Three dimensional finite element prediction of crack closure and fatigue crack growth rate for a corner crack

The effect of plasticity induced crack closure on fatigue crack growth rate has been examined for the case of a 2024 aluminium alloy plate with two lateral corner cracks emanating from a central hole. The plate was subjected to constant amplitude loading and a combination of constant amplitude and single overload loading prior to the measurement of the opening load. The opening load was measured by comparing the strain ahead of the crack tips with the far field strain as the load on the specimen was cycled. Measurements were also made of the growth rate of the crack in fatigue.A three-dimensional finite element quarter model was developed of the plate with a central hole and corner cracks. The geometry of the model forced the crack to assume a constant aspect ratio of bore length to surface length as the crack grew. Two different aspect ratios of 1:1 and 1.5:1 were considered. The load on the model was cycled and the crack extended by releasing nodes so as to predict the development of a plastic wake behind the advancing crack. The opening load was predicted from the finite element displacement versus load results. These opening load predictions agreed favourably with the experimental measurements.The finite element model was also used to predict the fatigue crack growth rate. This was achieved by first evaluating the effective stress intensity factor range from the stress distribution ahead of the crack tip. A Paris law fit to high R-ratio test data was then made to allow the corresponding growth rate to be calculated. Encouraging comparisons of the finite element predictions of fatigue crack growth rate with experimental measurement were obtained.