Image analysis – Applications – Surface texture or roughness measuring
Reexamination Certificate
1998-12-29
2002-07-02
Johns, Andrew W. (Department: 2621)
Image analysis
Applications
Surface texture or roughness measuring
Reexamination Certificate
active
06415044
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method for nondestructively determining the performance characteristics of a metal component. More specifically, the present invention relates to linking impact treatment process variables to surface texture and determining the impact process' reliability and reproducibility of process variables linked to fatigue life, stress corrosion resistance enhancement, or other related performance parameters of an impact treated or shot peened metal component.
Impact treatment processes, including shot peening, have long been known to be capable of developing large fatigue strength and/or stress corrosion resistance enhancements in cyclically stressed metallic workpieces. In addition, such processes are known to be capable of generating certain workpiece surface morphology characteristics which are advantageous in certain applications.
Impact treatment processes used to improve workpiece performance include shot peening, sand blasting, laser shock processing and any other such processes which produce surface plastic deformation resulting from the surface being impacted with relatively high energy. Such impact treatment processes enhance the performance characteristics of a component through modification of a component's surface and subsurface characteristics. During the shot peening process, a stream of shot or other near spherical particles typically made of a metal or glass travelling at high velocity is directed at a component surface. The shot is directed at the component so as to cause plastic deformation and strain of the component surface. Similarly, lasers which generate high energy are used to impact treat surfaces by generating plastic deformation and strain. Plastic strain is the mechanism where the surface of the component is put into a state of compressive residual stress, increased dislocation density, and other metallurgical microstructural changes, these being key to the life enhancing benefits of impact treatment processes. Depending on the nominal, range, and distribution of the values of certain critical impact treatment process variables or parameters such as in shot peening where shot size, shot velocity, shot impact angle, and workpiece shot peen process saturation are critical variables, the shot peen process induces different magnitudes and depths of residual compressive stresses and microstructural changes and varying degrees and types of surface morphology characteristic changes. In laser shock processing, variables such as laser intensity and the number of impacts or “shots” in a given area determine the level of the desired workpiece characteristics. Each of these impact process induced surface and subsurface changes and enhancements are quantitatively and reliably related to impact process variable's nominal, range, and distribution values for given workpieces of given chemical and physical characteristics. These relationships are well established in the art, particularly in shot peening.
Due to the shot peening processes total number of critical process parameters and the total number of ways changes in those variable values can be affected during processing, the shot peen process has developed a history of variability in process induced benefit when applied to workpieces in a production processing environment. This is true unless a very high degree of care is taken to positively and systematically control all shot peen process variables within relatively narrow tolerances.
The current best available method of defining what shot peen process should be applied to test workpieces and/or predicting whether a workpiece processed in a production application of shot peening would perform as well as expected is to determine the magnitude of the residual stresses introduced by the shot peen process, in profile and in depth, through x-ray diffractometer analysis, a technique well documented in the art. This is, however, a destructive inspection process as it requires extensive electrochemical milling of the surface being analyzed. It is also relatively costly and time consuming enough to be ineffective as a real time production inspection technique.
The standard industry accepted method of inspecting and calibrating a shot peen process involves the exposure of an SAE 1070 steel strip (Almen strip) to the blast stream of shot. This strip is mounted by four retaining screws on a thick block and placed in the blast in a manner which best represents the workpiece surface for the same exposure time the workpiece will experience. As an Almen strip is exposed to the blast stream, it arcs towards the stream. The amount of arc when stabilized over time (Almen saturation) indicates the total energy of the blast stream. The shot peen process cycle time is also typically determined by and/or calculated from the exposure time required to achieve Almen saturation utilizing Almen strips. This is a technique well established in the prior art.
While Almen intensity is an indicator of total blast stream energy, the method has significant shortcomings as a valid non-destructive inspection of the efficacy of a shot peen process in developing workpiece fatigue strength and/or other shot peen processes induced workpiece performance enhancements. First, the Almen strip is rarely of the same material or hardness as the workpiece. Second, it is only representative of the shape of a workpiece if that workpiece's area being shot peened is essentially flat. As such, an Almen strip, which is intended to be only a representation of the workpiece being processed, is also typically a very poor representation of the workpiece. Additionally, the total cumulative inherent measurement system variability of the Almen measurement system as specified in industry and U.S. military specifications has been documented in the prior art as being large enough to often exceed the total allowable manufacturing process tolerances for the shot peen process it is attempting to calibrate and measure.
While there are several other serious drawbacks and shortcomings to the Almen system of shot peen process measurement, which are well documented in the art, perhaps most importantly the height of an Almen strip at Almen saturation (Almen intensity) measures only the aggregate energy transfer to the Almen strip by the shot peen process in the form of residual compression. This aggregate residual compression is the aggregate effect of the nominal, range, and distribution values of all of a shot peen processes variables experienced during the processing of the Almen strip. An Almen intensity value as such cannot indicate what the nominal residual compression is at any given point on the surface and/or subsurface respectively. This effectively precludes the Almen system from measuring the individual level and effect of any or all shot peen process parameters. Thus, process parameters such as shot size, shot velocity, and shot angle of impact, cannot be predicted individually or quantitatively predicted by the Almen technique without unequivocally knowing the quantitative value of the other process parameters and the quantitative level of variability and measurement caused by cumulative measurement of variability. For example, one may deliver the same aggregate amount of energy to the workpiece by using large shot at a small velocity or small shot at a high velocity. In either case, the Almen strip can indicate the same effective measurement. As such, a given Almen intensity value can be arrived at through an infinite combination of process parameter nominal and range values, each combination having it's own unique pattern of residual stress magnitude in depth and in profile. Since the magnitude and depth of residual compression and other shot peen process induced workpiece changes are highly related to workpiece fatigue strength and other shot peen process induced workpiece benefits, the Almen measurement system cannot be counted on to quantitatively correlate a shot peen process with the desired level of benefits it is intended to induc
Cohen Donald K.
Simpson Roger S.
Advanced Material Processing
Johns Andrew W.
Nakhjavan Shervin
LandOfFree
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