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Both hardness testing and Profilometry-based Indentation Plastometry (PIP) can be used to obtain features of (tensile) stress-strain curves. The two tests are superficially similar, involving penetration (under a known load) of an indenter into the flat surface of a sample, followed by measurement of dimensional characteristics of the residual indent. The associated data handling procedures, however, are very different in the two types of test. Hardness numbers, which are commonly based on measurement of the lateral extent or depth of the indent, essentially give a semi-quantitative indication of the resistance to plastic deformation: going beyond this to infer features of the (nominal) stress-strain curve – notably the yield stress (YS) and Ultimate Tensile Stress (UTS) – can only be done via empirical correlations (often restricted to certain types of alloy). PIP testing, on the other hand, involves measurement of the complete indent profile, followed by (automated) iterative FEM modelling of the indentation, allowing the complete (true) stress-strain curve to be obtained. This paper covers application of both approaches to 12 different alloys, with inferred stress-strain characteristics being compared with those from tensile testing. Insights are provided relating to the very different levels of detail and reliability offered by the two procedures.

An important capability of PIP testing is that it allows detailed study of anisotropy (detectable via a lack of radial symmetry in the indent profile) and inhomogeneity (on a scale of a few mm or above). The technique was used in this study to clarify that a particular (additively manufactured) material was isotropic everywhere, but that its stress-strain curve changed significantly with increased distance from the original growth plate. PIP has unrivalled potential for study of AM products.

This work concerns use of the PIP procedure to obtain stress-strain relationships for a series of incremental near-surface layers in a steel that had been hardened by diffusional penetration of carbon (to a depth of about 1 mm). This was done via serial sectioning, with several layers about 200 µm in thickness being tested. Residual stresses were also measured and taken into account. No other testing technique allows such characterization to be carried out.