Conventional non-destructive evaluation of composite materials by techniques such as ultrasonic C-scan involves scanning a transducer over the whole area of structure to be tested. This type of testing is very time consuming, and hence expensive. In practice, composite laminates are often damage tolerant and it is frequently only necessary to find relatively large defects, for example 10 - 20 mm diameter delaminations. There is therefore a need to develop a testing technique that will detect defects of this type quickly and reliably; if a defect is found, conventional methods may, if necessary, then be used to characterize it in detail. Possible currently available techniques include thermography, holography, shearography and scanning laser ultrasonics. These have the potential to carry out relatively quick inspections, but are all very expensive to implement. Ultrasonic Lamb waves, which propagate in the plane of plate-like structures, have potential to be used for the long-range testing of composite laminates since they can propagate a long distance along the laminate and a large region can therefore be interrogated from each transducer position. Recent work at Imperial College has investigated the possibility of using these waves as the basis of a quick, relatively cheap method for the detection of delaminations in large structures. This paper reviews the currently available techniques and discusses their relative advantages and disadvantages, and then goes on to discuss the potential of the Lamb wave method.
