Combined effects of kink bands and hygrothermal conditioning on tensile strength of polyarylate liquid crystal co-polymer and aramid fibers

Translation of tensile properties from high-performance fibers to end-use fabrics is sensitive to weaving-induced filament defects and environmental exposure. In this effort, isolated and combined effects of hygrothermal exposure and curvature-induced kink bands on tensile strength of Vectran™ HT (polyarylate liquid crystal polyester fiber) and Kevlar® KM2 are studied. Hygrothermal conditioning was conducted at temperatures ranging from 40℃ to 100℃ in water for 30 days. Curvature-induced defects were created by wrapping tows around stainless steel rods of different diameters (0.25 mm to 5 mm) to create kink bands. Combined effects were evaluated by conditioning tows with kink bands at 100℃ for 30 days. All conditioned samples were dried and tested at room temperature. Hygrothermal aging showed that tensile properties for Vectran fibers were not appreciably affected below 100℃ (∼12% reduction), while KM2 fibers dropped continuously with increasing temperatures (∼48% at 100℃). The influence of curvature on kink band density was established for each fiber type. The isolated effect of kink band density on residual strength was approximately 15% for both Vectran 1670/600 and KM2-600. Combined effects of curvature-induced kink bands followed by hygrothermal exposure showed significant reductions in tenacity up to ∼96% for KM2 and 60% for Vectran HT1670/600. Inspection of the microstructure within the kink bands reveals extensive micro-cracking and fibril failure due to accelerated moisture ingress.