Effect of Martensitic Transformation on Local Stresses and Strains at a Notch of Cyclically Loaded Superelastic NiTi Ribbon

article in a collection out of WoS and Scopus

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Reversible and volume preserving martensitic transformation /MT/ proceeding in shape memory alloys such as NiTi differs from that in steels. Especially in superelastic NiTi largely used in medical devices, the effect of MT on the material response at stress risers and the role of MT in crack nucleation and propagation remain unclear and widely unexplored. Therefore, we studied the effect of MT on the stress and strain fields around a notch of a superelastic NiTi ribbon thus mimicking in a simplified manner mechanics at stress risers of geometrically complex medical devices such as stents or at crack tips of fatigue damaged NiTi elements. We used digital image correlation /DIC/ to track strains around a notch of a NiTi during superelastic. As for the stress evaluation, we used finite element analysis (FEA) coupled with a SMA material model implemented using a user subroutine. In summary, we found that the effect of MT proceeding in NiTi at a constant plateau stress is threefold. First, MT is triggered locally at the notch where it acts as a stress reliever as MT proceeds at a constant stress. Second, MT further propagates into the bulk that leads to high deformations of martensitic phase at already transformed material in the vicinity of the notch. Third, MT acts again as a stress reliever at the notch once it propagates macroscopically along the length of the sample through a shear band at a constant plateau stress.

Reversible and volume preserving martensitic transformation /MT/ proceeding in shape memory alloys such as NiTi differs from that in steels. Especially in superelastic NiTi largely used in medical devices, the effect of MT on the material response at stress risers and the role of MT in crack nucleation and propagation remain unclear and widely unexplored. Therefore, we studied the effect of MT on the stress and strain fields around a notch of a superelastic NiTi ribbon thus mimicking in a simplified manner mechanics at stress risers of geometrically complex medical devices such as stents or at crack tips of fatigue damaged NiTi elements. We used digital image correlation /DIC/ to track strains around a notch of a NiTi during superelastic. As for the stress evaluation, we used finite element analysis (FEA) coupled with a SMA material model implemented using a user subroutine. In summary, we found that the effect of MT proceeding in NiTi at a constant plateau stress is threefold. First, MT is triggered locally at the notch where it acts as a stress reliever as MT proceeds at a constant stress. Second, MT further propagates into the bulk that leads to high deformations of martensitic phase at already transformed material in the vicinity of the notch. Third, MT acts again as a stress reliever at the notch once it propagates macroscopically along the length of the sample through a shear band at a constant plateau stress.

Martensitic Transformation; Stress and Strain; Notch; Cyclic Loading; Superelasticity; NiTi Ribbon

18.07.2017

ICF

Rhodes, Greece

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