Acoustic emission from single carbon fibres and model microcomposites

It is difficult to interpret acoustic emissions generated within stressed fibre composites. To obtain a better understanding of the characteristic features of acoustic emission at source in carbon fibre reinforced plastics, a number of experiments have been performed to measure the emission from the fracture of single carbon fibres alone and of single fibres in microcomposites. An exponentially shaped steel cone was used as a waveguide to couple a single fibre to a transducer and tensile loading was achieved by slowly adding water to a container suspended from the sample. The microcomposites consisted of a single carbon fibre surrounded by a small number of glass fibres, all bonded by acrylic or epoxy resin. Measurements were made on five different types of carbon fibre. For freely mounted single fibres the amplitude of the acoustic emission produced at fracture was high, and it increased approximately linearly with strain energy in the fibre prior to fracture. A similar carbon fibre in a microcomposite suffered a succession of failures, and the amplitudes of the emission were lower, in qualitative agreement with the strain energy being lower at fracture. Results are also given for the acoustic emission produced during torsion tests on a microcomposite. In these samples the fibres remained intact and emissions were produced by debonding and/or matrix cracking.