Originally Posted by
fietsbob
An impact crushes the beaded styrofoam, the energy to crush that foam slows down the impact velocity. and is dissipated
the EPS stays crushed, it does nor rebound..
This is true.
Sometimes the plastic outer shell won't show much, if any damage, and the underlying EPS may be damaged. One could take a chance that there is either 1) enough energy absorption left or 2) you won't hit that same area again, but if a helmet is dropped or takes any significant impact, then I would recommend replacement.