This is a review of the current state‐of‐the‐art regarding a particular approach to extraction of the (quasistatic) stress–strain relationship of a metallic sample from an indentation experiment. It is based on the application of a relatively high load (kN range) to the sample via a large spherical indenter (≈1 mm radius), followed by measurement of the indent profile. This profile is then used as the target outcome for inverse finite element method (FEM) modeling of the test, aimed at converging on the best fit set of parameter values in a constitutive plasticity law (true stress–true strain relationship). This can then be used to simulate any specified loading configuration, including a conventional tensile test. Commercial products are now available in which the indentation, profilometry, and convergence operations are all automated and completed within a few minutes. The review covers the various conceptual and practical issues involved in implementation and optimization of these procedures, including both those related to the measurement system (experimental and FEM simulation) and those associated with the sample (such as anisotropy, inhomogeneities, and residual stresses). An attempt is made to convey an impression of the expected levels of reliability and also the scope for obtaining insights that are not readily obtainable using other types of test.
A comparison is presented between conventional tensile stress‐strain curves and those obtained via two methodologies based on (spherical) indentation. The first, termed Instrumented Indentation Technique (IIT), involves conversion of load‐displacement data to stress‐strain curves via analytical expressions. This has been done using loads below 1 N (“nano”) and in the kN range (“macro”). The other procedure, termed profilometry‐based indentation plastometry (PIP), is based on repeated finite element method (FEM) simulation, using the residual indent profile as the target outcome and obtaining the best fit set of parameter values in a constitutive stress‐strain law. This has been done on a macro scale only. The data from nano‐IIT tend to be very noisy and variable, whereas those from macro‐IIT are more reproducible and less noisy. With one of the two empirical formulations employed, the agreement of the macro‐IIT with experiment is close to being acceptable for the work hardening characteristics, although inferred values of the yield stress are in poor agreement with those from tensile testing. In contrast to this, the PIP procedure provides outcomes that are in close agreement with those from tensile testing, concerning both yield stress and work hardening. The causes of this are explored and discussed.
This paper concerns, the effect of (unknown) residual stresses in the near‐surface region of a sample on outcomes of an indentation plastometry technique for obtaining stress–strain relationships, using relatively large spherical indenters. The technique is based on the iterative finite element method (FEM) simulation of indentation, so as to infer the true stress–strain curve of the material (with the target outcome being the profile of the residual indent). It is expected that residual stress levels (if significant compared with the yield stress) are likely to influence the outcome, and indeed this forms the basis of many attempts to measure residual stresses in this way (using a known stress–strain curve). However, there are important issues of sensitivity here, which affect the reliability of such procedures and are also relevant to the accuracy of stress–strain curves inferred from measurements made on samples containing unknown levels of residual stress. Herein, both experimental work on samples with (equal‐biaxial) residual stresses created by the application of external loads and extensive FEM modeling to explore various scenarios are covered. The main conclusion is that the sensitivities involved are in general very low, particularly with relatively deep indentation. Inferred stress–strain curves are thus likely to be quite accurate, even in the presence of relatively high levels of residual stress. Conversely, the measurement of residual stress levels via this procedure is likely to be rather inaccurate, although the reliability is improved using shallow penetration (low ratio of depth‐to‐indenter radius). It is also noted that tensile residual stresses tend to influence outcomes more strongly than compressive ones.
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