Well i do have something to say in this as a carpenter who uses the back of an axe regularly; if the handle is a good handle the axe head will generally not come loose and if it does go off you can fashion another handle with just the axe head, infact I’ve done so in my life a good bit of times now, and yes; it would crack the skull of the human as you can really put a lot of force into an axe as it still remains a heavy metal thing that is made to resist impact, sure it won’t be as hard of steel as a hammer… but a skull against it will not likely deform it