A new low-frequency vibration technique for blind-side inspections

The application of low-frequency acoustic methods for nondestructive testing has a very long history, probably originating with a simple tap technique. These techniques have been successfully applied to a variety of materials and are widely used for the assessment of the quality of bonded sheet metals. They have also proved useful in the NDE of composite materials, particularly honeycomb panels. Three ‘low-frequency’ methods have been widely developed and implemented in commercial NDT instruments. These are commonly termed ‘Membrane Resonance’, ‘Mechanical Impedance’ and ‘Velocimetric’ methods, each of which has been carefully studied and placed on a more scientific footing by the work of Cawley and Adams(1-4) among others. In general, the minimum detectable defect size, or sensitivity, of each of these low-frequency methods is highly dependent on defect depth and size, limiting their application to relatively large or shallow defects. Since access to both sides of a panel is rare, single-sided inspection is usually a necessity. Whilst the three lowfrequency methods mentioned above have been found to adequately detect and size near-side defects in honeycomb panels, such as those arising from impacts and disbonds on the near-side skin, their ability to resolve equivalent far-side defects has been found to be limited, giving, at best, a diffuse image of the defect. This paper describes a new ‘Thickness-Resonance’ technique that is simple to implement and overcomes limitations of current methods in respect of far-side defects in honeycomb panels. This new technique is related to another low-frequency method, often referred to as the ‘Resonance’ method. As its name suggests, the resonance method excites the probe at its resonant frequency, while changes in the impedance of the probe, caused by the presence of defects, are used to locate damage. The principle of the new thickness-resonance technique is simply to locally excite the through-thickness resonance of the honeycomb panel, while monitoring the local response of the panel; the presence of a defect is indicated by a change in panel response, as the local resonant condition is highly sensitive to the presence of defects, irrespective of their depth within the panel. The first section of this paper introduces the analytical modelling used to predict the frequency and mode shape of thickness resonances associated with honeycomb panels constructed with carbon fibre reinforced plastic (CFRP) skins and Nomex® honeycomb core. Included in this section is the experimental validation of the analytical model. The second section presents the experimental results obtained using the thickness-resonance method, and provides an indication of defect detectability for this technique.