally defined by the intersection of intermediate wavefronts  960  and  962  of shockwaves  956  and  958 , respectively. The wing-type portion of the shockwave interaction zone (specified generally at  959 ) is generally defined by the intersection of leading wavefronts  961  and  963  of shockwaves  956  and  958 , respectively. The locus of the various interaction locations of the intersecting shockwaves  956  and  958  is defined generally by interaction locus curve  970 . 
     Referring now to FIG. 11B, the lateral offset between the laser beams has been increased relative to that of FIG.  11 A. As a result, the mid-plane portion of the shockwave interaction zone (specified generally at  972 ) has been shortened relative to the mid-plane portion  957  in FIG.  11 A. Additionally, the wing-type portion of the shockwave interaction zone (specified generally at  973 ) is steeper and rises more sharply towards surface  942 , as compared to the relatively flatter wing-type portion  959  in FIG.  11 A. The locus of the various interaction locations of the intersecting shockwaves  974  and  975  is defined generally by interaction locus curve  976 . 
     Referring now to FIG. 11C, the lateral offset between the laser beams has been increased relative to that of FIG.  11 B. As a result, the mid-plane portion of the shockwave interaction zone has virtually disappeared. Additionally, the wing-type portion of the shockwave interaction zone (specified generally at  978 ), which almost exclusively defines the whole of the shockwave interaction zone, rises even more sharply towards surface  942 , as compared to wing-type portion  973  in FIG.  11 B. The locus of the various interaction locations of the intersecting shockwaves  979  and  981  is defined generally by interaction locus curve  983 , which exhibits a weak residual stress overlap region. 
     The overlap between the compressive residual stress regions becomes progressively smaller throughout FIGS. 11A-C with an increase in lateral offset between the incident laser beams. 
     Referring to FIG. 13, there is shown an illustration of the mid-thickness plane  140  and the positioning of substantially opposite laser beam spots  142  and  144  (producing laser shock peened surface  143  and  145 ) for a workpiece portion  146  having non-parallel opposite surfaces  147  and  148 . 
     Referring to FIG. 17, there is shown a simplified block diagram illustration of a system for use in practicing the present invention. In its most elemental form, the system  160  includes a laser shock peening apparatus  162  and a controller  164  for selectively controlling the operation of laser shock peening apparatus  162  in conjunction with laser shock processing a specified object. 
     In a preferred form, controller  164  is selectively configurable to enable any type of laser shock operating sequence to be performed. For example, when controller  164  has a computer or microprocessor-based implementation, a suitable program code of instructions may be loaded into memory  166  and transferred to controller  164  for execution. The program code would fully define the series of control commands and instructions needed to execute, govern, and manage a corresponding laser shock processing operation as carried out by laser shock peening apparatus  162 . 
     A suitable user input device (not shows) may be optionally added to enable a user to input or change various operating parameters. 
     What has been shown and described herein are various laser shock processing sequences suitable for allowing a designer to design tailored or customized compressive residual stress distribution profiles in target workpieces. 
     For example, multiple neighboring spots may be selectively located on the same area of the workpiece surface in a simultaneous manner to enhance the laser peening effect, i.e., to increase the depth of the compressive residual stress. 
     Additionally, it is possible to vary the time interval (for example, within a range of less than one microsecond) between irradiation of multiple spots on opposite sides of a thin sections, there controlling the location (relative to mid-plane) where the shockwaves meet. 
     Moreover, different pulse lengths of the laser beams that irradiate opposite sides of a thin section will control the depth to which residual compressive stresses will extend below the processed surface. It also will determine the extent to which the shockwaves will interact at mid-thickness, e.g., the shockwaves generated by shorter laser pulses will attenuate much more rapidly with distance from the processed surface, as compared to longer pulses. Shorter pulses will therefore considerably weaken the shockwave interaction at mid-thickness and reduce mid-thickness compressive residual stresses, and increase the compressive stress gradient below the laser shock peened spot. 
     These methods may also be used to modify the shape of thin metal sections, such as by forming a processed section into gently curved shapes. 
     These methods also introduce an asymmetric residual stress distribution that may cause the thin section to curve inward towards the side having the less compressive stress. For example, in FIG. 9B, the thin section would being to curve concave upward as the compressive stresses and the laser peened area increased. In this way, by controlling the laser peening intensity and area of coverage, a desirable, controlled curvature of the thin section could be produced. 
     Finally, the neighboring beam concept could also be used in double-sided laser peening of thicker, thin sections to promote through-thickness compressive residual stresses that are present deeper into the material than would be possible using only a single beam on each side. The neighboring beam approach could also significantly enhance tangential compressive residual stresses around holes, further inhibiting crack initiation from the hole surface. This would happen because the intersecting planes would be radial to the hole, so that the major orientation of the compressive stress would be tangential to the hole, the most favorable compressive stress orientation, as shown in FIG.  7 . 
     To further explain portions of the invention, FIGS. 18 and 19 are provided. FIGS. 18A-E are figures in time sequence that show that with double sided laser shock processing when both laser pulses arrive at the opposing surfaces at the same time, the compression waves go through the workpiece and reflect off the opposite sides. When tensile waves travel back into the material, the waves meet at the mid-thickness of the workpiece or in that vicinity, and a very large increase in the tensile stress occurs in that region. If the material has low ductility, cracking can develop at that point, which is undesirable. The invention in one form, provides a means to minimize or eliminate a potential tensile wave interaction. 
     In FIGS. 18A-F, the Y axis measures pressure. The X axis is (on the left) distance to the left of the vertical line, it is distance into the material from the surface (surface  2 ), and to the right of the line, is the free surface. The negative X side shows the shockwave traveling from the opposing surface (surface  1 ). The position X side shows the shockwave being applied to the surface by the laser beam at the same instant. 
     In FIG. 18A, an in-material shockwave is shown approaching the free surface from the opposing surface, and a laser impulse is timed on the near surface to be initiated when the stress wave in the material is approaching that surface. The first shockwave (shockwave  1 ) is a solid line and the second shockwave (shockwave  2 ) to be applied is the dash line. In FIG. 18B, the first shockwave has just reached the surface and the second shockwave has already passed some distance into the surface and the solid line shows the actual pressure profile within the material from the superposition or interaction of the two. A beginning increase in the compression wave is shown higher than either of the shockwaves. In FIG. 18C, (another next time interval) shows a point in time when the first shockwave has begun to reflect off the opposing surface as a tensile wave which is shown below the line in the figure, coming back into the material. Reflected shockwave  1  (Tl) is the dotted dash line underneath the X axis, in the negative pressure portion quadrant. The second shockwave has traveled further into the material during that same time interval and the solid line now displays the combination. The solid line is the actual pressure profile that results in the combination of the three components of the shockwaves that are within the material (e.g., shockwave number  1 , shockwave number  2 , and reflected tensile shockwave number  1  (Tl)). The increase in the peak pressure at surface  2  will increase the magnitude and possible the depth of the compressive stress at surface  2 . 
     FIG. 18D shows another time interval where T 1  has progressed to a much greater degree and shockwave number  1  has basically reflected in large part off the opposing surface as a tensile wave with just a long compressive tail, that is still traveling towards the opposing surface, and shockwave number  2  has progressed substantially into the material. The solid line again shows the superposition effect of those three waves and demonstrates that one has a significant compression wave traveling into the material followed by a small tensile wave, its magnitude depending on the amount of attenuation that has already occurred in shockwave number  1  as it has gone through the material and reflected off the opposite surface. 
     FIG. 18E shows the same wave approaching the opposing surface (surface  1 ). Once the components separate from shockwave  1 , out of the combination of the compressive shockwave  2  plus the compressive tensile shockwave  1 , one has a compression wave with a following small tensile wave. Then, when such wave reflects from the surface as shown in FIG. 18F, then just a normal tensile reflective wave followed by a small compression wave exists. Thereby one does not get doubling of the tensile waves of the combination of the tensile reflective waves from the two opposing shockwaves. The end result is that one obtains compressive residual stresses on each surface, without getting the negative effect of the opposing shock or the tensile reflected waves of the opposing shockwave adding somewhere in the material and potentially causing cracking. In FIG. 18E the solid line again is the superposition or combination of the entire amount of force that it is traveling through the workpiece. The solid line clearly shows that the length of the combined shockwave is shorter than shockwave  2  would be without the interaction at surface  2 . This will increase the rate of attenuation of the peak pressure and reduce or eliminate its effect on the preexisting residual stress when it reduces surface  1 . 
     An important point in FIG. 18, is that shockwave  1  and shockwave  2  in FIG. 18A are not the same height because allowance has been made, and in the real world in made in fact, that in the time shockwave  1  approaches the opposing surface, there will be some attenuation at the peak pressure and some broadening of the pressure curve. This is illustrated by the difference in heights and slight difference in shape between the two. Therefore, there is no automatic match of the pressure waves if the applied beams are the same. 
     FIG. 19 illustrates double sided processing of thin surfaces or thin sections, where it is possible to achieve compressive residual stresses on one side without developing compressive residual stresses on the opposite side while conducting two sided laser peening. FIG. 19A shows a compressive wave passing through the material approaching the interior surface on side  2 , e.g., a compression wave  1  from laser shot peening  1 , and the dotted line the compression wave for shot  2 . The key aspect shown is that the shockwave  2  initiates at the time or nominally at the same time that shockwave  1  strikes the surface (side  2 ). Therefore there is a difference in terms of the timing relative to the initiation of pressure pulse  2  at side  2  between FIGS. 18A and 19A. 
     FIG. 19B shows (a short time later) when shockwave  1  is beginning to reflect off of side  2  and forming a tensile reflective wave T 1 . Shockwave  2  has just entered into the material. The composite or the combination of these three components, shockwave  1 , shockwave  2 , and tensile reflective wave  1 , is shown as the solid line, the actual pressure profile within the material. Note that there is a slight increase in compressive stress just inside the surface. FIG. 19C shows additional time has passed, and the peak of shockwave  1  has now reflected as a tensile wave, tensile wave  1 , back into the material and the peak of shockwave  2  has passed into the material. The solid line shows the combination of those three shockwaves. One can see that the peak pressure has now been diminished as shockwave  2  passes into the material. It is basically still on the tail of shockwave  1  passing through the material. 
     FIG. 19D shows a later delta time where shockwave  1  has passed mostly into the material and the tensile reflected wave of shockwave  1  is very nearly compensating for shockwave  2  so that the combination pressure of the three different components of the shockwave is a very low compressive residual stress and could be selected to be below the elastic limit of the material where it would not be developing additional plastic strain and contributing to the compressive residual stress. There may be some compressive residual stress at side  2 , but not nearly to the extent that would occur without the relative timing of the shockwaves as discussed above, because one is diminishing the peak pressure of shockwave  2  as shown in FIG.  19 C. 
     If the pressure is above the HEL limit for the material, then one will obtain dynamic yielding in plastic strain. The plastic strain will then be the source of the residual compressive stress. If the pressure is below the HEL, then one will not obtain yielding and plastic strain will not develop. Therefore no compressive residual stress will be created. It is the plastic strain int eh material that is the source of the compressive residual stress. 
     In FIG. 19, therefore, with the timing mismatched, with a particular timing, the sum of the pressures does not reach the HEL limit or there is less that is reaching the limit, and thereby reducing or eliminating the magnitude of the compressive residual stress at the surface. FIGS. 19D and 19E show a later time period. FIG. 19E shows the combined shockwave approaching side  1  and one can see that it is a very low magnitude compressive shockwave. This wave is what would be intended to be well below the HEL and would have no effect on the opposing surface. In essence what is shown between FIGS. 18 and 19 is that to place the highest amount of largest amount of compressive residual stresses and the deepest into the material, one would want to have some type of particular timing, between the first shot while it is in the material to when one applies or creates a second shot at the opposing surface of the material. So there would be some distance which would equal to some time between the two that D would be dependent upon the material. 
     In addition to the concepts explained above, what one can also accomplish is to adjust the magnitude of the intensity of laser pulse  2  compared to laser pulse  1  to further and contribute to decreasing the peak pressure. Therefore, instead of coming in at that same magnitude laser beams one could come in at a smaller magnitude. Therefore, the inventive system not only modifies the timing, but can also modify the relative intensities of the two pulses. In FIG. 19 the attempt is to develop a higher compressive residual stress no one side than the other side. However, in thin sections one can also obtain a significant deformation of the workpiece. What is shown is by applying a laser pulse from the opposite side, a minimizing of the distortion occurs since the system is sending in a compensating shockwave in the other direction. If everything is equal (as in conventional split beam processing) and the workpiece is equal distant away from the optics, and for example, the beams are coming in together at a midpoint, one would not want the workpiece at the midpoint between the place where the laser beams would actually cross or meet in the first instance. A physical setup for the effect of FIG. 19 could be accomplished by inserting (in conventional split beam processing) an appropriate difference in the beam path lengths to allow the one beam to arrive at a desired amount later than the other beam. In that case, one would be setting the workpiece and aligning it in the center and just change one beam path versus the opposite side beam path to have a longer length. So if it was needed to adjust the timing, in terms of a nanosecond or 5 nanoseconds, one would add in one or five additional feet of beam travel length with everything else still being centered. If relatively long delays between the two shockwaves were needed, i.e., approaching 1 microsecond, one would preferably use lasers with separate oscillators and then initiate either beam with the desired delay time. The delay time would depend on the material thickness. The time to travel through the material would be H over C as a time difference int eh beams of 5 to 20 nanoseconds depending on the pulse, probably in the range of 5 to 50 nanoseconds. The invention in this form prevents the reflected tensile waves from interfering between themselves. 
     In another embodiment, the center line cracking of workpieces may be avoided by alternate side processing where one avoids tensile stress wave interaction altogether by a longer delay between them. For example, FIG. 20 shows a drawing of a thin section having beam locations  1 ,  2 ,  3 , and  4  on both side  1  and side  2 . The novel example pattern foreseen is laser processing one side  1  location  1  and then after the shockwave traverses and rings out (e.g., self-attenuates within the part), side  2  location  2  is hit and rings out, and then side  1  location  3 , then side  2  location  5  and then so on going on to n length. At a later time, processing would start with side  2  location  1 , letting it ring out, then side  1  location  2  letting it ring and then continuing on again to side  2  location  2  and so forth with the pattern. 
     An additional thin section processing pattern could be side  1 , spot  1  let it ring out, side  2 , spot  1  let it ring out, side  1 , spot  2  let it ring out, side  2 , spot  2  let it ring out to spot n. Although the spots would typically co-align, the spots do no necessarily have to be aligned to avoid centerline cracking. 
     A calculation is possible to the kind of delay one would want for the shockwaves meeting off the mid-plane, and in this case, one would have a similar calculation relative to the intersection or midpoint between the spots. Five mm or less is a preferred spot separation because of the decay of the shockwaves in most metal workpieces. 
     In some embodiments (same side processing) a maximum delay time required, based on a maximum and preferred separation of 5 mm, is that distance (5 mm) divided by the sound speed within the material processed to give the number of nanoseconds of maximum delay. In other words, the maximum delay between the spots is the distance between the spots divided by the shockwave speed in that particular material, and that maximum delay, is the maximum delay allowed to have the effects. If one exceeds that delay time, shockwave two would be passing in material through which the shockwave one has already passed, and the workpiece therefore would have missed the interaction. 
     While this invention has been described as having a preferred design, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.