Numerical investigation of NiTi shape memory alloy structures for blast mitigation applications
Mundhekar, Amogh Ajay
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Super-elastic shape memory alloys (SMAs) undergo martensitic phase transformations upon being subjected to stress loading. They display hysteresis behavior under cyclic loading. SMAs possess superior strain recovery properties and within the recovery range they can dissipate a certain amount of energy without inducing a permanent strain. Plastic deformation also forms a means for energy dissipation. Thus, phase transformations, hysteresis and plastic deformation form three different mechanisms in the SMAs that are capable of dissipating energy upon being subjected to loads. Nickel-Titanium (NiTi) alloy, a type of SMA, which is widely used in industrial applications, has been proposed as a viable candidate to be used in blast mitigation application. In this thesis, NiTi alloy plates and NiTi based sandwich structures have been investigated for their dynamic response upon being subjected to blast loading. The material transformation characteristics, plastic deformation mechanisms, energy absorption and modes of failure for monolithic NiTi plates have been investigated using the numerical finite element code LS DYNA. Three different types of modes of failures were observed for plastic deformation and failure in monolithic NiTi plates. At low blast pressures, NiTi stays within elastic strain range and displays behavior similar to hysteresis. The potential of NiTi being used in sandwich composites as a core has been explored by designing two types of layered sandwich composites and by performing blast loading simulations. The first type of composite design comprises of a 5 mm NiTi layer sandwiched between two steel layers having 1 mm thickness each. The second type of composite sandwich uses a steel faceplate, 1 mm thick aluminum backplate and a 5 mm thick NiTi layer at the center. The results of all the NiTi structures have been compared with those for steel and aluminum. The second NiTi sandwich construction with aluminum as the backplate shows superior performance in terms energy statistics, maximum plate deflection and post blast vibrations over the other materials. Aluminum and steel plates having the same thickness as NiTi structures undergo plastic failure before the NiTi monolithic plates and sandwich structures.