Fracture mechanics and acoustic emission studies of thin-walled pressure vessels containing surface flaws

Aiming at a quantitative assessment of safety against crack extension or initiation, hydrotests have been carried out on small pressure vessels. The vessels contained surface flaws of equal length and different depth. They were equipped with a clip-gauge for measuring COD and with three A.E. sensors which allowed A.E. energy differences to be evaluated. For all tests the failure pressure and the critical crack opening displacement were measured, together with the A.E. energy, ΣniAi2, recorded by all the sensors. Vessels containing the deepest cracks exhibited a leak-before-break, while vessels with less deep cracks ruptured completely. Two types of F.M. formulations have been considered, namely, the Kc fracture toughness criterion and the plastic flow stress (f.s.) criterion, σ. It is shown that the operating failure criterion for through-thickness propagation is always the f.s. criterion, provided that the crack depth is assumed to be the characteristic dimension of the flaw, rather than the crack length. The leak-break discriminating line, as deduced from literature data, works correctly only if the f.s. criterion is operating for both axial and through-thickness propagation. No systematic correlation has been found between AE parameters and leak-or-break mode of failure. Very energetic AE pulses (∼20 × 10−2V2 have been observed, which indicate cleavage crack nucleation. Low-energy pulses (≦ 0.5 × 10−2 V2 point out simple, ductile fracture mechanisms. These conclusions are supported by SEM-micrographic observations. A considerable “anisotropy” in AE activity has been observed in some cases when comparing the energies recorded by different sensors during the same test, and by sensors in equivalent positions during different tests. It has been proved that this anisotropy was not due to structure geometry or the incorrect sensor attachment. Its appearance might be due to an effective emission directionally related to the specific nature of the emitting event. Possible practical implications of these results are discussed.