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Two kinds of carbon aramid/epoxy hybrid woven composite specimens with different fiber orientations were prepared. The progressive flexural damage behaviors of the composites were studied. The failure process was monitored in real time by acoustic emission during the test, and the characteristics of the acoustic emission signals originating from the damage were deeply studied. In addition, the internal damage initiation/evolution and failure mechanisms were characterized by X-ray micro-computed tomography. The results show that composite specimens exhibit higher strength and obvious quasi-brittle damage behavior when carbon fiber orients along the loading direction, and the macroscopic failure is mainly shear fracture, which propagates in the direction of thickness, and the damage is dominantly distributed above the neutral plane of specimen. When aramid fiber orients along the loading direction, composite specimens show high ductility and the failure modes are mainly ply fracture at the bottom, delamination (inter-ply delamination and intra-ply delamination) and tensile fracture of the tows. The combination of acoustic emission and X-ray micro-computed tomography analysis provides an insight for further elucidation of the progressive damage initiation/evolution and failure mechanism of composites.

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Accurate assessment for mechanical and damage behaviors of three dimensional carbon fiber braided composites by acoustic emission technique is difficult in practical application. In this study, a statistical analysis approach with calculated amplitude spectrum, probability distribution and entropy is proposed to process acoustic emission signals. Moreover, the tensile behaviors and damage characteristics of three dimensional four directional and five directional braided composites are investigated systematically by combining digital image correlation and acoustic emission methods. The results indicate that three dimensional five directional braided composites possess excellent load-bearing and deformation resistibility capacity, owing to more compact preform structure with additional axial yarns. The acoustic emission statistical analysis method can be used as criteria to distinguish and quantify various irreversible damage and failure mechanisms in three dimensional braided composites. Furthermore, the deformation, damage and failure criterion for two types of composites are fully described based on deformation fields and acoustic emission signals.