in chapter 11 what metaphor does remarque use to make a point about the men going mad?

I think you might be referring to this quote from Ch.11,

"We are little flames poorly sheltered by frail walls against the storm of dissolution and madness, in which we flicker and sometimes almost go out. "

The metaphor speaks to how fragile their sanity is in war. By now, nearing the end of the war, their psyches are fragile. THey are like a small flame in a storm.