Fatigue Crack Growth in Austenitic and Martensitic NiTi: Modeling and Experiments

Abstract

Fatigue crack growth of austenitic and martensitic NiTi shape memory alloys was analyzed, with the purpose of capturing the effects of distinct stress-induced transformation mechanics in the two crystal structures. Mode I crack growth experiments were carried out, and near-crack-tip displacements were captured by in-situ digital image correlation (DIC). A special fitting procedure, based on the William’s solution, was used to estimate the effective stress intensity factor (SIF). The SIF was also computed by linear elastic fracture mechanics (LEFM) as well as by a special analytical model that takes into account the unique thermomechanical response of SMAs. A significant difference in the crack growth rate for the two alloys was observed, and it has been attributed to dissimilar dissipative phenomena and different crack-tip stress–strain fields, as also directly observed by DIC. Finally, it was shown that the predictions of the analytical method are in good agreement with effective results obtained by DIC, whereas a very large mismatch was observed with LEFM. Therefore, the proposed analytical model can be actually used to analyze fatigue crack propagation in both martensitic and austenitic NiTi.

Analysis of the Local Functional Evolution in NiTi Shape Memory Alloys by Multicycle Nanoindentations

“…In addition, special and ad hoc investigation techniques were also developed and exploited to study the crack formation and propagation mechanisms under static [17][18][19][20][21] and cyclic loadings [22][23][24][25] by both local and global analyses [26,27]. An example is represented by synchrotron X-ray micro-diffraction (XRD) [18,19], infrared thermography (IR) [20,25], and Digital Image Correlation (DIC) [21,22,[28][29][30][31][32] that allow to demonstrate that phase transition phenomena significantly affect the fatigue properties of the material as well as the functional response, [26,27].…”