Expressions have been derived which describe the critical stress and pressure necessary to rupture oxide blisters which form on aluminum during the growth of corrosion pits. These expressions have been derived and evaluated for radial cracks in the oxide blister. The critical stress and corresponding pressure for rupture decrease with increasing blister radius and decrease with increasing crack length. The critical stress is independent of the ratio of oxide film thickness to blister radius, whereas the rupture pressure increases with this ratio. The critical stress is independent of Poisson's ratio for the oxide film whereas the rupture pressure is weakly dependent on Poisson's ratio for the oxide film.
The effect of biaxial stress cycling on the crack growth process in IN-718 plate specimens was investigated using a semielliptical surface flaw as a crack starter. The method of anticlastic bending was used to generate the biaxial stress field. This bending method generates two bending stresses: a tensile stress normal to the crack surface and a compressive stress transverse to the tensile stress at the tip of the crack. In a biaxial stress field, the linear elastic fracture mechanics (LEFM) theory predicts that the stress parallel to the crack direction has no effect on crack growth rate. However, in the past two decades, experimental studies have shown that a stress parallel (transverse) to the crack tip influences crack growth rate. In this investigation, the effect of biaxial stress ratio (β = − σ2/σ1) of − 1 on fatigue crack growth is presented. At a constant cyclic load, a crack growth transition from Mode I to Mode II was observed to take place when ΔK reached the value of 17.5 ksi √in. (19.2 MPa √m). Also, an acceleration in the fatigue crack growth rate was observed due to the compressive transverse stress.
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