Fatigue life and fracture mechanics of unconstrained Ni-Mn-Ga single crystals in a rotating magnetic field

Fracture resistance and long fatigue life are required to commercialize Ni-Mn-Ga ferromagnetic shape memory alloys. While Ni-Mn-Ga achieves high fatigue life at low strains, there is little data on the fatigue life of samples that are actuated near the theoretical strain limit. Furthermore, constraints imposed by clamping samples impact their magneto-mechanical and fatigue properties. Here, 10 M Ni-Mn-Ga single crystals were exposed to a rotating magnetic field while holding them with rubber O-rings so they were minimally constrained. Prior to testing, sample surfaces were prepared to various levels of finish. Fatigue life varied with some samples reaching more than 800,000 cyclic loads before fracturing and some samples failing after only 4000 cyclic loads. Long fatigue life and fracture resistance are achieved when avoiding crystal defects and surface imperfections. Short fatigue life was caused by high surface roughness and stress concentration at defects and notches.