Crack instability in a fully ductile pipe system

The problem of crack instability is examined, with the following results (a) the conventional SHA defined collapse locus is a conservative assessment of the pipe loading capability within the plastic range; (b) a large safety margin exists considering actual loads, even if secondary loads are included; (c) a parameter is defined which gives the rate of hinge rotation increment with respect to crack extension; (d) comparisons with tearing moduli are given, showing that the crack is generally stable. (orig.)