Acoustic attenuation for ultrasonic NDI detection of delaminations on composite laminates

The purpose of the paper is to show the importance of the acoustic attenuation and probe frequency for the reliable evaluation of delaminations on composite materials subjected to NDI ultrasonic investigation. An original ultrasonic NDI methodology has been applied to some composite materials of wide application, characterized by a high acoustic attenuation. The materials are based on Kevlar/polyester and glass/polyester systems; the fiber form is woven fabric, and the thicknesses are between 5 and 6 mm. All the specimens are made by RTM technology. The specimens have been subjected to low-velocity impact tests, and then the extension of delaminated areas has been evaluated. The ultrasonic test apparatus operates in the form of reflection, using two different probes at 5 and 15 MHz. The results point out how the higher frequency probe has a greater resolution and higher sensibility; nevertheless, the higher attenuation at 15 MHz causes a lower inspection depth along the specimen thickness. Thus, for materials with high attenuation the 5 MHz probe is preferable in order to have accurate ultrasonic maps along the thickness and reliable values for the delaminated areas.