I think that when the dry bulb gets to about -30 that wind chill is basically irrelevant, as exposed flesh is frozen within minutes. I remember in high school walking home from basketball practice after showering and having my hair freeze so hard that you could break it. Also, the plastics and pot metal in cars of the sixties were not designed for -40; door handles would snap off, and if your car wouldn't start and you hit the plastic dashboard they would shatter into a thousand pieces. I don't recall any actual frostbite, as if you got to that stage, such as a stranded motorist in a blizzard, the line between being prepared and dieing of exposure was a pretty crisp one.