The effect of phase transformation on fatigue crack growth of shape memory alloys
The fatigue crack growth of shape memory alloys remains a difficult challenge in the scientific community for years. Most of the works on fatigue crack growth are based on materials that don’t undergo phase transformation when an external force is applied. Consequently, the change of crack tip driven force due to phase transformation is not well understood. In this study, the modification of the crack tip driven force was characterized by the stress intensity factor due to tractions on the transformation zone surface. Through modeling, it was found that the modification became more significant at the maximum applied load than at the minimum in a single load cycle. The effective stress intensity factor range was also measured from the displacement fields upon regression. The predicted effective stress intensity factor range through modeling was later compared with the measured one. The results were found to be closed to each other. The closure effect was also measured and determined to be 30% of the maximum load which corresponded to the modeling results as well. All these agreements confirm the validity of the modeling and pointed to the major mechanical factors such as elastic moduli changes, the transformation residual strains, and the transformation domain dimensions that contribute to damage tolerance in shape memory alloys. The results are checked by conducting simulations for two other important shape memory alloys, NiTi and CuZnAl, where the reductions in stress intensity range were found to be lower than NiFeGa explaining the high levels of experimentally determined crack threshold stress intensity range in NiFeGa.
Shape Memory Alloys, Fatigue Crack Growth
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign