A study of interfacial aspects of epoxy-based composites reinforced with dual basalt and SiC fibres by means of the fragmentation and acoustic emission techniques

Fibre/matrix interfacial properties and interfacial shear strengths (IFSS) in epoxy composites reinforced by dual basalt and SiC fibres (DFC) were investigated by the fragmentation method combined with acoustic emission (AE) analysis. Statistical analysis of fibre tensile strength was performed in terms of statistical parameters. The tensile strength and elongation of basalt and SiC fibres decreased with increasing gauge length because of the size effect. Fibre tensile strengths above an optimum concentration decreased because of the stress concentrations at lumps in the coating. When an amino–silane coupling agent was used, the IFSS showed significant improvements of more than three times under dry conditions. The IFSS was also considerably improved under wet conditions. This environmental effect is probably due to chemical and hydrogen bonds as well as to interdiffusion effects in two different interphases in the fibre/silane-coupling-agent/epoxy-matrix system. In situ monitoring of AE during the straining of DFC specimens showed the sequential occurrence of two distinct groups of AE data. The first group may have come from fibre breakages, and the second mainly from cracking of the epoxy matrix. Characteristic frequencies coming from the different failure modes of the fibres and epoxy matrix were investigated by fast Fourier transform (FFT) analysis. By setting an appropriate threshold level, a one-to-one correspondence between the number of AE events and fibre breakages was established. This AE method could be correlated well with the fragmentation technique of obtaining the IFSS value. It can be also be applied to semi- or non-transparent composites where visual observation is not possible.