The fatigue crack growth behaviour in aluminium alloy sheets of 2024-T3 and 747ST761, subjected to standardized spectra (TWIST and FALSTAFF), was investigated using centre-cracked specimens. A strip crack closure model was used to interpret experimental data, and to make predictions for the crack growth.The strip model is based on the Dugdale concept, but modified to keep plastically-stretched materials on the crack surface so that the crack opening load can be determined, and the fatigue crack growth can be analysed according to Elber's crack growth assumption. Differing from other models of the same kind, a variable constraint factor was introduced to account for the gradual transition of stress state at the crack tip resulting from the crack growth. It has been shown that the transition of stress state at the crack tip causes the unusual behaviour of the fatigue crack growth in sheets. Both experiments and predictions show that a crack may grow faster at a low load than at a higher one in a certain applied load range due to the crack tip stress state transition. The crack tip stress state also contributes to the thickness effect observed for the crack growth in sheets. In agreement with experimental results, it has been shown that a plane stress state will prevail at the crack tip in a thin sheet compared to that in a thick sheet. The plane stress state results in a higher crack opening level which leads to a longer fatigue life for thin sheets. NOMENCLATURE a = crack length R = stress ratio S = gross stress T = plate thickness a = stress state constraint factor p = crack tip plastic zone size uy = yield stress K , AK = stress intensity factor, and range Subscripts c = critical eff = effective max = maximum min = minimum op = open th =threshold
A generalised approximate crack surface displacement solution for the two-dimensional part-elliptical mode I crack was developed. This solution includes the surface crack, corner crack and embedded crack, which is subjected to the arbitrary crack surface pressure. The crack surface displacement is derived from stress intensity factor solution and corresponding crack surface pressure distribution. Comparisons of the solution with accurate solutions showed that rather high accuracy has been achieved with the developed solution for various surface, embedded and corner crack problems. This solution can be used to derive three-dimensional weight functions as long as the stress intensity factor and the corresponding crack surface pressure are available for arbitrary mode I problems.
Investigations were performed for the round-ended straight attachment lug with a single crack emanating from the hole with the weight function method. The weight functions, covering the geometries from W/D = 1.5 to W/D = 4.0, were generated from the results obtained with a boundary element method using the approximate weight function technique. The results have been given both in the form of analytical weight functions and tabulated dimensionless stress intensity factors for simple normalized powers of the crack line loading. This is a simple straight forward procedure to calculate stress intensity factors once the crack line loading is approximated by a polynomial. The present method is also valid for deriving stress intensity factors and weight functions for general crack configurations.
A procedure has been developed to derive stress intensity factors (SIFs) for part-elliptical cracks based on an approximate crack surface displacement mode assumption for general configurations. The crack surface displacement mode is composed of available 2D crack surface displacement modes at intersections of the crack surface and boundaries, or in symmetry planes. Along with the obtained crack surface displacement mode, SIFs are determined by the magnitude of the crack surface displacement derived from energy release rate for virtual crack increments. The procedure was analytically verified with the exact solution for an embedded crack in an infinite body subjected to uniform crack surface pressure. Several examples show the obtained results in acceptable agreements with available solutions.
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