Standard Test Methods for Shape Memory Alloy Actuation

While there exists a broad range of potential applications for Shape Memory Alloy (SMA) actuators, their transition to production and commercial use is hindered by the lack of a broad range of accepted standardized test methods. Approved test methods are needed to support the development of design allowables, supplier qualification, and regularity approval of SMA materials and components. ASTM recently published two standard test methods for tensile testing of SMA materials and components (E3097 and E3098), however additional test methods are needed to fully support transition of SMAs to production use. Torsional SMA actuation has been demonstrated by Boeing and NASA as a practical actuator form including recent flight test of the Spanwise Adaptive Wing and Active Vortex Generators. Also an approved fatigue test method for SMA materials is needed to evaluate alloys, processing, and lot to lot variations. In this presentation two new standard test methods under development, torsional evaluation of tubes and bars and thermo-mechanical tensile fatigue cycling, will be described and preliminary test results shown. In addition to actuation fatigue, an understanding of the fracture toughness of SMA materials is crucial for evaluation of the limits of operational use and expected lifetimes. Most of the SMA fracture toughness values reported in literature are based on the premise of Linear Elastic Fracture Mechanics (LEFM), however the fracture behaviour of SMAs is complex compared to conventional linear structural metals. A methodology for measuring the fracture toughness of SMAs using Compact Tension specimens based on concepts of Elastic Plastic Fracture Mechanics (EPFM) employing the J-integral as the fracture criterion will be described. The proposed method relies on the ASTM: E1820 “Standard Test Method for Measurement of Fracture Toughness”, but differs by taking into account the mismatch among the apparent elastic properties of austenite, twinned, and detwinned martensite in SMAs.