Fatigue, Fracture, and Crack Arrest from Bending Induced Pre-strain in Superelastic Nitinol

Abstract

The present study evaluates the fatigue and fracture performance of electropolished Nitinol wire apex specimens subjected to a range of initial bending crimp/pre-strains to determine how the fatigue behavior may be modified by compressive strains and residual stress. Three extrados (tensile) crimp/pre-strain conditions (8, 12, and 14.5%) were investigated. Specimens were fatigue cycled in unidirectional bending up to 107 cycles at 37 °C and 1.5% mean strain. High bending crimp/pre-strains were observed to decrease the probability of fatigue fracture for higher strain amplitudes at 107 cycles. Bend intrados fatigue cracks were observed to initiate and arrest at crimp/pre-strains of 12 and 14.5%. These fatigue cracks were arrested at lengths corresponding to the amount of far-field compressive load applied. Finally bending fatigue initiation and fracture in Nitinol was observed to change from the tensile strain surface (extrados) to the compressive surface (intrados) depending on the amount of bending pre-strain applied.