An ultrasonic spectroscopy method for the roughness measurement of hidden corroded surfaces

Several NDE techniques are available for detecting the symptoms of corrosion such as pillowing, disbanding and metal loss. But, on real aircraft, each of these symptoms may not be caused by corrosion. For example trapped swarf can cause pillowing, sealant layers may never have been bonded, and metal can have been removed at the construction stage, or during a repair, sometimes without adequate documentation to inform the NDE inspector. Instances of all of these have been found in VC10 aircraft in a recent tear-down exercise. A method that measures the roughness of a hidden surface could provide that necessary confirmation that a disbond or region of thinning is actually corroded. This paper describes the development and evaluation of a technique which uses the frequency dependence of the reflection coefficient at a rough interface to measure the surface roughness. The method can be applied to full-waveform data files, or in real-time whilst scanning, using the ANDSCAN(R) system. The dependence of the method on analysis parameters, stand-off media and ultrasonic configuration is also discussed.