13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- ([20-21]). It appears that SIM transformation is rate-dependent so that the stress triggering the transformation process increases with strain rate. There are also a few papers that present some experimental results with tensile Hopkinson bar tests (strain rate up to 1.67 /s in[22] and up to 1200 /s in [15]). Unfortunately, all these results under impact loading rate are only based on macroscopic measurements which are the nominal strain or stress. The observation of the transforming regions of NiTi alloys is not yet reported, probably due to the experimental difficulty to get these results at such strain rates. Thus, the description of nucleation sites, number of bands, transformation front velocity under higher strain rate are almost unknown. There is no available shear testing results under impact loading. The present work wishes to provide a measurement of NiTi SMA behavior under tensile as well as shear loading up to an intermediate strain rate regime around 100 /s (3 decades higher than previous studies). In addition, heterogenous strain fields due to transforming regions is measured, coupled with a tensile Hopkinson bar system. For this purpose, strain fields of the sample are measured by Digital Image Correlation procedures using pictures captured with a high speed camera. 2. Experimental setup and macroscopic results 2.1 Tensile tests 2.1.1 NiTi specimens and quasi-static tests The material of this study is taken from NiTi polycristalline cold-rolling sheets, in the cold-rolling direction. The metallurgical composition is 50.7 wt.% Ni and 49.3 wt.% Ti ([23]). All the experiments are performed at room temperature, clearly above the characteristic temperature Af so that for stress-free condition the specimen is in a full austenite state. A single plate specimen, dog bone shaped, as schematically presented in the upper left corner of figure 1, is used for all the experiments. In its central part, the specimen is 60 mm long, 2.6 mm wide and 0.5 mm thick. Fig. 1: Static strain-stress curve for a prescribed velocity of 0.01 mm/s (1.4x10-4 /s), at room temperature For the fixation in the grips, the width is 10 mm so that the fillet radius of 10 mm leads to a stress concentration. It is observed that this triggers the SIM transformation at both ends of the central part of the specimen. This is acceptable because the paper is focused on propagation of the transformation, not on its nucleation. For all the experiments, the specimen is fixed between two
RkJQdWJsaXNoZXIy MjM0NDE=