A constitutive model for high temperature SMAs exhibiting viscoplastic behavior

Abstract

ABSTRACT In this work a 1-D constitutive model for high temperature shape memory alloys (HTSMAs) is presented,where the range of operating temperatures allows the appearance of creep mechanisms during transformation.The model aims to capture the coupled phenomenon, where the rate independent transformation and the ratedependent viscoplastic behavior coexist. Based on continuum thermodynamics, the Gibbs free energy and theevolution equations for forward, reverse transformation and creep are properly chosen. The generation of timeindependent irrecoverable strains during transformation is also taken into account. The calibration and validationof the model in the 1-D case is achieved with the help of experimental tests in Ti 50 Pd 40 Ni 10 , including isobarictests at selected stress levels with 2 dierent temperature rates.Keywords: High temperature shape memory alloys (HTSMAs), Martensitic phase transformation, Thermody-namics, Viscoplasticity 1. INTRODUCTION The need for reliable high force actuators, capable of working under high temperatures, enforces the usage of hightemperature shape memory alloys (HTSMAs). These types of shape memory alloys are usually produced byalloying conventional NiTi with a third element such as gold, platinum, palladium, hafnium or zirconium, whichincrease the transformation temperatures. Nowadays there is the capability of designing HTSMAs with trans-formation temperatures ranging from 100 C to 1100 C. However, the operating temperatures of the HTSMAsexposes them to temperature regimes where creep can aect their actuation behavior and it becomes exceedinglynecessary to understand their interactions.During the last three decades a signi“cant eort has been conducted on developing constitutive models forSMAs.