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The thermal induced martensitic phase transition in TiNiHf was exploited for bi-directional actuation with TiNiHf/SiO2/Si composites. When compared to free-standing films of similar thickness, films on a substrate exhibit a reduced fatigue effect upon thermal cycling and a smaller hysteresis width. Differential scanning calorimetry (DSC) and cantilever deflection measurements (CDM) results showed that the transition temperatures of fabricated TiNiHf films and TiNiHf/SiO2/Si bimorph composites decrease with thermal cycling. The change in transition temperatures after 40 thermal cycles is significantly reduced for TiNiHf films bound to a SiO2/Si substrate compared to the functional fatigue DSC results reported for freestanding films. The thermal hysteresis width is also reduced for TiNiHf films constrained by SiO2/Si and Si substrates compared to freestanding films of similar thicknesses. With proper composition selection and microstructural control, TiNiHf films can be promising SMA films for bistable actuators with PMMA/TiNiHf/Si composites.

In finite deformation inelasticity, one often has to deal with the incompressibility constraint. In the past, this was dealt with using, for example, an exponential mapping approach, which yields exact volume preservation in plastic deformations. In this article however, the special-linear update approach by Hurtado et al. The special-linear update: An application of differential manifold theory to the update of isochoric plasticity flow rules, Int J Numer Methods Eng. 2014;97(4):298-312, which utilizes a projection method to fulfill the incompressibility constraint is used. The model is applied to isotropic plasticity by a novel approximation of the logarithm and treats kinematic hardening without losing the symmetry of the internal variables. The model results are compared to models utilizing an exponential mapping approach in numerical experiments.