A new methodology for measuring residual stress using a modified Berkovich nano-indenter

In this paper, a new methodology, based on nanoindentation, is proposed to estimate the magnitude and orientation of the principal residual stress components, in a non-equi-biaxial residual stress field at the nanoscale. The proposed method exploits the force-penetration response provided by a modified Berkovich tip that, compared to the most traditional one, is characterized by a preferential elongated size. This feature allows in estimating non-equibiaxial residual stress field since the force-penetration response changes according to the indenter orientation. No observation of the residual imprint is required. Principal stress components and orientations are estimated thanks to theoretical and finite element analysis, establishing a linear relation between stress components and load variations respect to the stress-free sample. Finite element analysis is used to calibrate the model coefficients and to verify the accuracy of this approach for isotropic metals. Obtained results revealed that the method is well suitable in estimating both tensile and compressive residual stresses with an average error lower than 6% for the first case and lower than 10% for the second one.