Crack paths for mild steel specimens with circular holes in high cycle fatigue

Abstract

The most common current models for predicting the fatigue limit in notched solids use the stresses along a straight line, beginning at the notch root, to make the prediction. This line represents a simplification of the path of a real crack, which usually has a first part, known as stage I, in the direction of the maximum tangential stress, and a second part, known as stage II, in the direction perpendicular to the maximum normal stress. In this work, experimental crack paths for notched solids are analysed, with the objective of establishing the directions and lengths of stages I and II of fatigue crack growth from notches. The material was a mild steel, the geometry of the specimen was a thin-walled tube with a passing-through hole and the tests were axial, with R = -1. From the tests, the S-N curves were constructed and the fatigue limits were calculated. For the high cycle fatigue tests, the cracks paths were studied, with special attention to the crack initiation point and the crack direction along the first grains. The cracks paths on the specimen outer surface were studied with an optical microscope. In this surface, the crack initiation point was close to the maximum principal stress point at the hole contour. The direction of the crack in the first and second grain showed great variability. This variability noticeably decreased as the crack reached a length of 10-20 grains, approaching the direction of Mode I. However, the crack might actually start at an interior point on the surface of the hole, which has a depth of 1500 μm. In fact, the point of maximum principal stress of the entire specimen is not at the specimen outer surface but on the internal surface of the hole at 750 μm from the outer surface, that is, half the thickness of the specimen. The crack path in the plane transverse to the hole containing this point of maximum principal stress was analysed. For this, the fracture surfaces, at both sides of the hole, were analysed with a non-contact 3D optical profiler. The crack path in this internal transverse plane followed the trend described for the crack path on the specimen outer surface: the initiation point close to the maximum principal stress point at the hole contour, great variability in the direction of the crack along the first grains and tendency to Mode I direction when the crack gets longer.