OK, so I've just spent an afternoon researching the effects of armour materials.
In all of these tests, I've used the same opponent, Captain Feathersword, who has full iron armour and a featherwood training sword, but no skills. I've used a steel-armoured test dwarf who has GM skill level in his weapon (mostly hammers - I spent almost the whole afternoon just testing hammers) for all attacks. This is so that the test dwarf can attack Featherswordwithout worrying about reprisals. The high skill level is to minimise the effect of the rapid training that occurs in arena mode.
Firstly, to really determine the utility of plate armour against a particular weapon or material, we need full plate coverage, so I modified breastplates to have pauldrons. I did this because even with a mail shirt underneath sufficiently long fights would result in bruised/broken shoulders and arms, because of how blunt weapons are quite a bit more effective through mail than plate.
Secondly, something was causing the iron warhammer to bounce off the iron armour for aaaages, then suddenly start causing bruises. I looked more closely, and what happens by default is that an iron hammer bounces off everything except fingers and toes, which is invariably breaks. There's also occasional facial bruising, but that doesn't appear to have had much effect overall. Dwarves need to invent visored helmets, though!
In any case, after an age of getting broken fingers and toes, Feathersword would pass out from pain, causing him to fall down. This increases the effectiveness of an iron hammer to the point where it can *just* cause bruises! This was replicated and tested by creating a series of Featherpants, taking them over adventurer style and choosing to lie down against the attacks. It wasn't skill gain, it was attacking a prone opponent! Testing with a maul instead of a hammer revealed much more dramatic results - the maul would totally glance off a standing opponent, but suddenly start breaking bones on a prone one!
After a VERY long time, bruised-all-over Feathersword and his fifteen broken fingers/toes started getting broken hand/feet messages. Not very sure what happened there, it's possible that this was skill gain or something else, he'd been passed out on the ground for at least ten minutes of real-time arena fighting by this point.
Anyway, very silly stuff, but we've figured out some things. So, the strange things mostly explained, I was ready to do my real test - how do the impact_yield and impact_fracture material values influence armour against blunt attacks. First, some background. These values are set to 1080000 for every single metal! There's a note in one raw that says this is the "average for stainless steel". It looks a little like this might be placeholders that have stayed until release!
My hypothesis is this: These values are supposed to impact the effectiveness of various materials as armour, but since they're currently all set to stainless steel, they're effectively zeroing out that term of the equation and only density is having any effect. This creates the binary situation we see expressed in Zagibu's tests.
So, the first thing was to determine whether adjusting the values for iron hammer vs iron armour did anything, AKA "are these used in both the weapon and armour calculations?". Reducing them to 1 for iron seemed to confirm this - it did nothing. So far they either do nothing, or do equal things for both weapon and armour. Nex test, make pig-iron identical to pre-nerf iron and try "old iron" versus "nerfed iron". Bam. Both impact yield and impact fracture set to 1, a pig iron hammer goes through iron plate like it isn't there. Hooray, it looks like I'm onto something here.
Now I figured that maybe only the yield stat, not the fracture stat, was being used, because armour *damage* isn't in yet, and maybe impact_fracture was only for things taking damage rather than just absorbing like armour currently does. But leaving impact_fracture at 1080000 and setting just impact_yield to 1 produced the most interesting result yet: The armour was constantly letting through bruises to the entire body, serious deep bruises to the muscle and fat layers. But no breaks! Not a single one, where setting both values to 1 let everything go through and shatter bone like on an unarmoured dwarf!
So, it would appear, from these small initial tests, that my hypothesis is borne out: There are significantly more complicated calculations going on than just material density vs material density, but the fact that there are a load of placeholder values in the raws right now mean that they're getting cancelled out and we have armour as it is right now.
This is great news, I feel.
[EDIT: An amusing update: I left Captain Feathersword in his yield-nerfed armour being wailed on by pig iron guy, constantly getting yellow/brown bruises to his entire body (skin/muscle/fat bruises, but nothing to the bones), for about half an hour while I went and made some dinner. I came back and he had twenty broken fingers/toes and bruised lungs, throat and face. I watched, sort of giggling a little at how silly this was, and he died of *infection* right infront of me. Presumably they'd been fighting for days if not weeks of dwarf-time by this point.
This means the combat system is lacking a) Haemorrhaging (internal bleeding caused by ruptured blood vessels), this would make repeated blunt trauma a lot more deadly, as it's a serious side effect of tissue damage and its lack caused poor Captain Feathersword to get wailed on for weeks. Obviously in-game the results will not be so gratuitous, but it's through these edge cases that we can learn what's actually missing.
and b) Concussion. Getting hit in the head is bad, really, really bad, even if the skull is unharmed, the brain is smacked against the inside of the skull and that is bad. There is currently no damage at all to the brain even in cases where the skull shatters, unless it's a fracture and the skull is pushed into the brain, resulting in very swift death. This produces pretty amusing situations!
Anyhoo, I also wanted to say again that I'm not speaking ill of the new combat system at all, just pointing out where it currently falls short of awesome by violating reasonable suspension of disbelief. And it seems like it's actually very powerful under the hood, just currently in need of tweakage]