The development of an axial breach during pneumatic rupture of a thin-walled, mild steel pipe is investigated by high-speed photography. Data are presented for the variation of breach area with time, the velocity of the propagating breach tip and the velocity of the free edges. The results indicate that the initial stages of breach growth are controlled by inertia effects, but this is soon superseded by growth dictated by the straining of the free edges and the progress of the propagating crack. A “fully open” breach (breach area = twice pipe cross sectional area) is achieved when the breach length is three pipe diameters. At a breach length of eight pipe radii the breach geometry immediately behind the propagating crack has ceased developing and remains fixed in the subsequent breach development. The measured crack tip propagation velocity, which reaches a peak steady value well before the breach is “fully open,” is compared with a theoretical limit based on the propagation velocity of a displacement wave in a plastic membrane.

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