Criticality of degradation in composite materials subjected to cyclic loading

Degradation of composite materials subject to cyclic loading is a multi-step process involving micro-cracks formation and progression until failure occurs. In this paper, the signatures of acoustic emission (AE) response emanating from composite specimens subjected to fully-reversed bending fatigue are studied. The composite is glass/epoxy (G10/FR4) laminates and the experiments cover different frequencies and displacement amplitudes. Results are presented for AE energy, counts and amplitudes. It is shown that the distribution of the cumulative AE amplitude can be described by a power law. Further, examination of the evolution of the probability density function (PDF) of the AE energy (counts) reveals two scaling zones wherein the transition from the low energy (count) to high energy (count) regime is identified. The low-energy phase represents very low damage or damage-free state of the laminate characterized by a power law with an exponent of αE = 1.8 ± 0.05. In these series of experiments, AE energy release and AE counts follow the statistics and power laws that do not depend on the operational conditions.