An investigation of fatigue crack growth acceleration

The problem of fatigue crack growth acceleration was investigated in crack propagation studies and endurance testing. The study was driven by the needs of designers and researchers alike, to provide a better understanding of the mechanisms associated with accelerated growth, and recommendations on the use of Miner's rule to calculate fatigue life under variable amplitude loading. The study was conducted on S355 structural steel and 6082 T6 aluminium alloy using centre-crack tension (CCT) specimens, with and without additional welding, and longitudinal fillet welded specimens. Crack growth rates under simple sequence loading and more complex variable amplitude (VA) loading, all cycling down from fixed tensile stress levels, were determined using optical or direct current potential' drop methods and scanning electron microscope examination of fracture surface striations. Under simple loading sequences, comprising two magnitudes of stress range, the presence of tensile underloads resulted in accelerated growth rates compared with those based on constant amplitude (CA) loading. Various possible mechanisms to explain crack growth acceleration and factors that might influence it, notably crack closure and welding residual stress, were evaluated. The most promising outcome came from finite element analysis (FEA) of the crack tip stress and strain. This showed that whereas under CA loading the material near the crack tip cycled about zero mean stress, the mean stress was tensile after the application of a tensile underload, thus resulting in a higher crack growth rate. Fatigue endurance testing of welded joints performed under the same types of loading confirmed that Miner's rule overestimated the actual lives, consistent with the occurrence of acceleration. Thus, it was concluded that modification of the principle and application of Miner's rule is required to allow for stress interactions that cause crack growth acceleration. Preliminary design recommendations were made