Fibers

Short-fiber-reinforced thermoplastics are widely used in lightweight structural applications, but reliable prediction of their Young’s modulus remains challenging because classical analytical models usually neglect interfacial effects. In this study, the Cox–Krenchel model was extended by an adhesion efficiency factor derived from surface-energy-based interfacial tension to account for incomplete elastic load transfer at the fiber–matrix interface. Injection-molded polypropylene (PP) and polyamide 6.6 (PA 6.6) composites reinforced with basalt and glass fibers were produced and characterized experimentally. Model inputs comprised the matrix and fiber modulus, measured fiber volume fraction, experimentally determined individual fiber lengths incorporated through an effective Cox length efficiency factor, and experimentally determined fiber orientation factors. Interfacial tension was calculated from polar and dispersive surface-tension components using the Owens–Wendt–Rabel–Kaelble approach. The classical Cox–Krenchel model described the PA–basalt system with high accuracy but systematically overestimated the stiffness of the PP-based systems. Using a single globally calibrated proportionality constant of ω = 0.8, the adhesion-extended model reduced the mean absolute percentage error on the full condition-specific dataset of 55 data points from 14.2% to 2.7%. The results indicate that morphology remains the dominant basis of stiffness prediction, while surface-energy-based interfacial compatibility provides a relevant additional descriptor for residual system-dependent error. For the investigated systems, the proposed extension improved predictive accuracy while preserving the analytical simplicity of the Cox–Krenchel framework.

This review examines the ballistic resistance of fiber-reinforced cement composites (FRCs) and related cementitious systems for protective structures, with emphasis on projectile–target interaction, penetration and scabbing mechanisms, and the governing roles of material and structural parameters. The synthesis indicates that ballistic resistance is controlled not only by compressive strength but also by the combined effects of dynamic tensile behavior, fracture energy, crack-bridging efficiency, aggregate characteristics, target thickness, projectile characteristics, and structural configuration. Steel and hybrid fiber systems generally provide the most consistent improvements in scabbing suppression and residual integrity, while hard aggregates and multilayer hard–soft–tough arrangements enhance penetration resistance, stress-wave attenuation, and staged energy dissipation. The review also evaluates current numerical approaches, including rate-sensitive constitutive models, cohesive and continuum damage formulations, smoothed particle hydrodynamics, and data-driven methods. Several calibrated and experimentally validated simulations reproduced penetration depth and major damage trends with useful engineering agreement; however, significant challenges remain in representing multi-hit degradation, fiber-scale pull-out, fragment ejection, and interface debonding. In addition, the review highlights the limited suitability of existing ballistic standards for quasi-brittle cementitious systems and emphasizes the need for FRC-specific testing, large-scale validation, and more sustainable protective material design.

Plant-derived residues are promising renewable feedstocks for bio-based building materials, but their suitability depends on structural, thermophysical, fire-related, and mineral characteristics. This study characterises ten plant residues—corn stalk (CS), hemp stalk (HS), luffa (Lu), olive branches (OB), olive pit (OP), Posidonia oceanica (PO), rice husk (RH), rice straw (RS), sunflower stalk (SS), and wheat straw (WS)—to establish a first-stage screening framework for lightweight construction applications. The raw materials were assessed through morphological, density-related, microstructural, thermophysical, thermogravimetric, and microscale combustion analyses, while their calcined fractions were examined by ash-yield determination, X-ray fluorescence, and X-ray diffraction. Apparent bulk density varied by approximately a factor of 30, from 0.0275 g cm⁻³ for RS to 0.8279 g cm⁻³ for OP, while thermal conductivity ranged from 0.0436 to 0.1204 W/(m·K). Most residues remained within the range associated with lightweight insulation, whereas OP showed the highest conductivity. Peak heat release rate (PHRR) ranged from 45.7 W g⁻¹ for PO to 145.5 W g⁻¹ for Lu, with SS also showing low heat-release parameters. Ash yield varied from 1.66% to 26.23%, accompanied by silica-rich, calcic, alkali-rich, amorphous, and crystalline mineral profiles. The results provide preliminary criteria for categorising and prioritising residues for lightweight insulation-oriented applications or mineral-related valorisation, as a basis for future composite development and application-level validation.