Correlation of Acoustic Emission with Fractography in Bending of Glass–Epoxy Composites

Damage processes in glass fiber reinforced plastic composites have been examined extensively by many analytical and experimental methods, including acoustic emission. While damage phenomena in mezo- and macro-scale are well described, the subtle mechanisms in micro-scale are still under discussion. The goal of this work was to apply the acoustic emission to examine damage initiation in fiber reinforced epoxy resin composites with different continuous glass-fiber architectures. Basic lay-ups were used: unidirectional, cross-ply and angle-ply, each with varying fiber volume content in epoxy matrix. Detection of fine micro-cracks was possible owing to merging the mechanical monotonic and step-load tests in 4-point bending with acoustic emission monitoring and detailed scanning electron microscopy observations. Clear images of macroscopically undamaged samples after interruption of step-loading cycles revealed the initiation of micro-cracks by debonding of fiber-matrix interfaces, in agreement with acoustic emission signal analyses, featuring use of historic index and rise time/amplitude ratio.