Stress Measurement Repeatability in ESPI Hole-Drilling

Abstract

Repeatability measurements are useful for measuring residual stress by hole-drilling since single holes don’t provide a lot of information about measurement quality. Besides measurement procedure and statistics, they also allow assessing microstructural variability. This study uses Electronic Speckle Pattern Interferometry (ESPI) for making multiple stress measurements in samples made from aluminum and polycarbonate. Under bending, the aluminum samples show large variability in the stress depth profiles while the general trends fit the expected linear stress curves. The experiments suggest that the variability is primarily due to the microstructure rather than measurement errors. Deviations from the linear bending curves originate in residual stresses in the undeformed bar, which can change from location to location. In the polycarbonate sample, the measured stresses are reasonably close to those calculated when the holes were small. Larger holes produced stress depth profiles with significant shifts towards higher stresses. This effect is attributed to frictional heating during drilling and therefore, inappropriate drilling parameters.