Initial stages of damage in high stress fatigue in some pure metals

Abstract

The results to date are reported of an investigation into the initiation and propagation of fatigue cracks at high and low stresses in aluminium, nickel and copper. It is shown that crack propagation involving the formation of ripples on the fracture surface occupies, at high stresses, a large portion of the fatigue life and the whole of the fracture surface area, whereas at lower stresses a considerable part of the fracture surface may be covered with ripples if the specimen has been previously annealed, but only a few per cent of the total number of cycles are consumed in this type of fracture. Observations and measurements on the slow initial stages of crack growth at high and low stresses have been made, and it is shown that the variables of metallurgical condition of the specimen, environment and frequency of cycling affect this stage more significantly than propagation involving ripples. In discussion it is suggested that the environment assists the fatigue process by a corrosive action rather than by preventing welding of the crack tip during the compression part of the cycle.