Effects of extreme transverse deformation on the strength of UHMWPE single filaments for ballistic applications

Fibers used in both soft and hard body armor have very high longitudinal tensile strength and stiffness, but differ drastically in their transverse mechanical properties. Glass and carbon fibers are stiff and brittle in the transverse direction and easily shatter upon projectile impact unless they are cushioned within a soft matrix to disperse the load. In contrast, aramid fibers (e.g., Kevlar 29 and Twaron) and ultra-high-molecular-weight polyethylene (UHMWPE) fibers (e.g., Dyneema and Spectra) have quasi-plastic transverse behavior, with a low yield strength, and thus tend to flatten upon projectile impact, yet retain much of their tensile load-carrying capability. Thus, these polymer fibers are especially suitable for ‘soft’ body armor consisting of stacked sheets or fabrics, whereas the former glass and carbon fibers are useful mainly when aligned in a strong polymer matrix to form a thick plate. In this work, we report on a study of the tensile mechanical properties of single UHMWPE fibers (i.e., single filaments) that have been transversely deformed from their original cylindrical shape to form thin flat micro-tapes with a width-to-thickness ratio of up to 60:1. The deformed, ribbon-like fibers show very high retention in fiber strength, though with increased variability resulting from locally induced defects. Because transverse deformation resulted in more than a factor of three increase in surface area per unit length, the stress transfer length necessary to fully load a fiber near a break was found also to decrease by the same factor, as the corresponding interfacial shear stress remained the same. A Weibull probability analysis revealed that the increase in variability in fiber strength was consistent with a more pronounced length effect. These changes in fiber strength properties were understood through an alteration of the crystalline domains within the fibers due to the extreme deformation.