[Ben P.]

How well do they perform against plate? And other types of Armour for that matter,

[Colt Reeves]

While I've never gone and hit someone in plate with a hammer, I've hit many other metal objects with a hammer. Consider hammering a car body. I think that'll give you a basic idea of hammers on plate. I.E. The blunt side won't do much except dent it in a bit, while the backspike can be made to go through with a good hefty swing. Of course, just denting it will be hard on the guy inside, and it would be much harder to get a nice flat spot to stick the backspike in on armor. Also, blows will be dampened somewhat because a person in armor will have a lot more give than a 2+ ton car.

Against other types of armor? Well, that depends on what other types you mean. Maille for example, will be similar (blunt end doesn't go through, spike does), but its flexability means the hammer side will probably break bones and such. Coat of plates or lamellar will probably behave like a cross between the two. Padded armor would probably not be easily penetrated by the spike, but wouldn't really protect against the impact. Keep in mind these are just my opinions, take with a pound of salt.

[Bill Grandy]

The problem with this question is that not all plate is made the same, even on the same suit. Certain areas will be forged thicker and certain areas thinner (the sabatons, for instance, don't need to be as thick as the front of the breast plate). It also depends on the quality of the steel. Also, some plate was heat treated and some plate was not. A car is probably not the best analogy because not all cars are made of steel, and even the steel ones are generally made of a much thinner gauge than many parts of the armour.

In some cases a heavy strike with something like a poleaxe could definately compromise the armour, including things such as bending a joint so that the person inside could no longer move. While I suspect actually piercing the armour with the spike end wouldn't be the norm, it would probably be possible on certain parts of the harness. The spike of a poleaxe is quite often used more for hooking actions, though.

Even if you do not compromise the armour, though, a hard hit in the helmet with a heavy, blunt object can still "ring your bell", or even twist your neck and injure you. It can also knock you off balance enough to be grappled.

[Nathan Robinson]

The other faulty part of the the car analogy is that cars do not have design elements such as glancing surfaces, reinforced areas, ridges, rolled edges, fluting, separate overlapping pieces with "give", or any of the other elements plate armour utilizes to resist damage. It also does not have padding (or other backing, or the movement and resulting "give" a human body provides) that adds to the equation.

All in all, it's just not possible to simplify the answer to the point of taking a similarly thick metal material and whacking it in an attempt to reproduce the same results as would be achieved by hitting armour. Heck, it's not even possible to hit a piece of armour sitting on a table and achieve the same results as it would be when worn by a human with the other appropriate elements of his "kit". It's just not that simple.

Armour design is surprisingly complex with a lot of thought going into it.

[Colt Reeves]

I never said it was a good anology. As I mentioned, with the car you can get nice flat areas not present in armor and there is a lot more give to a person because of their weight and padding. I was just trying to use it as an example and couldn't think of a better anology.

[Nathan Robinson]

[J.D. Crawford]

It doesn't really matter what it does to the plate, does it; what matters is what happens to the man under the plate.

Imagine what happens when a blunt instrument like a war hammer hits a helm. That sucker is going to move real fast for a short period of time. Striking hard enough might break the neck, but the brain itself is likely more vulnerable. A high enough acceleration to the head (and thus the brain within) will cause a concussion, leading to anything from being temporarily dazed to being knocked out cold. Some padding between the helm and head is going to help, by absorbing some of the energy, but this can only go so far. Thus one could be knocked unconscious without having a scratch on the helm.

It's not hard to imagine a man-at-arms being knocked silly with a war hammer, and then either being apprehended for ransom, trampled underfoot and suffocating, or getting a dagger slipped through the visor before getting a chance to recover.

-JD