Stress-strain properties of polycrystalline graphites in tension and compression at room temperature

Longitudinal and transverse stress-strain curves were obtained by static tension and compression tests for several grades of polycrystalline graphite. The stress was applied cyclically either in tension or compression to observe hysteresis effects and residual strain as a function of stress. The stress, σ, was measured by an Instron load cell and the strains, ε, by strain gages. The longitudinal curves were fitted to Jenkins' equation (ε = Aσ + rBσ2); generally good fits were obtained in the region of low stress, and consistent deviations were found in the region of high stress. The transverse curves had different curvature in tension and compression, but the residual strain was positive in both cases. A series of cyclic tests in which the stress was alternately applied in tension and compression also demonstrated differences between the behavior of graphite in tension and compression. Some specimens were annealed after stressing and then retested; these experiments indicated that the residual strains were relieved and that the stress-strain properties were partially restored by a low temperature anneal and completely restored by an anneal to the graphitization temperature. A qualitative interpretation of the stress-strain properties of polycrystalline graphites, based on these results, is given.