Elastic State of Stress in a Stalpeth Cable Jacket Subjected to Pure Bending

Abstract

The stresses in the plastic jacket of a slightly bent telephone cable are analyzed within the linear theory of elasticity. The jacket is considered to be bonded to the underlying corrugated steel by a flooding compound. The constraining effect of the steel results in a three-dimensional state of stress that differs substantially from the predictions of elementary beam theory. For the thin jackets typically used on telephone cables it is found that the stress state is essentially biaxial, the axial and circumferential normal stresses being at least an order of magnitude larger than the others. On the tensile side, the stresses are closely approximated (at any given point) by those in the well-known biaxial strip experiment, in which the principal stresses are in proportion by the Poisson's ratio of the plastic. The compressive side is likewise in biaxial compression, and there the flooding compound is subjected to tensile stresses even before the onset of any jacket buckling. The results confirm the validity of previous approaches to the effects introduced by imperfections and indicate further that the probability of spontaneous cracking is increased by the adherence of the jacket to the soldered steel layer.