"Finite Element Simulations Of Localized Functional Fatigue In Pseudoelastic NiTi"

Pseudoelastic NiTi shape memory alloys are used widely in the medical device industry and their mechanical behavior has been studied extensively. Understanding the material behavior even under simple mechanical loading conditions is further complicated by a tendency for localized deformation, where martensite bands form and grow inhomogeneously throughout the specimens. Moreover, despite numerous experimental and theoretical studies, their fatigue behavior is still not fully understood today. Devices and components designed using shape memory alloys typically need to be studied numerically using the finite element method. However, there is a lack of material models that take into account both localized deformation and functional fatigue. In this study, we present results from finite element simulations that use a strain-softening approach (to simulate localized deformation) and internal variables (to track the number of transformation cycles). We study how seemingly complex, macroscopic behavior of pseudoelastic NiTi during mechanical cycling with varying amplitudes (i.e., formation of multiple plateaus and internal loops) can be described and fully rationalized using only a small set of parameters. We demonstrate that localization of transformation directly results in localization of functional degradation and therefore specimens with initially homogeneous microstructures become increasingly inhomogeneous with increasing number of cycles. Our results indicate that areas of pronounced functional fatigue can be both spread out or limited to relatively small volumes, depending on the geometry and load case, during the lifetime of complex NiTi components.