Luc Saint-Sulpice

Direct numerical determination of stabilized dissipated energy of shape memory alloys under cyclic tensile loadings

When shape memory alloys are subjected to cyclic loadings, the stabilized dissipated energy is an effective parameter in studying their performance, for instance, the fatigue life. However, thermomechanical coupling in the behavior of shape memory alloys under cyclic loadings causes the amount of stabilized dissipated energy to be obtainable once the responses of all transient cycles are determined. In this article, direct formulae are proposed to numerically evaluate stabilized dissipated energy of a shape memory alloy under cyclic tensile loadings as a function of maximum and minimum applied stresses as well as the loading frequency. A one-dimensional fully coupled thermomechanical constitutive model with a cycle-dependent phase diagram is utilized to be able to directly predict the uniaxial stress–strain response of a shape memory alloy in a specified cycle with no need of solving the previous cycles. The results are experimentally assessed for NiTi and CuAlBe specimens. Since the backward transformation in CuAlBe is realized to more gradually occur than that in NiTi, an enhanced phase diagram is proposed in which different slopes are considered for the start and finish of backward transformation strip. The numerical predictions of the present approach are shown to be in a good agreement with the experimental findings for cyclic tensile loadings.

Rotary bending fatigue analysis of shape memory alloys

In this work, a one-dimensional constitutive model is used to study rotary bending fatigue in shape memory alloy beams. The stress and strain distributions in a beam section are driven numerically for both pure bending and rotary bending to show the basic differences between these two loading types. In order to verify the numerical results, experiments are performed on NiTi specimens with an imposed bending angle using a bending apparatus. Since the specimens show significant stress plateau for forward and backward transformation in their stress–strain response, an enhanced stress–temperature phase diagram is proposed in which different slopes are considered for the start and finish of each transformation strip. In order to study low cycle fatigue of shape memory alloys during rotary bending, the stabilized dissipated energy is calculated from numerical solution. A power law for variations of the fatigue life with the stabilized dissipated energy is obtained for the studied specimens to predict their fatigue life. The numerical predictions of the present approach are shown to be in a good agreement with the experimental findings for rotary bending fatigue. Uniaxial tensile fatigue tests are further performed on the studied specimens to investigate effect of loading type on the fatigue lifetime.

A cyclic model for superelastic shape memory alloys

This study concerns the superelasticity of Shape Memory Alloys (SMA) under cyclic loading. A particular attention is paid to the evolution of residual strain with number of cycles (like ratcheting in cyclic plasticity of classical metals). To study the phenomenology of the cyclic behavior and to identify the origin of the developed residual strain a series of cyclic uniaxial tensile tests on copper based alloys wires has been realized. A macroscopic model describing the cyclic behavior of superelastic SMA has been proposed. The originalities of the model are, on the one hand, the definition of a particular elasticity domain when the material is in a two phased state and, on the other hand, an ad hoc kinetic of transformation strain taking into account a residual strain evolution. The proposed model has been identified using our experimental data base and has been used to simulate various cyclic multiaxial loadings.