Evaluation of Non-Equibiaxial Residual Stresses in Metallic Materials via Instrumented Spherical Indentation

Abstract

Residual stresses, existed in engineering structures, could significantly influence the mechanical properties of structures. Accurate and non-destructive evaluation of the non-equibiaxial residual stresses in these structures is of great value for predicting their mechanical performance. In this work, investigating the mechanical behaviors of instrumented spherical indentation on stressed samples revealed that non-equibiaxial residual stresses could shift the load-depth curve upwards or downwards and cause the residual indentation imprint to be an elliptical one. Through theoretical, experimental, and finite element (FE) analyses, two characteristic indentation parameters, i.e., the relative change in loading curvature and the asymmetry factor of the residual indentation imprint, were found to have optimal sensitivity to residual stresses at a depth of 0.01R (R is the radius of spherical indenter). With the aid of dimensional analysis and FE simulations, non-equibiaxial residual stresses were quantitatively correlated with these two characteristic indentation parameters. The spherical indentation method was then proposed to evaluate non-equibiaxial residual stress based on these two correlations. Applications were illustrated on metallic samples (AA 7075-T6 and AA 2014-T6) with various introduced stresses. Both the numerical and experimental verifications demonstrated that the proposed method could evaluate non-equibiaxial surface residual stresses with reasonable accuracy.

The Effect of Residual Stresses on Stress–Strain Curves Obtained via Profilometry‐Based Inverse Finite Element Method Indentation Plastometry

“…This requires care in the design of the sample and gripping system. Figure shows a schematic depiction of the loading frame, which is similar to that by Peng et al [ 35 ] The thickness of the frame was 40 mm. The loads were applied by tightening of the nuts (creating either compression or tension in the sample).…”

“…The most relevant of these for current purposes are those involving relatively large spherical indenters. [ 28,30,32–35 ] These include the work of Peng et al, [ 32,35 ] who showed that changes in experimental load–displacement plots induced by imposing “residual” stresses via externally applied loads were consistent with FEM predictions. Of course, load–displacement plots cannot be used to infer anisotropic residual stress states, as they provide no directional information.…”

“…Similar configurations have been used in previous investigations. [ 31,35 ] In the present work, samples were made by machining a 3 mm‐thick sheet of OFHC copper (grade C106), supplied by Doré metals. The dimensions of the design are shown in Figure .…”

“…The ill‐defined nature of hardness means that such approaches do not really have any potential in terms of experimental measurement of residual stresses. However, a number of others [ 23–35 ] have focused on monitoring how indenter penetration is affected by residual stresses. These investigations have all concerned metals, although similar procedures have been applied to polymers.…”

“…These investigations have all concerned metals, although similar procedures have been applied to polymers. [ 36 ] A number of these studies have been purely theoretical, but some have involved experimental indentation of samples in which controlled “artificial” residual stresses have been created by the external application of (equal‐ or unequal‐biaxial) forces [ 31,32,34,35,37 ] or via differential thermal contraction between a substrate and a surface layer. [ 26 ]…”