Rage increases both (critical hit damage and critical hit chance).
Formulas (from Russian fan manual) are:
CritDamageModifierByRage=1.3+CurrentRage/MaxRage*(1.7-1.3).
That means crit damage will be 1.3*"standart damage" if you have 0 rage and 1.7*"standart damage" if you have full rage. "standart damage" depends on many other parameters...
CritChance="standart chance"*ModCritByRage*ModCritByMorale
ModCritByRage=0.7+CurrentRage/MaxRage*(1.3-0.7)
ModCritByMorale:
-3 0
-2 0.5
-1 0.75
+0 1
+1 1.25
+2 1.5
+3 2
Look for english translation in
this thread (i know its automatic translation but still it is something (all thanks to Tempo))