Extraction of Inelastic Mecahnical Properties of Metals from Indentation Data
This work presents a detailed investigation into a methodology involving iterative finite element method (FEM) modelling of the indentation procedure, aimed at obtaining well defined mechanical properties of metals solely from experimental indentation data. Relatively large (1 2 mm radius) spherical indenters have been used which offer a number of advantages. The material properties must conform to an analytical constitutive law and part of the challenge of this type of work is to converge efficiently on the best fit set of values for the variables in this law. Comprehensive comparisons are made between outcomes of these procedures and properties obtained via conventional uniaxial testing. It is demonstrated that it is possible to extract the stress strain behaviour of metals that compare well with the macroscopically measured properties using an instrumented indentation load displacement plot, as long as the penetration ratio, δ/R, is approximately 40 %. The required penetration ratio to converge on the residual indent profile is much smaller which has significant benefits. Both of these approaches are applied to a coating system consisting of a sprayed superalloy overlayer (~2.5 mm thick) on a single crystal superalloy substrate (2.5 mm thick), in as received and annealed conditions. Time dependent creep properties were also investigated using recess instrumented indentation. This technique uses a pre machined spherical cap recess with the same radius as the indenter to ensure that the stresses during indentation do not exceed the yield stress. Indentation inferred properties compared well with those obtained from macroscopic (compression) uniaxial tests.
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