Towards Inferring Superelasticity Parameters from Instrumented Indentation Data

Using indentation to obtain material property parameters is highly attractive.  Specimens can be small, require minimal preparation and be obtainable (as a sliver) from components in service.  Interrogation of small volumes also allows mapping of properties over a surface.  On the other hand, the tested volume must be large enough for its response to be representative of “bulk” (continuum) behaviour.  While much recent indentation research has focused on the very fine scale, it is arguably on this “meso” scale (such that indents are large enough for representative material response, but small enough to allow small samples and mapping) that the main potential for increased industrial usage of indentation lies.  Considerable progress has already been made on obtaining plasticity parameters in this way, based on evaluating a “goodness of fit” parameter (g), converging on best-fit property parameter combinations and establishing the reliability and uniqueness of the solutions obtained.  While a similar approach can in principle be used to obtain superelasticity parameters, there is currently little information available about sensitivities, uniqueness characteristics or optimal methodologies for these parameters.  In this presentation, work will be described on uniaxial stress-strain testing of a NiTi shape memory alloy, obtaining best-fit values of the parameters in an FEM model describing this behaviour over a suitable strain range (up to about 10%) and then inverse procedures for inferring these parameters by searching for combinations that maximise the value of g associated with experimental load-displacement plots during indentation.  As with plasticity parameters, it is found that use of more than one indenter shape assists in converging on the best fit solution and optimal “g-scanning” procedures are identified.  Information is also presented on sensitivities and the probable reliability of superelasticity parameters obtained in this way for an unknown material.