Pulsed ultrasonic spectroscopy for roughness measurement of hidden corroded surfaces

Several NDE techniques are available for detecting the symptoms of corrosio n such as pillowing, disbonding and metal loss. However, 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 Instan ces of all of these have been found in VC10 aircraft in a recent teardown e xercise. A method that measures the roughness of a hidden surface could pro vide that necessary confirmation that a disbond or region of thinning is ac tually corroded This paper describes the development and evaluation of a te chnique which uses the frequency-dependence of the reflection coefficient a t a rough interface to measure the surface roughness. Several other workers have reported methods for determining top-surface roughness but hidden sur faces present many more complications, which are the subject of this paper: Hidden surfaces roughnesses from 5 mum to 40 mum can he measured with an a ccuracy of +/- 5 mum. The method can be applied to full-waveform data files , or in real-time whilst scanning, using the ANDSCAN(R) system. The depende nce of the method on analysis parameters, stand-off media and ultrasonic co nfiguration is also discussed.