Fast Cracking during Fatigue Crack Propagation in Metals

Abstract

FIG. 1 shows the fracture surfaces of an alloy steel specimen of 0.5 in. × 1 in. section which had been fatigued by bending in a cantilever machine¹ at the Royal Aircraft Establishment, stressing being of the form 1.1 σ ± σ. Cracking started from a 60° V notch 1/8 in. deep and having a root radius of 0.002 in. which was machined in the specimen. Measurements of the fatigue crack propagation rate were made using an electrical resistance method to measure the crack length². The light areas on the fracture surface are regions where the crack was advancing at a rate of about 10⁻⁶−10⁻⁵ in. cycle⁻¹, while the dark areas are regions in which the crack advanced by brittle cracking at a much faster rate, the rapid advance being accompanied by a cracking noise from the specimen. After its rapid advance the crack resumed its normal rate of about 10⁻⁵ in. cycle⁻¹. This type of failure is always obtained for this type of steel fatigued at room temperature. It was first thought that the brittle fracture advance occurred during one stress cycle, the crack then resuming its original rate. If this was the case, however, it is difficult to see why the crack, having acquired a high velocity, should stop before the specimen was completely broken.