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Abstract: This paper deals with a Finite Element (FE) behavior analysis of a shape memory alloy actuator for a micropump. It is composed of two membranes of NiTi shape memory alloy (SMA) in a martensitic state at room temperature. They have an initial flat shape and are bonded together with an intermediate spacer. The thermal loading allows the actuator to move up and down in the membrane normal direction. A detailed analysis of sensibility to material and geometric parameters of the SMA actuator is undertaken by FE method. The actuation capability and reliability are studied in order to lead to optimal parameters set providing a higher stroke (deflection) with a low heating temperature. The shape memory effect exhibited by these membranes is simulated by means of the phenomenological constitutive law based on Chemisky–Duval model , and implemented in the Abaqus® FE code. The obtained numerical results were detailed proving the ability of the proposed modeling to reproduce the actuator behavior under thermal loading. This analysis showed that it is possible to provide a large stroke for a minimal geometry of the actuator. [Copyright &y& Elsevier]

Abstract: Due to their thermomechanical properties (high mechanical work/volume ratio), shape memory alloys (SMA) are particularly interesting to be adopted in the design of micro-sensors and micro-actuators. Thus, various constitutive models have been developed and implemented in finite element codes in order to design such applications. If these ‘local’ models are well adapted to describe the behavior of the bulk material, they fail to satisfactorily describe phenomena such as transformation localization or size effects observed in small samples. A gradient constitutive model is presented in order to describe the localization of phase transformation in SMA structures. To achieve this development restricted to superelasticity, a non local variable (martensite volume fraction) is defined at a material point as the weighted average over the entire material domain of the local transformation variable. Using Taylor expansions, the non local definition is substituted by a gradient based equation, introducing a material length parameter which controls the size of the localization zone. The gradient and mechanical equilibrium constitutive equations have been numerically integrated by using the finite element method. Two kinds of finite elements have been developed (1D truss and 2D quadrilateral) and implemented in Abaqus®via a UEL subroutine. Several simulations have been performed which exhibit the localization phenomena of phase transformation in structures undergoing mechanical loading. [ABSTRACT FROM AUTHOR]

In this paper, the contact problem between a rigid indenter and a viscoelastic half space containing either isotropic or anisotropic elastic inhomogeneities is solved. The model presented here is 3D and based on semi-analytical methods. To take into account the viscoelastic properties of the matrix, contact and subsurface problem equations are discretized in the spatial and temporal dimensions. A conjugate gradient method and the fast Fourier transform are used to solve the normal problem, contact pressure, subsurface problem and real contact area simultaneously. The Eshelby's formalism is applied at each step of the temporal discretization to account for the effect of the inhomogeneity on pressure distribution and subsurface stresses. This method can be seen as an enrichment technique where the enrichment fields from heterogeneous solutions are superimposed to the homogeneous viscoelastic problem solution. Note that both problems are fully coupled. The model is validated by comparison with a Finite Element Model. A parametric analysis of the effect of elastic properties and geometrical features of the inhomogeneity is proposed. The model allows to obtain the contact pressure distribution disturbed by the presence of inhomogeneities as well as subsurface and matrix/inhomogeneity interface stresses at every step of the temporal discretization. [ABSTRACT FROM AUTHOR]