Multiscale insights into the stretching behavior of Kevlar fiber

Aromatic polyamide fiber becomes the most important material for ballistic protection purposes because of the high modulus and strength from the 1960s. To obtain an insightful understanding of the mechanical deformation of Kevlar, a detailed multiscale model is developed in this paper based on the structures of Kevlar from molecular scale to macroscopic scale. By analyzing the molecular structures of Kevlar, the high modulus of Kevlar fibers can be attributed to the high stiffness of aromatic polyamide chains and the massively distributed hydrogen bonds in the material. The damage initiation and evolution in Kevlar fiber can be captured at the microfibril scale. It can be found that the material strength in the grain boundary regions is only 1.9 GPa, resulting in much lower strength of the whole Kevlar fiber (2.5-3.3 GPa), compared to the theoretical strength of the aromatic polyamide crystals (> 30 GPa). The structural parameters of Kevlar fiber (pleated width, misorientation angle and skin thickness) are also studied to understand the contribution of these parameters to the stretching behavior of a Kevlar Bundle, which is significant to the future improvement of aromatic polyamide fibers for reinforcement purposes in advanced equipment.