Measurement of laser peening residual stresses

Laser peening is an emerging surface treatment for metals that imparts compressive residual stress near the surface by introducing cold work. The present paper describes two residual stress measurement techniques that are well suited for the measurement of residual stresses induced by laser peening. Since residual stress is a key outcome of the laser peening process, its measurement is an important process diagnostic. The two residual stress measurement techniques described are the slitting method and the contour method. In the present research, the slitting method is used to establish relationships between various laser peening parameters ( irradiance, pulse duration, number of layers, etc.) and the residual stress profile ( residual stress versus depth) in flat plate coupons that have been uniformly peened on one surface. This helps to identify an effective set of laser peening parameters for the given material. Next, laser peening is applied to a specimen(s) with representative geometry and the contour method is used to measure the spatial distribution of residual stress around critical geometric features. The information gathered from the contour method measurement( s) is used to identify an effective laser peening pattern ( coverage area) for the given geometry. This two step procedure is illustrated using recent measurements on sample Alloy 22 (UNS N06022) welds for the Yucca Mountain Project. Details of both measurement procedures are given including representative results for each.