Transformation Induced Structural Fatigue of Commercial Shape Memory Wires

Abstract

The present work investigates actuation fatigue of NiTi shape memory alloys (SMAs). Actuation fatigue relies on the accumulation of transformation induced defects, which affect the functional properties and the structural integrity of SMA components. When a NiTi SMA is subjected to thermal cycling at elevated constant stresses, small fatigue cracks form and grow, which eventually lead to fatigue failure. In the present work we investigate transformation induced fatigue of NiTi SMA wires which were obtained from two different commercial sources. The materials were subjected to heating/cooling cycles in dedicated functional test rigs. A special focus was placed on the effects of stresses on the evolution of functional properties and fatigue lives. The fractured wires were analyzed by scanning electron microscopy (SEM) in combination with quantitative image analysis. Differential scanning calorimetry (DSC) was used to analyze changes in transformation behavior. Our results show that higher stresses result in an increasing number of fatigue cracks and as expected in a reduced number of cycles to failure. It was observed that wires which showed short fatigue lives experienced a stronger change in transformation behavior during cycling. The fractographic analysis documents how different stress levels affect actuation fatigue damage accumulation.