Elasto-plastic fracture properties of an all-aluminium gas cylinder with different cracks

All-aluminium cylinders are used for on-board storage of compressed natural gas in vehicles. Besides being subjected to the maximum fill pressure, the se cylinders are subjected to fluctuating pressures, due to refuelling oper ations. In order to establish a relevant test method to ensure leak before break failure performance, elasto-plastic finite element stress analysis of the design containing various defects was carried out to obtain a theoreti cal basis for the establishment of the test method. Axial semi-elliptical c racks in the central portion of the cylinder and circumferential cracks in the bottom of the cylinder are modelled using 20-node hexahedron elements. Not only the cylindrical body but also the neck and transition areas of the cylinder are considered in the modelling. Slender cracks with lengths appr oximately five times the wall thickness of the cylinder, which often appear in applied all-aluminium gas cylinders, are considered. Crack depths varie d from 22.5% to 100% of the wall thickness. Through discussions about the c alculated J-integral and crack mouth opening displacement (CMOD) of the axi al and circumferential cracks, the effects of the different cracks on all-a luminium cylinders in the elasto-plastic deformation state are made clear. The analyses show that under the elasto-plastic deformation state, axial cr acks in the centre of the cylinder an more dangerous for the cylinder than circumferential cracks in the bottom of the cylinder, if these are of the s ame size and under the same conditions. The axial external crack is found t o be most severe among these different crack types. Finally, the CMOD of cy linders with an axial external crack have been measured by the experimental method and a good agreement between the calculated CMOD and the tested CMO D was reached. (C) 1998 Elsevier Science Ltd. All rights reserved.