Surface Crack Development in Transformation Induced Fatigue of SMA Actuators

Abstract

This paper is based on the study of a post mortem analysis of shape memory alloy (SMA) actuators undergoing thermally induced martensitic phase transformation fatigue under various stress levels. Fatigue life results were obtained for both complete and partial phase transformation cycles applied to the SMA actuators. The thermal cyclic loading was induced by forced fluid convection cooling in order to increase the cycling frequency to approximately 1Hz, resulting to corrosion assisted fatigue. The combination of corrosion and reversible phase transformation under stress led to the formation of circular cracks on the surface of the cylindrical SMA wire actuators, which eventually saturated in a periodical distribution. In order to understand the stress field contributions to the microcracking in the presence of eigenstrains, a shear lag model was developed. The model accounts for eigenstrains introduced by corrosion, plastic strain accumulation with the number of cycles and the cyclic phase transformation strain. Comparison of the model predictions with crack spacing reached at fatigue failure is carried out and the reduction of fatigue life of SMA actuators under a corrosive environment is discussed.

A review on Nitinol shape memory alloy heat engines

“…This is called a two-way shape memory effect [42,60]. This repeated loading causes gradual microstructural changes, which gradually lead to degradation of SMA behavior, causing low cycle fatigue [61][62][63].…”