Fatigue of NiTi shape memory wires

S. JAUREGUIZAHAR; MIRCO CHAPETTI; ALEJANDRO YAWNY

Catania

Congreso; 21st European Conference on Fracture; 2016

In the present work, an innovative methodology for characterizingstructural fatigue of NiTisuperelastic wires is proposed. It consists in firstlyperforming low speed pseudoelastic cycles in a limited region of the wirespecimen. Here, advantage is taken of the localized nature of the stressinduced transformation which proceeds by the movement of a finite number oftransformation fronts. This results in the stabilization of the pseudooelasticbehaviorwhichis accompanied by a decrease in the stress for forward transformation and amore moderate decrease in the stress for the reverse transformation. This firststage allows obtaining an equivalent to a geometric dog-bone shaped specimendue to the reduced transformation stresses associated with the precycled region.In a second stage, deformation speed is increased. By limiting the displacementrange to the precycled zone, pseudoelastic cycling can be conducted untilfracture avoiding any transformation activity outside that region. In this way,grip induced failures resulting in artificially shorter fatigue lives might becompletely avoided and the true structural life more accurately assessed. Additionally,strain controlledexperiments with the wires in fully austenite and fully martensite states havebeen performed. Resulting fatigue lives in these cases were at least two ordersof magnitude higher compared with the pseudoelastic fatigue.This indicates thedecisive role played by the stress induced transformation in determining fatiguelife. The influence of testing temperature and deformation rate on fatigue lifehas been also evaluated.