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"Flak armour comprises several layers of different ablative and impact absorbent materials which should absorb the majority of energy from a shot or blow. The protection it offers against a direct hit is somewhat questionable at the best of times, but it is more effective against proximity blasts and the shrapnel from explosions.
Flak armour does have the advantage of being vastly cheap and easy to produce, requiring a very low technology base."

"Carapace armour is made up of large rigid plates of armaplas or ceramite moulded to fit parts of the body. It may take the form of a complete armoured suit like those worn by the Adeptus Arbites or separate parts such as chest pieces, helmets etc.
Carapace armour is commonly found in all military arms of the Imperium such as the Imperial Guard and Navy. Indeed, the carapace breastplate often worn by Imperial Guard officers is as much a badge of rank as a piece of armour. Carapace can be tiring to wear in extended battles."

"Power armour is a completely enclosing suit of armour as worn by the Space Marines of the Imperium. It is made from thick ceramite plates and would be heavy and cumbersome but for electrically motivated fibre bundles implanted in the armour to replicate the movements of the wearer and supplement his strength. Power armour is fully sealed to provide protection against gas weapons and hard vacuum and it also commonly includes numerous auxiliary systems such as communicators, auto-senses, etc."

"Tactical dreadnought armour, or Terminator armour as it is more commonly known, is without doubt the toughest personal armour in the galaxy. The armour is massively bulky and contains a full exo-skeleton arrangement of fibre bundles and adamantium rods to support the heavy gauge plasteel and ceramite plates which form the outer carapace. [...] In addition to its considerable protective value, Terminator armour is equipped with a number of other devices. It always includes a targeter which is linked to whichever ranged weapon is carried, and a teleport homer to assist in teleport operations. Extensive communications gear and sensory apparatus is also fitted as standard, though this has little direct bearing on play and is subsumed into the Warhammer 40,000 game rules for convenience."

Keep in mind, however, that there are lots of different sources in existence all presenting their own ideas regarding such details, especially once you branch out into novels and other licensed material, so do not be surprised to see contradictions to the above. As always, there is no such thing as a singular truth in 40k on this level of detail.

I don't recall ever seeing a term for what material is used in Imperial vehicle armour (at least not in GW sources, it may well be that a novel or a FW army book might offer something there), but I'd suspect similar vague descriptors as the materials mentioned in personal armour, so plasteel or armaplas or something like that. The 2E IG Codex merely mentions the Leman Russ'es armour to be "not sophisticated by any means, but tough and rugged" to deal with both battlefield stress as well as the many different environments this tank is deployed in, and I think this sounds fitting.
It should also be noted that, at least as per WD #269, the composition of raw materials used in Imperial vehicle construction may vary from world to world all depending on what is available. Perhaps planet A uses 30mm of ceramite plating to build their Rhinos, but planet B tries a cheaper approach and thus uses 60mm of armaplas to achieve the same degree of protection (the 30-60mm is the range given in that article).]]> http://www.dakkadakka.com/dakkaforum/posts/preList/549531/6006384.page http://www.dakkadakka.com/dakkaforum/posts/preList/549531/6006384.page GMT http://www.dakkadakka.com/dakkaforum/posts/preList/549531/6006431.page http://www.dakkadakka.com/dakkaforum/posts/preList/549531/6006431.page GMT
You could also check out GW's Necromunda skirmish game or their Inquisitor RPG - the wargear sections in their (freely available) PDFs offer a few details as well, if not many:

http://www.games-workshop.com/MEDIA_CustomProductCatalog/m2360785a_m1330042_Necromunda_Rulebook.pdf
- page 48

http://www.games-workshop.com/MEDIA_CustomProductCatalog/m2350169a_m1320029_Inq_Rulebook_part_1.pdf
- page 74

I wish GW would write more background books on their stuff. I know that Black Library is doing this, but I'd much rather have the core studio authors tackle this like a Codex project.
They once published articles like the Index Astartes, Index Xenos, etc in White Dwarf ... if only they'd pick this stuff up again!]]> http://www.dakkadakka.com/dakkaforum/posts/preList/549531/6007158.page http://www.dakkadakka.com/dakkaforum/posts/preList/549531/6007158.page GMT http://www.dakkadakka.com/dakkaforum/posts/preList/549531/6008015.page http://www.dakkadakka.com/dakkaforum/posts/preList/549531/6008015.page GMT