Pressure at the Interface of Inner Conductor and Dielectric of Armorless Ocean Cable

In the process of designing new armorless coaxial ocean cables it is important to know the pressure at the interface of the inner conductor and the polyethylene dielectric. Determination of pressure at the interface would enable one to study the decay of tension, in a cable-laying machine, from the strength member to the holding surface, and to investigate the pressure effects on the resistance and capacitance of the cable. A suitable mechanical model of the armorless ocean cable is a polyethylene outer cylinder concentric with a steel inner cylinder. The cable is actually subjected, in three dimensions, to a pressure loading that varies with the depth of the ocean bottom. However, for a section of the cable on a relatively flat ocean bottom one may assume that the cable is subjected to a uniform external pressure. 963 964 T H E BELL SYSTEM TECHNICAL J O U R N A L , J U L Y 1960 One may obtain an expression for the pressure at the interface from a two-dimensional model (i.e., for plane stress or plane strain) using the theory of elasticity. However, the result is questionable, since polyethylene has a nonlinear stress-strain relationship in simple tension or compression. Moreover the value of Poisson's ratio for polyethylene, HP , ranges from 0.3 to 0.5, and it is not possible to determine nP accurately by a simple experiment. The value of the pressure at the interface, as obtained from such an equation, depends considerably upon the particular value of nP that is used. Hence, an experimental determination of the pressure at the interface is necessary, to investigate whether an elastic analysis is valid when one of the materials is potyethylene and to establish the value of Poisson's ratio for polyethylene.