Significant insights into the fundamental deterioration mechanism of the fibre–matrix interface under coupled thermal–mechanical actions

In the durability studies of FRP composites, accelerated laboratory exposure tests have been widely adopted, in which the specimens are commonly subjected to an elevated temperature to accelerate their deterioration. The results from the accelerated tests are often used to predict the long-term performance of FRP composites by adopting the Arrhenius equation. The use of the Arrhenius equation assumes that the fundamental deterioration mechanism (e.g., the activation energy) does not change in the accelerated tests, but this assumption has not been verified for FRP composites. Due to the characteristics of glass transition and entropic elasticity of the polymeric matrix, it is likely that the Arrhenius equation may only be valid for a certain temperature range for FRP composites. This temperature range, however, has not yet been clarified or rigorously examined. This paper presents a study to address this deficiency of existing studies using reactive force field MD simulations. Two models were established for the untreated and sizing-treated fibre-matrix interfaces, respectively, and performed debonding simulations over a wide range of temperatures (300 K ∼ 600 K) which allows the polymeric matrix to be investigated in both the glassy state and the rubbery state. Importantly, the molecular models are sufficiently large so that the macroscopic interfacial properties (e.g., the peak stress, modulus, and toughness) under various temperatures can be quantitively obtained as the statistical average of microscopic quantities. In this paper, the details of the methods are first presented, followed by interpretations and discussions of the simulation results with reference to the existing theories on polymers (i.e., Eyring’s theory and other kinetic theories). The simulations were validated with the previous experimental results in various terms and were used to quantitatively examine the effects of coupled thermal–mechanical actions on the key properties of the interfaces and their deterioration mechanism which involves the breakage of covalent bonds. The results shed light on the design and interpretation of accelerated tests and may be used in multiscale and multifield modelling of the durability of FRP composites in the future.