A Modular Nitinol Model With Lode Angle-Based Asymmetry and Improved Representation of the Superelastic Loops for Medical Device Applications

We describe a phenomenological constitutive model for representing Nitinol in medical device applications. The intent is to capture some of the response features important to medical device applications that are neglected in prevalent state of the art models such as the Abaqus superelastic model, without losing any of the major capabilities that are already widely adopted. These features include better representation of the superelastic loops under compression which is needed to accurately assess bending—a primary response mode in slender medical devices. This model captures tension/compression asymmetry as a function of Lode angle rather than pressure as in some existing models. Test data show Lode angle functionality is superior. We will refer to this implementation as the Thornton Tomasetti (TT) model to distinguish it from other existing implementations that have evolved from the baseline underlying theory. We implement the model in a modular form suitable for use in commercial finite element software and describe initialization from laboratory test data. This model improves fidelity in representing the mechanical response of Nitinol, but as will be shown below, there is more to be done. Higher fidelity models lead to improved medical device simulations and hence better designs.