Three-dimensional (3D) printing continuous carbon fiber-reinforced polylactic acid (PLA) composites offer excellent tensile mechanical properties. The present study aimed to research the effect of process parameters on the tensile mechanical properties of 3D printing composite specimens through a series of mechanical experiments. The main printing parameters, including layer height, extrusion width, printing temperature, and printing speed are changed to manufacture specimens based on the modified fused filament fabrication 3D printer, and the tensile mechanical properties of 3D printing continuous carbon fiber-reinforced PLA composites are presented. By comparing the outcomes of experiments, the results show that relative fiber content has a significant impact on mechanical properties and the ratio of carbon fibers in composites is influenced by layer height and extrusion width. The tensile mechanical properties of continuous carbon fiber-reinforced composites gradually decrease with an increase of layer height and extrusion width. In addition, printing temperature and speed also affect the fiber matrix interface, i.e., tensile mechanical properties increase as the printing temperature rises, while the tensile mechanical properties decrease when the printing speed increases. Furthermore, the strengthening mechanism on the tensile mechanical properties is that external loads subjected to the components can be transferred to the carbon fibers through the fiber-matrix interface. Additionally, SEM images suggest that the main weakness of continuous carbon fiber-reinforced 3D printing composites exists in the fiber-matrix interface, and the main failure is the pull-out of the fiber caused by the interface destruction.

The ability to control wave propagation in highly deformable layered media with elastic instability-induced wrinkling of interfacial layers is presented. The onset of a wrinkling instability in initially straight interfacial layers occurs when a critical compressive strain is achieved. Further compression beyond the critical strain leads to an increase in the wrinkle amplitude of the interfacial layer. This, in turn, gives rise to the formation of a system of periodic scatterers, which reflect and interfere with wave propagation. We demonstrate that the topology of wrinkling interfacial layers can be controlled by deformation and used to produce band gaps in wave propagation and, hence, to selectively filter frequencies. Remarkably, the mechanism of frequency filtering is effective even for composites with similar or identical densities, such as polymer-polymer composites. Since the microstructure change is reversible, the mechanism can be used for tuning and controlling wave propagation by deformation.

Inspired by the imbricated scale-tissue flexible armor of elasmoid fish, we design hybrid stiff plate/soft matrix material architectures and reveal their ability to provide protection against penetration while preserving flexibility. Indentation and bending tests on bio-inspired 3D-printed prototype materials show that both protection and flexibility are highly tunable by geometrical parameters of the microstructure (plate inclination angle and volume fraction). We show that penetration resistance can be amplified by a factor of 40, while flexibility decreases in less than 5 times. Different deformation resistance mechanisms are found to govern flexibility (inter-plate matrix shear) versus penetration resistance (localized plate bending) for this microstructural architecture which, in turn, enables separation of these functional requirements in the material design. These experiments identify the tradeoffs between these typically conflicting properties as well as the ability to design the most protective material architecture for a required flexibility, providing new design guidelines for enhanced flexible armor systems.