I have had interest for roman history for about two years now. All since I started reading about the romans, one question always remained in my head though; Where is the archeological evidence? Btw, I'm not some nutter who thinks it never existed or something like that. I simply find it strange that we only have some few scattered finds from different locations around Europe. We see The Lorica Hamata and Lorica Segmentata in numerous reliefs, we also know the roman army was HUGE for large periods of time, and a lot of times in history these armour types were quite standarised? Same thing also goes for helmets etc.
I'd actually say that there is quite a lot of archaeological evidence for the Roman army, particularly in Britain. Then again, Britain had a lot more forts than most provinces. Many Roman troops were billeted in civilian towns, meaning that it can be difficult to differentiate civilian finds from military finds.
But as for what you asked regarding armour: it's a bit of a misconception that archaeologists dig up lots of weapons and armour. Iron and steel don't survive nearly as well as you might think, especially in acidic soils. It's also important to remember that most archaeological finds are things that people have lost or thrown away. This doesn't usually happen to weapons and armour because, even when they are damaged beyond use, they're easily big enough to be worth melting down. So when we find weapons and armour, they're usually items that were deliberately deposited, in hoards, as grave-goods or as offerings to deities. For instance, there are several examples of English helmets from 400-800, but none at all from next four centuries, even though helmets were more common in the latter period. The Romans didn't usually deposit weapons and armour deliberately, hence why so few examples have been found.
edit: typo