Assessment of advanced additively manufactured composites for structural applications

Fibre-reinforced polymer matrix composites (FRPs) are materials which generally have superior properties to conventional materials such as stiffness and specific properties at a reduced weight. Fused filament fabrication (FFF) is the most widely used additive manufacturing (AM) technique to produce FRPs, due to their low wastage, geometric flexibility and ease of use. Traditional modelling and manufacturing of composite materials does not allow for the geometric flexibility facilitated by FFF, due to the need for post-fabrication material removal and its associated wastage and cost. However, composites manufactured by FFF are highly susceptible to defects such as high void content and poor bond quality at the fibre and matrix interfaces. Detection of such defects has created a new challenge in terms of quality control and assessment. This study assessed the FFF AM printing process for polymers and short and continuous FRPs. A novel method of in-line monitoring was developed, for the in situ early identification of issues and defects in the printing. This method of in-line monitoring combined infrared thermography (IRT) and acoustic emission (AE) was benchmarked against offline assessment through micro-computed tomography (micro-CT). This is the first time IRT and AE methodologies have been combined for the in-line monitoring of the FFF process. Structural assessment including tensile testing was also performed as well as material characterisation including differential scanning calorimetry (DSC), melt flow rate (MFR) and x-ray diffraction (XRD). Utilising this in-line monitoring methodology, multiple matrix materials including polylactic acid (PLA) and polyamide (PA), single nozzle and dual nozzle printing, print settings and print parameters were explored to determine their effects on the print quality and structural properties. The layer height, line width and infill pattern were explored specifically and were identified major impacts on the quality and mechanical behaviour of the printed parts. Specifically decreasing the layer height led to a reduction in porosity and an improvement in mechanical properties, and the lines infill pattern recorded the highest mechanical properties compared to grids and triangles. The developed methodology successfully led to the early identification of printing defects, which can be employed to successfully optimise the printing quality, ultimately leading to improved structural properties.