A comprehensive fracture mechanics study has been carried out on the fatigue crack growth under various biaxial stress conditions. Fatigue tests have been conducted on a high-cycle biaxial fatigue test facility and a cruciform flat specimen of a weldable structural steel with a center crack parallel to one load axis or a slant crack.

The effects of biaxial stresses have been investigated on the fatigue crack growth properties ΔK_I-da/dN, the threshold conditions ΔK_Ith, and the crack closure behavior.

If a crack is large and stress level is low, no effect of biaxial stress appears. However significant effects of biaxial stress appear where a crack is small and stress level is high.

The mixed mode cracks were realized by a series of fatigue tests started from a slant crack in a cruciform specimen, where a longer bent crack or a branched crack appeared. It is found that such a nonstraight crack follows along the direction where ΔK_II = 0 and the ΔK_I-da/dN relations coincide with those for a straight crack under single K_I mode.