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

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.

Effects of indenter geometry on indentation-induced densification of soda-lime glass

Hardness of glass is known to be related to the resistance to permanent deformation. However, the mechanism of permanent deformation of glass under a sharp diamond indenter is not clear yet. One of the deformation modes of oxide glass at room temperature is permanent densification. In this study, the indentation-induced densification of soda-lime glass under diamond indenters was evaluated from the volume recovery of indentation imprint by thermal annealing. The volume change of the indentation imprint by annealing corresponds to the densified volume under the indenter. Using some kinds of diamond indenters, which have different inclined face angles, the ratios of densified volume to the total “lost” volume under the indenters were determined. With an increase in the inclined face angle, the densification contribution decreased and the shear-flow contribution increased. This indenter-shape dependence of densification in glass is discussed in terms of the stress dependence of the deformation mechanisms in glass.

The Relation of the Vickers Hardness With the Loading Stiffness of Instrumented Vickers Indentation of Metal Substrates

The present paper presents a relation between the vickers hardness HV and the loading stiffness C of instrumented vickers indentation of metal substrates. The relation is based on the fact that the nonaxisymmetry of the plastic deformation of the vickers indenter leaves the corners of the indentation imprint at the surface of metal substrates after complete unloading. This relation can transform available HV data for metals to C data. It is also shown that the strain hardening details are important in the estimation of material properties and investigators should be cautions when using power-law strain hardening in all cases.

Effect of temperature on metastable phases induced in silicon during nanoindentation

Indentations were performed on silicon using a Berkovich indenter at loads up to 12 mN, at temperatures from 20 to 135 °C. Transmission electron microscopy revealed crystalline silicon phases in the residual indentation imprint at and above 35 °C. Also, the first reconfirmation of the occurrence of Si-VIII during unloading was observed at temperatures of 100 and 125 °C. Interestingly, at 125 °C a cavity was also observed, and an unidentifiable phase was observed at 135 °C. The observations show the strong effect of temperature on pressure-induced phase transformation in silicon.