Effects of matrix inelasticity on the overall hysteretic behavior of TiNi-SMA fiber composites

The effects of the inelastic deformation of the matrix on the overall hyste retic behavior of a unidirectional titanium-nickel shape-memory alloy (TiNi -SMA) fiber composite and on the local pseudoelastic response of the embedd ed SMA fibers are studied under the isothermal loading and unloading condit ion. The multiaxial phase transformation of the SMA fibers is predicted usi ng the phenomenological constitutive equations which can describe the two-s tep deformation due to the rhombohedral and martensitic transformations, an d the inelastic behavior of the matrix material using the standard nonlinea r viscoplastic model. The average behavior of the SMA composite is evaluate d with the micromechanical method of cells. It is observed that the inelast ic deformation of the matrix due to prior tension results in a compressive stress in the matrix after unloading of the SMA composite and this residual stress impedes the complete recovery of the pseudoelastic strain of the SM A fibers. This explains that a closed hysteresis behavior of the SMA compos ite is no longer observed in contrast with the case that an elastic behavio r of matrix is assumed. The predicted local stress-strain behavior indicate s that the cyclic response of matrix is crucial to the design of the hyster etic performance of the SMA composite under the repeated loading conditions . (C) 2000 Elsevier Science Ltd. All rights reserved.