Values of the fracture toughness KIc and the elastic-plastic toughness parameter Ji or KJi were determined for a medium-strength pressure-vessel steel (Type 22Ni-Mo-Cr3 7-SW similar to A508 Class 2) and a high-strength quenched and tempered steel 30Cr-Ni-Mo8 following the rules of ASTM Test for Plane-Strain Fracture Toughness of Metallic Materials (E 399-81) and ASTM Test for JIc, a Measure of Fracture Toughness (E 813-81). Proportional wedge-open-loaded X-type (WOL X) specimens with thicknesses 25, SO, 100, and 200 mm and compact (C) specimens with thicknesses 20, 40, and 80 mm were tested. This allowed for both cases to cover a wide range of toughness and temperature in which the results from specimens of different sizes and different evaluation procedures could be compared.
A single specimen J-evaluation procedure was developed and checked by compliance calibration experiments, and onset of stable crack growth was registered by a d-c potential method and interrupted loading tests. The linear elastic fracture toughness KIc (T)-curve from large specimens for both materials is in agreement with the KJi (T)-curve from small specimens up to a level of about 150 MN/m3/2.
Extensive scanning electron microscope investigations of the failure phenomena at the crack front showed that for all specimens of one steel tested at a given temperature. the width Δs of the stretched zone is about the same and is proportional to the square of the critical stress-intensity factor. This is true all through the transition regime and it is there where Δs seems to be the controlling parameter for fracture initiation regardless whether the subsequent fracture is of cleavage type, is stable crack growth followed by cleavage, or is fully stable crack growth. The equality of the toughness parameters KIc and KJi can then be expected.