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.
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".
I think his point is that things very rarely experience purely compressive forces. Just being compressed induces tension in other directions, like water being squished out between your clapping hands. So even though water has great compressive strength, in practice this isn't very useful.
Many materials would have compressive strength easily, just by being relatively uncompressible.
But most loads have a (troublesome) tensile component. Fundamentally, the ability of a rigid material to resist deformation (in the most general sense) is what is most important, and that requires tensile strength.
Look up the Wikipedia definition [1] of compressive strength:
> In mechanics, compressive strength (or compression strength) is the capacity of a material or structure to withstand loads tending to reduce size (compression). It is opposed to tensile strength which withstands loads tending to elongate, resisting tension (being pulled apart).
Google search AI summary states:
> Compressive strength is a material's capacity to resist forces that try to reduce its volume or cause deformation.
To be fair, compressive strength is a complex measure. Compressibility is only one aspect of it. See this Encyclopedia Britannica article [2] about how compressive strength is tested.
I'm not saying water meets the strict definition of a material with high compressive strength (it does meet some, since it resists forces that attempt to decrease its volume well). I am just using as an extreme example of the issues with the concept of compressive strength.
Nothing that you wrote here indicates you understand what is being discussed.
Water has very low compressive strength, so low that it freely deforms under its own weight. You can observe this by pouring some water onto a table. This behavior is distinct from materials with high compressive strength, such as wood or steel.
(I say "very low" instead of "zero" because surface tension could be considered a type of compressive strength at small scales, such as a single drop of water on a hydrophobic surface)
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.
> 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.
> This is true, but neither is "tensile strength" really a thing for the same reason.
Well, not the same reason. Shear strength is in many ways more important because failure usually propagates from an origin like a rip- by shearing.
But fundamentally, tensile strength and shear strength are both much more empirical than compressive strength. Tensile strength can be used with ~99% accuracy on a wire that is microns long or miles long. If you double the relative width of a testing sample it will dramatically change the measured compressive strength even without buckling.
You could take that further and say that shear does not exist. It's a construct we have created to deal with tension failures in other planes. Everything basically comes down to a truss model. The failure you idealize as a shear stress is really a failure of a tension element in a miniature truss. This is true in bolt shear, scissors, etc.
This is why the code forces you to use strut and tie model deep concrete beams. As much as you may want to idealize a shear stress, really what's happening is the beam is arching over the span.
"All failure is shear failure" - this is a simple explanation of Tresca's Yield Criterion. For materials with higher compressive than tensile strength, the equivalent is the Mohr-Coulomb failure criterion.
For a simple example try twisting a ductile aluminum bolt (or clay), twisting a brittle piece of chalk (or concrete), or twisting a composite (twig). They all fail differently (and the first two at 45deg to each other. Mohr's circle is interesting, and fatigue failure more interesting still.
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.
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).
Tress of the Emerald Sea is set on quite a unique world. The planet is covered by oceans of magical "spores" which react violently to water.
For example the spores in the Emerald Sea, where the hero is from, instantly grow into massive vines that destroy everything in their path. That makes sailing rather dangerous.
The story is whimsical, perhaps an adult fairy tale (or just a fairy tale?), so I don't know if it fits your taste.
A long time ago, Harry Harrison wrote a series (https://en.wikipedia.org/wiki/West_of_Eden) where dinosaurs weren't wiped out, evolving for millions of years before primates showed up. The dinosaurs have a genetic-engineering based industry.
I second the recommendation of Children of Time! A cracking fun novel that goes in some weird directions. Very well realized "aliens" that have their own culture and technology.
You might like some of the Paolo Bacigalupi windup world stuff. Some great belivable ideas, some that go too far beyond belivability for my taste, but I enjoyed it a lot. The basic idea is that there's an advanced society where for some reason electricity & electronics tech was never developed, so mechanical mechanism technology progressed instead.
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.
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.
Positing that your research could contribute to sustainability/DEI/etc. is sort of the researchers equivalent of describing the pile of if statements in your software product as AI. Meaningless but largely harmless box checking to make sure that someone who might give you money doesn’t decline to give you money because someone else did a better job of looking trendy. That’s not to say the research isn’t useful; you’ve just finished reading the interesting part of the abstract. If this were a resume, this would be the obligatory “proficient in Microsoft Office” item.
The iron snail, Chrysomallon, inhabits undersea volcanic vents. It sticks to magnets. When probed with a diamond tip indentation test, it doesn't indent.
If you're going to post ChatGPT output then at least bother to click on the links. If people wanted ChatGPT output, especially uncurated output, then they'd just go to ChatGPT themselves.
I know you have not clicked on the links as the article that "reports tensile strengths up to ∼4.9 GPa" is just referencing the original article from 2015 to get that number.
The comparison with kevlar and titanium is weird, as they don't compete in the same category of strength and they are not the strongest in their categories. "I heard kevlar is used in flak vests, so it must be strong" is not a scientific argument.
Kevlar is an appropriate comparison- it has one of the highest tensile strengths known. It is not just lightweight and tough, it is also extremely strong in absolute terms. The strongest kevlar is somewhere between as strong as this material and half as strong.
Titanium is a pretty bad comparison. Its 10-20x weaker, and is also weaker than fiberglass, nylon, most steels, sapphire, many other types of metals and fibers...
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
65 comments
[ 0.19 ms ] story [ 134 ms ] threadThere also is no such thing as strength per volume.
I also noticed that it’s from 2015, although it was still new to me and interesting.
Are you confusing "compressive strength" with compressibility?
Many materials would have compressive strength easily, just by being relatively uncompressible.
But most loads have a (troublesome) tensile component. Fundamentally, the ability of a rigid material to resist deformation (in the most general sense) is what is most important, and that requires tensile strength.
See this comment elsewhere in this sub-thread that explains it probably better than I did: https://news.ycombinator.com/item?id=43904800
> In mechanics, compressive strength (or compression strength) is the capacity of a material or structure to withstand loads tending to reduce size (compression). It is opposed to tensile strength which withstands loads tending to elongate, resisting tension (being pulled apart).
Google search AI summary states:
> Compressive strength is a material's capacity to resist forces that try to reduce its volume or cause deformation.
To be fair, compressive strength is a complex measure. Compressibility is only one aspect of it. See this Encyclopedia Britannica article [2] about how compressive strength is tested.
[1] https://en.wikipedia.org/wiki/Compressive_strength
[2] https://www.britannica.com/technology/compressive-strength-t...
These are well understood terms in the field. Unfortunately, this illustrates the bounds of ai in subfields like materials: it confuses people.
Water has very low compressive strength, so low that it freely deforms under its own weight. You can observe this by pouring some water onto a table. This behavior is distinct from materials with high compressive strength, such as wood or steel.
(I say "very low" instead of "zero" because surface tension could be considered a type of compressive strength at small scales, such as a single drop of water on a hydrophobic surface)
See this comment elsewhere in this sub-thread that explains it probably better than I did: https://news.ycombinator.com/item?id=43904800
> 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...
Well, not the same reason. Shear strength is in many ways more important because failure usually propagates from an origin like a rip- by shearing.
But fundamentally, tensile strength and shear strength are both much more empirical than compressive strength. Tensile strength can be used with ~99% accuracy on a wire that is microns long or miles long. If you double the relative width of a testing sample it will dramatically change the measured compressive strength even without buckling.
This is why the code forces you to use strut and tie model deep concrete beams. As much as you may want to idealize a shear stress, really what's happening is the beam is arching over the span.
"All failure is shear failure" - this is a simple explanation of Tresca's Yield Criterion. For materials with higher compressive than tensile strength, the equivalent is the Mohr-Coulomb failure criterion.
https://en.wikipedia.org/wiki/Material_failure_theory
https://youtu.be/1YTKedLQOa0?t=533
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.
For example the spores in the Emerald Sea, where the hero is from, instantly grow into massive vines that destroy everything in their path. That makes sailing rather dangerous.
The story is whimsical, perhaps an adult fairy tale (or just a fairy tale?), so I don't know if it fits your taste.
And I think there is one more similar spider civilization book that's also popular – A Deepness in the Sky by Vernor Vinge.
So many questions and quests.
Has there been any progress since then?
First viable airplane shell is anticipated to hit the market in 2250.
/s
"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
https://www.astralcodexten.com/p/only-about-40-of-the-cruz-w...
I know you have not clicked on the links as the article that "reports tensile strengths up to ∼4.9 GPa" is just referencing the original article from 2015 to get that number.
Though the pop article is light on details.
Titanium is a pretty bad comparison. Its 10-20x weaker, and is also weaker than fiberglass, nylon, most steels, sapphire, many other types of metals and fibers...