Here is some info from a Quora question asking the same thing:
https://www.quora.com/How-thick-was-...l-and-the-skin
And here, from MyArmoury.com:
http://myarmoury.com/talk/viewtopic.php?t=9475
Thicknesses ranged from 1-3mm on helmets, 1.5-2mm on the thickest parts of the breastplates (the front, tapering a bit on the sides).
Armor was built to protect; plate armor was good enough that historically shield use started to disappear, both because it wasn't as necessary and because if you are fighting another plate armor wearer a sword isn't going to be the best weapon to use--a poleaxe is. Swords are sidearms, not the primary weapon. You don't cut through armor. You bypass it by going for slits/chinks, hence estocs, rondels, and half-swording. Or flipping the sword around and doing a murder stroke, if you don't have a mace or warhammer handy.
__________________
Warmest regards,
StevenH
My current
worldbuilding project. You can find the Adventure Logs of the campaign
here. I try to write them up as narrative prose, with illustrations. As such, they are "embellished" accounts of the play sessions.
Link of the moment:
Bestiary of Plants. In a world of mana, plants evolved to use it as an energy source.
It is also the new home of the
Alaconius Lectures, a series of essays about the various Colleges of Spells.