An Experimental Comparison of the In-Air and Underwater Damping Properties of Composite Cylinders

Acoustic radiation and self noise are potentially problematic in many underwater applications since both can interfere with the performance of onboard sensors. This is especially prevalent when the structural material for the pressure hull is metallic. In an effort both to reduce radiated noise and minimize structural vibrations, experiments were undertaken to investigate the use of composite materials for cylinders which would be subjected to hydrostatic pressure loading. The damping characteristics of cylinders with various designs and material combinations were obtained initially from modal analysis testing in air. This technique very quickly provides very consistent results. The results desired for this venture, however, were the damping characteristics of these cylinders underwater, which significantly is a more complex experimental configuration.

This paper presents the test procedure and results of modal analysis testing conducted in air and underwater on some composite cylinders. The experimental results, which are presented and compared herein, include the resonant frequencies, damping factors, and mode shapes at resonance. In order to ensure a valid comparison, the natural frequencies and damping factors are compared on a mode-by-mode basis, that is, the result for two cylinders are compared when the resonance has the same mode shape. Despite the increased complexity of the experiments required to obtain these underwater measurements, it is shown that the in-air and underwater levels of damping observed at relatively low frequencies are comparable.

The results show that an appropriate choice of composite materials can increase the levels of damping by at least an order of magnitude over conventional metals. The damping can be increased additionally by using a double hollow core or triple-shell configuration with reinforced polyurethane ribs. The paper discusses this new reinforced polyurethane as a structural component of the cylinder, and proposes various mechanisms of damping that this material form provides.