Multi-scale characterization and modeling of notched strength and translaminar fracture in hybrid thin-ply composites based on different carbon fiber grades

Owing to their high specific stiffness and strength, Carbon Fiber Reinforced Composites (CFRP) are ideal candidates for the development of lightweight high-performance structures. Within this family, thin-ply composites allow for wider design freedom and present superior mechanical properties as failure is reached at nearly the ultimate strain of the fiber, in contrast with regular composites, due to the delay or suppression of transverse cracking, micro-cracking, and delamination. However, this results in a very brittle failure, and a low translaminar toughness. Thus, thin-plies are not tolerant to stress intensity concentrators, preventing a damage-tolerant design approach, and thus restricting their wider use.Fiber hybridization is a possible route to reach a trade-off between the translaminar toughness and tensile properties in thin-ply composites. The present thesis work focuses on the combination of two different types of carbon fibers, one with a high strain to failure and lower modulus, and the other with a high modulus, and low strain to failure. Various types of fiber hybridization are explored, from an interlayer configuration to interyarn and intrayarns architectures.A thorough experimental analysis was conducted to evaluate the Energy Release Rate in Cross-ply laminates, and the tensile properties in unnotched and open-hole tensile mode for Quasi-iso laminates, and identify hybrid effects, as a function of the low-strain fiber volume fraction, ply-block thickness and symmetry. A novel J-integral implementation to derive the experimental mode I translaminar toughness from experimental displacement fields of Compact Tension (CT) specimens measured by Digital Image Correlation (DIC) was proposed and benchmarked for three different formulations.Results highlighted the large design space opened by hybridization, with a significant change in the damage sequence. The interlayer hybridization yields a substantial positive hybrid effect with respect to the ply-thickness effect. The positive hybrid effect observed for the best-performing arrangements resulted from the presence of secondary damage in asymmetric ply-blocks. In contrast, symmetric ply-blocks were found ineffective as a proper fragmentation of the low-strain plies could not be triggered, due to a lower experimental strain to failure of the high-strain fiber, as compared to the datasheet.Interyarn and intrayarn configurations were then explored as a means to increase fiber dispersion and mitigate the ply-block thickness increase resulting from layer-by-layer hybridization. A similar experimental test campaign was conducted, highlighting the possibility of obtaining a positive hybrid effect through alternative mechanisms as compared to pull-out, such as crack bridging by low-strain tows.A phenomenological study was then conducted to account for the change in translaminar toughness, by quantification of the pull-out length in fracture surfaces, completed by a shear-lag model implemented in a finite element model to propose a method for prediction of these changes as a function of microstructure. Results showed that the pull-out bundle height and width distributions are strongly impacted by the ply thickness in agreement with previous studies. More importantly, quantitative data was gathered about the influence of fiber hybridization and its architecture on the pull-out bundle distribution, leading to identify two distinct dissipative mechanisms.