Study of Ultralow-Cycle Fatigue of Iron-Based SMA in Triaxial Stress States

Abstract

An experimental study of the ultralow-cycle fatigue of iron-based shape memory alloy (Fe-SMA) in triaxial stress states was conducted. Monotonic tensile tests of specimens with different geometries were carried out to investigate monotonic mechanical properties of Fe-SMA under various triaxial stress states in terms of stress triaxiality and Lode angle, followed by ultralow cyclic tests (cycles <100) which were carried out to study the ultralow-cycle fatigue of Fe-SMA under various loading systems at triaxial stress states. Fe-SMA had nonlinear combined hardening under cyclic loading. A parameter identification method based on an optimization-based algorithm was established to calibrate the constitutive model of Fe-SMA. The initiation and propagation of cracks in the fracture process were different for Fe-SMA under monotonic and cyclic loading. The influences of stress triaxiality and Lode angle on the fracture ductility of Fe-SMA were investigated. An ultralow cyclic fracture criterion, the cyclic Bai–Wierzbicki (CBW) criterion, was established based on the accumulation of ductile damage and the attenuation of ductile damage threshold. The applicability of the calibrated constitutive model and the proposed fracture criterion was validated by simulating the ultralow-cycle fatigue and fracture failures of Fe-SMA.