Analysis of acoustic emission signals resulting from fiber breakage in single fiber composites

Acoustic emission techniques are able to sense micro failures in loaded structures for a wide range of materials. Moreover, a spread application of such techniques to structural health monitoring requires further efforts in the signal analysis in order to assess the relationships between the micromechanics of the failure and the signal features as recorded by a transducer. In the case of polymer matrix composites, the main sources of acoustic emission signals are fiber breakage, matrix cracking, and fiber/matrix debonding. In this work, single fiber composites have been tested to produce fiber breakage acoustic emission events. Three different fiber/matrix systems have been investigated: PCA polyester matrix/carbon fiber (6 ?m diameter), ECA epoxy matrix/carbon fiber (6 ?m diameter), and ECB epoxy matrix/carbon fiber (12 ?m diameter). Acoustic emission signals recorded through a wide band transducer have been analyzed in the frequency domain in order to study the amplitude spectra characteristics. The irregular shape of the amplitude spectra suggested the fractal dimension as a feature. The Richardson fractal dimension resulted in a key parameter in the signal identification among the different fiber/matrix systems: DPCA = 1.28±0.02, DECA = 1.46±0.03, DECB = 1.42±0.02. On the basis of this finding, the shape of the amplitude spectra has been analyzed. An algorithm able to extract shape similarities from a class of spectra has been developed. It permits the definition of the representative spectra together with the identification of the relevant frequencies for the fixed fiber/matrix system.