Synergistic Enhancement of Multi-Mode Fracture Toughness in Carbon Fiber-Reinforced Ultra-High Performance Concrete By Sialon Nanoparticles: Numerical İnsights and Experimental Validation

Abstract

Although various nanomaterials have been widely investigated for enhancing the mechanical and fracture properties of Ultra-High Performance Concrete (UHPC), the specific role of Sialon nanoparticles (SNPs) in modifying the multi-mode fracture resistance of carbon fiber-reinforced UHPC (CFR-UHPC) has not yet been addressed.This study examines the effects of SNPs on the fracture toughness of CFR-UHPC through a combined experimental–numerical framework. A high-fidelity three-dimensional multi-scale finite element model was developed, incorporating randomly distributed carbon fibers and an interfacial transition zone (ITZ) represented via traction–separation laws calibrated using a Genetic Algorithm (GA). The model was validated against cracked straight-through Brazilian disk (CSTBD) experiments conducted at crack inclination angles of α = 0°, 29°, and 45°, demonstrating strong agreement in both load–displacement responses and crack propagation patterns. Parametric simulations revealed that SNPs enhance fracture toughness across all fracture modes. The most pronounced improvement was observed in Mode II, where KIIc c increased by approximately 34 % at 2 wt% SNPs, while mixed-mode conditions (α ≈ 15°–45°) exhibited the highest ductility. Although SNP addition improved resistance to crack propagation, compressive ductility decreased at SNP contents above 4 wt% due to nanoparticle agglomeration, which introduced defects and promoted brittleness. The optimal SNP dosage for enhancing multi-mode fracture toughness was found to lie between 2 and 4 wt%, where matrix refinement induced by SNPs and fiber-bridging effects act synergistically without compromising ductility. Compressive strength also improved by ∼27 % at 4 wt% SNPs, albeit accompanied by reduced post-peak deformability. SEM analysis corroborated these findings by revealing improved matrix densification, more uniform microstructures, and enhanced fiber–matrix bonding at optimum SNP levels, whereas higher dosages produced visible clusters and micro-defects. These microstructural observations directly support the measured improvements in Mode I, Mode II, and mixed-mode fracture toughness. Collectively, the results provide practical guidelines for tailoring CFR-UHPC formulations with enhanced fracture resistance for demanding structural applications.