Acoustic emission and fractographic analysis of seamless steel pressure cylinders with artificial flaws under hydrostatic burst testing

Abstract

Eight 34CrMo4 high-pressure seamless steel cylinders (HPSC) with a nominal water-containing capacity of 80 l for a working pressure of 300 bar and an artificially manufactured flaw in each cylinder were subjected to a hydrostatic burst test with an additional monitoring of the acoustic emissions throughout the pressurization process. The use of water as a pressurizing medium makes the post-processing of acoustic emission data more difficult compared to pneumatic burst tests. Therefore, an artificial flaw has been utilized as a stress concentration element with the intent to situate the material damage process with the associated activity of the acoustic emission into a specific part of the geometry. With regards to the cylinder dimensions and the acquisition of an acoustic emission (AE) signal, the artificial flaw was, in all cases, milled in the middle of the cylindrical section and the remaining wall thickness under the flaw was equal to 1 mm. The presented results show the existence of three regions within the material damage process, namely void nucleation by cavitation, including the initiation of micro cracks, the stabilization of the cracks and the region with a further propagation of the previously stabilized cracks resulting in the final rupture of the cylinder wall under the notch. A complex analysis of the AE data, recorded at two different distances from the center of the flaw, in conjunction with the results from fractographic examination, enables the authors to perform a complex study of the material damage process evolution.