Bulge tests on ferroelastic and superelastic NiTi sheets with full field thermal and 3D-kinematical measurements: experiments and modelling

Superelastic and ferroelastic behaviour of polycrystalline Nickel-Titanium Shape Memory Alloys have been extensively studied, mostly via uniaxial tension and compression, simple shear and more recently in combination of tension (compression)-torsion tests. This paper reports on a study of tensile tests and thin plate bulging tests of NiTi. The bulging tests were performed on a circular diaphragm in both superelastic and ferroelastic states at room temperature. Thermal and 3D kinematical full-field measurements were simultaneously conducted owing to the use of one Infrared camera and two visible cameras. The bulging loading creates plane tensile deformation state along the rim of the circular bulge specimen and equi-biaxial deformation state at the centre. In this region, measurements of the curvature radius and of the bulging pressure allow to determine the equi-biaxial stress values. Occurrences of deformation localisation during tensile and bulge tests are studied as function of the deformation mechanisms and of the type of loading. Experimental results are compared with simulated results obtained using “Herezh” Finite Element software in which an “elastohysteresis” constitutive equation is implemented. The parameters of the law are determined from tensile tests on the two sheets and analysis of the equibiaxial deformation behaviour in the region close to the centre of the bulge specimens