The pure logic is:
Damage thing == 1
Note that Damage is an analogue value 0-1, not a digital state 0 or 1, so I tend to use:
Damage thing > 0.9
or whatever seems a suitable figure. The AI does tend to keep engaging nearly dead targets until they are really dead, but I don't trust it to. In your case, a [very] badly placed charge may only damage not destroy your crates and not trip the trigger.