Experiments to relate carbon-fibre surface treatments to composite mechanical properties

The single-fibre Broutman test was originally used to study fibre/matrix interface debonding behaviour. A competition between two failure mechanisms was observed during the Broutman test: fibre failure under compressive stresses and fibre/matrix interface debonding under transverse tensile stresses. For systems presenting a good fibre/matrix adhesion, e.g. carbon-fibre/epoxy composites, it was not possible to observe interface damage before compressive failure of the fibre. Consequently, the test appeared to provide interesting compressive fragmentation data. Interfacial shear strength values derived from this test were compared with those obtained from traditional tensile fragmentation tests. In addition, the results allowed some discussion on the compressive strength of the PAN-based carbon fibres. During testing, damage was detected using visual observation under polarised light and acoustic emission (AE) monitoring. Failure mechanism separation based on AE events was performed by using time domain parameters (amplitude and event width). In particular, fast fourier transform frequency spectra of the AE waveforms were used as fingerprints allowing the discrimination of fibre failure, matrix cracking, and parasitic ‘noise'. In addition, fibre bundle transverse tensile tests were performed with the PAN-based carbon fibre/epoxy systems. The transverse strength, characteristic of the interface transverse properties, showed the same trends as the compression fragmentation results. The interface adhesion quality assessed through scanning electron microscope fractography could be qualitatively related to the transverse strength data.