Thats where my knowledge ends unfortunately! although you won't go too far wrong with an exponential relationship. As long as stronger hits spread out more than weaker ones!
For a better answer than what I can give read "The new science of strong materials: or why don't you fall through the floor?" by Jim Gordon, its a masterpiece and contians all the theory you need to know to explain this. Also, fair warning this may be a long reply.
Things underlined are things...