Abstract This paper focuses on the criterion for the plastic rupture of a thin ligament associated with a part-through crack in a tensile plate subjected to an applied tensile stress. With a very wide (infinite width) plate, the critical crack length for ligament rupture is shown to decrease with increasing applied stress. At low applied tensile stress levels, i.e. less than 50% of the yield stress, ligament rupture requires a very thin ligament and a very long crack. With a finite width plate, the required crack length for ligament rupture is less than the infinite width plate value, provided that the crack and plate dimensions are such that general yield occurs. The implications of these conclusions with regard to the development of a leak-before-break condition are discussed.