Temperature changes in specimens in microplasticity tests

Abstract

A tensile specimen and a gage assembly were designed to measure plastic strain at room temperature. The assembly mounts three capacitance strain gages 120 deg apart, and is held by ribs with a 45 deg included angle so as to define a 2 in. (5.08 cm) gage length. This novel construction eliminates spurious effects due to the fillets and shoulders of the specimen. The strain sensitivity is ±1 × 10⁻⁷. The cooling and heating of a specimen, accompanying the loading and unloading, respectively, in the elastic range, are shown to affect plastic strain readings in a manner not considered by other investigators of microplasticity. The thermal contributions to apparent strain in specimens of normalized 4340 steel and annealed Invar were determined and were found to be significant at strains less than 10^-5. Strain measurement techniques necessary for the exclusion of the thermal dimensional changes of the specimen from the measurement of very small plastic strains are discussed.