Crack Initiation, Slow Crack Growth, and Unstable Propagation of Ductile Fracture in Mild Steels in a Laboratory Experiment

A laboratory test for the unstable ductile fracture of mild steels was developed. A shear crack was initiated from a very deep notch. Some devices such as “pipe tabs” were used to reduce the stiffness of the system enough to allow the shear crack to continue to grow in an unstable manner. Four kinds of pipe materials were tested, and three of them with low ductility showed unstable ductile fracture. The fracture toughness for the onset of unstable fracture in these steels was evaluated and correlated successfully to upper shelf energy in the Charpy test. A slow crack growth test was performed with a high stiffness tensile system, ductile crack growth resistance R curves being obtained experimentally. A model of ductile crack growth was proposed by the modification of the Dugdale model, elastic/perfectly plastic crack analysis, by which R curves were finely described. Critical energy release rate values for the onset of unstable ductile crack propagation were shown to compare well with the values estimated from the R curves. Thus, ductile crack growth, both slow and rapid, is well characterized by the values of notch ductility.