There are a couple of different kinds of plate armour and the effect would inevitably vary depending on what you were wearing. There are also several factors to consider when taking an impact from a weapon. My expertise is pretty limited with regards to types of war hammer and mace which would affect this as well so sadly I'll be neglecting that. I'm not familiar with any good works on that last subject either, sadly.

When we're talking about breastplates we really have three distinct groups. We have the first wave of breast plates which historian Claude Blair refers to as White Plate. This was so called because apparently it was often worn without any covering tabard so the light could reflect off of the breastplate. This would be mid-late 14th century and generally the weakest of the types. Suits of it are relatively rare, however, so we know less about how durable this armour was. Italian Plate is the next kind, this was primarily made in Milan, and then the third and final is German Plate, generally made around Nuremburg and Innsbruck. German Plate came last, sometime around the middle of the 15th century, and is the best of the lot. Alan Williams has done extensive metallurgical testing of suits of German plate that show they were quenched and tempered to be extremely sturdy. Most suits of Medieval Armour weren't tempered, instead they were slack quenched (quenched in a liquid other than water, often oil) which makes for a weaker steel but is easier than tempering. Poor tempering could ruin the steel and when working on a piece as expensive as a breastplate you generally didn't want to risk ruining it just to learn a new technique. Italian plate was further weakened in the late 15th century as it was increasingly gilded and experiments with medieval plate have shown that reheating the steel to gild it weakens the overall structure.

So that said, all three kinds were worn in battle and Italian plate certainly didn't go out of style when German started being made. If you were to take a blow from a war hammer onto the chest you would have a few things to worry about.

1. Does the weapon penetrate the armour and just kill you? Probably not. Hammers and maces aren't made for penetration, your armour is far more likely to buckle than it is to simply collapse in. Breastplates were also designed with curvature and grooves on it partly to look nice and partly to make landing a solid blow difficult. Landing a solid blow straight on to someone's breastplate without glancing off was surprisingly difficult.

2. Does your armour absorb the impact? Even if your breastplate buckles instead of collapsing you're taking a lot of force straight on to the chest. You might actually want your breastplate to collapse some and absorb the force of the blow as much as possible rather than just transmit the force to your ribcage. Better a broken breastplate than a few broken ribs. You would also be wearing several layers of padding beneath your breastplate (as much for comfort as protection) so that would help.

3. Is the force sufficient to damage you anyway? Just taking an impact of sufficient force can cause damage to you even if it doesn't obviously break your armour. This is most often observed when people test bullet proof vests and while a war hammer has nowhere near the force of a modern firearm it's still a concern to some extent. This force will hardly ever kill you at the relatively low impact we're talking about but it could cause serious bruising or possible crack a rib in a really bad situation. That kind if injury could make avoiding a follow up blow much more difficult, though.

The breastplate is the place with probably the thickest layer of steel in a suit of full plate, though, so that is worth keeping in mind. The only place that could arguably compete is the helmet but there seems to be a lot more variation between helmets than with breastplates. A blow to a limb would often be a lot more disabling than one to the breastplate even if there's a lower chance of it being fatal. In either case, though, killing someone in full plate with a single blow is highly unlikely. That's not to say it couldn't be done, Robert Bruce famously cleaved an enemy charging knights head open through his helmet with an axe at Bannockburn in 1314. Bruce did have the advantage that his opponent was riding at him full tilt on a horse, though, so that's some serious extra force and it did shatter his axe handle.

If you move into the 16th century as gunpowder becomes more common the extra limb protection of full plate becomes rarer and instead extra durability is placed on the breastplate. 16th century breastplates are often much thicker than medieval ones and designed to take a much hardier blow but at the cost of being a lot heavier. It was a very different kind of warfare they were concerned about.

When talking about the strength and durability of armour the book to get is Alan Williams' The Knight and the Blast Furnace: A History of Metallurgy of Armour in the Middle Ages and Early Modern Period but that book is staggeringly expensive. I haven't even been able to afford a copy (we're talking $300 here, try and find a library with it) so I haven't exactly gotten to read it but I have read lots of Williams' research that led up to the publication of that book and it's all excellent. He's done extensive metallurgical analysis of medieval suits of armour as well as research on medieval forging practices. Claude Blair's book European Armour: Circa 1066 to Circa 1700 is much easier to get your hands on but is more of a chronology of how armour changed over that time period than an account of how durable it was. His work is still probably the best general history of medieval armour though.

It really depends on how good your armour is. A nice expensive southern German suit of armour c. 1500 would have been carefully tempered and "proofed" by having every large plate shot by high draw-weight bow or crossbow, leaving a characteristic round dent which could be shown to the customer before being lightly pressed back out. The real point of armour in my opinion having made and worn quite a bit is to defend from the attacks you aren't expecting, like arrows and bolts. You would hope to defend from melee weapons primarily with your own weapons by parrying and such. A really nice armour would probably have been strong enough to shatter the shaft of any arrow that wasn't of the highest quality. However, being struck by a mace on the chest would probably deliver more force than an arrow and would probably have been purposefully directed at striking a particular point. I have never tried hitting my armour that hard as I have put a lot of time into making it nice and it isn't tempered, but I assume that it would leave a deeper dent from where the flanges hit the armour. The force would probably be multiplied greatly if the horse's rearing follows the motion of the hit, or if the horse is charging you. If he wasn't using a mace but instead a longer blunt weapon, this would also contribute to more force.

Now, what would that do to your body? if the attacking edge of the weapon was spiked and passed through your armour at all you would probably be dead. There is a modern conception that knights wore many layers of armour on top of each other. This is not the case, as tempered thick plate like that would be really heavy, only supplemented by maille at joints. Beneath you would have a padded jacket, but even these couldn't have been to thick as they constrict motion a lot and are really hot! So you would only have maybe half an inch of cloth and maybe so air space between the plate and your actual body, so basically any breach in the armour is a KO. If got hit the force would also probably break your bones and seriously bruise your body, because that is a lot of force that the armour isn't going to absorb, but is going to transfer to your padding and then to your body. On a horse, a knight would most easily hit you on the head, and that would be the most advantageous to him, as you would hopefully be knocked out. Then he could kill you, but since you have really expensive armour, you would probably get dragged out of the battle by your opponents so they could ransom you.

In the end, anything other than the best armour would mean death if you were attacked by a mounted knight.

TL;DR Your armour would be dented if not wrecked if they hit you with excessive force from being on horseback or using a longer weapon, you would be seriously bruised if not dead, and the blow would probably knock you to the ground.

Hi, I'm developing a realistic combat system for a board game and I've done a lot of research of this stuff.

Unlike edged or pointy weapons, which try to take the path of least resistance to slice or put holes in important parts of the body, blunt weapons are all about kinetic energy, mv^2 and all that. Blunt force is difficult to defend against for this reason- helmets for football players only have a bit of foam that actually absorbs energy, the rest of its effect is to spread out the force of impact over a larger area of skull. The brain is still accelerated into the skull at the same rate, which is the damaging thing.

So armor can protect against a blunt injury in a few ways:

1. Deflect the blow. Curved metal surfaces are great at deflecting edged or pointed weapons but can have some benefit at deflecting a blunt weapon. But in game terms you might think of this as more of a "miss" than a "hit with reduced damage."

2. Spread the impact out. Plates and padding underneath will spread the force over a larger surface area. The total force is the same but instead of fracturing a bone you might just get a big ol hematoma.

3. Absorb the force. Padding won't really do much here. Imagine being hit full force in the ribs with a baseball bat. Ok, now you can wear a padded gambeson. Doesn't matter. Gambeson is nice if you are getting hit with a blunt sword (blunted by the plate above it) but not gonna make a big difference against a hammer strike. The only thing that is going to absorb a lot of force is a "crumple zone" like in a car accident- the energy of the impact is partially expended deforming the metal. This could certainly happen with a big curved cuirass, but strikes to the limbs or head don't really have any chance to form a crumple zone since these plates are flatter.

rears back and smashes me full in the chestplate

Think about how you swing a hammer. It's mostly overhead strikes. You are rarely going to thrust against someone heavily armored. Big sweeping horizontal swings are unwieldy. So your hammer blows are going to fall on the head, shoulders, and arms of the target. (There was a thread with pictures of bodies excavated from a medieval battlefield. The victims mainly either had penetrating injuries- arrows, poleaxe, etc.- to the torso or their skulls were smashed in.)

In short, to answer your exact questions, a hammer blow to your chest would be kind of unusual but depending on the construction of your armor it might crumple and save you from having your chest caved in. But more realistically the strike is going to come from up high and hit you in the head/shoulders/upper arms and unless it deflects off of a curve surface its probably going to be bad.

War hammers were used to counter heavy plate armor and while the armor still afforded decent protection against blunt weapons, the hammers were very effective especially against the head. A helmet would barely do anything against a hammer strike, unless the strike glanced off the helmet's curves. (See sugarloaf helm vs. the cylindrical great helm).

The ultra short version with plate armor is that its invulnerable to edged weapons, can be pierced by powerful/specialized pointed weapons like a poleaxe or crossbow, and is effective but somewhat vulnerable to blunt weapons. Blunt weapons to the head are devastating, and it's easier to land an attack on the head of a knight who is loaded down with armor and in a helmet that half blinds him.

Edit: Here's a really awesome bunch of photos of a guy testing various weapons against different kinds of armor: http://www.myarmoury.com/talk/viewtopic.php?t=11131 If you want more sources for this sort of stuff feel free to ask.

I'm a medieval knight in full plate armor, dismounted, in the middle of a battle. An enemy knight walks up, rears back and smashes me full in the chestplate with the blunt end of his heavy iron war hammer.

Why would he smash your breastplate? I mean, that might dent it in somewhat, and maybe break a bone or two, but he's probably going to be aiming for your head, because bashing you there will make you stop fighting back. He's going to smite your noggin, because that will win him the fight. Maybe he misses that downward arc, and "only" hits your shoulder--well, that's still a broken shoulder, and you're again not fighting back.

Schola Gladiatoria is a decent source for learning the hows of various weapons being used:

https://www.youtube.com/watch?v=OnveFLcgoG0

https://www.youtube.com/watch?v=otCfAuuG92g

https://www.youtube.com/watch?v=bf16NgeEI_U

https://www.youtube.com/watch?v=vA5PCDKJS2k

Note how warhammers look a rather lot like your average hammers. There's a good reason for that--how much damage can be done isn't the primary concern in designing a weapon. Things like "can I actually use it for an extended period of time", "is it too expensive to even get", and "how much of a pain is it to carry it around when I'm not using it to beat someone to death" are pretty high on the list. A giant sledgehammer would be a terrible weapon, even though it would be very lethal--it's too unwieldy, too burdensome to carry around, and it's actually overkill. A smaller head will still do lethal damage, be cheaper, be easier to use, and you can carry it for hours on end without being wearied by it.