Effects of Processing Residual Stresses on Fatigue Crack Growth Behavior of Structural Materials: Experimental Approaches and Microstructural Mechanisms

Fatigue crack growth mechanisms of long cracks through fields with low and high residual stresses were investigated for a common structural aluminum alloy, 6061-T61. Bulk processing residual stresses were introduced in the material by quenching during heat treatment. Compact tension (CT) specimens were fatigue crack growth (FCG) tested at varying stress ratios to capture the closure and Kmax effects. The changes in fatigue crack growth mechanisms at the microstructural scale are correlated to closure, stress ratio, and plasticity, which are all dependent on residual stress. A dual-parameter ΔK-Kmax approach, which includes corrections for crack closure and residual stresses, is used uniquely to connect fatigue crack growth mechanisms at the microstructural scale with changes in crack growth rates at various stress ratios for low- and high-residual-stress conditions. The methods and tools proposed in this study can be used to optimize existing materials and processes as well as to develop new materials and processes for FCG limited structural applications

Microstructural mechanisms and advanced characterization of long and small fatigue crack growth in cast A356-T61 aluminum alloys

Fatigue crack growth-based design is a significant modern engineering consideration for the transportation sector, and its implementation requires accurate characterization and understanding of crack propagation mechanisms with respect to microstructure. To support this goal, long and small fatigue crack growth studies were conducted on widely used A356-T6 cast aluminum alloys in various microstructural conditions. Microstructural variations were created through processing and chemistry means in order to systematically investigate the individual and combined effects of the materials’ characteristic microstructural features on fatigue crack growth at all growth stages. Crack growth mechanisms and failure mode transitions are identified with respect to the eutectic Si morphology/distribution and grain structure by fractographic techniques and electron backscatter diffraction. Crack-microstructure interactions were investigated in depth across all crack sizes, and the respective roles of microstructural features were identified experimentally and further corroborated by numerical models. It is concluded that the eutectic Si phase enhances the alloys’ fatigue crack growth resistance in early growth stages (by transferring stresses off of the α-Al matrix), and progressively decreases due to damage localization. In later growth stages, the eutectic Si phase becomes increasingly detrimental to fatigue crack growth resistance because of its inherently low debonding strength and brittle fracture, as evidenced by the crack selectively following eutectic Si colonies