The strong interfacial interaction between carbon fiber and epoxy matrix plays a key role in ensuring the performance of carbon fiber reinforced polymer (CFRP). During a prolonged service-life, CFRP is inevitably exposed to the hygrothermal environment and the integrity of fiber/matrix interface is most vulnerable, but the microscopic behavior of the interface under the environmental exposure remains elusive. Here an atomistic analysis is presented on mode I and mode II traction-separation behavior between carbon fiber and epoxy matrix, which provides insights into how the surrounding water molecules at different temperature levels impact the interfacial behavior. It is found that the water molecules at the interface reduce the contact area between fiber and matrix and weaken the epoxy properties by disrupting the molecular interactions, which consequently lowers the energy barriers to interfacial separation and sliding, and the elevated temperature level further degrades the interfacial mechanical response as the epoxy becomes softened. The research findings demonstrate that the presence of water drastically deteriorates the integrity of carbon fiber/epoxy interface, and the derivation of cohesive laws based on tractionseparation simulation results provides a paradigm of deriving the fundamental inputs for a multiscale modeling of the interface at the continuum level by considering the environmental effect.

Introduction

Carbon fiber reinforced polymer (CFRP) is a type of remarkably resilient composite material possessing outstanding properties, such as high specific stiffness- and strength-to-weight ratios, good thermal stability, and strong corrosion resistance. CFRP composite has emerged as a viable alternative to the conventional materials in construction industry, such as applications in concrete infrastructures as external confinement/reinforcement and internal rebar [1–۳]. Despite the promise as reinforcement of existing infrastructures and structural building-block in new constructions, CFRP composite exhibits a certain degree of property degradation under the environmental exposure, which shortens the intended service-life [4–۹]. These problems are usually attributed to the deterioration of interfacial integrity between carbon fiber and epoxy matrix, which is crucial to the performance of these macroscale applications involving CFRP composite [9,10].