Increased Robustness of Residual Stress Measurements Using Two-Axis ESPI Hole-Drilling

Background: Single-axis hole-drilling ESPI measurements give direct indications of the residual stresses in the sensitivity direction, but can identify the transverse residual stresses only through Poisson’s ratio. This gives a significant directional bias to the measurements. Objective: It is desired to develop a method to make hole-drilling ESPI residual stress measurements without directional bias and also to enhance the measurement robustness. Methods: A two-axis hole-drilling ESPI residual stress measurement system was proposed and was made by adding a second measurement axis to a commercial single-axis ESPI residual stress measurement device. Hole-drilling measurements were made in a calibration standard specimen and the results compared with expected values. Results: Good correspondence with the expected residual stresses was achieved. The expected directional bias in stress evaluation quality was observed when each axis was considered individually, with the stress evaluation noise in the direction perpendicular to the sensitivity direction ~ 1.5 to 2.0 times greater than in the parallel direction. The two-axis approach was successful in achieving low noise in both axial stresses. The stress evaluations also showed a good ability to identify modest sized residual stresses, even within much larger surface stresses. Conclusions: The use of two-axis measurements successfully achieved good stress sensitivity in both axes. The redundancy of the data from the two measurement axes increases measurement robustness and measurement security. Measurement time is not significantly increased over the single-axis technique.