Fracture of thin-ply composites : effects of ply thickness

The demand for carbon/epoxy laminated composites is continuously increasing due to their high specific modulus and strength. Among them, laminates made of particularly thin layers (from t=0.015 to 0.150 mm) and called thin-ply composites are gaining in interest for the larger design space they offer. Moreover, they have outstanding strength properties, thanks to a significant ply thickness effect reflected by a delayed onset of damage. However, the improvement in strength is accompanied by higher brittleness and reduction of fracture toughness. Therefore, this thesis focuses on explanation, characterization, and modeling of ply thickness effects on the inter-, intra-, and trans-laminar fracture of M40JB/TP80EP composites in the range t=0.150 to 0.030 mm. In mode I inter- and intra-laminar fracture, while the critical energy release rate (ERR) at initiation is found practically independent of ply thickness, it is decreased by 50% (interlaminar) and 23% (intralaminar) at steady-state with decreasing ply thickness. During crack growth, fiber bridging develops in the wake of the crack. The heterogeneous microstructure of the thick-ply laminates promotes distributed micro-cracking around the crack surface, crack surface waviness and the development of large bundles of bridging fibers, leading to higher fracture resistance. In contrast, the homogeneous microstructure in thin-ply composites reduces significantly the creation of bridging bundles leading to lower ERR. The results suggest that bridging fibers are better anchored in intra- than inter-laminar fracture due to fiber waviness and misalignment and thus exert larger closing forces that explain the much higher steady-state ERR measured in intra- than in inter-laminar fracture. The translaminar fracture toughness of cross-ply (CP) and quasi-isotropic (QI) laminates is significantly decreased between t=0.150 and 0.030 mm plies at both the initiation (-70%) and steady-state propagation (-77%). The translaminar fracture toughness scales linearly with ply thickness and is found ~25% lower in the QI specimens compared to the corresponding CP specimens. Quantitative morphological studies of fracture surfaces show that the ply thickness effect is directly correlated to the height of the pull-out fiber bundles, which are much shorter in thinner-ply specimens. The inter-, intra- and trans-laminar fracture are simulated by cohesive elements models, with linear stiffness degradations corresponding to the initiation ERRs and identified non-linear relations representing the ERRs associated with the toughening mechanisms. In inter- and intra-laminar fracture, these traction-separation relations related to fiber bridging are identified with an efficient R-curve based method developed in this work. In translaminar fracture, the pull-out mechanism is proposed to be modeled by a constant traction level. For each fracture mode, scaling strategies are suggested to account for the ply thickness effects. Potential solut