Crack Closure Level Research Articles

Fatigue Crack Propagation under Periodic Overstressing with Special Attention to Crack Closure and Dislocation Structure

Fatigue crack propagation by periodic overstressing, i. e. by intermittent application of a very small number of cycling of overstress during a very large number of cycling of understress below Kth, was studied with plate specimens under the in-plane reversed bending stress condition. This condition is a more sophisticated test condition from the standpoint of fracture mechanics than the rotating bending test previously reported. Remarkable acceleration of crack propagation amounting to more than one hundred times was observed in the present study as in the previous ones, indicating the validity of the previous results. Crack closure levels were not affected appreciably by the periodic overstressing in most of the tests. An extra-ordinarily well developed dislocation cell structure was observed in thin foil specimens containing a fatigue crack by means of ultra-high voltage electron microscopy. The acceleration could be attributed not to mechanical causes such as crack closure but to microscopic processes such as the change of dislocation structure observed in the present study or the change of crack propagation mechanism revealed by fractography in the previous studies.

Four lectures on fatigue crack growth: II. Fatigue cracks, plasticity effects and crack closure

Concepts introduced are residual plastic deformation, residual stress, reversed plastic deformation, plastic deformation in the wake of the crack and crack closure under tensile load. COD measurements as a method to determine the crack closure level are discussed. The significance of crack closure for fatigue crack growth is analysed and illustrated by several examples, including effects of yield stress, stress ratio and delayed crack growth after a peak load. Finally some attention is paid to three dimensional aspects following from thickness effects, shear lips and curved crack fronts.

繰返し曲げによる非貫通疲労き裂の進展

Experiments were conducted on the propagation of part-through cracks (semi-elliptical surface cracks) by alternating bending stresses with sheet specimens of an aluminum alloy (2017-T3). Crack closure levels were determined by means of the compliance method as well as by the surface observation. The propagation rate of part-through cracks could not be correlated with that of through-thickness cracks in terms of the stress intensity, but it could be in terms of the effective stress intensity based on closure measurements by the compliance method. The effective stress intensity based on closure measurements by the surface observation was applicable neither to crack propagation in the interior nor to that on the surface of specimens.

FEM analysis of crack closure and delay effect in fatigue crack growth under variable amplitude loading

An analysis of fatigue crack closure under variable amplitude loading was made by using the finite element technique. Two basic types of variable amplitude loading were selected for the analysis; constant amplitude loading with a single overload and block loading. A characteristic variation of a crack closure level was found to exist for both types of loading: the trace of the crack closure level vs crack length rose to a maximum value and then decreased asymptotically. The characteristic behavior was explained in terms of the residual stress which had been induced by an overload or a load preceding to the variation. The predicted fatigue crack growth behavior which was obtained analytically was consistent with the experimental results, and it was concluded that the retardation and acceleration phenomena are closely correlated with the crack closure.

Crack closure at positive stresses during random load fatigue crack growth

Fatigue crack closure at positive stresses during broad band, random load fatigue crack growth, has been investigated. Tests were performed on centre-notched specimens made from an aluminium alloy (B.S. 2L 71). The closure stress was found to be dependent upon the material thickness, the mode of fracture and the stress ratio Q. Under plane strain fracture mode conditions it was found that the closure stress for 22 mm thick specimens was greater than that found with 6 mm thick specimens. This difference in closure behaviour produced a slower fatigue crack growth rate for the thicker specimens. In addition it was found that for a given r.m.s. stress the fatigue crack closure level only impinged upon the dynamic range at low mean stress levels.