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Comment by analog31 | Hacker News Reader

>>> ...rotating layers to change the bulk performance of materials composed of anisotropic plies is very old technology developed originally for aerospace / racing applications.

Indeed the original engineered composite material: Plywood.

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I believe plywood has piles/fibers orthogonal to each other, in this video they were laid over diagonal to the underlying concrete.

Some higher-quality plywood does have layers whose grain is at 45 degrees to the adjacent layers: https://www.globalpp.co.uk/blog/why-do-plywood-layers-run-in...

That seems like a typo. The title says “opposite”, and just before specifying 45 degrees, the article says the layers are at a “right angle” to one another. Looking at their other products, they seem to differentiate on bonding strength and surfacing, which is pretty common.

Isn't concrete an engineered composite material to begin with? It's ancient. AFAIK plywood is an 19th century invention.

In engineering parlance composites are materials with high tensile strength bonded to materials with high compressive strength.

Concrete is compressive strength materials (rock, sand) bonded with cement. OTOH - We could say reinforced concrete, (steel, fiberglass, etc reinforcement) is a composite.

Plywood was used for shields by the Romans, I'm fairly certain the Vikings, and wikipedia mentions the Greeks and Egyptians used it as well. From what I can tell from a quick google, it does seem to be a casualty of the Dark ages though.

Maybe not. The Dutch police appear to be using wicker riot shields in 2021. Good for the environment.

https://www.bostonglobe.com/2021/01/25/nation/weekend-riotin...

I believe the implication was it was lost in the Dark Ages and re-discovered in 19thc. No reason to suggest plywood is not in use today

I remember reading in Gates of Fire [0] that the Spartans used heavy, round oak shields which suited there phalanx style of warfare.

The Persians (and their allies) used wicker shields that chose light weight over protection [1].

More recently, there was also a video floating around recently of how protesters were building Viking shield walls to protect themselves [2]

0 - https://amzn.to/39n23Jx 1 - https://en.wikipedia.org/wiki/Sparabara 2 - https://twitter.com/joshuapotash/status/1291962889323020288?...

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