If normal wear is considered breakage, then I'd have to say the string. Serving wear renders string potentially weak and dangerous. And same goes for strand to strand chaffing from failure to wax properly. Wax the string frequently and keep an eye on the serving so that the nock does not wear through it into the string.
On my recurve crossbows, I rotate the string end for end to extend serving wear. Also get rid of string after 3 re-servings.