NiTiHf high temperature shape memory alloys (HTSMAs) are being used in an ever-growing array of applications, specifically in the aerospace and automotive industries. One of the difficulties facing further implementation is ensuring the actuation fatigue lifetime is sufficiently long as to prevent the HTSMA components from being a limiting factor to the mean time between failures of a system. Another potential problem for widespread use is the deterioration of actuation stroke during lifetime, which can be problematic when attempting to have a high-fidelity repeatable design. One way of solving these issues is to optimize the microstructure through careful control of composition, processing, and heat treatments. Current research shows composition of large-scale productions is incredibly difficult to control, and small deviations in composition (~0.1 at.% Ni) can result in changes in transformation temperature by 50?C or more. Four NiTiHf compositions were investigated. The initial goal to simply extend the actuation fatigue lifetime and provide a stable actuation response morphed into determining material factors that influence the actuation response of partially cycled samples.

Standard test methods for Uniaxial Constant Force Thermal Cycling (UCFTC) and Uniaxial Pre-strain and Free Recovery (UPFR) have been published under ASTM as E3097 and E3098, respectively. These two test methods capture the two most important responses in shape memory alloys (SMAs) for actuation, which is load-biased shape memory effect and classical shape memory effect. These test methods measure the transformation properties such as transformation temperatures, actuation strain, and residual strain to provide data for the characterization and selection of SMA materials, quality control, design allowables, and actuator design. In light of this recent development, additional test methods are still needed to fully support the transition of SMA actuators to production use. First, there is a recognized need for a test method that defines and evaluates the evolution of actuation properties and actuation lifecycle during repeated UCFTC. Second, rotary SMA actuation has been demonstrated as a practical and valuable actuator form as highlighted recently during flight test of the spanwise adaptive wing and reconfigurable vortex generators and in remote controlled actuated wind tunnel models. To this end, two new test methods have been proposed to ASTM that extend the applicability of E3097 to repeated UCFTC and torsional testing, respectively. This paper provides an update on the development of SMA standardized test methods for actuator applications highlighting their need and envisioned approaches.