Effect of Short Carbon Fiber Content on Shear Performance and Mode II Fracture Behavior of Epoxy Adhesives

ABSTRACT This study utilizes the V‐notched beam method and asymmetric short bend beam (ASBB) fracture testing to investigate the effect of short carbon fiber (SCF) content on the shear properties and Mode II fracture behavior of SCF/epoxy composites. Results show that increasing SCF content led to enhanced shear strength and modulus, with a 22.1% strength improvement at 30 wt%. However, fracture toughness peaked at 20 wt% and then declined by 6.3% beyond this point, revealing a typical strength–toughness contradiction. SEM analysis revealed that fiber pull‐out and rupture synergistically enhanced both strength and toughness below 20 wt%, while excessive fiber content (30 wt%) induced interfacial defects that compromised toughness despite continued strength gains. A predictive model for Mode II fracture, based on the Tresca yield criterion with T‐stress correction, was proposed. It predicted crack initiation angles with less than 5% deviation and demonstrated greater sensitivity to SCF content variations than the traditional maximum tangential stress (MTS) model. These findings can guide the design and optimization of epoxy adhesives with high shear strength and fracture toughness.