1978 Volume 64 Issue 7 Pages 947-957
A laboratory test for unstable ductile fracture of mild steels was developed. Shear crack was initiated from very deep notch. Some devices such as “pipe tabs” were employed to reduce the stiffness of the system enough to make shear crack continue to grow in an unstable manner.
Four kinds of pipe materials were tested using the present test method and three kinds of them with low ductility showed unstable ductile fracture. Fracture toughness for the onset of unstable ductile fracture in these steels was evaluated and was correlated successfully to Cvshelf, upper shelf of absorbed energy in Charpy test.
Slow crack growth test was also performed by means of high stiffness tensile system, “R-cruve” to ductile crack growth being obtained experimentally. A model to ductile crack growth was proposed by the use of the modification of Dugdale model, by which R-curves were finely described. Critical G-values for the onset of the unstable ductile crack propagation were shown to be well compared to the values estimated from the R-curves.
Thus, both the ductile slow growth and its rapid propagation are shown to be well characterised by the values of notch ductility.
