Ab initio shock loading on poly (p-phenylene terephthalamide) (PPTA) and its implications for Kevlar and other aramid-based fibers performance

Ab initio molecular dynamics simulations using the multi-scale shock technique are applied in the study of the dynamic response of poly (p-phenylene terephthalamide) (PPTA) crystals to shock loading. PPTA crystals form the bulk of para-aramid fibers, such as Kevlar and Twaron, and are responsible for their outstanding strength-to-weight ratio. Strong shock loadings describe the shock response from elastic to polymer decomposition. Results reveal an anisotropic response for shocks perpendicular to the crystal symmetry axis (aramid fiber axis) including stress release mechanisms combining structural phase transformation (SPT) and production of paracrystallinity. SPT is observed for shocks along the [100] direction and are triggered by shock-induced coplanarity of amide and phenylene groups resulting in reorganization of PPTA sheet stacking. Generation of paracrystallinity is triggered by [010] shock-induced scission of hydrogen bonds, trans-cis polymer conformation change, and disruption of chain sheets. While the SPT preserves crystalline order and PPTA properties the generation of paracrystallinity strongly affects PPTA strength. The simulation results provide an atomistic view on the effects of shock in para-aramid fibers.