you have it backwards, no helmet will protect forever, and traditional helmets only protect for one impact. multiple hard hat helmets will protect for many impacts but no where near as well per impact, they have a soft foam that you can squish and it will squirm back into shape (think foam bed foam), traditional helmets have hard foam that you cant squish unless you press very hard and then the dent is in the foam and its useless. (think Styrofoam but more durable and harder) really to be on the safe side, replace any helmet after any real impact, your noggin is to important to risk.