Dynamic Stress and Displacements in a Two-Material Cable System Subjected to Longitudinal Excitation

Abstract

This paper deals with the analysis of the dynamic stresses and displacements in a cable subjected to longitudinal excitation simulating ocean-wave motion. The cable is considered to be made up of two segments of different materials and different physical dimensions. External damping due to fluid action and internal damping due to viscoelastic material properties are considered. The payload mass attached to the end of the cable is considered to be attached to a rigid foundation by an elastic spring and dashpot. This model covers a large number of possible oceanographic applications ranging from salvage recovery of objects immersed in the ocean sediments to suspension of instrumentation packages from buoy systems. Numerical results are presented for a hypothetical system consisting of the payload attached to a steel-nylon elastic cable exposed to sinusoidal excitation at the upper end. Forcing frequencies approaching the second natural frequency are shown to cause forces in the steel portion which are significantly higher than those predicted by a single degree of freedom discrete system. It is further shown that external damping reduces the amplitude response of the payload; but can cause an increase in the cable stress in the steel segment when compared to the undamped case.