Effect of Processing and Loading on Equiatomic NiTi Fatigue Life and Localized Failure Mechanisms

Shape Memory Alloys (SMAs) have many promising applications in the aerospace, automotive, and energy industries. However, due to a lack of understanding of their actuation fatigue, applications are sometimes limited to non-structural or non-critical components. This paper addresses the actuation fatigue characteristics of a specific SMA, equiatomic Nickel-Titanium (NiTi), with varying heat treatments, as well as different methods for assessing actuation fatigue response, including improved testing procedures and distributed extension measurement methods. Heat treatments ranged from 350°C to 400°C for one to three hours. Dogbone specimens processed from heat treated NiTi sheets were mechanically loaded on test frames which provided resistive heating and forced convective cooling with dry air via vortex tubes. Two mechanical loading schemes were utilized: constant uniaxial load (initial stress of 200MPa) and a linear or spring load centered at 200MPa (and ranging from approximately 150MPa to 250MPa). Linear loading schemes were introduced in order to better simulate actuation in an aerospace application, such as the morphing of semi-rigid surfaces. Specimens were thermally cycled to full actuation with a time-based control scheme developed in LabVIEW. Fatigue responses varied widely as a result of different heat treatments and loading schemes. Due to the main failure mechanism being high localized extension (necking) for the constant loading schemes, additional hardware and software were developed to visually capture extension distribution over specimen length. By analyzing actuation characteristics (e.g. transformation strain) and fatigue mechanisms, the ideal post-processing for actuator applications was determined. Utilizing the local extension distribution evolution over the fatigue life of NiTi specimens as well as postmortem analysis of the failure surfaces allowed for the failure modes to be determined for each heat treatment.