Structure-property relations of Kevlar 49 fibers

The relation between the physical structure and deformation and failure processes of Kevlar fibers is presented. The fiber fabrication processes that affect the critical structural parameters controlling deformation and failure are considered. The deformation and failure processes are discussed in the light of fracture topography studies of epoxy composite strands and non-etched and HCl-etched bare yarns together with deformation studies of bare yarns deformed directly in the optical microscope. A physical model is suggested for Kelvar 49 in which there is an amorphous skin and crystalline core. The core consists of periodic transverse defect planes spaced about 200 nm along the fiber. Chain ends are assumed to cluster within the vicinity of these planes. The non-crystalline skin in which the chain ends are arranged essentially randomly relative to one another does not contain such transverse weak planes.