The relationship between the physical structure and the microscopic deformation and failure processes of poly(p‐phenylene terephthalamide) fibers

Relations between the physical structure and the deformation and failure processes of poly( p ‐phenylene terephthalamide) (PPTA) fibers are reported. The effects of the physical processes involved in fiber fabrication, including the crystallization of PPTA‐H 2 SO 4 dopes under stress, are considered in relation to their effect on the structure of the fiber. The deformation and failure processes together with the structure of the fiber are discussed in the light of fracture‐topography studies of fiber‐epoxy composite strands, single filaments, HCl‐etched and unetched yarns, and transmission optical microscopy studies of stressed and unstressed yarns. In view of these observations, the physical structure of PPTA fibers is discussed in terms of pleated H‐bond sheets, the macromolecular chain‐end concentration and distribution, and the presence of impurities. The structural parameters that affect the failure processes of these fibers and how such parameters can be modified by service environment conditions are also addressed.