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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.

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