It is our purpose to review fracture characteristics of heavy-walled pressure vessels in relation to the plane-strain crack toughness known under the term,
K
Ic
. As a starting point, suppose that direct measurement of the strength of a full-scale pressure vessel containing a specific crack is contemplated. An initial crack of approximately the desired size can be introduced in several ways, for example, by inserting a sharp groove and then vibrating that region until a fatigue crack develops. However, full-scale testing is often impractical either for reasons of expense or because the introduction of in-service damage, say by nuclear irradiation, is not feasible at the full-scale size. Furthermore, valid test results can usually be obtained at much smaller scale. Small specimen fracture tests Crack extension behaviour observed in a small specimen test can be regarded as representative of full-scale fracture behaviour so long as the stresses carried by the surrounding material into the region containing the crack receive adequate representation. Since the specimen size desired for irradiation purposes is quite limited, we consider next whether crack extension of a large part-through crack in a thick-walled pressure vessel can be modelled by testing just the slice of material indicated in figures 108 (
a
) and (
b
). The calibration and use of test specimens similar to the one shown in figure 108(
b
) are described by Sullivan (1964).