Finite Element Analysis on the Driving Force for Fatigue Crack Formation near Particle/Void Assemblies in NiTi

It was recently observed that inclusions strongly affect fatigue life of pseudoelastic NiTi shape memory alloys once surface defects (e.g. die marks resulting from wire drawing) are removed through electropolishing. In most cases, crack initiation occurs at particle/void-assemblies (PVAs). These PVAs come about typically as a result of thermomechanical processing, which detaches inclusions from the matrix and breaks up larger inclusions into several partially debonded fragments. In the present work, the unit cell method is employed to create a periodic hexagonal prism microstructure containing a single PVA. This microstructure is represented as an axisymmetric cylindrical finite element model in Abaqus. By incorporating geometry taken from SEM images of a PVA which had initiated a microcrack, an analysis of an isothermally strained unit cell brings to light the mechanical fields and stress induced transformation in the surrounding regions. In this analysis, the particle is simulated as linear elastic while the SMA uses a constitutive law adopting the classical rate-independent small-strain flow theory for the evolution equations of the transformation strains. Also, a parametric study based on an elastic matrix was conducted, quantifying the effects of differing PVA geometries and varying elastic mismatches between the inclusion and matrix. It is shown that the elastic mismatch, particle to void size ratio, and particle aspect ratio each play a significant role in stress concentration buildup around the PVA.