How to simulate tank shell impacts?

This might not be of too much help, but here goes.  It's been years since I modeled any kind of anti-tank fire damage, but when I did, I used heat and a pointed tool.  I had the tool marked at the spot to give me the scale hole I needed.  Study pictures of actual combat damage and note the scaring and areas around the impact site.  My theory with using the pointed tool was that it would replicate the projectile in real life.  The heat just made it easier to get through the plastic. And by re-heating the tool point a bit I was able to sculpt gouges in the surrounding impact zone. The work I did at the time I was happy with,  but today I would probably do more study on what the actual damage looks like before doing it.      

"shell impacts" can mean a whole range of things, modeler1.  Do you mean hull/turret penetrations?  glancing blows or gouges? What's the scene you're trying to replicate?  It was rare for a tank to get multiple shots and not be KO'd.  Plus, after the first hit, you can bet that the driver threw the thing into reverse to avoid subsequent hits.

this can range from a Tiger 1 with multiple hits and no penetrations to Japanese light tanks who were drilled by US 75mm AP shells which went completely through them and they continued to fight until hit by HE rounds.

Perhaps if you can tell us what's your goal and what's your subject tank, we can lend greater assistance.