When Did The Roman Army Standardize Using Plate Armor?

[Nathan Ross]

segmentata wasn't replaced by anything; it simply ceased production.

But the Roman army did not, as far as we know, get any smaller during this period. So the troops formerly equipped with segmentata must later have been equipped with... something else. Therefore we can speak of it being replaced. I just don't think we can be sure it was necessarily mail or scale that replaced it.

it is quite hard to demonstrate that segmentata is cheaper than mail

The decorative hinges, brass edging, enamelled studs and washers that appear on some sets of segmentata demonstrate that this was not (or not always) cheap, poor-quality or poorly-made armour.

the centralized armories... more interested in fulfilling orders than in sending quality products.

Since we have imperial orders for the fabricae to produce large amounts of gilded and silvered helmets, they were clearly producing 'quality products'.

But the myth that the late Roman state was poor and its military equipment was cheap and substandard simply refuses to die!

Segmentata has been introduced following the battle of Carrhae

As there was at least a generation between Carrhae and the earliest known finds of segmentata, and this type of armour was far more commonly used in the west than the east (where the only find, I think - Gamla - was being worn by a western legionary) then a defence against Parthian arrows can hardly have been a reason for its introduction.

[CaesarAugustus]

For the helmets we have a dedicated topic. Some helmets with a bit of silver say nothing about the cost of building standard helmets and comparing them to previous standard helmets. If you want, we can restart where we were, with a structural composition weaker in newer helmets and a general quality recognized as lower from archaeologists who studied the helmets. I am still in contact with the one who has studied them.
Same for the armor. It is a matter of fact that segmentata structure was more suitable to dissipate the impact of arrows. And is something we know that the Hamata used at Carrhae demonstrates its low utility in the theater. Probably, segmentata is coming from Seleucid experience against the same Parthians. Still remaining at Carrhae, Surenas had total faith that his arrows would penetrate Roman armour. Despite Romans had already faced similar threats, they were shocked from Surena's arrows/bows. Why a segmentata armor was not immediately adopted after Carrhae? Armies take time to adapt, especially changing a central piece of the panoply. We can see how long it took in the middle ages to go again from the mail to the plate. And, honestly we don't have a full picture, so it is difficult to say.
But, probably it is a matter of organization. Mail was logistically better, and for late republic legions it was difficult to have the logistic for a more stronger, but more complex armor. Something similar happened with siege/field weapons, fully developed with the golden age in the high empire, and progressively abandoned/reduced following the third century crisis.

[Dan Howard]

But, I think you have forgotten by purpose the real advantage of the Segmentata: the greater protection level on covered parts.

This is a myth that needs to be squashed. All types of metal armour - mail, scale, lamellar, segmetata, etc. provided similar levels of protection.

[CaesarAugustus]

But, I think you have forgotten by purpose the real advantage of the Segmentata: the greater protection level on covered parts.

This is a myth that needs to be squashed. All types of metal armour - mail, scale, lamellar, segmetata, etc. provided similar levels of protection.

Do you want to squash material science books? They will explain you in detail why you are wrong on the full topic. Different materials offer different levels of protection. And different shapes and assemblies offer different levels of protection. Each discontinuity reduces the ability to distribute the impact, that is, it concentrates its force. The mail is the one that has the greatest discontinuities, that is, it is the worst in dissipating the impact.

[Dan Howard]

The armours constructed of weaker materials were made to be thicker and heavier. The result is that they all stopped the same weapons.

[CaesarAugustus]

The armours constructed of weaker materials were made to be thicker and heavier. The result is that they all stopped the same weapons.

So the solution is to have an heavier and so less efficient soldier? This means your soldier will combat for less time, will be able to cover less distancy, will be injured quicly and with less effort from the enemy. Just in case it is true... and it is quite absurd. It is not a mistery that mail was replaced with plate armor several times in the history. When an army had enough organization/logistic to replace it, the plate armor replaced it. 
Also, are you able to calculate how much a mail has to be thicker to resist impact Segmentata was able to afford?

[Hanny]

The armours constructed of weaker materials were made to be thicker and heavier. The result is that they all stopped the same weapons.

Odd way to look at it, a human can only carry a certain amount and be expected to perform well in combat, so plate replaced mail as it gave more protection than mail, but was more weight per sq metre of coverage, what did not happen was to get the same protection from mail, sq metres of weight protection, was to wear multiple layers of mail and become immobile.

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But, I think you have forgotten by purpose the real advantage of the Segmentata: the greater protection level on covered parts.

This is a myth that needs to be squashed. All types of metal armour - mail, scale, lamellar, segmetata, etc. provided similar levels of protection.

You already wrote "Nobody would wear lamellar if plate was available."

Its not a myth, its just your unable or unwilling to accept ballistic properties 
reality.