The effect of impact induced strain on the SIM transformation of superelastic NiTi shape memory alloy wires

The aim of this work is to study the effect of maximum strain achieved during an impact event on the SIM transformation of superelastic NiTi shape memory alloy wires. Instrumented tensile impact tests have been carried out in order to obtain stress-strain curves at impact strain rates with different energies to obtain different maximum strains not only with incomplete but also with complete SIM transformations. Moreover quasi-static tensile tests for the same maximum strains have also been carried out in order to compare the results at low and high strain rates.

Direct SIM transformation stress is independent of the maximum strain achieved and is higher at impact than at quasi-static strain rates. Reverse SIM transformation stress depends mainly on maximum deformation achieved until unload path starts. When SIM transformation is not completed, reverse SIM transformation stress keeps constant and is strain independent. However, when impact energy is enough to complete the SIM transformation, this trend changes and reverse SIM transformation stress diminishes as the maximum strain achieved is increased. This may be attributed probably to the high stresses generated during the loading path, which originate internal stresses favorable for the reverse SIM transformation. This behaviour is similar for both, quasi-static and impact strain rates, but the stresses at impact are higher.Results from the impact tests show that the deformation energy and the recoverable strain energy increase with the maximum strain achieved and are higher at impact than at quasi static strain rates. The dissipated energy also increases in all the strain range studied but in this case is lower at impact reaching close values to those obtained at quasi-static strain rates.