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A numerical study of “functional fatigue” of closed-cell NiTi shape memory foams

Shape Memory Foams (SMFs) are an emerging class of materials, providing a wide spectrum of applications due to the shape retention capabilities that they possess. Although recent research has focused on the fabrication of metallic SMFs, the effect of porosity on their superelastic recovery remains a contentious issue. Three 3D FE models, representative of NiTi foam specimens with 50, 60 and 70% porosity, are examined. Due to the high complexity of their closed-cell structure and the corresponding computational effort required to simulate a purely stochastic geometry, a CAD-based approach employing Non-Uniform Rational B-Splines (NURBS) is used to develop the 3D models. The exact closed-cell volume is approximated by combining parametric Voronoi cell volume partitioning with sophisticated CAD algorithms, thus ensuring a fail-free volume discretization. A verified/ validated constitutive material law for NiTi Shape Memory Alloys accounting for TRansformation-Induced Plasticity (TRIP) is assigned to the porous geometries and used to model their cyclic thermomechanical response to uniaxial compression. The results provide refined insight on the mesoscopic response of metallic SMFs to external nominal isothermal cyclic mechanical loading, while yielding interesting results as to the effect of porosity on local phase transformation, irrecoverable strain, residual stresses, and retained martensite.