Materials that are strong under compression aren't necessarily strong under tension, and vice-versa. I would think teeth (just) need to be really strong under compression, and spider silk really strong under tension.
> The tensile strength of discrete volumes of limpet tooth material measured using in situ atomic force microscopy was found to range from 3.0 to 6.5 GPa
Also "compressive strength" is not really a thing, in that it's only a metric that is useful for practical applications. It's proportional to tensile strength, and unlike tensile strength it does not generalize well to things like modeling stress. Tensile strength is a much more fundamental quality than compressive strength.
Strength of a material is force per area. In ideal terms it is measured over an infinitely short length; if you measure over a long distance then the sample is stretched and becomes thinner, changing the measurement. If you test on a shorter and shorter sample you get closer and closer to the ideal value.
The same is not true for compressive strength tests. If you measure compressive strength by pressing on a very very thin disc of material it will just resist all force; it has effectively infinite strength. The actual failure mode of compression is always tensile strength in the radial direction, or buckling or something. You press the sample and it stretches sideways until it exceeds the sample's tensile strength in that direction. The shorter the sample is, the less it can expand radially and the stronger it appears to be. There is no "ideal" compressive strength, only useful test setups.
>Also "compressive strength" is not really a thing.
This is true, but neither is "tensile strength" really a thing for the same reason. A simple uni-axial tensile test is not really uni-axial, but a combination of orthogonal normal stresses that ultimately results in shear failure. I've heard it said that "all failure is shear failure", and I think that's true. When you look closely at the ductile fracture surface of a ruptured tensile specimen, the characteristic "cupping"[0] appearance consists of various surfaces at 45 degrees from the direction of the applied load. Principle shear stresses are always oriented 45 degrees from the principle normal stresses.
The "infinite" compressive strength for a sample that cannot expand laterally is only an approximation valid for small pressures.
At high enough pressures, all materials change their molecular and crystalline structures into structures with higher densities of atoms per volume and the volume of the tested samples diminishes, so the samples collapse at certain pressure thresholds.
The well known transformation of graphite into diamond is just an example of what happens with any substance at high pressures. Diamond is a more unusual example just because it remains stable even after the pressure that has created it is removed.
Moreover, for non-homogeneous materials, like concrete or many natural rocks used in construction, which are composed of harder particles cemented in a weaker matrix, it is normal to have a tensile strength that is many times smaller than the compressive strength, because when subjected to tension the weaker matrix allows pieces to detach, but in compression the strength may be determined mostly by the threshold where the harder particles break.
The snail teeth are also made of composite materials, mineral crystals in a protein matrix, so they are also likely to have different strengths depending on what kind of stresses are applied and in what directions.
But how much time to grind is needed and is there close by spawn point for sea snails and spiders or do you have to first get loot from snails and then travel to farm spiders.
If you’re into this kind of fantasy bioengineering I highly recommend reading The Tainted Cup and the sequel, A Drop of Corruption. And if anyone has read these, please tell me about any other books in this similar bioengineering genre, or even just highly unique fantasy worlds (I’m just so sick of books about dragons and boring magic).
I was curious about what they meant by strength, and the link at the bottom of the article says this is tensile strength. So the comparison to spider silk was actually appropriate.
I also noticed that it’s from 2015, although it was still new to me and interesting.
To be fair tensile strength is more impressive and to me is the only true strength. Water has great compressive strength, and yet is it difficult to think of it as "strong".
It is useless until you are in a movie gun fight next to a pool / river. At that point jumping into the water is both life saving and cinematic with turbulent bullet trails follwing you in the water but falling just short of you.
From a 2022 study in Nature (1) where researchers grew limpet teeth:
"The proof-of-concept presented in this study can be scaled up using made-to-measure chitin sheets and synthetic substitutes for limpet cell-conditioned media. Given that chitin is currently a waste by-product of the fishing industry⁴⁴, our approach would allow its repurposing into a novel composite material that could substitute for many existing synthetic materials that are manufactured in a polluting or unsustainable manner, and could help solve environmental challenges such as the ocean plastics crisis. Furthermore, as chitin is itself biodegradable, this bioinspired composite meets the key modern engineering challenge of sustainability. In short, this new material has the potential to be manufactured and disposed of without generating harmful waste products."
The Nature article's motivation seems not very well thought out to me, considering the fishing industry is among the most unsustainsbly and polluting industries on earth. It's even an especially large source of ocean plastics.
The strength quality of the mineral in question, goethite, is only good at nano scale (400-800nm). If the mineral fibers get bigger they are not as strong. Thus presents one of the challenges in replicating this for human use
Readit News
> The tensile strength of discrete volumes of limpet tooth material measured using in situ atomic force microscopy was found to range from 3.0 to 6.5 GPa
Also "compressive strength" is not really a thing, in that it's only a metric that is useful for practical applications. It's proportional to tensile strength, and unlike tensile strength it does not generalize well to things like modeling stress. Tensile strength is a much more fundamental quality than compressive strength.
Strength of a material is force per area. In ideal terms it is measured over an infinitely short length; if you measure over a long distance then the sample is stretched and becomes thinner, changing the measurement. If you test on a shorter and shorter sample you get closer and closer to the ideal value.
The same is not true for compressive strength tests. If you measure compressive strength by pressing on a very very thin disc of material it will just resist all force; it has effectively infinite strength. The actual failure mode of compression is always tensile strength in the radial direction, or buckling or something. You press the sample and it stretches sideways until it exceeds the sample's tensile strength in that direction. The shorter the sample is, the less it can expand radially and the stronger it appears to be. There is no "ideal" compressive strength, only useful test setups.
This is true, but neither is "tensile strength" really a thing for the same reason. A simple uni-axial tensile test is not really uni-axial, but a combination of orthogonal normal stresses that ultimately results in shear failure. I've heard it said that "all failure is shear failure", and I think that's true. When you look closely at the ductile fracture surface of a ruptured tensile specimen, the characteristic "cupping"[0] appearance consists of various surfaces at 45 degrees from the direction of the applied load. Principle shear stresses are always oriented 45 degrees from the principle normal stresses.
[0]https://upload.wikimedia.org/wikipedia/commons/1/1b/DuctileF...
At high enough pressures, all materials change their molecular and crystalline structures into structures with higher densities of atoms per volume and the volume of the tested samples diminishes, so the samples collapse at certain pressure thresholds.
The well known transformation of graphite into diamond is just an example of what happens with any substance at high pressures. Diamond is a more unusual example just because it remains stable even after the pressure that has created it is removed.
Moreover, for non-homogeneous materials, like concrete or many natural rocks used in construction, which are composed of harder particles cemented in a weaker matrix, it is normal to have a tensile strength that is many times smaller than the compressive strength, because when subjected to tension the weaker matrix allows pieces to detach, but in compression the strength may be determined mostly by the threshold where the harder particles break.
The snail teeth are also made of composite materials, mineral crystals in a protein matrix, so they are also likely to have different strengths depending on what kind of stresses are applied and in what directions.
So many questions and quests.
I also noticed that it’s from 2015, although it was still new to me and interesting.
There also is no such thing as strength per volume.
Has there been any progress since then?
"The proof-of-concept presented in this study can be scaled up using made-to-measure chitin sheets and synthetic substitutes for limpet cell-conditioned media. Given that chitin is currently a waste by-product of the fishing industry⁴⁴, our approach would allow its repurposing into a novel composite material that could substitute for many existing synthetic materials that are manufactured in a polluting or unsustainable manner, and could help solve environmental challenges such as the ocean plastics crisis. Furthermore, as chitin is itself biodegradable, this bioinspired composite meets the key modern engineering challenge of sustainability. In short, this new material has the potential to be manufactured and disposed of without generating harmful waste products."
1: https://www.nature.com/articles/s41467-022-31139-0
First viable airplane shell is anticipated to hit the market in 2250.
/s
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