Discussion Forum

Hello,

I've a shape memory alloy wire, Flexinol, trained to bend upon heating. I'm trying to model this behavior in COMSOL Multiphysics 5.4 using the shape memory alloy material model of the nonlinear structural mechanics module in Structural mechanics physics. I'm doing a stationary study with custom mesh by having a transverse point load acting at the center. The wire is point constrained with fixed constraint and prescribed displacement at the end points. Such that one end is fixed and other end can move freely along an axis (y-axis). So that when the load is applied, the boundary surfaces can rotate about a point and one end can slide along y-axis thereby making a U shape. The load is applied and removed gradually with auxillary parameter sweep. Currently, I'm using the Lagoudas model with phase transformation properties, same as the one given in uniaxial loading of shape memory alloy tutorial. The model input temperature is user-defined so that T>T_Af (austenite finish temperature) in order to have superelastic behavior.

The study works fine for small loads that causes stress to stay within the martensite start critical stress for that particular temperature thereby exhibiting elastic behaviour. The study FAILS when the material undergoes plastic deformation under loading or while it retrives the residual inelastic strain during unloading by throwing ERROR that computed plastic multiplier is negative.

The model parameters works fine for uniaxial loading. Since the model has both tension and compression during bending, I'd like to know if the shape memory alloy model in Nonlinear structural material module is capable of calculating both tension and compression equation seperately for calculating the strain especially during phase transformation. I believe that Lagoudas model has two sets of equation for calculating strain in tension and compression.

Thank you

Regards, Ramprasad