A Model for the Propagation of Fatigue Cracks in Aluminium

Abstract

Based on the results obtained by a series of X-ray and transmission electron microscope studies on the structure near fatigue cracks, a vacancy absorption model is proposed for the propagation of a fatigue crack. In the course of stress cycling, a great number of vacancies are generated in the plastic zone and in the stress-concentrated region around a fatigue crack. Those vacancies diffuse towards a region near the tip of the fatigue crack due to the compressive stress gradient at the crack tip. It appears that the growth of a fatigue crack occurs as a result of the absorption of the vacancies.
The migration rate of vacancies in a field of stress gradient and the total volume of vacancies absorbed by a fatigue crack in one stress cycle was calculated to give an equation for the propagation rate of a fatigue crack. Comparison of the propagation rates calculated from the above equation with those obtained experimentally leads to the following conclusions. The short circuit migration of vacancies along dislocations should be taken into account to discuss the fatigue crack propagation. The stress intensity factor and the frequency of stress cycling would have an important effect upon the vacancy mechanism mentioned above.