I'm basically going to agree with many other answers that say "no" to the question of bows & arrows / crossbows making a comeback, but for a simple reason that I think everyone else glanced over:
You are correct that some kinds of armor perform better against high-energy projectiles and it's (in the loosest sense) "plausible" that ...
Henry VII's foot combat armor for competitions held during the 1520 Field of Cloth of Gold Conference held with the King of France.
Note the Tonlet or skirt that replaces the conventional armor pieces covering the things. The advantage of this type of protection is that it increases mobility when fighting on foot in tournaments and possibly increases ...
It's not as terrible as some people are suggesting, but it could definitely use some improvement.
No head protection. Going to put helmets as a separate thing maybe? Hopefully? A helmet is really important.
No neck flute. The rolls and ridges in armor serve to catch weapon points and control where they go. You really want an outward-facing lip around ...
TAKE ADVANTAGE OF THE BENDYNESS!
The material might work. It seems to be similar to cataphract armor.
The form-fittingness is silly though. Your wearer needs this extreme figure to fit.
For comparison lets see what that looks like on the world famous Olympic swimmer Micky Mick Mick Phelps. Here is it scaled to fit at the armpits.
Even for a suitably ...
Your armour will be awkward and uncomfortable. Sitting down will be a real pain. Riding a horse is probably out of the question. And with no neck protection or groin protection, you have 2 massive vulnerabilities. Unless that skirt thing is just cloth. Then it won't be uncomfortable and a problem for horses. It will just offer no real protection.
If I ...
There are many examples
Medieval Armor and Shields
Armour suit made with riveted chainmail (for full body) as shown here.
Flexible Armor with steel plates
A flexible armor suit made of steel plates can be found in this video, where the knight can make all kinds of movements wearing this suit.
Overlapping plates; think of horizontal window blinds, for example.
The plates will be like 3" wide, with 1" of overlap. Standing, each plate provides 2" of coverage. At maximum compression, each provides 1.25" of coverage. At maximum extension, each provides about 2.75" of coverage.
Make the plates smooth and oiled so they can slide ...
If it were practical to do this, someone, somewhere, would have done it. Instead you have to trade off between flexible armor, and nice solid protective plate that doesn't flex at all.
In the real world everyone seems to have been happy to make that compromise in favor of protection, but obviously you're operating under slightly different ...
Here are two things that I'd take issue with:
Abs of Steel
The idea of flexible plates like this is lovely, but it is noteworthy that when people were able to wear decent amount of metal plating then they made sure that their abdomen was well protected.
Consider this greek bronze breastplate (from 600 BC):
And this Italian plate from 1450:
Note that over ...
Aside from the fact that only a few kinds of women would actually be able to fit that specific shape/size, it may be comfortable enough(I don't know the cloth material), but protective?
Hardly. The only things that are more or less okay for protection are the pauldrons and the backpiece, everything else(corset piece) would fail at protecting someone from ...
Whatever is handy
Demons with claws and teeth and spiked tongues, rending and tearing and... tonguing. Been done done done. Demons with spiky weapons and glowing swords! Done. Demons with special little poison demon knives. BEEN SO DONE!
None of that tired stuff. Your demon just goes around as he is. He considers it gauche to be toting a weapon and ...
roughly in the 1880s with firearms and a small amount of modern industry
Careful, now. By 1880, armies had access to things like rifled, breech-loading artillery firing explosive shells, the classic Gatling gun was invented in the 1860s and the Hiram machine gun was developed in the 1880s (though there were quite fast firing guns even before that).
Put a LID on it, then lock it tight.
This is a case where you need to look at the "alien" species technology being developed for them by them. If they have this glaring "touch this and I'm dead" weak spot, you bet it would be a major focus of their martial technology development.
No gorgets. They don't need to protect around their necks. ...
Full plate (with related padding), since their bones can be broken. It becomes much more difficult to prevent an attacker from bypassing your armor when your arms are broken. Though the armor might not need to be quite as robust as what a human might wear, since a drullahan is already more robust in their anatomy. And like Lemming said, the Gorget is ...
Your plate armor does not contribute to the wearer's temperature at all.
Aside from heating by the sun, the metal will rapidly acquire the same temperature as the air surrounding it.
What then matters is what you wear under the plate.
The reason you associate wearing armor with heat is because you typically wear a great many layers of chain, leather,cloth ...
Weapons against them
Anything that can stab the neck without a specific angle being required from both the dullahani and its assailant would suffice. It's one of the few cases where I'd recommend using a scythe(the one with the horizontal blade, not the specifically combat-adapted one with a straight blade), though a pickaxe or something similar whose ...
No, I do not think it does. Typically many layers are worn and full plate is one of the most outern. Plate will not get in contact with body.
Often the setup goes like: first you put thin normal clothing, next you put jacket, next you put chainmail, next you put coat, then plate and then additional outer layers can also go on top of that if needed.
It all depends on how cold. If it is very cold, like -20°C or less, this is starting to head into temperature regimes where normal armour steels may suffer from brittle failure. This means that cold steel may shatter when made very cold and subjected to a strong impact.
Some steels are more likely to become significantly more brittle when cold, and ...
There's generally a certain amount of padding under armor. In extremely cold weather, the choice of adding more cloth over or under the armor would be determined by:
Is there enough space underneath? If not, then under the armor isn't a choice (unless you are wealthy enough to have a second suit of armor sized correctly for just such occasions). If under is ...
Just pay attention that no skin touches the metal
Maybe additional fur where there are gaps in the armour.
But apart from that, the usual gambeson you wear underneath should be enough.
The real danger is plate armour in really hot weather/in direct sun
Note that even in really hot weather, the issue is the same, you have to wear something underneath to ...
Protection from enemy not from weather
Full plate armour is for protection from the enemy's sword or javelin. It is worn over the normal dress. The soldier will wear normal dress according to the weather and wear the armour over it. Therefore same armour can be used in different weathers.
Depends on style of armor.
If a crafter wanted to substitute jade for the steel of full gothic plate? No. This would be exceedingly difficult and impractical.
If it was a lamellar armor that used jade for the stitched together plates. Then yes. The Wikipedia article suggest some sets used stone.
Alternatively insetting/inlaying jade into another armor type ...
The chiton has decentralized eyes - many tiny eyes distributed in and among its scales. The loss of one or several eyes will not impair vision.
So too your helmet. There will not be goggles to look out of. There will just be helmet over the face. The helmet will be covered with tiny eyes.
The point (as I see it) of the chiton eye helmet is ...
No Armor is Perfect
Let's get this out there: no armor is perfect. There will be gaps and it will have to come off at some point as well as be maintained. Nevermind practical considerations like relieving oneself, nursing babies or taking care of children.
Such as that worn by Native Americans from the plains. Yes, this will need to be ...
The balance of Offence vs Defence swings the other way
Right now, the problem is that an armor that can stop a bullet in its tracks is expensive, heavy, inflexible and generally only good for one or two hits with low-calibre weapons.
Weapons technology has vastly outstripped personal body-armor to the point where we no longer defend against direct attack.
I recently read Seven Eves by Neal Stephenson. Potential spoiler:
Combat in that universe takes place mostly in space habitats. Conventional weapons are not ideal in that environment.
So they developed a kind of weapon that shoots tiny robots that avoid penetrating habitat structures. When landed on humans they would crawl around to find an opening in the ...
It's already returning.
E.g. this is the Sotnik infantry armor that Russia is developing, planned to be done 2025(picked it as an example, because it looks the most sci-fi ish, there are plenty of others):
You must remember, that body armor didn't go away due to ineffectiveness ...
The armor incorporates a powered exoskeleton. It allows the soldier to carry the armor, and its power supply, without increasing their burden.
It will also allow the soldier to carry additional ammunition and supplies and even heavier weaponry. It could also include environmental protections (possibly including heating or cooling) that would not be ...
As you stated: the main reason that soldiers today only wear light partial armor is due to weight. The encumbrance of full-body armor outweighs the benefit of full protection.
To get back into full armor, one or both of that must change: either full body protection must become a definite benefit, or the weight of the suit must be reduced (or both).
One way ...
Your military strategy emphasizes sheer numbers and brute force over everything else. Remember that the Stormtrooper example is based on what is essentially a limitless clone army. Not all of them will require this level of full-body protection, but some will, and the full body armor is standard issue to all. It's the mass production of troops, without any ...
Armor Simply Needs to Become Lighter
Full body armor is always better than partial armor as long as it can meet the following two criteria:
It must stop all common threats when hit.
It must not be so cumbersome as to make one combat ineffective.
If you make your armor too thin trying to get full body coverage, it will not stop any common attacks; so, it is ...
Psychology + Uniform
Even light plastic armor with some kevlar under it goes a way to protect from discontent civilians. At least if the solders are in compact group. Also it improves their morale, as they see brave friendly solders around, even if they are scared inside. It protects from inconvenient scratches, or in case of falling from speeding motorcycle....
One New Thing (Name Your Problem):
All you need to justify head-to-toe armor is ONE new attack form that requires armor to protect from it. It doesn't matter WHAT that
is, as long as the logical defense is armor. It doesn't even have to
be an attack currently in use - even knowing the enemy WILL use it if
you don't defend against it will require everyone to ...
Secret police or collaborators from an occupied country.
Occupying another country is hard. Doing it with just my troops from home? Now that's tough. So I need some locals to help out, but the local population isn't on-board yet with my leadership and I don't want my people hassled when they're off the clock.
What better way to protect them than to conceal ...
Who are the officers? Who is telling people what to do? Who are the high value targets?
If soldiers have their entire bodies concealed it is harder for an enemy to figure out which soldiers warrant more attention. Additionally, if you are a soldier, uniforms of this sort will conceal the fact that some of your fellow soldiers are aliens, or ...
Body armor is designed to protect mostly against the type of bullets we use.
These bullets produce localized damage. They don't fly that fast. Some body parts are more vital than others. Armor is heavy and cumbersome. The enemy will aim for the center of the chest because it's a lot easier to hit than the head. So it makes sense to only put armor on the most ...
Their armour looks shiny..
Modern armour material can be reflective, that is why it may return.
In Star Wars, the stormtroopers have a light weight, reflective white armour.
In real life, strong laser beams will damage mirrors, same would happen to armor. A shiny armor would be most effective compared to clothes, when the angle of the ...
There exist already today full body armors: hazmat suits
In a scenario where the enemy can use biological, chemical or nuclear weapons, you want to avoid that those substances enter in contact with the body of the soldiers. The only solution is therefore to fully cover the body and leave 0 exposed surface.
I'd say that the previous answers have covered most of the reasons why armor could return.
Effectiveness against commonly-used weapons
To add to the above, if the materials from which armor is made prove to be sufficiently effective against "short range" weapons (i.e., machine guns, pistols, shotguns, grenades) they would normally encounter during ...
You've already answered your question
The material the armor is made of is light and strong enough that it does not burden the solider much, or at most requires assistance in either a passive/very minor assistive exoskeleton to operate with
If the material is so light and unencumbering, then there would be little need to leave out armour pieces for weight ...
Space is a dangerous place. You may find that you're more at risk of dying from radiation or lack of oxygen or heat than by being shot by the enemy. Sure you might get instantly obliterated if one of their ship artillery hits wherever you are on the ship but should you survive you want to be protected from dangers such as ...