I did the same thing with one I originally bought in 1993 - my first helmet. A couple years ago, I crashed while wearing it, hitting the ground hard enough to put a nasty, deep scuff and a small crack in it. The pads were all shot anyway, and I had several other helmets by then, so decided to experiment. With a shopping bag half full of sand (which I deemed a suitable stand-in for my head*) in it, I swung it by the strap, smacking it into the pavement in my carport, upping the force of the blow a bit each time. Turns out I had four more crashes left in that foam hat, but the fifth one would have killed me, probably.

(*my wife would likely agree with this assessment )