Fatigue Crack Initiation from Stress Concentrations in Cast Steels

Abstract

A range of plain carbon, carbon-manganese and low alloy cast steels were tested in fatigue in bending with machined notches of radii from 0.125 mm to 25.4 mm. The number of cycles to produce a detectable fatigue crack correlated well with both the range of stress intensity factor divided by the square root of the notch root radius and with the product of the stress concentration factor and the net section stress. The results obtained were compared with the initiation lives of specimens with notches of 12.5 mm and 25.4 mm with sand cast surfaces. This permitted the calculation of the effective notch radius arising from the cast surfaces, which was, in all cases significantly less than the macroscopic radius. The results indicated that the crack initiation behaviour of the steels examined could be predicted from a knowledge of the material yield stress. Fatigue crack propagation rates were also measured and shown to be anomalously high for fatigue cracks of 1 mm in length emanating from notches. This was shown to be caused by both rapid crack propagation through the region of the Neuber particle and by growth enhancement due to corrosion fatigue. The influence of water vapour, either adsorbed on the metal surface or present in the gaseous environment, significantly affected the initiation and propagation behaviour of all the steels examined. The evolution of hydrogen from specimens tested in the presence of water vapour indicated that a hydrogen embrittlement mechanism may be responsible for the effects observed. The results provide an adequate engineering method for design against fatigue failure from a wide range of stress concentrations for the cast steels examined.