Is the tensile test relevant to characterize NiTi Shape Memory alloy behaviour?

Tensile tests are widely used to characterize NiTi shape memory alloy superelastic behaviour. They are used by manufacturers in industrial production for quality control and by researchers to characterise the superelasticity and fatigue properties. Most models implemented in finite element software to aid design of NiTi applications and to predict their performances in use are also based on tensile test results. Analyses of tensile tests are often under the assumption of homogenous deformation. However, it is well known that NiTi may exhibit strong localisation during deformation via martensitic transformation.

This study carried out a comparative investigation of tensile and shear tests performed on Ti-50.8at% Ni plates using both thermal and kinematical full-field measurements. The kinematical measurement allows full-field detection of strain heterogeneity. Sample temperature changes due to the latent heat effect of the phase transformation were monitored during deformation. This measurement allows full-field detection of transformation heterogeneity.

It was found that in both tests the strain and temperature fields on the samples exhibited some level of heterogeneities. These heterogeneities are due to different origins. In tension the alloy exhibited some strongly localised bands, which were detected in both temperature and strain fields measurements. Homogeneous phase transformation precedes localised deformation. High strains observed in the localised bands are associated with high localised stress-induced phase transformation. In shear the heterogeneous strain fields are found to be more related to thermal and mechanical boundary conditions together with thermomechanical coupling. In this regard, the heterogeneities are reduced in shear when reducing the thermal effect by decreasing the deformation rate. Similar effect should be obtained by performing the test in a liquid bath.

To conclude, shear test performed at low deformation rate appears in a liquid bath to be more suitable for characterising mechanical behaviour and developing constitutive equations for superelastic NiTi.