Determination of Acoustic Attenuation in Composites by the Time Frequency Method

Measurement of the porosity of a composite is a necessity for the aerospace industry. Composites for structural parts are considered acceptable when their porosity is less than 2%. In some cases, ultrasonic attenuation measurement may be used as an alternative to X-ray tomography. Work on the measurement of porosity by acoustic methods has already been published, but they generally implement laboratory techniques that are not easy to export in an industrial production context (simple, fast and reliable measurements). For industrial samples of the RTM type with known average porosity, we propose a procedure for measuring the ultrasonic attenuation that combines proven techniques: measurement of the reflection on the plate of the waves emitted by a plane transducer ("Pulse echo" method) and then signal processing by calculation of the Fourier transform with sliding window ("Short Time Fourier Transform"). These results are compared with those obtained by a so-called "standard" method giving exact results but for which the processing of signal is more delicate because it is not possible to accurately identify the start and end of interface signals when they are mixed with those of internal reflections. The results obtained by both methods are in good agreement and show that an affine relationship between attenuation, frequency and porosity is acceptable for this type of plate. In addition, we show that if porosity increases, the evolution of the attenuation-frequency relationship clearly differs from those obtained furthermore for unidirectional composites.