The Phase I study demonstrated that lightweight foam materials with air void content up to 80% could be fabricated using flexible Expancel macrospheres and a variety of polyethylene matrix materials. These materials possessed acoustic impedance properties which would theoretically attenuate underwater shock waves and hull plating response by approximately a factor of 10. However, the static stiffness of these materials as measured by compression testing indicated that the materials are too flexible to sustain submergence pressures for deep diving submarines. The Phase II effort will initially concentrate on increasing the static stiffness of the high void materials by means of introducing chopped Kevlar into the matrix material, use of high molecular weight polyethylene and epoxy while maintaining the low acoustic impedance which is required for high shock attenuation. The fabrication process will be improved with specially designed low shear mixing head and dispensing nozzles. After verification of adequate static stiffness, high strain dynamic tests will be performed to measure the shock wave transmission properties in a laboratory environment. The measured properties will be used to describe the foam layer in an underwater shock analysis of a representative submarine/equipment configuration. The final task will be to conduct an underwater shock experiment to verify the shock attenuating performance of the material for components which are external to the hull and components which are directly attached to the inside of the pressure hull. Test data will also be used to validate the shock design computer model. A design guidance document will be prepared which will enable survivability engineers to design and implement foam shock attenuation materials in combatant ships and submarines.
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