https://www.youtube.com/watch?v=76mbOMFjlu0

I found this video where a guy uses a high-powered crossbow against a piece of frontal plate/cuirass. As I never found any related video, I would like to hear from you about the test precision: the breastplate is a close contemporary quality, or at least munition-grade quality? How about the bolts?

1.5mm is fairly thin for a 16th-century breastplate but within the historical range. It's not clear exactly what work-hardened means (how hard?), and given the bolt velocity without the bolt weight makes calculating precise kinetic energy impossible. I assume the 1.2mm plate is mild steel, which would take around 75 J to pierce 40mm according to Alan Williams. So the standard bolt apparently delivers somewhat more than 75 J (as it certainly should, coming from a 1,000lb crossbow). Piercing 1.5mm mild steel to a depth of 40mm would take about 108 J according to the same formula (assuming the optimal angle of impact). That fact that it's work-hardened should increase this. Assuming a liner force curve, a 15.24cm (6in) powerstroke, and 50% efficiency, a 1,000lb steel crossbow should manage around 170 J. The one in the video seems to be performing a bit worse than that, which is common enough for steel crossbows. 50% efficiency is perhaps being too generous.

Alan Williams presents a 2.5mm low-carbon-steel breastplate as the 16th-century standard for heavy infantry (pikers and such). Such a breastplate would require around 185 J to defeat, assuming a perpendicular angle of impact. (Williams gives a 45-degree impact as the default because of the breastplate's keeled form. Williams also adds 50 J for defeating padding, which I think is a bit excessive. Williams apparently didn't test the padding against his arrowhead simulator.) Thus, according to Williams, many or most 16th-century Western European pikers/halberdiers had their torsos well-protected from crossbow bolts. Some heavy infantry and many cavalry possessed even better protection, made of air-cooled or hardened medium-carbon steel.

Ben,

I'd be wary to say 1.5 mm is low for average still. Alan Williams when I asked about this said that to his knowledge no one had looked at enough of the medieval breastplates with thickness in mind to make a firmer estimation than when he made the estimate of 2mm but that even he had not done all needed to be more definitive in this. IN the end it is a guess. An educated one but an estimate.

As well by the time we are getting into 16th it is very likely things are changing as we know more and more firearms are playing a part and the trend is increasing thickness for helmets and breastplates.

I wish more could be done to really take a look. I have seen breastplates with places approaching and at 1mm on them.

RPM

The breastplate in the video is listed as being a "16th century chest plate." Only five of the ninety (5.55%) 16th-century breastplates listed in The Knight and the Blast Furnace are 1.5mm or under.

Now, I'm not sure why they're testing a 15th-century crossbow against a 16th-century breastplate. Crossbows saw less and less military use past 1525. Some or many 15th-century and early breastplate may have been 1.5mm or thinner. 1.5mm of hardened steel would require around 160 J to defeat with a arrowhead, assuming a perpendicular impact and ignoring padding.

Ben,

I can sort of see it but you are right, honestly the 15th century crossbow to 16th century breastplate is just odd. To be fair I think the crossbow hung on pretty well toward 1550 but largely based on inventories and late musters. Who knows.

As for 15th century breaplates I have handled many are within that range. The one 15th century suit at the Wallace the breastplate is 1.1-1.8mm from Alan's work. Several of the RA breastplates from the second half of the 15th are all right around that, I suspect a scan would reveal most of the breastplate was less than that but I could only take edge thickness up to 4-5" with the calipers I had. I really think thickness would be a worthwhile study but trying to get funding for such research not always the easiest. And finding the right machines to do the job the best are pretty tricky. There was a 16th century breastplate that was scanned but the person who wrote the article was able to get things lined up through associates. I doubt many people would be able to scan so many pieces though.

RPM