I was reading some of James Turner's Pallas Armata (1683) I came across this passage in a section about armor:

Quote:

if the armour of the Horsemen be not Musket-proof, either the Bullet pierceth through, or beats the Iron into the Horsemans body, which is equally dangerous

I know one curious development of the 16th-17th centuries is that the production of higher-quality steel armors seems to to disappear and instead armor starts to be made of softer iron, relying primarily on thickness to stop projectiles.

Could a possible reason for this be that harder armors were considered more likely to shatter or splinter when struck by a high-energy bullet and send fragments of steel into the wearer? I'm not an expert in metallurgy so I'm not sure if that's how it works or not.

I remember that in Bert Hall's description of the 1988 Graz tests he does mention that 16th century horse's brestplate which was tested seemed to perform better than modern mild steel and didn't result in splintering like the modern steel did.

https://journals.lib.unb.ca/index.php/MCR/article/view/17669/22312

Quote:

13 The most dramatic of the Austrian tests involved a pistol shot fired at a 16th-century breastplate from a distance of 8.5 metres. The breastplate was a fragment of a piece meant to protect horses; it was manufactured in Augsburg between 1570 and 1580, and made of 2.8-3.0-mm thick cold-worked mild steel (hardness 290 HB).4 It was mounted on a sandbag covered with two layers of linen (meant to simulate a normally clothed wearer). The pistol was RP 2895, with a shot weighing 9.54 gm and a calculated energy/surface ratio of 838 J/cm² at the muzzle, and 550 J/cm² at 30 metres. (This figure expresses the energy in die shot in a manner independent of the ball's size.) At the instant of impact, the ball was travelling at a calculated speed of 436 m/s and with a kinetic energy of 907 Joules.5 The breast-plate was completely penetrated by the bullet, but the shot lost all its kinetic energy in piercing the armour. The ball was highly deformed, lost 24 per cent of its initial mass, and was found lodged in the linen. It had not penetrated the sandbag. There were no secondary splinters from the armour plate to cause damage either. The experimenters judged that a human being struck in the same manner would have survived with only bruises to his chest. The fact that modern mild steel failed to absorb all the bullet's kinetic energy, while the 16th-century breastplate did, can probably be attributed to the early armourer's skill at cold-working the breastplate and hardening its surface.

Spalling is always a potential problem with armor designed to protect against high velocity impacts. Even during the world wars flaking of chips from the interior surface opposite impact continued to be a major issue. So of course old armor would have had similar problems with no known fix at the time.

I think that any bullet that pierces steel armor, be it hardened or 'regular' steel, is going to produce steel shards---even if very small. If the bullet has sufficient power, it will most likely go through any padded layers underneath also, before entering the body. Then, you have metal fragments AND cloth fragments--probably dirty cloth--that only compounds an already hard to treat , if not fatal wound. I'm far from expert on this, but I don't think I'm too far from wrong. ;) .....McM

2.8–3 mm is pretty thick for a person to be wearing, even if it is part of a gradient in thickness across the armor, though late armors definitely got thicker than that in places. Ideally, we’d know the size of the tested sample and the thickness across the whole of it. My admittedly limited experience with ferrous metals makes me suspect that the steel is less likely to splinter or shatter like glass or wood, and more likely to shear or split, and therefore not likely to produce large fragments that would enter a person. Slaggy steel isn’t likely to make small fragments either, unless it has been homogenized fairly well and hardened without tempering.

The behavior of projectile-resistant armor plate is a very complex subject. Anyone who didn't spend a lifetime working for Krupp has a lot of information to absorb just to understand how many different competing properties must be balanced to produce the right kind of plate to resist a particular sort of projectile. Even knowing what terminology to use to describe why a particular piece plate will or won't do the job can make your head swim.

http://navweaps.com/index_nathan/metalprpsept2009.php

The reason armor manufacturers did proof tests with live ammo was the inability of any theoretical prediction of effectiveness to be relied upon. More art than science, resulting in lots of interesting ways for armor to produce unwanted results. It's certainly all way too complicated for me. All I know is that a lot of careful testing was absolutely necessary to produce materials that did what they were supposed to do. Sometimes the armor actually made things worse for the guy hiding behind it.

I don't remember reading about any work hardened iron or steel breastplates in 'The Knight and the Blast Furnace;. When heat treating of some kind was not done, the endless refrain was 'heated and allowed to air cool' - in other words, normalized. Also, the laminate nature of most period irons and steels actually makes it harder to penetrate than a homogeneous modern steel, due to the way they resist crack propagation through all the layers (there is another thread that explains this, and there was an article on the Royal Armouries website about laminated breastplates), which is why modern heavy tank armour is laminated.