Fatigue and Tensile Properties of Aramid Fibers Measured by Single Fiber Mechanical Tests

Aramid fibers, with superior mechanical properties, have been widely used in a lot of industrial fields. From a safety point of view, long-term durability is required for these aramid fibers. In this study, the fatigue and tensile properties of two types of single aramid fibers, poly(para-phenylene terephthalamide) (PPTA) and poly(para-phenylene-co-3,4′-oxydiphenylene terephthalamide) (PPODTA), were investigated. Single fiber tensile and fatigue tests, cyclic tensile tests, tensile tests for post-fatigue test specimens, and optical microscopic observations were conducted. The stress–strain curves of the cyclic tensile tests indicated that PPTA and PPODTA fibers could be deformed above the elastic limit, and the strain at 2.0 GPa stress and the initial tensile modulus in subsequent tensile tests were changed by the cyclic stress. In fatigue tests, the cumulative probabilities of fatigue failure increased with the number of stress cycles for both the PPTA and PPODTA fibers. The fatigue behavior was influenced by the stress conditions and gauge length of the specimens. The initial tensile modulus in subsequent tensile tests was increased by the fatigue tests for both the PPTA and PPODTA fibers. However, regarding the tensile strength, significant decreases due to the fatigue tests were not observed. Fibrils detached from the fiber surface were observed in the post-fatigue test PPTA and PPODTA fiber specimens by optical microscope observations, with the loss of these fibrils contributing to the changes in modulus.