Reduction of mode I fatigue crack growth rate due to occasional mode II loading

The effect of mode II loading on subsequent mode I crack growth is experimentally investigated on steel AISI 01. The results show that the mode I crack growth rate decreases after a single mode II load cycle. This effect remains also after the crack has propagated through the mode II plastic zone. The crack growth reduction is shown to be caused by crack closure due to tangential displacement of crack-surface irregularities. The durability of this reduction has a decisive influence on the fatigue life when the mode I R-ratio is not as high as to keep the entire load cycle above the closure level.