Investigation of Accurate Imaging of the Defects in Composite Materials Using Ultrasonic Air-coupled Technique

The modern aircrafts are built using various composite materials such as carbon/glass fibre reinforced plastics, GLARE and honeycombs. Manufacturing technologies of composite materials are complicated and include many stages. Small deviations from the technology parameters may cause relatively small defects, but influence of these defects on a total strength of a construction may be essential. So, due to safety reasons they should be inspected during manufacturing process and exploitation, using accurate non destructive testing techniques. Conventional ultrasonic inspection techniques require liquid acoustic coupling, but wetting of composites with coupling liquids in different manufacturing cases is unacceptable or even not allowed because they may cause additional technological failures. Because of that the ultrasonic air coupled technique is very attractive for inspection of composite materials. On the other hand, the complex structure of composite materials causes big losses and scattering of ultrasonic signals. Additionally, the attenuation of ultrasonic waves in air increases with the frequency. Due to these facts the ultrasonic air coupled measurements are performed at lower frequencies, what leads to reduction of a spatial resolution and accuracy of ultrasonic imaging. So, the objective of this paper was to present results of the investigation of accuracy of ultrasonic air coupled imaging of defects in composite materials.