FACTORS Causing COMPRESSIVE DAMAGE-INDUCED Cracking IN Nitinol

Compressive damage-induced cracking is observed when Nitinol is subjected to significant compressive strain, such as occurs when bending a wire to a small radius of curvature. This study builds on the data published by James, et al. The effects of tension while bending the Nitinol wire, the shape of the final component and critical strains on Nitinol were examined. Finite Element Analysis models were also created for each of these scenarios to predict the critical strain amplitude and locations. Compressive damage-induced cracking was observed at strain levels above 34% (strain calculated from neutral axis); this effect was magnified when higher tension was used to bend the wire. For comparison three different strains were used 24-50%, along with three part geometries (0 – 60º) and tensions (6700 – 18000psi) needed to bend the wire. The results show that compressive damage may be minimized by reducing the strain applied to the wire during forming to below 34% and lowering the wire tension. Etching the wire after forming was partially successful in removing the damage to the surface. These results indicate that forming tension and geometry play a significant role in the sensitivity of Nitinol to compressive damage during bending.