An Instrumented Sharp Indentation Method for Measuring Equibiaxial Residual Stress without Using Stress-Free Specimens

The presence of residual stresses in materials or engineering structures can significantly influence their mechanical performance. Accurate measurement of residual stresses is of great importance to ensure their in-service reliability. Although numerous instrumented indentation methods have been proposed to evaluate residual stresses, the majority of them require a stress-free reference sample as a comparison benchmark, thereby limiting their applicability in scenarios where obtaining stress-free reference samples is challenging. In this work, through a number of finite element simulations, it was found that the loading exponent of the loading load-depth curve and the recovered depth during unloading are insensitive to residual stresses. The loading curve of the stress-free specimen was virtually reconstructed using such stress-insensitive parameters extracted from the load-depth curves of the stressed state, thus eliminating the requirement for stress-free reference samples. The residual stress was then correlated with the fractional change in loading work between stressed and stress-free loading curves through dimensional analysis and finite element simulations. Based on this correlation, an instrumented sharp indentation method for measuring equibiaxial residual stress without requiring a stress-free specimen was established. Both numerical and experimental verifications were carried out to demonstrate the accuracy and reliability of the newly proposed method. The maximum relative error and absolute error in measured residual stresses are typically within +/- 20% and +/- 20 MPa, respectively.