Fatigue crack growth under mixed-mode loadings

The adoption of damage tolerance design concept along with increased demand for accurate residual structure and component life predictions have provided growing demand for the study of fatigue crack growth under mixed mode loadings in mechanical components. In this study, four-point bending and shear specimens made of SAE 1045 steel asymmetrically loaded under different initial KSeveral special tests were also conducted to study the effects of mode I compressive load, fatigue precracking, and prior overloads on fatigue crack growth. When the mode I load component was in compression under mixed mode I and II loadings, no fatigue crack growth was observed. Also, under mode II loading, when the specimen was not fatigue precracked, fatigue crack growth did not occur. Based on limited test data, it was found that initial mode I overloads result in accelerated fatigue crack growth rates on the subsequent mode II load cycles. Initial mode II overloads, however, retard fatigue crack growth rates during the subsequent mode II load cycles. Under large mode II loading, a crack propagated in mode II for a short distance before it changed its growth direction to mode I.