RELATING ACOUSTIC-EMISSION SIGNAL PARAMETERS TO THE STRENGTH OF FIBERS USED IN THE MANUFACTURE OF POLYMERIC COMPOSITES

A study has been made of the use of acoustic emission (AE) to monitor the strength of fibres used in composite manufacture. When testing materials using acoustic emission, the relationship between the source excitation and the detected stress wave is complex. In the case of the testing of fibres, the test situation is simpler since the mode of failure of parallel fibres is mainly of one kind (fracture transverse to the fibre axis). Bundles of different types of fibres, and fibres subject to various surface treatments, have been strained to failure in order to investigate the relationship between acoustic emission response and fibre strength. In general, the AE signal parameters employed increased with fibre failure stress and strain. For different fibre systems, the magnitude of the AE parameter and the associated AE source event related better to the average elastic strain energy released. Single-fibre composites have been tested and AE has proved an effective method of monitoring fibre fragmentation, which is usually used to investigate and quantify interfacial shear stress between the fibre and composite resin. When fibres are exposed to ultrasound, this exposure has a dramatic effect on the condition of the fibre surface, resulting in modification to the mechanical and associated AE emanating from fibre fracture. Trends in AE data are interpreted in terms of simple AE models. Where appropriate the statistical distribution of mechanical failure is characterized using Weibull statistics combined with AE measurement, which provides two parameters able to characterize material failure.