Enhanced Measurement of Residual Stress by Speckle Correlation Interferometry and Local Heat Treating for Low Stress Levels

The measurement of residual stresses by laser annealing and electronic speckle pattern interferometry has been successful for moderate to high stress levels. However, for lower stress levels, say less than 80% of yield, submicrometer deformations are encountered that result in subfringe interferograms. Subfringe measurements are not a serious problem with modern optical procedures such as phase shifting interferometry. However, such subfringes are comparable to noise levels resulting from air turbulence, environmental thermal variations, and so forth. Stress relief from local heating is also subject to theoretical lower stress limits that result from the competing effect of thermal expansion in the case of tensile stresses. Thus, for both practical and fundamental reasons the technique was modified such that several fringes will result at lower stress levels. These experimental techniques are shown to substantially expand the stress measurement range for this technique.