The growth of short fatigue cracks in a medium carbon steel
Short crack growth behaviour was studied using a method of plastic replication on hour-glass shaped specimens of a medium carbon steel which were subjected to push-pull fatigue testing at ambient temperature. Crack lengths were measured from replicas using an optical microscope from which the growth rate could be calculated. A theory for short crack growth is presented which may be expressed mathematically by the equation:- = (for a < d) da dN where (a) is crack length, (d) is a characteristic dimension between adjacent microstructural obstacles to crack propagation, C1 is a function of stress or strain range and a is a constant. For the medium carbon steel used in this study (d) was equated to the ferrite band length which contained the growing crack. This theory was used to model short crack growth in an Aluminium alloy T6-7075 Al and for the medium carbon steel used in this project. iii By using a second equation to describe "long" crack growth of the form:- da dN = where C2 is a function of the applied strain range and 0 is a constant, it was then possible to describe the complete history of crack growth. By obtaining short crack growth data for different stress levels, quantitative expressions of these two equations were calculated from which fatigue lifetime predictions could be made by integration of the equations for any stress level. Using this method an estimate could also be made of the percentage of fatigue lifetime spent in the initiation and growth of short cracks.