Experimental methods for evaluating strain rate dependency of shape memory alloy materials under quasistatic and impulsive loading
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Shape memory alloys (SMAs) are innovative materials that have great potential in structural engineering because they can provide significant energy dissipation capacity and introduce considerable re-centering ability to structures. The stress-strain relations of SMAs are dependent on loading rates, and the responses of SMAs under intermediate strain rates are hard to obtain using conventional experimental techniques. This research developed an innovative high-loading-rate tensile testing system to test specimens under intermediate strain rates, bridging the gap between quasistatic strain rates from conventional servo-hydraulic techniques and high strain rates from typical Kolsky bar techniques. This testing system converted impacts from a high-speed actuator into high-loading-rate tensile forces and elongated specimens under relatively constant deformation rates. This testing system is capable of testing not only prismatic material specimens to evaluate stress-strain behavior but also non-prismatic structural components composed of different materials to evaluate force-deformation behavior. The testing system was verified and calibrated through a series of validation tests on aluminum tensile specimens. Experimental results were compared with theoretical estimations and finite element simulations to confirm this system obtained reliable force-deformation measurements in a repeatable and controllable manner. This research conducted two types of experimental tests on SMA specimens: a quasistatic cyclic loading test on a seismic bracing system based on an SMA ring and a series of high-loading-rate tensile tests on various SMA tensile specimens. The test results corroborated that this new testing system is capable of assessing the behavior of material specimens and structural components under intermediate strain rates.