MICROSTRUCTURAL AND MECHANICAL ASPECTS OF FATIGUE CRACK INITIATION AND GROWTH IN A 2048 ALUMINUM ALLOY.

摘要

This thesis is concerned with a study of the micro- and macro-mechanics of fatigue crack initiation and crack propagation in an age-hardenable 2048 aluminum alloy, which is the high purity version of 2024 or 2124 aluminum, and their relationship to localized plastic deformation. Special attention was given to the importance of micro-structure, including grain size and ageing treatment in cyclic response and low cycle fatigue (LCF) behavior, crack growth along with the phenomena of crack tip closure, and on overload retardation effects. Crack closure was studied in detail throughout the course of this research program and its use in describing crack growth was explored. Fracture toughness tests were performed and the results were compared with the K(,IC) value reported in literature. Discussion is also included on the experimental techniques used in this research since they are novel and should have quite general applicability in fracture mechanics theory.; The results show that the LCG performance and thus fatigue initiation life based on the plastic strain control tests do not show any appreciable differences between different temper materials, but crack closure levels, crack growth rates and overload retardation are well influenced by microstructure. The lowest crack growth related to highest crack tip closure loads and the largest amount of overload retardation are associated with underaged microstructures which are prone to display non-uniform deformation pattern characterized by localized shearing. The closure model is also shown to provide an accurate description of overload retardation following single cycle and multiple cycle overloads in that we found a very good correlation between crack growth rate and "effective stress intensity range" (DELTA)K(,eff).