The effect of crack angle on fracture toughness

The effect on fracture toughness of a crack inclined at an angle β to the plane of a plate, so that Mode I and Mode III crack surface displacements were present, was investigated using ASTM-type single-edge notch tension specimens made from two aluminium alloys and a tool steel. It was found that for β = 45° plane stress conditions prevail even in thick plates, and that for DTD 5050 aluminium alloy the apparent value of K IC ( K AC ), determined ignoring the crack angle, is approximately equal to the K c value obtained from thin sheets. For values of β between 90 and 45°, crack extension starts when the resolved value of the opening mode stress intensity factor K I equals K IC ; the shear mode stress intensity factor K III , apart from increasing the size of the plastic zone at the crack tip does not affect the start of crack growth. By contrast, tests on DTD 5050 centre crack specimens with cracks inclined at angle α so that Mode I and II crack surface displacements were present showed that as the resolved value of the edge sliding mode stress intensity factor K II increased, the value of K I at the start of crack growth first increased slightly and then decreased sharply. Some comments on the estimation of stress intensity factors and plastic zone sizes for inclined cracks are given in an Appendix.