Measuring and testing – Vibration – By mechanical waves
Reexamination Certificate
2000-11-27
2002-07-23
Williams, Hezron (Department: 2856)
Measuring and testing
Vibration
By mechanical waves
C073S011020, C073S632000, C073S646000, C073S602000
Reexamination Certificate
active
06422082
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to quality assurance methods used for quality assurance for laser shock peening and, more particularly, for ultrasonic testing and statistical analysis of laser shock peened surfaces for quality assurance of a production laser shock peening process.
2. Discussion of the Background Art
Laser shock peening or laser shock processing, as it is also referred to, is a process for producing a region of deep compressive residual stresses imparted by laser shock peening a surface area of a workpiece. Laser shock peening typically uses multiple radiation pulses from high power pulsed lasers to produce shock waves on the surface of a workpiece similar to methods disclosed in U.S. Pat. No. 3,850,698, entitled “Altering Material Properties”; U.S. Pat. No. 4,401,477, entitled “Laser Shock Processing”; and U.S. Pat. No. 5,131,957, entitled “Material Properties”. Laser shock peening, as understood in the art and as used herein, means utilizing a laser beam from a laser beam source to produce a strong localized compressive force on a portion of a surface by producing an explosive force by instantaneous ablation or vaporization of a painted or coated or uncoated surface. Laser peening has been utilized to create a compressively stressed protection layer at the outer surface of a workpiece which is known to considerably increase the resistance of the workpiece to fatigue failure as disclosed in U.S. Pat. No.4,937,421, entitled “Laser Peening System and Method”. These methods typically employ a curtain of water flowed over the workpiece or some other method to provide a confining medium to confine and redirect the process generated shock waves into the bulk of the material of a component being LSP'D to create the beneficial compressive residual stresses.
Laser shock peening is being developed for many applications in the gas turbine engine field, some of which are disclosed in the following U.S. Pat. Nos.: 5,756,965 entitled “ON THE FLY LASER SHOCK PEENING”; U.S. Pat. No. 5,591,009, entitled “Laser shock peened gas turbine engine fan blade edges”; U.S. Pat. No. 5,569,018, entitled “Technique to prevent or divert cracks”; U.S. Pat. No. 5,531,570, entitled “Distortion control for laser shock peened gas turbine engine compressor blade edges”; U.S. Pat. No. 5,492,447, entitled “Laser shock peened rotor components for turbomachinery”; U.S. Pat. No. 5,674,329, entitled “Adhesive tape covered laser shock peening”; and U.S. Pat. No. 5,674,328, entitled “Dry tape covered laser shock peening”, all of which are assigned to the present Assignee. These applications, as well as others, are in need of efficient quality assurance testing during production runs using laser shock peening.
LSP is a deep treatment of the material and it is desirable to have a quality assurance test that is indicative of a volumetric LSP effect. It is also desirable to have a QA method that is compatible with a dual sided or simultaneous dual sided LSP process wherein substantially equal compressive residual stresses are imparted to both sides of a workpiece, i.e. along the leading edge of a gas turbine engine fan blade.
One laser shock peening quality assurance technique previously used is high cycle fatigue (HCF) testing of blades having leading edges which are LSP'd and notched in the LSP'd area before testing. This method is destructive of the test piece, fairly expensive and time consuming to carry out, and significantly slows production and the process of qualifying LSP'd components. An improved quality assurance method of measurement and control of LSP that is a non-destructive evaluation (NDE), inexpensive, accurate, and quick is highly desirable. It is also desirable to have an NDE quality assurance method that is relatively inexpensive and sufficiently economical to be used on each workpiece instead of a sampling of workpieces. LSP is a process that, as any production technique, involves machinery and is time consuming and expensive. Therefore, any techniques that can reduce the amount or complexity of production machinery and/or production time are highly desirable.
Interferometric profilometry method and apparatus to obtain volumetric data of a single laser shock peened test dimple created with a single firing of a laser used in the laser shock peening process is disclosed in U.S. Pat. No. 5,948,293 “Laser shock peening quality assurance by volumetric analysis of laser shock peened dimple”. Other QA methods are disclosed in U.S. Pat. No. 5,987,991 “Determination of Rayleigh wave critical angle”; U.S. Pat. No. 5,974,889 “Ultrasonic multi-transducer rotatable scanning apparatus and method of use thereof”; and U.S. Pat. No. 5,951,790 “Method of monitoring and controlling laser shock peening using an in plane deflection test coupon”.
SUMMARY OF THE INVENTION
A method for quality control testing of a laser shock peening process of a production workpiece includes (a) ultrasonically scanning at least a portion of a laser shock peened surface on the workpiece wherein a region having deep compressive residual stresses imparted by the laser shock peening process extends into the workpiece from the laser shock peened surface, (b) digitizing a signal derived from the scanning and forming a digitized image of intensity values from the scanning, (c) calculating at least one statistical function value for a plurality of points of the digitized image of the workpiece based on the intensity values, and (d) comparing the statistical function value to a pass or fail criteria for quality assurance of the laser shock peening process or accepting or rejecting the workpiece.
In one exemplary embodiment of the invention the plurality of points of the digitized image (
44
) are delineated by a group of circles corresponding to laser shock peened dimples within the portion of the laser shock peened surface. The statistical function comprises at least one of four statistical properties of the digitized image defined by four equations, a Mean Matrix MM(k) for each kth dimple, a Dimple Standard Deviation Matrix SDM(k), a Mean Vector MV(x) of all the points in the group of circles, where x is the number of pixels in each dimple, and a Standard Deviation Vector SDV(x) of each of the group of circles. Three types of the statistical function are a Mean of Dimple Mean Matrix F
1
, a Mean of Standard Deviation Matrix F
2
, and a Mean of Standard Deviation Vector F
3
.
In another exemplary embodiment of the invention the plurality of points of the digitized image are delineated by a rectangle around laser shock peened dimples within a portion of the laser shock peened surface and the statistical function is a Sobel Function F
4
including a Sobel operator.
The pass or fail criteria is based on a pre-determined correlation of test piece statistical function data and high cycle fatigue failure based on high cycle fatigue tests of test pieces that were laser shock peened in the same or similar laser shock peenin apparatus. Each of the test pieces has a failure precipitating flaw within a laser shock peened area of the test piece that was laser shock peened in the same or similar laser shock peening apparatus.
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Ramaswamy V. G.
Rosen Steven J.
Saint-Surin Jacques
Williams Hezron
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