Microstructure and damage evolution in short carbon fibre 3D-printed composites during tensile straining

Short-fibre thermoplastic composites offer a balance between cost, processability, and performance, as well as providing a use for recycled fibres making them attractive in various industrial applications. However, the fibres tend to be misaligned due to their low aspect ratio, which can impact mechanical performance. This work examines the as-manufactured microstructure of a chopped carbon fibre-reinforced nylon composite made by material extrusion additive manufacturing in terms of fibre misalignment, void content, shape and distribution before going on to determine its effect on damage evolution under tensile straining by in-situ time-lapse synchrotron computed tomography (CT). To this end, CT scans have been acquired at various stages throughout straining. A high degree of fibre alignment is observed with ≈ 86% within 14 ∘ of the extrusion axis, giving a Krenchel orientation factor of 0.75. The time-lapse CT image sequence reveals that because the mean fibre length ( ≈ 98 μ m .) is below the critical fibre length fibre fracture does not take place during plastic straining. Instead, failure occurs during straining from pre-existing voids and newly nucleated ones mainly located at fibre ends, their growth and coalescence. The experimental elastic modulus and strength are compared against the Cox-Krenchel and Kelly-Tyson analytical models that take into account fibre misalignment and length, which demonstrates that the fibre orientation is sufficient and future improvements in properties could be achieved by reducing the initial void content ( ≈ 2.3%) and increasing the length and volume fraction of the reinforcing fibres. • Microstructure characterisation of short-fibre 3D printed composites. • Damage accumulation tracked by X-ray CT scanning during straining. • Void nucleation, link-up and coalescence identified as the dominant failure mechanism. • Future composite design guided by analytical model predictions.