Strength and its variability in 3D printing of polymer composites with continuous fibers

Additive manufacturing (AM) of polymer composites with continuous fibers could play a major role in the future of aerospace and beyond but will require printed materials to achieve new levels of reliability. This study characterized the strength distribution of selected thermoplastic matrix composites as a function of printing via fused filament fabrication (FFF). Experimental and commercial composite filaments of continuous carbon or Kevlar fibers were printed with volume fraction (V f ) ranging from approximately 28 to 56 %. The strength was evaluated under uniaxial tension after specific stages of printing and Weibull statistics were applied to characterize the strength distribution. There was a significant reduction in strength of the printed material with respect to the unprinted condition, regardless of reinforcement type, fiber volume fraction or printer used. Damage introduced by feed extrusion of the filament, and fiber failures induced at material deposition were most detrimental. For carbon fiber filaments, the reduction ranged from approximately 10 % for an experimental material to over 60 % for a commercial filament. There was no correlation in the strength degradation or variability with V f . The prevention of process-related fiber damage is key to advancing AM for continuous fiber composite and application to designs intended for stress-critical applications.