Carbon Nanotubes Strengthen the Interaction of Carbon Fiber-Reinforced Polymers with Steel for Improved Mechanical Properties

Carbon fiber-reinforced polymer (CFRP) composites are often bonded to strengthen damaged steel structures by using epoxy adhesives. However, the inferior mechanical properties of epoxy adhesives compared to those of steel and CFRP prevent the use of the full potential of the steel/CFRP material systems. Herein, we introduce carbon nanotubes (CNT) into the epoxy matrix and evaluate their performance using theoretical and experimental methods, in which the adhesive is analyzed at multiple length scales to determine the changes in its thermomechanical behavior. The results show that the epoxy molecules are unable to form adequate bonds with steel and CFRP laminates because they lack sufficient nucleophile and electrophile species and an interconnected structure to endure elevated temperatures. Ab initio calculations demonstrate that while epoxy molecules do not effectively interact with laminates, especially CFRP, π electrons in the CNT structure help form strong bonds with both CFRP and steel surfaces. These electronic absorptions lead to interactions between the steel/CFRP laminates and modified adhesives, leading to significant improvements in slip resistance, adhesive cohesion, and thermal stability. The experimental results show a ∼100% increase in the bond strength at moderately elevated temperatures, as well as increased stress distribution throughout the joint, higher fracture energy, and improved interlayer bonding at moderately elevated temperatures.