I think it would depend on the type of composite. Yet, most composites, such as carbon fiber, are fiberous similar to fiberglass and may fray at the break point. I have yet to see carbon fiber fail. It is extremely strong, like kevlar. But much much lighter. Not being an engineer, I can't really give any definite answer on how it would break.
The thing about composites is the fact that even if they do crack, they will retain their regidity and strength, in most cases. Where metal would shear, if it were to crack and continue under pressure, as the picture in one of the earlier posts, composite would most likely keep it's integrity. Much safer, much lighter, and most likely cheaper in the long run.