Microstructurally sensitive short fatigue crack propagation behaviour in a 7010-T7451 aluminium alloy under variable amplitude loading conditions

摘要

Fatigue studies have been carried out on an aluminum 7010-T7451 alloy, under constant and variable amplitude loading conditions (CA and VA), using cylindrical plain hour-glass specimens. The plastic replica method was used to obtain information regarding surface crack growth. Load ratio effects have been examined under constant loading conditions, which show that increasing the load ratio accelerates crack growth rate, reducing fatigue life. The experimental data of variable amplitude loading show that while repeating single underloads increases cumulative damage, leading to a reduction in fatigue life, repeating single overloads has a retarding effect on the crack growth, prolonging fatigue life. Additionally, the higher the overload ratio the larger the retardation. The results of two step tests and two step block tests all showed a trend of longer than expected fatigue life in terms of Miner's cumulative damage summation values, irrespective of High-Low or Low-High sequence but particularly during the High-Low sequence. This interesting phenomena concerning the case of the High-Low sequence, which is totally opposite to observations on other materials, may be rationalised in terms of increased work hardening following the high stress step, giving rise to higher fatigue resistance. Results of variable amplitude tests show that they can be satisfactorily modelled in terms of the Navarro-Rios model, implying the importance of using microstructural fracture mechanics principles in modelling short fatigue crack growth behaviour. They also show that consideration of work hardening within the crack tip plastic zone as the crack advances, and of crack closure, is required for a better understanding of variable amplitude loading effects on short fatigue crack growth behaviour.