A THERMODYNAMICAL CONSTITUTE MODEL FOR SHAPE-MEMORY MATERIALS .1. THEMONOLITHIC SHAPE-MEMORY ALLOY

Pseudoelasticity and the shape memory effect (SME) due to martensitic transformation and reorientation of polycrystalline shape memory alloy (SMA) materials are modeled using a free energy function and a dissip ation potential. Three different cases are considered, based on the nu mber of internal state variables in the free energy: (1) austenite plu s a variable number of martensite variants; (2) austenite plus two typ es of martensite; and (3) austenite and one type of martensite. Each m odel accounts for three-dimensional simultaneous transformation and re orientation. The single-martensite model was chosen for detailed study because of its simplicity and its ease of experimental verification. Closed form equations are derived for the damping capacity and the act uator efficiency of converting heat into work. The first law of thermo dynamics is used to demonstrate that significantly more work is requir ed to complete the adiabatic transformation than the isothermal transf ormation. Also, as the hardening due to the austenite/martensite misfi t stresses approaches zero, the transformation approaches the isotherm al, infinite specific heat conditions of a first-order transformation. In a second paper, the single-martensite model is used in a mesomecha nical derivation of the constitutive equations of an active composite with an SMA phase. Copyright (C) 1996 Elsevier Science Ltd