Assessment of the out-of-plane behaviour of a 3D-printed carbon fibre reinforced composite material

ENG- This doctoral thesis presents a comprehensive investigation on the out-of-plane mechanical behaviour of a 3D-printed continuous carbon fibre reinforced polymer (c-CFRP) composite, manufactured using fused filament fabrication (FFF) technology from Markforged®. It addresses a critical literature gap regarding the response of these novel materials to out-of-plane loading conditions, such as indentations and low-velocity impacts (LVI), which are highly relevant for structural applications in aerospace, automotive, and other engineering sectors. Experimental and numerical approaches were employed to evaluate and predict the impact damage resistance of these 3D-printed composite materials. An extensive literature review revealed that key mechanical properties had not been adequately studied for this material. In particular, accurate characterisation of the material fracture toughnesses is essential for implementing reliable damage models in simulations. The thesis begins with the development of a novel double-tapered compact tension (2TCT) specimen to determine the translaminar fracture toughness and corresponding cohesive law. This geometry was designed and validated via finite element parametric analysis to ensure crack propagation while avoiding undesired failure mechanisms like buckling or premature compressive failure, which are common in standard compact tension specimens for composite materials. The 2TCT design enabled accurate experimental determination of the translaminar fracture toughness of the 3D-printed c-CFRP material without additional damage. Using the inverse problem methodology, the laminate bi-linear cohesive law and the related fibre tensile cohesive law were derived from the experimental data, ready for implementation in a numerical damage model. The results yielded an average laminate fracture toughness of 17.4 N/mm and a fibre tensile fracture toughness of 33.1 N/mm. Next, the thesis explores the damage mechanisms under out-of-plane quasi-static indentation (QSI) considering two laminate configurations: a conventional quasi-isotropic layup and a dispersed layup with non-standard fibre orientations. Sequential mechanical testing combined with high-resolution X-ray micro-computed tomography (µCT) revealed a five-stage damage sequence. Initial stiffness loss was caused by void-induced micro-delaminations, which grew until fibre breakage and matrix cracking occurred, eventually leading to full perforation. The results highlighted the significant role of manufacturing-induced voids in damage initiation and demonstrated that dispersed laminates tend to develop fewer but larger delaminations, while conventional laminates exhibit more distributed through-thickness damage.To complement the QSI findings, detailed finite element models were developed to simulate LVI events. The models use a meso-scale approach using conventional shell elements and zero-thickness cohesive elements. Intralaminar damage was modelled using a continuum damage mechanics (CDM) framework, and interlaminar delamination via a cohesive zone model (CZM). The models incorporated the experimentally derived cohesive laws and were validated against drop-weight impact tests at two energy levels (12 J and 15 J). Infrared thermography (IRT) inspections were conducted to quantify the projected delaminated areas and compare them with the numerical predictions. The simulations successfully reproduced the global impact response and damage evolution. Dispersed laminates showed earlier damage initiation and greater impactor displacements, while conventional laminates exhibit higher peak loads and more fibre breakage. Unlike QSI results, LVI tests revealed intralaminar damage preceding significant delaminations, suggesting strain-rate dependent behaviour. While the predicted delamination patterns and energy dissipation trends validated the modelling approach, high computational costs due to element distortions indicate for improvement in the developed models