Fatigue of NiTi shape memory wires

S. JAUREGUIZAHAR; M.D. CHAPETTI; A. YAWNY

Procedia Structural Integrity

Elsevier

Lugar: Amsterdam; Año: 2016 vol. 2 p. 1427 - 1434

2452-3216

In this work the fatigue behaviourof a NiTi shape memory wire is study. A new specific method is proposed to testsmall section wires under pull-pull loading that allows obtaining a ?virtualdog-bone? specimenavoiding grip induced failures. This is obtained applying a given number ofpseudoelastic cycles to stabilize the stresses of transformation andre-transformation in a specific region of the free wire. Uniaxial pull-pullfatigue tests are then carried out by applying pseudoelastic cycling up tofracture. The localized nature of the transformation, the number of activetransformation fronts, the maximum local temperature due to thermal effectsassociated with the first order martensitic transformation and the resultingmaximum stresses can be then controlled.Strain controlledexperiments were also performed of wires in fully austenite and fully martensite.In these cases, in comparison with pseudoelastic tests, obtained lives are inall cases at least two order of magnitude higher. These results show thedecisive role played by the stress induced transformation in determiningfatigue life. It was further found that for the pseudoelastic tests the resultsdo not depend on the test temperature or the associated stress level. Finally,a clear influence of strain rate on the fatigue life is shown an Coffin-Mansontype models usually used to study the fatigue life of superelastic materialsare critically analyzed.