Surface selection at the gripping interface for tensile testing per ASTM F2516 affects the pseudoelastic stress-strain response of Nitinol

Abstract

Complications such as premature failure and sample slippage associated with testing uniform cross-sectioned samples using conventional gripping systems are well known within the testing community. While these complications can be avoided for most materials using dogbone-shaped samples, the same does not apply to Nitinol due to the strong dependency of its response on sample geometry. This limitation significantly affects Nitinol product design and engineering, as most commercial Nitinol applications have uniform cross sections. To improve such testing, we evaluated the effects of grip-surface texture on the tensile behavior of Nitinol wire samples. Tests were conducted in general accordance with ASTM F2516 and ASTM E8/E8M. A novel set of criteria was introduced to assess testing performance. Rough, waveform, and knurled textures were investigated. The waveform texture captured nearly the entire stress–strain response without evidence of grip-related artifacts and yielded the clearest and most consistent measurements of the characteristic material properties. The rough texture enabled similar test progression, but evidence of microslip coincided with distorted transformation plateaus and greater variability in strain-related properties. In contrast, the knurled texture promoted grip-induced damage and local multiaxial loading, resulting in brittle-like fracture before the onset of necking, and substantially altered the measured transformation response.