Mechanical characterisation of AM nylon-matrix carbon-fibre-reinforced composite in tension

The ability of additive manufacturing (AM) to produce structures with complex geometry has gained significant attention from researchers and fused deposition modelling remains a popular AM method to fabricate a layered structure using thermoplastic polymer. However, the application of these materials remains limited to prototyping because of weak performance associated with manufacturing defects; porosity and weak interfaces. Stronger composite structures may be produced through optimisation of processing parameters and addition of reinforcing fibres in the polymer to expand the application of products, e.g., in medical prosthetics and protective gear. This study investigates the fracture behaviour of AM nylon matrix composites reinforced with short and continuous carbon fibres under tensile load. Additionally, the impact of fibre volume fraction on mechanical properties of the composites is explored through comparison with literature. Tensile testing on unidirectional solid specimens loaded in the fibre direction revealed that nylon reinforced with 5% of continuous carbon fibre had significantly higher tensile strength and Young's modulus but lower elongation at break compared to nylon reinforced with 20% short carbon fibre thanks to the superior performance of continuous fibres. Post-experimental fracture surfaces of both materials revealed brittle failure with fibre pull-outs for the composite with continuous fibres, indicating weak bonding between the nylon matrix and the carbon fibre. The mechanical properties (stiffness and tensile strength) of both materials for different levels of fibre volume fractions are presented based on this study and literature data. The tensile strengths and moduli changed in a similar manner for both materials, but an almost linear relation was obtained for nylon reinforced with continuous carbon fibre at higher fibre content as the properties of the composite structures were governed mostly by fibres. On the contrary, for the composite reinforced with short fibres at lower fibre content, no direct relationship was observed.