The effect of voids on quasi-isotropic carbon-fibre reinforced plastic laminates under quasi-static loading is compared with that under cyclic tension loading. Emphasis is placed on following damage development at the non-crimp fabric ply-level by investigating the influence of voids on damage accumulation, most notably transverse cracking and delamination. Details from experiments include micrographs of voids taken in both scanning-electron and light microscopy, measurements of void content and crack density using light microscopy, and stiffness plots from both quasi-static and cyclic tests. The stiffness results are compared with theoretical predictions accounting for transverse cracks. Voids have a significantly more detrimental effect on the mechanical properties in cyclic loading compared with quasi-static loading. Specifically, the stiffness reduction development, the underlying transverse cracking in layers and the number of cycles to failure are affected. Quality control by only quasi-static testing for void-containing composite materials to be used in components subjected to fatigue cannot therefore be recommended.

This study presents a novel methodology integrating high-resolution X-ray computed tomography, digital volume correlation and statistical visualisation mapping, to perform microscale observations and correlate delamination fracture mechanisms in heterogeneous materials. To demonstrate the utility of this integrated approach, it is applied to study the damage behaviour of aerospace carbon/epoxy composite laminates with an open hole, subjected to quasi-static tension and fatigue at a load ratio of 1:10. The study also explores the applicability of a Paris law type relationship to determine effective macroscopic fatigue delamination resistance in the load-bearing plies. The X-ray imaging for both load cases revealed extensive formation of delaminated fracture surfaces surrounding both glass fibre interlacing weaves and entrained voids within them, acting as preferential sites for localised strain hot spots. It is demonstrated that local energy dissipation is governed by the recurring weave pattern and topological order, which can help explain the typical damage state in quasi-static behaviour, establishing a direct link between microstructural features and macrostructural material response.