The influence of sheet thickness on delayed retardation phenomena in fatigue crack growth in HT80 steel and A5083 aluminium alloy

Abstract

Three types of delay tests were carried out on HT80 steel and A5083 aluminium alloy in order to investigate the differences in the retardation behavior of fatigue crack growth in the central and edge regions following a single application of an overload; the first type were tests on specimens of four different thicknesses, the second type were tests on specimens whose surface layers were machined away just after the overload, and the third type of test used variations in the loading to put beach-marks on the fatigue fracture surface. In the tests on the specimens of four different thicknesses, the retardation effect was almost constant in the predominantly plane strain region and increased drastically in the plane stress region for the steel, while it became a minimum at the transition from plane strain to plane stress for the aluminium alloy. Machining away the surface layers following the overload, in the type 2 tests, resulted in a drastic decrease in the retardation for the aluminium alloy, but had little influence on the retardation for the steel. The beach-marking tests revealed that the crack front was slightly curved before the overload was applied. Following the application of the overload the curvature initially decreased, and then increased until the retardation was over at the surface. Thereafter, the curvature decreased toward the equilibrium curvature characteristic of the preoverload fatigue conditions. This difference in retardation behavior between the surface and the interior was much larger in the aluminium alloy than in the steel. This behavior was explained theoretically on the basis of the test results both on the specimens of varying thicknesses and on the surface-removal specimens.