Considerable effort has been devoted to identifying the microstructural factors that govern the tensile strength of carbon fibers. Here, ten commercial polyacrylonitrile (PAN)-based carbon fibers of comparable modulus (200–330 GPa) but widely varying tensile strength (3.84–6.36 GPa) were characterized by atomic force microscopy, scanning electron microscopy, wide-angle X-ray diffraction, Raman spectroscopy, and small-angle X-ray scattering. Surface roughness and cross-sectional morphology showed no correlation with tensile strength. Crystallite dimensions exhibited threshold behavior at approximately 4.3 GPa, above which larger crystallite thickness was associated with higher strength. However, this trend did not hold across the full-strength range, and the Raman features showed no consistent correlation. Microvoid misalignment likewise could not explain the strength variation, as all fibers were already highly oriented. Building on our previously reported two-population void model, we separately quantified small and large voids. Among the void parameters, the small void volume fraction showed the strongest correlation with tensile strength (R2 = 0.904). Because all void parameters are mutually collinear, we adopt it as the primary structural indicator of tensile strength rather than its sole cause. The correlation between the volume fraction of small voids and tensile strength remained strong after accounting for the contribution of tensile modulus (partial R2 = 0.784), indicating that it is not merely a proxy for tensile modulus. The relationship was further examined against four carbon fibers excluded from the analysis, whose strengths extend modestly beyond both ends of the calibration range, and their measured values were reproduced to within 0.5 GPa.
Improving the recyclability of carbon fibers is attracting increasing attention because of their excellent mechanical properties that are paid with high production costs and significant CO2 footprint. Current recycling processes often result in a significant reduction in material performance due to degradation of the ordered microstructure and loss of the original fiber length. In this context, staple fiber yarn processing represents a promising approach, in which discontinuous recycled carbon fibers and polymer fibers are carded and spun into yarns with improved fiber alignment. However, quantitative descriptors linking processing parameters to composite microstructure remain limited. This study aims to establish X-ray computed tomography (XCT) - derived parameters that characterize micro- and mesostructural quality as a function of yarn stretch ratio of 1%, 20%, and 50% during processing. Both tape and laminate specimens were investigated. XCT imaging combined with semantic and instance segmentation of the single carbon fibers was employed to derive quantitative descriptors, including fiber volume content, fiber orientation distribution, and homogeneity estimations. The results indicate that increasing stretching values lead to higher fiber volume content and improved microstructural homogeneity measured through a Voronoi cell-based analysis. Moreover, using the Homogeneity Index of Mixture, a difference in homogeneity was measurable between tapes and laminates in unprocessed reconstructed images.
Fiber-reinforced polymer composites (FRPCs) are pivotal in aerospace and automotive sectors; however, their limited thermal stability under extreme conditions remains a challenge. In this study, the thermomechanical behaviour of unidirectional basalt fabric-reinforced unsaturated polyester (BFPC) composites was enhanced through the incorporation of silica nanoparticles with varying morphologies, namely nonporous, semiporous, and porous structures, as secondary reinforcements. Silica nanoparticles with a flower-like hierarchical structure were synthesized via a modified Stöber sol-gel process, and an optimized loading of 1.0 wt% of different morphologies was used to incorporate into the composite system. The flower-like SiO2 exhibited a hierarchical micro-/meso-/macroporous structure with a BET surface area of 759 m2 g−1 and a total pore volume of 0.71 cm3 g−1, and XPS revealed a ∼1.5 eV shift of the Si 2p peak to lower binding energy in the porous-silica composite, indicating Si–O–C interfacial bonding with the polyester matrix that persists throughout the 10–50 nm depth profile. Then, porous silica nanoparticles with different loadings (0.5%,1%,1.5%) were analysed in detail for different properties. Thermomechanical and thermal stability via dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC), and coefficient of thermal expansion (CTE) measurements revealed a 40-50% reduction in thermal expansion, a 19 °C increase in glass transition temperature (Tg), and enhanced thermal stability compared to neat composites in the case of the composite incorporated with porous nanoparticles at 1.0 wt% loading. Mechanical characterization revealed that all properties showed substantial enhancement for 1.0 wt% loaded composites. The tensile strength reached 250 MPa, representing improvements of 25-30% over semiporous variants and 40-50% over the neat composites. Flexural strength reached 315 MPa, representing an improvement of 15-20% over the semiporous composite and over 30-35% over neat composites. The impact strength also improved by almost 20-25% over the neat composites. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) confirmed robust interfacial adhesion and a high surface area, i.e. 50-60% increase due to the porous nanoparticles’ architecture, facilitating mechanical interlocking and crack deflection.
