VARIABLE AMPLITUDE LOADING OF SMALL FATIGUE CRACKS IN 6261-T6 ALUMINUM-ALLOY

Constant and variable amplitude (VA) loading fatigue studies were carr ied out on a 6261 aluminium alloy using cylindrical plain hour-glass s pecimens. Crack growth was monitored via surface replication using cel lulose acetate. Crack growth results at constant amplitude loading sho w the typical intermittent high and low periods of growth rate associa ted with crack-microstructure interactions. Acceleration in growth rat e during an overload block depends on crack length and stress amplitud e ratio. It appears to pass through a maximum at a crack length corres ponding to the first microstructural barrier. Microstructural-based mo delling is therefore required for small fatigue cracks, rather than so lely closure-based modelling. The Navarro-de los Rios model of short f atigue crack growth appears able to provide good indications of crack growth rates under VA block loading, and gives reasonable life predict ions. For short cracks (surface length < 80 mu m) and a small overload ratio (6.7%), crack growth may show severe retardation during the ove rload block. This is ascribed to crack tip blunting being more importa nt than the increase in stresses when closure is low. It appears from a Miner's rule type exercise, that VA block loading has its major effe ct on growth at a surface crack length of 20 mu m. This means that the crack initiation period cannot be ignored in life prediction models f or small fatigue cracks.