Mixed Mode Brittle and Ductile Fracture of a High Strength Rotor Steel at Room Temperature

The mixed mode fracture of a high strength rotor steel has been investigated at room temperature using single edge notched specimens. In mode I, and for limited amounts of shear loading, the steel exhibited cleavage fracture. For conditions near mode~II ductile fracture occurred. A transition from brittle to ductile fracture occurred for mixed mode loading. Finite element analysis provided estimates of the extent of near crack tip yielding and elastic-plastic stress intensity factors. Test results agreed with the maximum tensile stress (MTS) criterion for small scale yielding for limited amounts of shear loading. The load for mode II fracture was lower than predicted from the MTS criterion, but higher than predicted from plastic collapse predictions. Observed fracture angles where in broad agreement with the predicted fracture mechanisms. The load for the transition from brittle to ductile fracture was found to agree approximately with the predicted load.