Weibull Fatigue Strength Distribution for Composite Fibers
Polyvinyl-2,6-bensobisoxazole fibers are by far the strongest polymer fibers in existence, even stronger and more heat/humidity/oxygen resistant than high-modulus aramid fibers. But to guarantee a fatigue strength safety factor of 2 would require a beyond-fatigue-limit design safety factor of 3, with overstress testing up to this level, and destructive scrapping of 50% of the material, and doubling the cost of the end-product.
(just an interesting fact)