Voids, the most studied type of manufacturing defects, form very often in processing of fiber-reinforced composites. Due to their considerable influence on physical and thermomechanical properties of composites, they have been extensively studied, with the focus on three research tracks: void formation, characteristics, and mechanical effects. Investigation of voids in composites started around half a century ago and is still an active research field in composites community. This is because of remaining unknowns and uncertainties about voids as well as difficulties in their suppression in modern manufacturing techniques like out-of-autoclave curing and parts with high complexity, further complicated by increased viscosity of modified resins. Finally, this is because of the increasing interest in realization of more accurate void rejection limits that would tolerate some voidage. The current study reviews the research on formation, characterization, and mechanical effects of voids, which has been conducted over the past five decades. Investigation and control of void formation, using experimental and modeling approaches, in liquid composite molding as well as in prepreg composite processing are surveyed. Techniques for void characterization with their advantages and disadvantages are described. Finally, the effect of voids on a broad range of mechanical properties, including inter-laminar shear, tensile, compressive, and flexural strength as well as fracture toughness and fatigue life, is appraised. Both experimental and simulation approaches and results, concerning voids' effects, are reviewed.

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Digital Image Correlation monitoring of the surface strains, microscopic in-situ observations of the micro-damage on the specimen edge and Acoustic Emission (AE) are utilized simultaneously during tension tests of quasi-isotropic carbon fibre reinforced polymer composites. It is found that the cluster analysis for characteristic parameters of AE events (the main being the signal amplitude and frequency) does not unambiguously identify the type of damage which causes the event. With optical instruments, it is observed that the signatures of AE events depend on the position of the ply where damage happens and on the ply orientation (90° vs 45°). Robust evidences for the variations in AE characteristics of damage modes in different lay-ups are presented. AE events, originated from surface cracks, have high amplitude and low frequency, whereas AE events, originated from transverse cracks in the inner plies, have low amplitude and high frequency characteristics. Any conclusion for fibre breaks are not reached in this study. Therefore, measurements in this study rather point out that the AE events, which could be interpreted as fibre breaks because of their high frequency characteristics, as optical observations prove, correspond to other damage types in quasiisotropic laminates.

We employ X-ray micro-computed tomography to generate an extensive dataset of voids' characteristics in carbon/epoxy laminates with different stacking sequences. The voids are deliberately induced by modifying the cure cycle. The three-dimensional images with voids are segmented and quantified using an in-house software package, VoxTex. The characteristics of voids, i.e. shape, size parameters, orientation, and spatial location distribution, are obtained. The majority of voids have slightly flattened cross-sections. The voids prefer to localize in the inner plies of the laminate, rather than in the outer ones. Near the laminate mid-thickness, they become more elongated. The stacking sequence affects the size and shape of voids. The planar location of voids is not random and tends to create a pattern, corresponding to the the ply orientations in the laminate. The data is reported in detail, ready to use, for example, as input for modeling studies of similar composites with voids. Response to Reviewers: The response to the reviewer's comments is provided in a separate MS Word file, which is uploaded along with the submission.

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