Abstract
The ASTM E3097 standard for constant-force thermal cycling provides a useful framework for measuring properties of shape memory alloys relevant to actuation applications, namely transformation strains and transformation temperatures. The standard allows for operator discretion when selecting a heating and cooling rate for the thermal cycling, provided that the specimen temperature is maintained uniformly within ±3°C across the specimen throughout the course of the thermal cycling. With larger test specimens such as dog-bones, for example, this can be ensured via thermocouples affixed to the specimen. Substantial work has been done to quantify the ruggedness of the standard for such specimens. However, it can be difficult to verify specimen temperature in-situ during testing of wire specimens due to their small diameter. Contact methods, such as thermocouples, can be difficult to affix to the specimen due to its small size. It is also difficult to get accurate measurements using contact methods, due to the often- substantial size and mass disparity between the temperature sensor and the wire specimen and the tendency of the temperature sensor to act as a heat sink for the test specimen. Non-contact methods such as infrared (IR) thermography are possible, but not always practical. A high image resolution is crucial for IR measurement of small diameters, and calibration adjustments may be required from one test to the next, depending on surface characteristics and resulting emissive properties, to ensure accurate measurements. For these reasons, one common practice is to thermally cycle the wire specimens by means of a temperature-controlled environment, rather than heating the specimens directly, and change the temperature of the environment at a rate such that the wire specimen remains equilibrated in temperature. It is, therefore, especially important when testing wire according to the E3097 standard for the test operator to understand the potential influence of heating and cooling rates on test results. This work presents results from ASTM E3097 tests performed on various wire diameters using different thermal ramp rates, as well as different testing media.
