Thermoelectric Nondestructive Evaluation of Residual Stress in Shot-Peened Metals

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This paper presents a noncontacting thermoelectric method that can be used to characterize the prevailing residual stress in shot-peened specimens. This novel method is based on magnetic detection of local thermoelectric currents in the compressed near-surface layer of metals when a temperature gradient is established throughout the specimen. Besides the primary residual stress effect, the thermoelectric method is also sensitive to the secondary “material” effects of shot peening (local texture, increased dislocation density, hardening), but it is entirely insensitive to its “geometrical” by-product, i.e., the rough surface topography. Our experimental results in copper indicate that the developed method is more sensitive to residual stress effects than to the secondary material effects, but unequivocal separation of residual stress relaxation from the parallel decay of secondary cold-work effects is generally not feasible. However, since the ratio of residual stress to cold work is primarily determined by the material and the specific surface treatment used, the thermoelectric method still offers the unique capability of nondestructively monitoring thermomechanical relaxation below the treated surface. Preliminary results on IN100 nickel-base superalloy are also presented to demonstrate that the proposed method might be applicable to a wide range of alloys including high-strength, high-temperature engine materials.