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Effect of pressure sensitivity on spherical indentation response of shape memory alloys

Instrumented indentation has proved to be an invaluable tool to determine the small volume mechanical behaviour of shape memory alloys (SMAs). Multi-axial experiments show that SMAs can exhibit pressure sensitive martensite phase transformation, which manifests in terms of tension-compression asymmetry of the associated stress and strain. Since high magnitude of pressure can prevail during indentation, the spherical indentation response of SMAs is investigated in this work by using a constitutive model that captures the above behaviour. Finite element (FE) simulations are first performed at two temperatures (close to A s and above A f ) and corresponding to different values of pressure sensitivity parameter γ 1 for a Ni-Ti SMA. It is found that the indentation load and mean contact pressure enhance at a given depth, while the residual depth at complete unloading (for temperature below A f ) reduces with increase in γ 1. Further, the transformation zone size at a fixed load drops dramatically with increase in pressure sensitivity index. An expanding cavity model (ECM) is also developed to predict the mean contact pressure for a pressure sensitive SMA and validated against FE simulations as well as available experimental data.