Evolution of the martensitic transformation in shape memory alloys under high strain rates

D Saletti,H Zhao,S Pattofatto

doi:10.1051/epjconf/20100629008

Abstract

The speciﬁc properties of the shape memory alloys are mainly due to the martensitic transformation occuring in the material when mechanical or thermal loadings are applied. Here, the eﬀect of strain rate on the transformation on an NiTi SMA is studied in tension. Diﬀerent tests were performed at diﬀerent strain rates in the range of 0,0001 /s to 15 /s. Two distinct methods were used to measure the extension rate of the martensitic phase region in the specimen: digital image correlation technique and infrared thermography (IR during quasi-static tensile tests only). For the dynamic tensile tests, a Split Hopkinson Tensile Bar set-up was used with a fast camera recording at 45’000 fps. A superimposition of DIC and IR measurements in time and space can be done during quasi-static tests and results show that the temperature peak, as expected, follows the transformation front. As a consequence of the former validation of the DIC procedure, the velocity of the transformation front at high strain rate is deduced from space-time ﬁgures. As a conclusion, in the range of strain rates investigated in this paper, no strain rate sensitivity is observed for dynamics of extension of the transformation region.

Highlights

Due to two of their specific properties, namely superelasticity and the shape memory effect, shape memory alloys (SMA) have a great potential of applications in a lot of innovative technologies
Superelastic properties of the SMA are shown on a stress-strain curve, which is decomposed in three parts: first the elastic response of the material in a full austenite phase, generally a plateau where martensitic transformation is occuring and propagating, which is visible through imaging techniques ([1]), and the homogeneous elastic response of the material in a full-martensitic phase
Assumptions have been made to explain the change in high strain rate response ([3]) and refer to the adiabatic regime at the transformation front, preventing the austenite phase from tranforming unless the level of stress is raised

Summary

Introduction

Due to two of their specific properties, namely superelasticity and the shape memory effect, shape memory alloys (SMA) have a great potential of applications in a lot of innovative technologies. Assumptions have been made to explain the change in high strain rate response ([3]) and refer to the adiabatic regime at the transformation front, preventing the austenite phase from tranforming unless the level of stress is raised. Methods were used to measure the velocity of the propagation of the martensitic phase in the specimen: digital image correlation (DIC) techniques and infrared thermography (IR, during the quasi-static tensile tests only)

Experimental techniques

Specimen characteristics

Measurement of dynamic nominal stress-strain curve

Measurement of the effect of prescribed velocity on the SIM growth rate

Results and conclusion