## 5 Fiber-Reinforced Composites  
Most polymers employed in Fused Filament Fabrication (FFF) exhibit inherently low mechanical properties; one effective strategy to enhance their performance is reinforcement with fibers. Integrating fiber reinforcements into thermoplastic matrices significantly improves mechanical strength, dimensional stability, and thermal performance in FFF [67; 68]. Short fiber-reinforced composites (SFRCs), commonly carbon, glass, or natural fibers, improve stiffness and moderately increase strength, but their properties remain anisotropic due to random fiber orientations. Continuous fiber-reinforced composites (CFRCs) provide superior mechanical properties comparable to polymeric materials but require precise fiber alignment and controlled deposition, complicating the printing process. [PERSON] et al. (2017) [69] developed a coupled Smoothed Particle Hydrodynamics (SPH) and Discrete Element Method (DEM) model to simulate the 3D printing process of fiber-reinforced polymers. Their study explored the behavior of both short glass fibers in ABS and continuous carbon fibers in Nylon during extrusion. As shown in Figure 19, short fibers tend to align with resin flow, but interactions near the nozzle walls can introduce chaotic movement and reduce alignment. In contrast, continuous fibers remain centered early in the extrusion but undergo significant bending deformation and stress buildup as printing progresses. This deformation, along with fiber-nozzle contact, can lead to tool wear and performance reduction. The model also demonstrated how nozzle geometry and extrusion velocity influence flow fields and fiber positioning. Although the model lacked thermal and shrinkage considerations, it provided valuable insights into fiber-resin interactions and established a foundation for multi-physics simulations. [PERSON] et al. (2022) [70] further studied fiber breakage and orientation experimentally using X-ray micro-computed tomography (uCT), finding that smaller nozzles increased fiber breakage and larger layer heights improved alignment but increased porosity, thus decreasing tensile strength. Their results highlighted critical trade-offs between fiber integrity and mechanical performance. Topology optimization has emerged as an effective approach for improving composite performance. [PERSON] et al. (2022) [71] developed a topology optimization method aligning fiber paths with stress distributions, improving stiffness-to-weight ratios and manufac  
Figure 19: Simulated velocity profiles of resin flow and fiber behavior during FFF extrusion. (**a**) Short glass fiber/ABS composites show chaotic flow-induced reorientation; (**b**) Continuous carbon fiber/Nylon composites exhibit bending and stress concentration due to interaction with nozzle walls. Adapted with permission from [69]. turability. Experimental validations confirmed improvements but emphasized the need for better fiber continuity and optimized path planning. [PERSON] et al. (2024) [72] similarly demonstrated that topology optimization improved stress distribution and structural performance compared to shape optimization, although defects and fiber misalignments from manufacturing processes indicated that models should better reflect practical constraints. These observations are further supported by Figure 20, which presents numerical stress analysis, crack initiation zones, and \(\upmu\)CT scans of failed specimens. Broader reviews by [PERSON] et al. (2018) [68] emphasized that fiber alignment increases stiffness but reduces transverse strength, highlighting interlayer adhesion as critical. Large-scale systems, such as Big Area Additive Manufacturing (BAAM) and Large-Scale Additive Manufacturing (LSAM), improved production speeds but still face challenges in accurately predicting fiber orientation, improving interlayer bonding, and modeling residual stress. [PERSON] et al.