LAUSR

Epoxy adhesives are pivotal in industries ranging from aerospace to automotive due to their high load‐bearing capacity and versatility. However, their susceptibility to mechanical failure and limited fracture toughness under mixed‐mode loading remains a critical challenge. Cellulose nanocrystals (CNCs)—a sustainable, renewable resource with high‐strength potential—offer untapped potential as eco‐friendly alternatives to conventional nanomaterials. Unlike prior studies on CNC‐reinforced epoxies, this work uniquely evaluates mixed‐mode fracture energy alongside elastoplastic properties. Tensile tests, double cantilever beam (DCB), and end‐notch flexural (ENF) experiments were coupled with a cohesive zone model (CZM) to quantify enhancements in Young's modulus (9%), tensile strength (16%), mode I (21%), and mode II (53%) fracture energy at 1 wt% CNC—a threshold beyond which agglomeration degrades performance, offering critical guidelines for CNC usage. Scanning electron microscopy (SEM) validated uniform CNC dispersion up to 1 wt%, while finite element simulations confirmed the CZM's predictive accuracy. These findings advance CNC‐epoxy design for aerospace joints or automotive adhesives, combining sustainability with superior mechanical resilience.