Porosity Characterization in Thin Composite Plates by Ultrasonic Measurements

Summary

Owing to their excellent weight/stiffness ratio, Aeronautics uses more and more composites/nomex (honeycomb) sandwich type structure. The non destructive testing of the plates used as “skin” of these structures is done by measures of attenuation of ultrasonic waves and there is no problems encountered for plates from 3 to 10mm thick. In the case of thin plates (about 1mm thick), this testing is much less precise and moreover it is better to achieve the control with the plate bonded to its nomex structure. We propose a method for the measure of ultrasonic attenuation in a plate bonded on its structure. This method is based upon a comparison of the different echoes reflected by the plate. Its uses a modelization of echoes spectra as well as ultrasonic attenuation which permits to separate two parameters, one of them characterizing the attenuation, free from frequency, due to impedance gaps at material interfaces and the other characterizing the attenuation, variable with frequency, due to porosity. We show a quite good correlation between this last parameter and the porosity measured with a destructive method, for plates bonded on nomex structure as well as for unbonded plates.