Specimens of cold worked and annealed polycrystalline Nickel 270 were tested in strain controlled cycling followed by monotonic tensile loading to failure. The fracture surfaces of the failed specimens were examined using scanning electron microscopy. The effects of the initial condition of the material and precycling on the tensile response and the failure morphology of the specimens were investigated. A better understanding of the relationships between loading history, stress-strain response and damage morphology will be useful in developing improved predictive capability for simulated in-service structural response and multi-scale modeling of damage evolution under mechanical loading.
When a piece of metal is plastically deformed, much of the mechanical energy absorbed by the specimen is manifested in the form of heat. It is generally accepted that the portion of the mechanical energy of plastic deformation not dissipated as heat goes into raising the internal energy of a metal. The energy balance associated with plastic deformation may be quantified by the First Law of Thermodynamics: U = W - Q (l) where 0 is the rate of change in the internal energy of the system, W is the rate of irreversible mechanical work done on the system and Q is the heat dissipation rate. The internal energy may be partitioned into a part due to a temperature rise and a part due to "other" processes (Raghavan and Wagoner, 1987). These "other" processes include kinetic energy imposed by external loading, which is negligible in the present study, and changes in material structure; for example, the formation of point defects such as vacancies and interstitials, the generation and rearrangement of dislocations, and the formation of internal surfaces, specifically voids and cracks. These changes in material structure define the term damage as it is used in this paper.
