Properly designed, steel will have an infinite fatique life, i.e. with normal loads it will never crack. Aluminum does not have an infinite fatigue life, i.e. after enough rounds it will crack. Polylmers can also display infinite fatigue life though as noted they can be damaged by means other than shock loading that will weaken them which will result in failure. There also appears to be an aging process where polymers loose strength over time even if not exposed to UV or chemicals that would damage the hydrocarbon bonds and weaken the polymer.
