Fatigue of NiTi shape memory wires

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

Procedia Structural Integrity

Elsevier

Año: 2016 vol. 2 p. 1427 - 1434

2452-3216

In this work the fatigue behaviour of a NiTi shape memory wire is study. A new specific method is proposed to test small section wires under pull-pull loading that allows obtaining a ?virtual dog-bone? (VDB) specimen avoiding grip induced failures. This is obtained applying a given number of pseudoelastic cycles to stabilize the stresses of transformation and re-transformation in a specific region of the free wire. Uniaxial pull-pull fatigue tests are then carried out by applying pseudoelastic cycling up to fracture. The localized nature of the transformation, the number of active transformation fronts, the maximum local temperature due to thermal effects associated with the first order martensitic transformation and the resulting maximum stresses can be then controlled.Strain controlled experiments were also performed of wires in fully austenite and fully martensite. In these cases, in comparison with pseudoelastic tests, obtained lives are in all cases at least two order of magnitude higher. These results show the decisive role played by the stress induced transformation in determining fatigue life. It was further found that for the pseudoelastic tests the results do 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-Manson type models usually used to study the fatigue life of superelastic materials are critically analyzed.