Environmental Acceleration of Fatigue-Crack Growth in a High-Strength Steel

Abstract

With precise control of the environmental species, water vapour, oxygen, and hydrogen are observed to increase fatigue-crack growth rate in a high-strength steel. The gas-pressure-dependence of the increase is similar for water vapour and oxygen, as the two species generally exhibit little or no change in crack growth rate at high and low pressures and an intermediate transition region of rapid increase. Although oxygen and water vapour produce individual non-additive effects, water vapour causes a 20% larger Increase In crack growth rate and is the dominant species in mixtures of these two gases. The increase is much greater with hydrogen but does not start until appreciably higher pressures are reached than with either water vapour or oxygen and there was no sign of levelling off at the highest pressure of this investigation. Calculations based on the rate of gas molecule impingement and the generation of fresh crack surfaces indicate that the rate-controlling mechanism of accelerated crack growth is adsorption for oxygen and water vapour but not for hydrogen.