The Effects of Programme Loading on Fatigue Crack Propagation

Abstract

The object of the present research was to find and evaluate the effects of programme loading on fatigue crack propagation in EN24 steel. A potential drop technique was developed to measure very small changes in the crack length. The constant load amplitude test results, analysed on the basis of fracture mechanics, served as a basis for comparison with the results of the programme loading tests. The block programme loading schedules were designed to find the effects of load interaction and stress levels on the crack propagation rate. Microscopic and fractographic examinations were conducted to understand the influence of micro-structures and strength levels on the fatigue fracture process. The results showed that the micro-structural features of the three heat-treatments considered exerted a secondary influence on the crack growth rate. The capacity of the low strength ductile steels to accommodate large amount of strain at the crack-tip by plastic deformation and their ability to dissipate energy by numerous crack-branching and island formation were associated with the higher crack growth resistance of these steels as compared with the high strength martensitic steel. At any ΔK level, the crack growth rate was initially constant followed by a gradual slowing down which depended on the loading variables. With a growing fatigue crack at a constant ΔK, a decrease in the maximum stress caused a transient slow growth period whereas an increase in σmax did not show any appreciable acceleration of the crack growth rate. Based on a proposed mechanism of fatigue crack growth, an equation of the type da/dn= A(ΔK)M/R’, where R’ = σys/σmax, was found to unify the results of different loading conditions considered. This equation does not account for the slow growth periods and would, therefore, yield a conservative estimate of fatigue life.