Fear should break on 5-15%
On yellow attacks, it has higher chance on breaking 21%-100% . On white attacks, it has MUCH lower chance, 0.2%-20%
But with dots, it has LOW chance breaking.
http://www.arenajunkies.com/topic/239335-what-is-the-threshold-of-dmg-taken-before-fear-breaks/
http://wowwiki.wikia.com/wiki/Diminishing_returns