Fiber reinforced composites offer exceptional directional mechanical properties, and combining their advantages with the capability of 3D printing has resulted in many innovative research fronts. This review aims to summarize the methods and findings of research conducted on 3D-printed carbon fiber reinforced composites. The review is focused on commercially available printers and filaments, as their results are reproducible and the findings can be applied to functional parts. As the process parameters can be readily changed in preparation of a 3D-printed part, it has been the focus of many studies. In addition to typical composite driving factors such as fiber orientation, fiber volume fraction and stacking sequence, printing parameters such as infill density, infill pattern, nozzle speed, layer thickness, built orientation, nozzle and bed temperatures have shown to influence mechanical properties. Due to the unique advantages of 3D printing, in addition to conventional unidirectional fiber orientation, concentric fiber rings have been used to optimize the mechanical performance of a part. This review surveys the literature in 3D printing of chopped and continuous carbon fiber composites to provide a reference for the state-of-the-art efforts, existing limitations and new research frontiers.

Fiber volume fraction is a driving factor in mechanical properties of composites. Micromechanical models are typically used to predict the effective properties of composites with different fiber volume fractions. Since the microstructure of 3D-printed composites is intrinsically different than conventional composites, such predictions need to be evaluated for 3D-printed composites. This investigation evaluates the ability of the Voigt, Reuss, and Halpin–Tsai models to capture the dependence of modulus and strength of 3D-printed composites on varying fiber volume fraction. Tensile coupons were printed with continuous carbon fiber-reinforced Onyx matrix using a Markforged Mark Two printer. Specimens were printed at five different volume fractions with unidirectional fibers oriented at either $0°,\mathrm{ }45°,\mathrm{ }\mathrm{or}\mathrm{ }90°$ to obtain longitudinal, shear, and transverse properties, respectively. It is shown that the Voigt model provides an excellent fit for the longitudinal tensile strength and a reasonable fit for the longitudinal modulus with varied fiber content. For the transverse direction, while the Reuss model fails to capture the transverse modulus trend, the Halpin–Tsai model provides a reasonable fit as it incorporates more experimental parameters. Like conventional composites, addition of fibers degrades the transverse strength, and the transverse strength decreases with increasing fiber volume fraction. The shear modulus variation with fiber content could not be fitted reasonably with either Halpin–Tsai model or Reuss model.

In this study, the effects of stress concentration on the tensile properties of a 3D printed carbon fiber-nylon composite were investigated. The samples were 3D printed with continuous carbon fiber and chopped fiber reinforced nylon. Samples were manufactured with four different open hole sizes as 3. 175 mm (⅛ in), 6.35 mm (¼ in), 9.25 mm (⅜ in), and 12.7 mm (½ in). Five samples were manufactured for each hole size group. Continuous carbon fibers were printed in the longitudinal direction. Additional reinforcements were placed around the periphery of the open hole. Samples were tested under uniaxial tension. The results were compared with the prediction of fracture mechanics theories namely Average and Point Stress Criteria. The results show that failure was initiated at the stress concentration region but the progression into the hole was prevented with the presence of continuous fiber. The experimental findings show that the samples with larger holes are more sensitive to discontinuity than the ones with smaller holes. The results confirm that 3D printing can be used to strengthen the parts at the discontinuity region to mitigate the effect of stress concentration.

Mechanical behavior and reliability of composites are driven significantly by microstructural variability. Such variability can be present in the form of both morphological and constituent property variability. To understand the effect of this variability on macroscopic mechanical behavior, many statistically equivalent microstructures must be evaluated. This requires the ability to generate such microstructures. In this work, morphological variability was quantified by image analysis of actual microstructures. To reproduce this variability, a methodology was developed in which random microstructures are generated and subsequently adjusted to simultaneously match both short- and long-range statistics of actual microstructures. Synthetic microstructures were generated at a length scale of 70 µm, corresponding to the length scale at which fiber volume fractions of adjacent microstructures are uncorrelated. The utility of this methodology was also demonstrated for larger microstructures containing defects such as alignment fibers, voids and resin seams.

Additive manufacturing (AM) has been used widely to produce three-dimensional (3D) parts from computer-aided design (CAD) software. Traditional Fused Deposition Modeling (FDM) 3D printed polymer parts lack the necessary strength to be used for functional parts in service. The potential of printing continuous fiber reinforced composites has resulted in parts with better mechanical properties and enhanced performance. Very few studies have investigated the impact energy absorption of continuous fiber reinforced 3 D printed composites. The purpose of this work is to investigate the effect of different fiber patterns (unidirectional versus concentric), different stacking patterns (consolidated versus alternating layers), and fiber orientations (0^°, 90^°, 45^°) on the impact energy absorption of 3 D printed continuous Kevlar fiber reinforced Onyx composites. Charpy impact testing was used to determine the impact energy absorption of the specimens. It was concluded that alternating the fiber and matrix layers as opposed to consolidating all the fiber layers in the center of the specimen results in lower impact energy absorption. Additionally, the specimens with unidirectional 90° fiber orientation had the lowest impact energy absorption among the specimens with alternating stacking pattern and those with consolidated [ ]45° angle-ply fiber orientations had the highest impact energy absorption.