13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Experimental observation of phase transformation front of SMA under impact loading He Huang1, Dominique Saletti2, Stephane Pattofatto1, Feifei Shi1,4, Han Zhao1,* 1 LMT-Cachan, ENS Cachan/CNRS 8535/UPMC, 61 Avenue du president Wilson, 94235 Cachan cedex, France 2 Laboratoire de Biomécanique, Arts et Métiers ParisTech, 151, boulevard de l’Hôpital,75013 Paris, France 3 School of Aeronautics, Northwestern Polytechnical university, 710072 Xi’an, China * Corresponding author: zhao@lmt.ens-cachan.fr Abstract Pseudoelasticity is one of the main characteristics of shape memory alloys (SMAs), allowing them to recover their initial state after undergoing large deformation. This is due to the martensitictransformation (MT) occurring in the material, turning the austenitic phase into a stress-induced martensitic phase when a mechanical load is experienced. Even if such an effect was largely studied in the past decades under quasi-static loading, the dependence of this effect of the loading rates was rather poorly documented. Only some studies have reported the strain rate dependence of the macroscopic behaviour of SMAs, but no detailed observation of the transformation process under impact loading is available. This paper investigates the influence of the loading rate applied to a NiTi SMA at the level of the MT, providing experimental data of not only the macroscopic stress-strain curves, but also the corresponding observation of the heterogeneous strain field during the test. Main testing results are obtained under tensile loading. Experiments were conducted at three different levels of prescribed velocity : 0.01 mm/s, 1 mm/s and 5000 mm/s, using a classical loading machine for the quasi-static cases and a Split Hopkinson Tensile Bar (SHTB) for the dynamic cases. The observations of the heterogeneous strain field during the tests were made using a digital image correlation (DIC). Additional results under double shear test at various loading rate will be also presented. Keywords Impact testing, Phase transition; NiTi shape memory alloy; Domain patterns and domain spacing; Strain rate. 1. Introduction Shape memory alloys (SMA) have a great potential of applications in a lot of innovative technologies owing to two specific properties: the shape memory effect and pseudoelasticity [1]. In particular, NiTi-based SMA are widely used as biomaterials (to manufacture heart artery stents for instance) or in mechanical applications such as actuators, smart materials in the automotive industry, household appliances, and so on. The ability of NiTi alloys to undergo large strains is due to the transformation, under mechanical loading, of initial austenite to stress-induced martensite (SIM). This specific stress-induced martensitic transformation is associated to a mechanism of propagation of bands into the specimen as in localization phenomena (Luders bands, Portevin-Le Chatelier effect [2], compaction of foams ([3-4]), which leads to a non-homogeneous strain state ([5-7]). Technically, the observation of the bands and the influence of strain rate on the propagation of the martensitic transformation has been initiated fifteen years ago ([6-8]). New measuring techniques, such as Digitial Image Correlation (DIC) that catches the measurable difference of strain between elastically loaded austenite and mechanically transformed martensite([9-10]), as well as the Infrared thermography (IRT) that takes advantage of the fact that the transformation of austenite into martensite is exothermic ([11-12]), have been used in the last decade. In the open literature, particular results are reported about the nucleation sites and the number of bands ([13]), the propagation of the transformation front, or the effect of stress concentration ([14]). Under higher strain rates (more than 10 /s), some authors have studied the strain rate sensitivity of NiTi in compression ([15-19]) with Split Hopkinson Pressure Bars (SHPB), a system commonly used for accurate measurement of dynamic and macroscopic stress-strain responses of materials
RkJQdWJsaXNoZXIy MjM0NDE=