X-ray scattering study on the damage in fibers used in soft body armor after folding

The goals of the research effort described in this article are to develop a framework to evaluate improvements in next-generation fibers used in soft body armor and to anticipate long-term performance and potential fiber deterioration. This effort to date has included the effect of folding on the fibers and exploring the interaction between the specific fiber strain energy and their sound velocity. Previous work in this lab noted a severe drop-off of tensile strength and strain-to-failure in poly(p-phenylene benzobisoxazole) (PBO) fibers when subjected to repeated folding. Subsequent work on poly(p-phenylene terephthalamide) (PPTA) fibers showed at most a slight drop-off in these mechanical properties. Results from wide angle X-ray diffraction indicated that both PPTA and PBO fibers showed no significant changes in the d-spacing and the apparent crystal size. However, with small angle X-ray scattering, it was found that the void and fibril sizes within PBO fibers may decrease after folding. Environmental scanning electron microscopy showed no damage to the fiber surfaces upon folding, and confocal microscopy revealed extensive internal damage to the PBO fibers that tracks well with the SAXS and mechanical testing results. POLYM. COMPOS., 2012. (c) 2012 Society of Plastics Engineers