Estimating the damage state of composites is essential since carbon fibre reinforced polymers (CFRP) have a complex damage and propagation behaviour. Accordingly, a comprehensive study of the effects of matrix cracks – e.g. their size and crack density – and the influence of different damage types and their interaction on the vibro-acoustic modulation (VAM) behaviour of CFRP specimens under fatigue loading was carried out. These effects were evaluated in two sets of commonly employed laminate layups. Firstly, in a cross-ply (CP) layup, the number of 90°layers was varied, with a constant areal weight (268 g/m2). Secondly, quasi-isotropic (QI) specimens were tested, where the areal weight was reduced from thick-ply to thin-ply (30–268 g/m2), changing the failure mode from a delamination-dominated to a brittle failure. After damage introduction under tension–tension loading (R=0.1), high-frequency Lamb waves are introduced via piezoceramic actuators and modulated by a low-frequency vibration using a servo-hydraulic testing machine. VAM correlates with the different damage behaviour and damage conditions in CFRP. For this purpose, the evolution of the sideband amplitudes, the modulation index, the signal in time domain and X-ray scans are used. Regardless of the layup, the degree of stiffness-reducing inter-fibre fractures and large-scale delaminations can be detected.
