Ductile iron has nodular graphite, so it bends and absorbs impact instead of cracking. Against grey iron it gives roughly twice the tensile strength and far higher impact resistance — which is what lets a [40] mm cover take a 400 kN test load. Against steel it does not dent and does not rust through from the inside. Against composite covers it keeps its shape under sustained wheel loads and does not creep on hot asphalt.