Composites are very sensitive to the load in the direction of thickness, especially out-plane low velocity impact (LVI). In this article, the detailed failure mechanisms of composite laminates with four material systems (CCF300/Epoxy, CCF300/Bismaleimide, CCF800/Epoxy, and CCF800/Bismaleimide) under the loading of LVI and compression after impact (CAI) were studied by experiment and finite element analysis. In finite element model, a newly proposed multi-scale failure criterion (MMF3) and cohesive elements are used to determine the intralaminar damage and interlaminar delamination, respectively. Through experiment and finite element analysis, it can be concluded that the damage tolerance performance of epoxy resin composites are better than that of bismaleimide resin composites, and the damage tolerance performance of CCF300 carbon fiber composites are better than that of CCF800 carbon fiber composites. The proposed multi-scale failure analysis method can effectively simulate the impact and CAI damage process of composite laminates.

Computational mechanics has been carried out to study the microscopic failure mechanisms of unidirectional fiberreinforced polymer composites. A representative volume element of fiber random distribution based on molecules random collision model is established, with two dominant damage mechanisms, matrix plastic deformation and interfacial debonding included in the simulation by the extended Drucker-Prager model and cohesive zone model, respectively. The simulation results clearly reveal the damage process of the composites and the interactions of different damage mechanisms. It can be concluded that the transverse tension fracture initiates as interfacial debonding and evolves as a result of interactions between interfacial debonding and matrix plastic deformation, while the compression failure is dominated by matrix plastic damage. The longitudinal tension and compression are both dominated by fiber breakage, but longitudinal tension initiates as matrix plastic damage and longitudinal compression initiates as fiber microbuckling. Finite element method appropriately simulated the process of progressive damage of the fiber buckling failure, which is consistent with the observed result under scanning electron microscopy.

Tailored fibre placement (TFP) is an embroidery-based technology that allows the fibre tows to be placed exactly where they are most needed for structural performance and stitched into position on a compatible textile or polymer substrate. In this study commingled carbon-nylon fibre tows were utilised to produce thermoplastic cross-ply net-shaped preforms using TFP. Four TFP composite plaques were manufactured; baseline (blank), machined-hole, tailored-hole-1 and tailored-hole-2. Steering the tows was used to create the hole in tailoredhole-1 and tailored-hole-2. In comparison to the design of tailored-hole-1, a different fibre trajectory, with a circular reinforcement around the hole, was suggested for the tailored-hole-2. Fibre volume fraction, optical microscopy, X-ray-CT scans, tensile and open-hole tests were carried out. With the exception of the baseline sample, the modified design of tailored-hole-2 composite exhibited the highest axial strength and modulus compared to the machined-hole and tailored-hole-1 composites. Only the tailored-hole-2 specimens exhibited less than 10% reduction of the notched strength compared to the un-notched strength. This study highlights the importance of the stress/load-paths and associated fibre-orientations. While TFP can be an extremely valuable design tool for composite preforms and resulting structural components, a deep understanding of stress distributions is inevitable to achieve optimal TFP-design.

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In order to analysis thermal residual stress and its influence on the strength of composite, the hierarchical multi scale simulation method is applied. A microscopic computational model of single fiber composite with thermal residual stress is built to research the stress distribution. Then the damage initiation discipline details of unidirectional composite are researched, and the effects of different fiber arrangements on thermal residual stress distribution, damage initiation and the different final failure behaviors of fiber regular distribution and random distribution under tension and compression are researched in details. It shows that in fiber regular arrangement, damage initiation in interface appears evenly and in matrix it appears at somewhere randomly. But in fiber random arrangement, initial damage focuses at the resin pockets between closely packed fibers with both interface and matrix damage. The maximal thermal residual stress in fiber random arrangement model is larger than that in fiber regular arrangement model. And it reaches the normal strength of the interface and thus causing the initiation of interface damage. Also the failure modes of composites under transverse tension and compression with and without residual stress are quite different from each other. The strength and failure path of different RVE and loading are showing respectively in this paper.