8 ms·

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 c

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.

potato3732842 1y ago
Strength per weight vs strength per volume are an issue as well.
- hwillis 1y ago
  It's tensile strength, not specific strength. Strength over an area. There also is no such thing as strength per volume.
e28eta 1y ago
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.
prmph 1y ago
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".
- sk5t 1y ago
  Water being mostly incompressible is not the same as having high compressive strength. Liquid water makes for a poor tooth or structural column.
  - prmph 1y ago
    That's my point; think about it deeply.
    - jmillikin 1y ago
      You wrote "water has great compressive strength", sk5t directly (and correctly) refuted that claim. What is there to think about? Are you confusing "compressive strength" with compressibility?
      - stormfather 1y ago
        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.
        prmph 1y ago
        Exactly. 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 https://news.ycombinator.com/item?id=43904800
      - prmph 1y ago
        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. [1] https://en.wikipedia.org/wiki/Compressive_strength https://en.wikipedia.org/wiki/Compressive_strength [2] https://www.britannica.com/technology/compressive-strength-test https://www.britannica.com/technology/compressive-strength-t...
- blitzar 1y ago
  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.
hwillis 1y ago
https://royalsocietypublishing.org/doi/10.1098/rsif.2014.1326 https://royalsocietypublishing.org/doi/10.1098/rsif.2014.132... > 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.
- hydrogen7800 1y ago
  >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. [0]https://upload.wikimedia.org/wikipedia/commons/1/1b/DuctileFailure.jpg https://upload.wikimedia.org/wikipedia/commons/1/1b/DuctileF...
  - hwillis 1y ago
throwawee 1y ago
Then the ideal armor must be sea snail teeth woven together with spider silk. This advancement could save millions in mythril and adamantine.
- AndrewOMartin 1y ago
  Finally the Druids get a viable endgame armour set!
- VladVladikoff 1y ago
  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).
  - yreg 1y ago
    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.
  - dekhn 1y ago
    A long time ago, Harry Harrison wrote a series (https://en.wikipedia.org/wiki/West_of_Eden 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.
  - doorpheus 1y ago
    Try the children of time it's a great scifi book about a spider civilization evolving with all the different tech they create
lawlessone 1y ago
so we need both to build a space elevator?

potato3732842 1y ago

Strength per weight vs strength per volume are an issue as well.

hwillis 1y ago
It's tensile strength, not specific strength. Strength over an area. There also is no such thing as strength per volume.

e28eta 1y ago

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.

prmph 1y ago

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".

sk5t 1y ago
Water being mostly incompressible is not the same as having high compressive strength. Liquid water makes for a poor tooth or structural column.
- prmph 1y ago
  That's my point; think about it deeply.
  - jmillikin 1y ago
    You wrote "water has great compressive strength", sk5t directly (and correctly) refuted that claim. What is there to think about? Are you confusing "compressive strength" with compressibility?
    - stormfather 1y ago
      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.
      - prmph 1y ago
        Exactly. 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 https://news.ycombinator.com/item?id=43904800
    - prmph 1y ago
      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. [1] https://en.wikipedia.org/wiki/Compressive_strength https://en.wikipedia.org/wiki/Compressive_strength [2] https://www.britannica.com/technology/compressive-strength-test https://www.britannica.com/technology/compressive-strength-t...
blitzar 1y ago
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.

hwillis 1y ago

https://royalsocietypublishing.org/doi/10.1098/rsif.2014.1326 https://royalsocietypublishing.org/doi/10.1098/rsif.2014.132... > 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.

hydrogen7800 1y ago
>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. [0]https://upload.wikimedia.org/wikipedia/commons/1/1b/DuctileFailure.jpg https://upload.wikimedia.org/wikipedia/commons/1/1b/DuctileF...
- hwillis 1y ago

throwawee 1y ago

Then the ideal armor must be sea snail teeth woven together with spider silk. This advancement could save millions in mythril and adamantine.

AndrewOMartin 1y ago
Finally the Druids get a viable endgame armour set!
VladVladikoff 1y ago
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).
- yreg 1y ago
  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.
- dekhn 1y ago
  A long time ago, Harry Harrison wrote a series (https://en.wikipedia.org/wiki/West_of_Eden 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.
- doorpheus 1y ago
  Try the children of time it's a great scifi book about a spider civilization evolving with all the different tech they create

lawlessone 1y ago

so we need both to build a space elevator?