A damage integral approach for low-cycle isothermal and thermal fatigue

A damage integral approach for low-cycle isothermal and thermal fatigue is examined in this thesis. This approach involves the calculation of the stress, estimation of damage rates and integration of the damage to determine the low-cycle fatigue life. The stress profile is dependent on the time profile of the applied displacement, the effective stiffness and the inelastic deformation properties. The rate of damage accumulation is influenced by the stress state, the environment and the temperature.; A phenomenological approach based on a state variable concept has been found adequate in describing the stresses for a variety of test methods and loading profiles. The considerations involved in the application of a similar approach to damage rate modelling is considered here. The ramifications of the environment and damage mechanisms on a phenomenological damage rate relation are examined.; The application of a damage integral approach to low-cycle high temperature fatigue based on such phenomenological formulations is demonstrated using solder joint fatigue as an example. Solder joints with eutectic compositions have been emphasized. First, the application of published data is reviewed and it is shown that phenomenological constitutive relations may be used to accurately calculate the stresses in the solders and solder joints during isothermal mechanical displacement and thermal displacement cycling. The determination of the crack growth rate from fatigue data is demonstrated. The experimental portions of this study are directed towards conditions for which additional data are required. The phenomenological model for crack growth is used to describe the damage rate. The effects of temperature and applied loading frequency on the crack growth rate are discussed. Finally, the considerations involved in the integration of the crack growth rate formulation, and effects of the failure criterion on the apparent fatigue life are described.