Sensitivity of Nitinol Fatigue Strain on Material Inputs in Finite Element Analysis

Finite Element Analysis requires a UMAT to describe its stress-strain relationship. This in turn determines how each element interacts with its neighboring elements.  Typical UMAT inputs for superelastic Nitinol include the elastic moduli of martensite and austenite, loading and unloading plateau stresses and strains, and Poisson’s ratio, plus a single material data point for the compressive side of the stress.  Lifetime prediction in Nitinol is generally based on mean and cyclic stain calculations.  While it is often assumed that strain calculations are primarily geometric in nature and largely independent of the UMAT inputs, that is not the case—far-field behavior is displacement controlled, but near-field calculations are force controlled (stress-based) and thus dependent upon the UMAT inputs.  The current paper will present several case studies highlighting the sensitivity of FEA strain calculations to various UMAT inputs, showing a particularly powerful influence on stress hysteresis.

Three implications of this sensitivity will be discussed.  First, the need to accurately determine precise material properties form the material actually used.  Second, the difficulty in using published Strain Limit Diagrams constructed using FEA.  And perhaps most importantly, the difficulties brought about by strain localization and cyclic changes in properties that are not currently considered in establishing UMAT in put parameters.