Analytical solutions are derived for the isothermal pseudoelastic response of a shape memory alloy (SMA) thick-walled cylinder subjected to internal pressure. The Tresca transformation criterion and linear hardening are used. Equations are given for the radial and circumferential stresses, transformation strains and radial displacement at various steps of loading and unloading. A structural pressure-temperature phase diagram is provided for the cylinder, analogous to the stress-temperature phase diagram of SMA materials. Pressurization of an initially 100% austenitic cylinder causes the martensite to initially form at the inner radius. For
Shape memory alloy pipe couplers use the shape memory effect to apply a contact pressure onto the surface of the pipes to be coupled. In the current research, a shape memory alloy pipe coupler is designed, fabricated, and tested. An in-house developed NiTi alloy is used for the coupler. The coupler is initially designed using finite element as well as analytical modeling to accommodate the required actuation temperature and contact pressure properties. The coupling pressure is then measured using strain gages mounted on the internal surface of an elastic steel ring. The thermal actuation response of the coupler is measured under both stressed and stress-free conditions. In addition, the state of anisotropy is investigated in the coupler by characterizing samples in the longitudinal and transverse directions. The NiTi coupler demonstrates a multiple-actuation response and a high contact pressure. The results show no change in transformation temperatures with respect to two perpendicular transverse and longitudinal directions. However, the material can undergo higher transformation strains in the longitudinal direction.
In this work, the influence of the latent heat of transformation and heat transfer on the performance of shape memory alloy (SMA) actuators is numerically explored. A 1D analytical model is first considered and used to perform parametric studies on the effects of geometry and heat transfer conditions on SMA wire responses. In order to consider the response of SMA structures, a recent SMA constitutive model is expanded to include the effects of the latent heat of transformation. The enhanced model is then implemented in a 3D finite element framework to solve the coupled and transient thermomechanical problems. The resultant model is used to explore the isothermal and adiabatic assumptions commonly used for quasi-static SMA modeling by considering the response of SMA structures. The responses of an axial SMA actuator heated from one end is evaluated and it is shown that the generation of latent heat during forward transformation and its absorption during reverse transformation decreases the actuation response when compared to a case neglecting thermal effects. It is concluded that the latent heat of transformation must be considered for the design of SMA components unless their operation can reasonably be approximated as isothermal.
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