Elastic fracture properties of all-steel gas cylinders with different axial crack types

All steel cylinders are being used for on-board storage of compressed natur al gas in vehicles. Typical maximum fill pressure for these cylinder is 25. 85 MPa (3750 psi). These cylinders are subjected to fluctuating pressures, due to the refueling operation. In order to establish a relevant test metho d to ensure leak before break failure performance in the event of a through -wall cracking, the finite element stress analysis of the design containing various defects has to be firstly carried out to get some theoretical basi s for the establishment of the test method. External and internal axial sem i-elliptical surface cracks are modeled. Crack front regions are modeled us ing singular elements, whereas the rest of the cylinder is modeled using tw enty-node hexahedron elements. Not only the cylindrical body but also the n eck and transition areas of the cylinder are considered in the modeling. Sl ender cracks with approximately 10 times the wall thickness of the cylinder , which often appear in the engineering application of all steel gas cylind ers, are considered. The crack depths varied from 25% to 100% of the wall t hickness. Analysis is also carried out for the cylinder with through-wall a xial cracks, which have similar crack lengths with external and internal su rface cracks. The cylinders are assumed to be in the elastic deformation st ate. Stress intensity factor, Kr, and crack mouth opening displacement, CMO D, as the functions of internal pressure, crack size, location (external ve rdus internal) and shape (elliptical versus straight-fronted), are establis hed. Calculated results are compared with published results. Deep axial ext ernal cracks are found to be more severe than axial internal surface cracks having similar crack lengths. Crack driving force for a semi-elliptical th rough-wall crack is found to be significantly less than that of a straight- fronted through-wall cracks, which have the same crack length. So, the esta blishment of a relevant test method to ensure leak before break failure per formance in the event of through-wall cracking is of high practical value f or the engineering design and application of these cylinders. (C) 1999 Else vier Science Ltd. All rights reserved.