Fatigue Crack Growth under Random Loading (3 rd Report)

Fatigue crack growth under random loading has been extensively studied since nearly a decade in relation with the design of aircrafts, offshore structures, ship hulls, pressure vessels and so on.For the sake of simplicity, random loads in service conditions are often simulated by the block programed loads in laboratory fatigue tests. However, very few studies have been performed on the influence of sizes and sequence of load blocks, and on the difference between crack growth behavior under block programed loads and that under real random loads.The authors previously presented an experimental analysis on fatigue crack growth under block programed loading for simple cases where the stress ratio R= σmin/σmax=0.An analytical model was also presented to interprete the features of fatigue crack growth observed in the experiments.This paper presented an analysis on fatigue crack growth under more generalized block programed loads with varying stress ratio R. An analytical model similar to the previous one was presented to interprete the experimental evidences on crack growth behavior. A good agreement was obtained between experimental results and the calculations by the model. The linear damage law, an application of Palmgren-Miner hypothesis would give a non-conservative estimation for the crack growth life when the mean stress was high and/or the block size was small.