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 [Formula: see text] 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.

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 [Formula: see text]45° angle-ply fiber orientations had the highest impact energy absorption.

To improve the properties of additively manufactured parts to be used in high-end applications, intrinsic defects occurring during the printing process need to be minimized. Defects such as void can significantly degrade the mechanical properties of the resulted parts. The presence of void is more evident in composite printed parts due to the inhomogeneity of the specimen. In this study, composite rectangular coupons printed with a Markforged Mark Two printer were manufactured with different fiber orientations and stacking sequences. A void content reduction/consolidation process, consisting of applying pressure at different temperature levels, was developed and implemented to remove the voids in form of air bubbles trapped in the specimen. A two-part mold with female and male components with the same dimensions as the rectangular specimen was designed and machined to be used in a hot press process. The success of the approach was evaluated by calculating the density of the specimen pre- and post-consolidation. The void content reduction results were highly dependent on fiber orientation; however, the density increased for all tested specimens, confirming the reduction in porosity.

No abstract

The mechanical properties of polymers highly depend on the loading rate, however, the effect of loading/strain rate with the addition of Carbon Nanotube is not well understood. In this study, the effect of Carbon Nanotube (CNT) content on the rate dependence of polymers was studied. Injection molded mini-tensile samples with CNT content ranging from 0 to 15wt% at strain rates of .0006, .0013, .0019 and .0025 s−1 were tested. It was found that as strain rate increased, the ultimate strength and Young s Modulus of the tensile specimens increased. It was also shown that addition of CNT will lower the chain mobility of polymer and lower the polymer dependence of properties to strain rates.