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.
Highlights
Residual stresses, which could significantly influence the mechanical properties, such as fracture, fatigue, wear, and corrosion, commonly exist in engineering structures, especially in coatings and films due to the mechanical or thermal mismatch between coatings/films and substrates
The first stage is focusing on the measurement of equibiaxial residual stresses, which is relatively straightforward because it is not necessary to determine the directionality of principal residual stresses [14,15,16,17,18,19,20,21,22,23,24,25,26,27]
Through numerical and experimental investigation of the spherical indentation behavior on elastic-power-law strain-hardening solids with non-equibiaxial residual stresses, it was revealed that the load-depth curve and the top-view morphology of the elliptical residual indentation imprint had good sensitivity to residual stresses at an optimal indentation depth of 0.01R (R is the radius of spherical indenter)
Summary
Residual stresses, which could significantly influence the mechanical properties, such as fracture, fatigue, wear, and corrosion, commonly exist in engineering structures, especially in coatings and films due to the mechanical or thermal mismatch between coatings/films and substrates. Ahn et al [33] developed an indentation method to determine the directionality of residual stresses through two orthogonal wedge indentations An inconvenience of such methods is that it needs to conduct multiple indentation tests to evaluate non-equibiaxial residual stresses. With the aid of dimensional analysis and finite element (FE) analysis, two sensitive parameters, i.e., the loading curvature of the load-depth curve and the asymmetry factor of the elliptical residual indentation imprint, were correlated with the non-equibiaxial residual stresses. Based on these correlations, a spherical indentation method was established for the evaluation of non-equibiaxial residual stress. It should be noted that to determine the plastic parameters via spherical indentation, the indentation tests on the unstressed sample must be deep enough (in the fully plastic state) to achieve good sensitivity [39,40,41,42]
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