Conventional ultrasonic or eddy current inspection of structures requires a probe to be scanned over the whole area to be tested. This is extremely time consuming, and hence costly, when large areas such as aircraft wings or pressure vessels are to be covered. A further disadvantage of ultrasonic inspection is that a coupling fluid between the transducer and the structure is generally required. The most reliable coupling method is to use immersion, the testpiece being fully immersed in a water bath. However, with large structures this is frequently not practical and they are often tested using jet probe assemblies, the ultrasound being propagated down jets of water directed at the structure. However, this is generally only practical at the manufacture stage and field inspection is often carried out manually using contact transducers, coupling being achieved by applying gel to the surface of the structure. Many structures also have critical areas which are difficult to inspect because access is impeded. For example, spars and other stiffeners in aircraft pose problems because once the aircraft is built they are covered by the fuselage or wing skin.
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