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7. Non Metallic NDE, Acoustic Microscopy

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By using a rubber window acoustically matched to sea water, the Sonar system on a destroyer can be made considerably more sensitive. However, if the layered construction of the window develops delaminations while in service, the hydrodynamic characteristics of the structure may become modified and acoustic noise can be generated during high-speed operations. In order to inspect for these delaminations while the ship is tied up to its dock, a pulse-echo ultrasonic scan performed by a diver using a sea-water coupled transducer would appear to be ideal. However, the choice of acoustic parameters suitable for inspecting rubber were unknown. Laboratory studies sho1~ed that by utilizing very short time duration pulses whose center frequencies lie between 0.5 and 1.0 Mhz, it was possible to detect water-filled pockets within 0.2 inches of the outer surface of the window. A prototype instrument suitable for shipboard and dry dock operation has now been constructed. This instrument features optimized pulse excitation of low-frequency broad band transducers when attached to 50 to 100 feet of coaxial cable. A signal light is also incorporated to provide the diver with information on the condition of the rubber window under his transducer.



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