Effect of variable amplitude blocks ordering in the functional fatigue of superelastic NiTi wires

Accumulation of superelastic cycles in NiTi uniaxial elements generates changes on the stress-strain response. Basically, there is an uneven drop of martensitic transformation stress plateaus and an increase of residual strain. This evolution associated with deterioration of superelastic characteristics is referred as functional fatigue and occurs due to irreversible micro-structural changes taking place each time a material domain transforms. Unlike complete cycles, for which straining is continued up to elastic loading of martensite, partial cycles results in a differentiated evolution of those material portions affected by the transformation. It is then expected that the global stress-strain response would reflect the previous cycling history of the specimen. In the present work, the consequences of pseudoelastic cycling of NiTi wires using blocks of different strain amplitude interspersed in different sequences are analysed. The effect of successive increasing, successive decreasing and mixed strain amplitudes, on the evolution of the pseudoelastic response is characterized. The feasibility of postulating a functional fatigue criterion, similar to the Miner´s cumulative damage law used in structural fatigue analysis is established. Experiments were simulated using a previously developed 1‑D model which describes the pseudoelastic behaviour under dynamic conditions taking into account the movement of localized domain interfaces. In that way, the relation of the observed stress-strain response with the transformational history of the specimen could be rationalized.