Nanoscale origin of bilinear stress-strain response of polyethylene terephthalate fiber in tension

Polyethylene terephthalate (PET) fiber-reinforced polymer composites have emerged as a promising strengthening material for improving the ductility of concrete structures due to their large deformability. In particular, PET fibers exhibit a unique bilinear tensile stress-strain response, which is significantly different from other conventional linear fibers like carbon and glass. However, the mechanism responsible for this bilinear behavior remains elusive. In this study, molecular dynamics simulations are performed to provide insights into the nanoscale origin of the bilinear behavior of PET fibers. The atomistic model of the lamellar structure of the PET is constructed to contain the crystalline and amorphous phases. Simulation results show that the molecular chains of PET fiber change from entanglement to ordered alignment in the amorphous phase under tension at 300 K. As a result, the order, radius of gyration, and density in PET are changed. The structural evolution results in the transition in the tensile stress-strain curve and yields a bilinear response. In addition, it is found that the moisture and elevated temperature cause the loss of bilinear characteristics due to the weakened interaction between molecular chains of PET. This comprehensive analysis provides unknown mechanisms of the bilinear response of PET fibers. © 2024 Elsevier Ltd.