5 ms·

I’ve often heard that spider silk is stronger per kg (or is it per m^3?) than steel. Are they talking about pulling, compressing, sliding, or all of the above?

TylerE 5y ago
Plenty of things are stronger than steel on a per pound basis. For instance, many aluminum alloys have about 60-80% the tensile strength of steel, but only 1/3rd the weight.
- lazide 5y ago
  Titanium even more so. For those, they are actively swapped around where cost vs weight trade offs happen. steel vs aluminum vs magnesium, vs titanium in engineering application, where for example engine blocks, airplane parts, car parts, battery components, etc. all have a long history of this. It’s a complicated process because the trade offs are not simple cost/weight/strength. Steel has an nearly infinite fatigue lifetime for instance, so steel springs are great. Aluminum does not, so aluminum springs are terrible - among other things. No amount of weight savings can likely fix that problem in a useful way. These pose big challenges in aircraft in particular where aluminum skins and fuselages make flight doable/economic, but means pressurized aircraft in particular have a finite lifespan in pressurization cycles/takeoffs and landings before they fall apart, no matter how nicely you treat them. Several major accidents (including the top of an airliner coming off and sucking a flight attendant out over the pacific on the way to Hawaii) happened before this was fully understood. Titanium is in theory much better, but is incredibly difficult to work with(requiring forgings in most cases, and being almost unmachinable), and very expensive as the bond it forms with oxygen is so strong the normal fluorine based processing used with Aluminum won’t work. Yeah, you read that right. Fire danger (such as magnesium engine blocks burning) is also a non trivial thing to mitigate. Titanium can be one of the worst offenders here (powdered titanium fires can burn SAND used to try to put it out as an oxidizer), which makes working with it hazardous in some cases. Iron, which will also burn, is generally so mellow when it does that burning it is a normal operation while scrapping and cutting it and you can’t get a runaway from doing so except in truly difficult to achieve circumstances (it’s what an oxy-acetylene cutting torch is doing).
  - inglor_cz 5y ago
    Pressurization cycles is what killed the reputation of the first commercial civilian jet, the De Havilland Comet. The British were good in early jet design and actually introduced jet aircraft into the non-military world, but the early hulls would fail catastrophically after a certain, relatively low # of cycles, tearing the fuselage apart mid-flight and killing everyone on board. After several such incidents in short order, the entire fleet was grounded and scientists came up with solutions, but by then, the reputation of Comets was tarnished and Boeing came with a competing 707 model. These days, the UK does not have a domestic jet manufacturer anymore.
    - TylerE 5y ago
      That wasn't really a cycles issue exactly, it was they used square windows rather than round, leading to stress concentrations. Per your last point, BAE and Hawker are still around. The UK also does plenty of Airbus work.
      - lazide 5y ago
        The stress concentration became a big problem because of the fatigue cycles though didn’t it? The manufacturers you mentioned are all military or regulatory sop manufacturers no? Though I guess Boeing isn’t much better, so meh.
        TylerE 5y ago
        Hawker still makes bizjets.
  - ghaff 5y ago
    >Several major accidents (including the top of an airliner coming off and sucking a flight attendant out over the pacific on the way to Hawaii) It was actually an inter-island flight so lots of short flights (and therefore pressurization cycles relative to flight hours or miles). The amazing thing was that the plane was able to make an emergency landing. https://en.wikipedia.org/wiki/Aloha_Airlines_Flight_243 https://en.wikipedia.org/wiki/Aloha_Airlines_Flight_243
- Ma8ee 5y ago
  Even wood is supposedly quite much stronger than steel for the same weight of material.
lazide 5y ago
Pulling (tensile), which is also the direction steel is unusually strong in (from a materials perspective). Compression wise it’s close to a wet noodle the way it’s manufacturable (though that has more to do with cost/practical production/gathering methods - if we could get a solid chunk of it I imagine it would be pretty strong in compression). Sliding, it’s a fiber, so very weak in that sense (barring the same scenario above). It actively flexes, so isn’t strong’
- ghaff 5y ago
  As I recall my material science, it's not so much that steel is weak in compression but, if you have a rod of steel (i.e. much longer than it is thick), if you compress it, it will buckle and shear. As you suggest--I'd have to look up the numbers--but a large cube of steel probably has similar strength with tensile and compressive loads. ADDED: In fact, if you deform a supported I-beam you get similar amounts of tension and compression on the bottom and top respectively at the mid-point of the beam (given a variety of assumptions).
  - user_7832 5y ago
    > but a large cube of steel probably has similar strength with tensile and compressive loads. Not just similar, but pretty much identical from what I recalled from my uni classes. Wikipedia for structural materials (https://en.wikipedia.org/wiki/Structural_material https://en.wikipedia.org/wiki/Structural_material) also has this bit - >Steel is equally strong in tension and compression.
    - ghaff 5y ago
      As I wrote in another comment, my material science and mechanical engineering is very rusty at this point. :-) But, yeah, I-beams basically wouldn't work if steel were weak in compression because, assuming a straightforward loading of a supported beam you're basically putting one flange of the beam in tension and the other flange in the equivalent amount of compression. So the fact that one part of the I-beam is strong would be pretty much irrelevant if the other part were weak.
kragen 5y ago
Tensile, mostly, but it's not stronger, it's tougher. The other answers to your question so far are all wrong or missing the central point. Spider silk has ultimate tensile strength comparable to some high-strength steels, around 1.4 GPa. Everyday steels are significantly weaker, around 0.4 GPa. Some steels have higher tensile strengths, like 2.6 GPa maraging steel. There are many materials with similar or higher tensile strengths: E-glass (3.5 GPa), carbon fiber (typically 4.1 GPa, 7 GPa for Toray T1100G), kevlar (3.8 GPa), zylon (5.8 GPa), boron (3.1 GPa), sapphire (1.9 GPa), diamond (2.8 GPa), graphene (130 GPa), dyneema (3 GPa). Some of these, like dyneema, are even comparable in density to spider silk. So what's so special about spider silk? Where spider silk is outstanding is that it combines high strength with high extensibility. Think of rubber: cast iron or diamond are much stronger than a rubber tire in the sense that they can bear heavier loads on the same cross section, whether in tension, compression, or shear; but if you hit a cast iron pot or a diamond with a sledgehammer they will break, while a rubber tire will be unharmed. This is because the rubber deforms under the blow, slowing down the hammer without ever experiencing a very high force. If you integrate the force on a spring over a distance (∫ F · dx), you get the change in the energy of the spring. If you integrate it over the spring's entire working range, you get its energy capacity as a spring. Every solid object is a spring, so you can do this for any solid object. "Stress" is force divided by cross-sectional surface area, and "strain" is extension divided by length, and the presumption of linear elasticity theory is that the stress–strain relation of a material, as well as its yield stress and breaking stress, are properties of the material rather than of particular objects made from it. If, instead of integrating force over distance, you integrate stress over strain, you get the amount of deformation energy the material can absorb per unit volume without breaking, for example in an impact. That's the material's theoretical toughness. All the other high-strength materials in the above list either deform plastically or break at under 10% extension; many of them break at under 0.1% extension. Spider silk breaks at 35%–500% extension. This makes some spider silks 10 times tougher than even kevlar. Usually compressive, tensile, and shear strengths increase and decrease together; the biggest exception is when the material is full of cracks, like glass and concrete.

TylerE 5y ago

Plenty of things are stronger than steel on a per pound basis. For instance, many aluminum alloys have about 60-80% the tensile strength of steel, but only 1/3rd the weight.

lazide 5y ago
Titanium even more so. For those, they are actively swapped around where cost vs weight trade offs happen. steel vs aluminum vs magnesium, vs titanium in engineering application, where for example engine blocks, airplane parts, car parts, battery components, etc. all have a long history of this. It’s a complicated process because the trade offs are not simple cost/weight/strength. Steel has an nearly infinite fatigue lifetime for instance, so steel springs are great. Aluminum does not, so aluminum springs are terrible - among other things. No amount of weight savings can likely fix that problem in a useful way. These pose big challenges in aircraft in particular where aluminum skins and fuselages make flight doable/economic, but means pressurized aircraft in particular have a finite lifespan in pressurization cycles/takeoffs and landings before they fall apart, no matter how nicely you treat them. Several major accidents (including the top of an airliner coming off and sucking a flight attendant out over the pacific on the way to Hawaii) happened before this was fully understood. Titanium is in theory much better, but is incredibly difficult to work with(requiring forgings in most cases, and being almost unmachinable), and very expensive as the bond it forms with oxygen is so strong the normal fluorine based processing used with Aluminum won’t work. Yeah, you read that right. Fire danger (such as magnesium engine blocks burning) is also a non trivial thing to mitigate. Titanium can be one of the worst offenders here (powdered titanium fires can burn SAND used to try to put it out as an oxidizer), which makes working with it hazardous in some cases. Iron, which will also burn, is generally so mellow when it does that burning it is a normal operation while scrapping and cutting it and you can’t get a runaway from doing so except in truly difficult to achieve circumstances (it’s what an oxy-acetylene cutting torch is doing).
- inglor_cz 5y ago
  Pressurization cycles is what killed the reputation of the first commercial civilian jet, the De Havilland Comet. The British were good in early jet design and actually introduced jet aircraft into the non-military world, but the early hulls would fail catastrophically after a certain, relatively low # of cycles, tearing the fuselage apart mid-flight and killing everyone on board. After several such incidents in short order, the entire fleet was grounded and scientists came up with solutions, but by then, the reputation of Comets was tarnished and Boeing came with a competing 707 model. These days, the UK does not have a domestic jet manufacturer anymore.
  - TylerE 5y ago
    That wasn't really a cycles issue exactly, it was they used square windows rather than round, leading to stress concentrations. Per your last point, BAE and Hawker are still around. The UK also does plenty of Airbus work.
    - lazide 5y ago
      The stress concentration became a big problem because of the fatigue cycles though didn’t it? The manufacturers you mentioned are all military or regulatory sop manufacturers no? Though I guess Boeing isn’t much better, so meh.
      - TylerE 5y ago
        Hawker still makes bizjets.
- ghaff 5y ago
  >Several major accidents (including the top of an airliner coming off and sucking a flight attendant out over the pacific on the way to Hawaii) It was actually an inter-island flight so lots of short flights (and therefore pressurization cycles relative to flight hours or miles). The amazing thing was that the plane was able to make an emergency landing. https://en.wikipedia.org/wiki/Aloha_Airlines_Flight_243 https://en.wikipedia.org/wiki/Aloha_Airlines_Flight_243
Ma8ee 5y ago
Even wood is supposedly quite much stronger than steel for the same weight of material.

lazide 5y ago

Pulling (tensile), which is also the direction steel is unusually strong in (from a materials perspective). Compression wise it’s close to a wet noodle the way it’s manufacturable (though that has more to do with cost/practical production/gathering methods - if we could get a solid chunk of it I imagine it would be pretty strong in compression). Sliding, it’s a fiber, so very weak in that sense (barring the same scenario above). It actively flexes, so isn’t strong’

ghaff 5y ago
As I recall my material science, it's not so much that steel is weak in compression but, if you have a rod of steel (i.e. much longer than it is thick), if you compress it, it will buckle and shear. As you suggest--I'd have to look up the numbers--but a large cube of steel probably has similar strength with tensile and compressive loads. ADDED: In fact, if you deform a supported I-beam you get similar amounts of tension and compression on the bottom and top respectively at the mid-point of the beam (given a variety of assumptions).
- user_7832 5y ago
  > but a large cube of steel probably has similar strength with tensile and compressive loads. Not just similar, but pretty much identical from what I recalled from my uni classes. Wikipedia for structural materials (https://en.wikipedia.org/wiki/Structural_material https://en.wikipedia.org/wiki/Structural_material) also has this bit - >Steel is equally strong in tension and compression.
  - ghaff 5y ago
    As I wrote in another comment, my material science and mechanical engineering is very rusty at this point. :-) But, yeah, I-beams basically wouldn't work if steel were weak in compression because, assuming a straightforward loading of a supported beam you're basically putting one flange of the beam in tension and the other flange in the equivalent amount of compression. So the fact that one part of the I-beam is strong would be pretty much irrelevant if the other part were weak.

kragen 5y ago

Tensile, mostly, but it's not stronger, it's tougher. The other answers to your question so far are all wrong or missing the central point. Spider silk has ultimate tensile strength comparable to some high-strength steels, around 1.4 GPa. Everyday steels are significantly weaker, around 0.4 GPa. Some steels have higher tensile strengths, like 2.6 GPa maraging steel. There are many materials with similar or higher tensile strengths: E-glass (3.5 GPa), carbon fiber (typically 4.1 GPa, 7 GPa for Toray T1100G), kevlar (3.8 GPa), zylon (5.8 GPa), boron (3.1 GPa), sapphire (1.9 GPa), diamond (2.8 GPa), graphene (130 GPa), dyneema (3 GPa). Some of these, like dyneema, are even comparable in density to spider silk. So what's so special about spider silk? Where spider silk is outstanding is that it combines high strength with high extensibility. Think of rubber: cast iron or diamond are much stronger than a rubber tire in the sense that they can bear heavier loads on the same cross section, whether in tension, compression, or shear; but if you hit a cast iron pot or a diamond with a sledgehammer they will break, while a rubber tire will be unharmed. This is because the rubber deforms under the blow, slowing down the hammer without ever experiencing a very high force. If you integrate the force on a spring over a distance (∫ F · dx), you get the change in the energy of the spring. If you integrate it over the spring's entire working range, you get its energy capacity as a spring. Every solid object is a spring, so you can do this for any solid object. "Stress" is force divided by cross-sectional surface area, and "strain" is extension divided by length, and the presumption of linear elasticity theory is that the stress–strain relation of a material, as well as its yield stress and breaking stress, are properties of the material rather than of particular objects made from it. If, instead of integrating force over distance, you integrate stress over strain, you get the amount of deformation energy the material can absorb per unit volume without breaking, for example in an impact. That's the material's theoretical toughness. All the other high-strength materials in the above list either deform plastically or break at under 10% extension; many of them break at under 0.1% extension. Spider silk breaks at 35%–500% extension. This makes some spider silks 10 times tougher than even kevlar. Usually compressive, tensile, and shear strengths increase and decrease together; the biggest exception is when the material is full of cracks, like glass and concrete.