Theory and Application of a Bi-Metal Ring-Stiffened Cylinder Under External Pressure

Abstract

Ring-stiffened pressure vessels have historically been constructed of one material only (for example, steel); this is especially true in naval submarines. Existing codes for the design of pressure vessels subjected to external pressure such as ASME and the British Standard BS5500 refer explicitly to unimetal designs only where the stiffening rings and the shell are made of one material only. Bi-metal structures are not accommodated in the design codes. A bi-metal structure would have one type of metal for the shell plating and another type for the ring stiffeners, possibly differing both in Young's modulus and material yield stress. An analytical study of the axisymmetric elastic deformations and stresses in a bi-metal ring-stiffened cylindrical shell under external hydrostatic pressure is given. A closedform solution to the governing differential equation for the bi-metal case is obtained and compared with the uni-metal case. A parametric study is conducted by varying the Young's moduli ratio for the shell and stiffeners of a typical design. Trends in the vessel's critical collapse pressure, shell deflection, and principal stresses are analyzed and the significance of different failure criteria is examined. An approach for modifying current uni-metal design methodologies to accommodate bi-metal structures is suggested. Preliminary comparisons of resulting bi-metal designs with "equivalent" uni-metal designs indicate that a bi-metal pressure hull can have significant advantages. Finally, recommendations for further research studies are made.