An investigation on ductile crack initiation in structural steel has been made, based on the concept of Gurson's yield function for porous material.
In the first part, the condition of ductile crack initiation in a uniform stress field was investigated. The condition of ductile crack initiation under various stress triaxiality obtained from the tests on axisymmetric notched tensile specimens was found to be well expressed by the condition of constant void-volume-fraction obtained analytically from the Gurson's model. This result means that the condition of constant void-volume-fraction may be used as the local criterion of ductile crack initiation.
In the second part, the behaviors of void growth and ductile crack initiation in the area near the notch tip under mode I and mode II loading were investigated. Under mode I loading, the increase in void-volume-fraction around the notch with an increase in applied load agreed well with the behavior of porous material predicted by FEM analysis based on Gurson's yield function, and the ductile crack initiation could be predicted by the criterion of critical void-volume-fraction as in the case of uniform stress field given above. The same criterion was not applicable for the crack initiation under mode II loading and further study is needed.