Mechanical behavior and failure mechanisms of α-MnO ₂ nanorods/epoxy adhesives in structural bonding of CFRP composites

In recent years, nano-reinforced composite materials have emerged as alternatives to traditional materials, particularly for their high mechanical strength, low weight, environmental stability, and extended service life, which are critical in the aerospace and automotive industries. For this purpose, α-MnO2 nanorods were synthesized by the hydrothermal method and subsequently homogeneously incorporated into epoxy resin at weight percentages of 0.25%, 0.50%, 0.75%, and 1.00%. Tensile tests were conducted in accordance with ASTM D638, and fracture toughness evaluations were performed in accordance with ASTM D5045. Shore D hardness tests adhered to ASTM 2240 standards, while the adhesive performance of single lap joints was assessed following ASTM D5868 guidelines. The prepared nanocomposite adhesives were used in single-lap joints (SLJs) of carbon fiber-reinforced epoxy composite panels and subjected to mechanical testing in accordance with ASTM standards. Characterizations were performed using SEM, XRD, FTIR, TGA-DSC, and BET analyses. The results showed that a 0.75% additive content provided optimum performance in terms of tensile strength (∼9% increase), fracture toughness (∼20% increase), and SLJ shear strength (∼81% increase). Hardness values increased with the additive content, reaching a Shore D value of 88.5. At higher additive content, partial decreases in mechanical properties were observed due to agglomeration. Microstructural analyses revealed that at the optimal additive content, the nanorods inhibited crack propagation and increased energy absorption; however, the matrix became brittle under overload. These findings suggest that α-MnO2 nanorod/epoxy composites are promising candidates for structural adhesives in advanced engineering applications requiring high bond strength and fracture resistance.