13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- tabs that can further be screwed (i) in the bars for dynamic tensile experiment, (ii) in a clamping system for static experiments. This allows to be more confident in the comparison of experimental results obtained with different devices. We could ensure that there is no sliding of the specimen during monotonic static or dynamic tension experiments due to the presence of a screw that fits into one hole machined into each end of the specimen. Quasi-static tensile tests were performed on an electromechanical Instron device so there was no heating of the specimen through the gripping system. Experiments were done at two different prescribed velocities : 0.01 mm/s (1.4x 10-4/s) and 1 mm/s (1.4x 10-2/s). Each experiment was repeated twice in order to check repeatability of the measurements. The nominal stress-strain curve obtained at 0.01 mm/s is shown in figure 1. Three domains appear on the curve. First the elastic tension of the initial austenite phase for strains up to 0.01. Then the stress plateau associated to the pseudoelastic domain of interest in this study begins. It corresponds to the stress-induced martensitic (SIM) transformation for strains up to 0.06. One unloading-reloading cycle has been performed in the pseudoelastic domain in order to show that the reverse transformation is effectively reversible. It can be noticed that two serrations are visible at the beginning of the pseudoelastic domain. This phenomenon is due to the heterogeneity of the martensitic transformation ([12]). At a third stage, the supposed "fully" martensitic specimen is submitted to elastic tension. The vocabulary "fully martensitic" refers in fact to the end of the SIM transformation process even if it has been documented that probably all austenite has not been transformed yet ([24]). In this experiment, maximal strain is under 0.08 and there is no evidence of plastic deformation during tension. 2.1.2 The tensile Hopkinson bar experiment The use of Hopkinson bars as a measurement technique has been introduced at the end of the 19th century and has led to many developments in the field of analysis of material behaviour under dynamic loading ([20], [25]). Hopkinson bar measurement allows to obtain an accurate measurement of dynamic forces and velocities at both ends of the specimen. The device used in this study is a Split Hopkinson Tensile Bar (SHTB) as schematized in figure 2. The apparatus is composed of two bars of 10 mm diameter, called the input and the output bars, made of maraging steel. The specimen (equipped with its end tabs) is tightly screwed at the end of each bar. The tabs are manufactured in the same material as the bars and designed so as to minimize any mass effect by matching the same acoustic impedance as the bars. The loading principle is the following: first a tensile elastic energy is stored in the input bar along a length L between points A and B of the input bar, then this energy is suddenly released from point B so that an incident tension pulse reaches the specimen ([26]). The initial elastic energy controls the subsequent strain and strain rate applied to the specimen. The initial load N0 is applied with a hydraulic jack, and, to perform a test at an initial velocity of around 1000 mm/s, an initial load of around 5 kN is applied.
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