MATERIALS SCiEWCE & ENGINEERING ELSEVIER Materials Science and Engineering A222 (1997) 45-57 Shear and tensile thermomechanical behavior of near equiatomic NiTi alloy Pierre-Yves Manach”, Denis Favierb “Laboratoire G&e Mtkztziqtte et MatCriaux, LkioersitP de Bretagtte Sud, Cetrtre de GEnie Itzdtcsriel, 56520 Guidei: Frame ‘Laboratoire Sols-Solides-Slrltctltres, titlir$ Mine de Recherche CA:RS 5521, Ittstitur Xationnl Polvtechnique de Grenoble, Linicersirfi Joseph Fourier, BP53X, 38031 Grenoble, Ftmce Received 20 Map 1996 Abstract The industrial development of devices using the intriguing properties of shape memow alloys involves accurate prediction of their thermomechanical behavior. This may be achieved using Computer Aided Design together with Finite Element programs. Reliabie constiiutive laws are needed for the execution of such programs. Several tensorial constitutive laws have been proposed to model the unusual thermomechanical properties of shape memory alloys. However, for all these tensorial models. it is necessary to make assumptions which cannot be verified when only tensile property data are available. The purpose of this paper is to present a new set of experimental mechanical data, including tensile and simple shear tests performed on sheet samples of near equiatomic NiTi allcy. Both mechanica! behaviors are compared for 2 !arge iemperature range {from below .bIf to above ,4f). It is shown that the von Mises assumptions usually made in the establishment of tensorial constitutive equations are not always valid. Other yield locus forms are proposed to model the typical tension and simple shear thermomechanical behavior of shape memory alloys. Ke~~words: Shape memory alloys; Equiatomic nickel titanide; Thermomechanical behavior 1. Introduction The development of new materials that can be ap- plied in contemporary engineering structures deter- mines progress in modern technology. One set of such promising materials are shape memory alloys @MA). Though experimental evidence has been gathered for material like NiTi SMA for forty years [I], theoretical models intended to describe their mechanical behavior appeared only at the beginning of the 1980s. Implemen- tation of these models into finite element code started only two or three years ago [2,3]. Several authors modeled the thermomechanical be- havior of SMAs, using mostly one-dimensional models [d-7]; such models are only able to describe the behav- ior of simple bodies (e.g. wires) for one type of stress state (e.g. tension). Thus they do not predict behavior of bodies having more complicated shapes or undergo- ing complex stress states. The few theoretical three-di- mecsionz! models which have been proposed [8-lo] consider isotropic materials and generally assume that 0921-5093!96,.515.00 0 1996 - Elsevier Science S.A. All rights reserved PII SO92i-5093(96)105iO-4 the shear behavior is independent of the pressure, thus neglecting the volumetric deviatoric coupling effects. Furthermore, most of the models which use the notion of transformation strain [ 1l] are based on the defini- tions of equivalent strain and stress leading to a behav- ior conforming to a von Mises yield criterion. Experimental studies concerning the mechanical behav- ior of SMA focus essentially on the behavior in tension and are not sufficient to assess the validity of such constitutive laws and their related assumptions. Thus, in order to establish reliable constitutive lairs for their integration in finite element programs [3], thermome- chanical tests under several stress states are ‘needed. In that respect, experimental thermomechanical ten- sile and shear tests have been performed on a near equiatomic KiTi alloy,, which is the most widely used SMA. These two mechanical tests have been chosen for several reasons. First, tensile and shear tests can be performed on the same sample form,. such as sheet. Second: the sheet shape is particularly suitable for investigating the infuence of different thermomechani-