Thermomechanical model for NiTi wire structures

Optimal design of NiTi wire structures (NiTi knitted braided self-expanding stents, NiTi plain or hybrid textiles, complex NiTi wire supported actuators) requires reliable knowledge and modelling of thermomechanical behaviour of NiTi wires which is still quite difficult task, particularly due to the multiple deformation mechanisms involved. This work presents SMA model describing both the superelastic and shape memory responses of NiTi wires. The model was developed especially for NiTi wire structures and textiles. It captures i) the strains due to multiple deformation mechanisms in NiTi transforming via B2-R-B19’ sequential martensitic transformation, ii) reorientation processes in martensite and R-phase, iii) internal cycles and iv) asymmetric behavior in tension and compression. The model is parametrized by realistic physically based material parameters and implementable into finite element codes in a relatively simple way. Numerical simulations of thermal, mechanical or thermomechanical actuator cycles are performed in strain and temperature controlled mode. Experimental and simulated behaviors of the knitted self-expanding stent and NiTi wire actuator are shown and discussed to demonstrate the early practical applications of the model.