How to calculate impact/stopping power energies
If you want to do that, I am afraid I don't have the know-how to help you step by step. However, I think you should try and look for graduate papers on the matter, they are usually very helpful when you take the time to read and understand them.
I can recommend you to start easy and then gradually add more effects in your analysis. For example, start with the energy involved. Assume the kevlar is a thin sheet and see how much it is deformed, then start playing with it's thickness. You might want to pick up some books on material resistance. Sure, it will take more time to achieve your goal, but you learn much more this way as you each parameter's impact on the phenomenon.
I do not know how much you know about engineering and a little info on your knowledge background would help.
Hope this helped!