This depends, put bluntly. However, I'm going to use my own area of expertise, the Hundred Years War, and basically answer 'how much damage did percussive weapons do to plate armour worn by combatants in the Hundred Years War from 1400 onwards?' because this is an area which is very close to what we see as 'THE' armoured knight.

For reference, this is the type of armour we're discussing. Fire-hardened steel between 1.5 and 2.5 milimetres thick, with a riveted ringmail shirt beneath and a padded gambeson beneath that.

And the blunt weapons of the day were well designed to inflict damage. However, many warhammers had penetrative elements - the bec de corbin, the lucerne hammer, and the pollaxe all being good examples of this. Still, the weapons of this period were expertly designed to inflict damage. At the Battle of Verneuil, 1424, Lord Bedford using his pollax 'smashed open an expensive armour like a modern tin can, the body underneath being mangled and crushed before even the blade sank'. Analysing this comment, we can assume that the force of Bedford's blows distorted the steel so as to break it open, while 'mangling' his foes. It is highly likely this involved significant soft tissue damage, but the prime focus would have been the breaking of bones. This is obvious from the way blunt force trauma works; impacts rarely tear or slice because the kinetic force is evenly spread across a (relative to the edge of a blade) area.

So that is a good source for what a blunt instrument could do against plate armour, found in Desmond Seward's The Hundred Years' War.

Certainly, a firm solid strike from a warhammer could, and did, fracture skulls through helmets, but this is not to say helmets were useless. Only very powerful strikes at the correct angle to do anything more than concuss; a spike would generally be preferred for getting through the element. However, when applied to the extremities, hammerheads became invaluable; applied to the elbow joint or the knee, where gaps might weaken the armour, a warhammer would permanently cripple an opponent. Archaeological finds from Visby shows evidence of this, and it correlates with modern estimates as to the strength of medieval fighters.

So, I'll summarise. Wielded correctly, a blunt weapon can and did fracture bones and skulls through armour. Two-handed weapons were particularly deadly in this regard. One handed percussive weapons were often fitted with a penetrative element to puncture through the most formidable armour, but generally were more than suitable for causing significant bodily harm through armour.