Role of Intergranular Damage‐Induced Decrease in Young's Modulus in the Nonlinear Deformation and Fracture of an Alumina at Elevated Temperatures

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Abstract
The effect of the time‐dependent decrease in Young's modulus due to damage accumulation by pore growth and intergranular cracking on the stress‐strain behavior of a coarse‐grained polycrystalline alumina deformed under conditions of displacement control at elevated temperatures was investigated. Considerable nonlinearity in stress‐strain behavior, which increased with decreasing strain rate, was noted. At the higher strain rates, the failure stress was found to be independent of strain rate, thought to be due to a strain‐rate‐dependent fracture toughness due to the growth of microcracks at the tip of the failure‐initiating macrocrack, which offsets the expected strain‐rate sensitivity due to the growth of a single macrocrack only. Pore growth and intergranular cracking, accompanied by major reduction in Young's modulus by as much as a factor of 5, was most pronounced at the intermediate values of strain rate. This decrease in Young's modulus, under conditions of displacement‐controlled loading, results in a decrease in stress, referred to as strain softening, which contributed to the observed nonlinear deformation. This conclusion was confirmed by a theoretical analysis, which showed that in addition to diffusional creep, time‐dependent decreases in Young's modulus (elastic creep) by crack growth can make significant contributions to nonlinear deformation.

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