Experimental and numerical study of the superelastic behaviour on NiTi thin-walled tube under biaxial loading

A 3D thermo-mechanical constitutive model for the polycrystalline shape memory alloy (SMA) has been used to predict the superelastic response of the textured thin-walled NiTi tubes under combined tension–torsion loading. This model is based on the work of Lim et al. and Wang. Considering the difference of Young’s modulus between austenite and martensite phase, some formulas have been modified. The new developed model has been implemented into ABAQUS as UMAT. In order to validate the model, thin-walled tube has been tested under biaxial loading. From the comparison between the simulated and experimental results, it is found that the model is quite good in predicting the overall loading–unloading process of the SMA. Both the experimental and simulated results demonstrate the significant differences in the superelastic responses under different biaxial loading paths. For both biaxial loading paths, at relatively low level of tensile strain, the overall behaviour of the tubes is smooth. But with the increase of the tensile strain level, the equivalent stress–strain curves fluctuate. For torsion–tension loading program, corresponding to about 4% strain, stress decrease upon tensile loading and stress increase upon tensile unloading are observed due to the change of shear stresses. For Type II loading program (tension followed by torsion, unload in reverse order), a stress decrease upon loading and a stress increase upon unloading are observed but the corresponding strains become big with the increase of the tensile strain level. In addition, according to the simulated results, the rate of martensite formation under tensile loading is higher than that under shearing loading.