Materials Science and Engineering A 524 (2009) 108–111 Contents lists available at ScienceDirect Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea Instrumented tensile–impact test method for shape memory alloy wires J. Zurbitu a,∗ , S. Kustov b , G. Castillo a , L. Aretxabaleta a , E. Cesari b , J. Aurrekoetxea a a Mechanical and Industrial Production Department, Mondragon Unibertsitatea, Loramendi 4, 20500 Mondragon, Spain b Department of Physics, Universitat de les Illes Balears, Cra Valldemossa, km 7.5, 07122 Palma de Mallorca, Spain article info Article history: Received 25 March 2009 Received in revised form 3 June 2009 Accepted 5 June 2009 Keywords: Impact test NiTi Martensitic transformation Young’s modulus Strain rate abstract New instrumented tensile–impact test method is proposed for the characterization of shape memory alloy wires in the strain-rate range from 1 to 10 2 s -1 . The force and the velocity evolution during the impact are registered and, based on these curves, the stress–strain response at impact may be obtained. This method is able to measure strain-rate dependent parameters, such as the direct and reverse stress-induced martensitic transformation stresses or the dissipated energy. Moreover, the accuracy of properties mea- sured with this method, such as the Young’s modulus of the austenitic phase or the permanent strain after load–unload deformation, is higher than those calculated exclusively from the integration of the force–time curves. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Shape memory alloys (SMA) are interesting for impact appli- cations due to their unique superelastic behaviour [1,2]. SMA wires embedded in polymer matrix composites have shown to be effective in improving their impact behaviour [3]. The stress- induced martensitic (SIM) transformation is exothermic, whereas the reverse transformation is endothermic. Characteristic stresses and strains of these transformations depend on the temperature, and since the strain rate could change the heat-transfer phenom- ena, the temperature could change during the loading–unloading path. So, the knowledge of the strain-rate effects on the mechanical properties of superelastic SMA is necessary. Standard tensile-test methods are confined to strain rates below 0.1s -1 [4] and around 1s -1 when servo-hydraulic test machines are used [5]. Impact stud- ies have been carried out at strain rates higher than 10 3 s -1 [6–9] using Split Hopkinson Pressure Bar (SHPB) technique. Thus, in the intermediate range, from 1 to 10 3 s -1 , relevant to many applica- tions such as crashworthiness [10], there is a lack of experimental data. The attempts hitherto made to study the SIM transformation at these intermediate strain rates using SHPB have failed because the initial velocity of the striker bar must be set at such a low level that the amplitude of the loading pulse is not large enough to load the specimen beyond its initial transformation strain [8]. However, instrumented tensile–impact method has been applied successfully to polymer characterization in this strain-rate range ∗ Corresponding author. Tel.: +34 943 79 47 00; fax: +34 943 79 15 36. E-mail address: jzurbitu@eps.mondragon.edu (J. Zurbitu). [11], and in this study, it has been proposed as a useful technique for the impact characterization of the SMA wires in the intermediate range from 1 to 10 2 s -1 . Moreover, the conventional instrumented tensile–impact technique has been modified and the new instru- mentation is able to measure with higher accuracy parameters that are strain dependant such as the effective elastic modulus, the dis- sipated energy or the permanent deformation. 2. Experimental method and materials In tensile–impact tests, the sample is fixed between the mobile grip and the fixed grip. When a pendulum impactor reaches the lowest point achieving the impact velocity, it hits the mobile grip and the tensile force is transmitted to the sample (Fig. 1). This force is measured at the fixed grip by a piezoelectric sensor. The most common impact-test instrumentation is based on the registration of the force–time curve and the initial impact veloc- ity [12]. The stress is calculated by dividing the impact force curve by the initial area of the wire. Using Newton’s second law, the displacement ı(t) integrated , Eq. (1), is calculated by two successive integrations of the force data (F(t)): ı(t ) integrated =  t 0  V 0 - 1 m  t 0 F (t )dt  dt (1) V 0 is the measured initial impact velocity and m is the mass of the impactor system which is accelerated during the deformation, that includes the impactor, the mobile grip and the sample mass. The sample mass (in this case <1 mg) and the mobile grip mass (30 mg for this case) are usually neglected because they are much smaller 0921-5093/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2009.06.012