Electric heating – Metal heating – By arc
Reexamination Certificate
2002-07-29
2003-12-16
Heinrich, Samuel M. (Department: 1725)
Electric heating
Metal heating
By arc
Reexamination Certificate
active
06664506
ABSTRACT:
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
Clauer Allan H.
Dulaney Jeff L.
Lahrman David F.
Toller Steve M.
Heinrich Samuel M.
Knuth Randall J.
LSP Technologies Inc.
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