Two-Dimensional Thermomechanical Model for Combined Loading of NiTi Wire Structures

Thin NiTi shape memory wires with unique thermomechanical properties are considered to be promising material for a wide range of new engineering applications. Based on broad experimental research on behavior of NiTi wires, which reveal a significant anisotropy in the transformation behaviors between tension and torsion, a new two-dimensional NiTi SMA material model was developed, particularly for the purpose of simulating NiTi wire structures behavior under combined mechanical and thermal loading. Two mutually perpendicular phase interfaces between autenite and martensite are formed during combined tension/torison loading. The first one, induced by tension, is perpendicular to symmetry axis, whereas the torsion-induced one is parallel to this axis. The model is parameterized by realistic physically-based material parameters and describes motion and evolution of these interfaces. Some numerical simulations — NiTi wire actuator, knitted self-expanding stent, shape memory fastener hook and shape memory helical spring — are presented and discussed to demonstrate the early practical applications of the model.