Miniature Thermomechanical Ramping Tests for Accelerated Material Discrimination

摘要

Conventional thermomechanical fatigue tests are usually performed under conditions of close control of strain amplitude, temperature cycle, loading rates, and frequency-mainly for the purposes of developing data for design. However, in some cases, it is helpful to have a rapid assessment of the likely response to specific thermomechanical environments of those materials, particularly when new materials are being developed, or when material is in short supply, or when material production processes have been changed. For this purpose a miniature test system, developed at the National Physical Laboratory, has been evaluated for its ability to discriminate between different materials under these general circumstances, but with conditions of ramping load or temperature to accelerate material response and failure characteristics. The miniature test system uses D-C current to heat test pieces to appropriate temperatures, up to 1300℃ in under a minute if necessary, using small cross-sectional (2 x 1 mm) rectangular samples. Strain was measured using changes in electrical resistance of the central 2-3 mm of the gage length (16-20-mm grip separation), where temperature was known to be acceptably uniform. Several material types were studied, including a powder metallurgy high speed steel, cemented carbides, a refractory metal, Ta, and a Ni-base superalloy. Tests were performed under combinations of either ramping load or temperature ramping. The system was shown to discriminate well between the different materials under various conditions of the thermomechanical environment with respect to the effects of accumulated strain.