4 ms·

TIL that creep can even happen at room temperature (albeit Caribbean room temperature) if the stresses are great enough and the time span long enough. I always

by WJW 3y ago

TIL that creep can even happen at room temperature (albeit Caribbean room temperature) if the stresses are great enough and the time span long enough. I always thought that it was more a thing for objects like jet engine turbine blades, that get very hot as well as having very great forces acting on them.

h2odragon 3y ago
Trying to think about the forces on the ends of the cables; when the wind is blowing and the big weight they support is bucking and the resonances of all these waves are interacting among all those cables... I kinda suspect there were instantaneous loads at times that were several multiples of the static weight. kinda expect that would count for more. Not that I know anything about it :) It stood for as long as it did, and had more weight added than the original design called for: thats a pretty good win for the original design, i think.
- sandworm101 3y ago
  At these scales, at this length of cable, they would stretch enough to soften most instantaneous loads.
HPsquared 3y ago
Failures due to creep deformation are pretty common in solder joints - driven by residual stresses created during assembly which persist over a long time, and eventually lead to cracks forming (stress relief - but not so good for the connection). Usually not an issue at room temperature, but it can happen - solder has a low melting point. In the case of the observatory, the failure happened in zinc components, which has a fairly low melting point. Turbine blades are just one of the most extreme examples, it's really hard to make a metal that resists creep in that environment.
thsksbd 3y ago
Transport properties all "happen" at any temperature. The rate is exponential with T, though, so at some point you dismiss it. Kinetics usually (always? Who remembers? ) is continuous with T. Phase changes, on the other hand can happen abruptly wrt T (first order, melting/boiling ice), or gradually (second order, curie transition, boiling mixtures). Of course, first order transitions require a pure substance that doesn't actually exist since the chemical potential is infinite at infinite dilution. But that's entirely academic. EDIT: I forgot to mention that creep has many proposed mechanisms and their relative contribution is not fully understood; however most depend on a transport phenomena, typically diffusion.
- WJW 3y ago
  Diffusion of atoms through solid materials is one of those things I intellectually know happens, but still don't really grok. Sintering is one of the weirdest things. Why does squishing things together AND warming them up cause the particles to stick together more than when you just squish them together or just warming them up?
  - imchillyb 3y ago
    > Why does squishing things together AND warming them up cause the particles to stick together more than when you just squish them together or just warming them up? >> The equations describing these laws are special cases of the ideal gas law, PV = nRT, where P is the pressure of the gas, V is its volume, n is the number of moles of the gas, T is its kelvin temperature, and R is the ideal (universal) gas constant. >> https://pressbooks-dev.oer.hawaii.edu/chemistry/chapter/relating-pressure-volume-amount-and-temperature-the-ideal-gas-law/ https://pressbooks-dev.oer.hawaii.edu/chemistry/chapter/rela... The laws of thermodynamics describe how gases work, those laws cause the effects you’re asking about. The above is a bit much for a primer on the topic, but provide enough key words for a Google-dive into the subject.
    - WJW 3y ago
      Did you miss the part where I was talking about sintering and diffusion of atoms in materials in their solid phase? A quick search in the article you linked shows zero results for either "sintering", "diffusion" or related terms. The ideal gas law is very nice but hardly applicable to describe things like creep in metals under stress, let alone things like the grain boundary diffusion mechanism for sintering metal particles together.
      - salawat 3y ago
        What is a solid but a, relatively speaking, very, very cold gas? Put two solids together with no intervening medium, and heat everything up, and you'll see some mixin' and mingling. Often, the reason you don't see more of that sort of thing is because everything is immersed in an oxygen rich fluid with a tendency to create large, relatively reluctant to mix and mingle oxide coatings before anything fun can happen. Hence why another word for cold welding is vacuum welding. Materials, when you really look at em' are not as 'solid' and 'stable' as you may think. Hence why I have a mechanics of materials book I peruse from time to time.
mensetmanusman 3y ago
https://youtu.be/EXbiEopDJ_g?si=pXwPNdH8d-AHAf2b https://youtu.be/EXbiEopDJ_g?si=pXwPNdH8d-AHAf2b Dislocation motion looks cool too. Fun math behind these strings of holes in the atomic structure.
aidenn0 3y ago
Different metals behave differently. Zinc exhibits creep at a much lower temperature than most (all?) steels, for example.

by WJW 3y ago

h2odragon 3y ago

Trying to think about the forces on the ends of the cables; when the wind is blowing and the big weight they support is bucking and the resonances of all these waves are interacting among all those cables... I kinda suspect there were instantaneous loads at times that were several multiples of the static weight. kinda expect that would count for more. Not that I know anything about it :) It stood for as long as it did, and had more weight added than the original design called for: thats a pretty good win for the original design, i think.

sandworm101 3y ago
At these scales, at this length of cable, they would stretch enough to soften most instantaneous loads.

HPsquared 3y ago

Failures due to creep deformation are pretty common in solder joints - driven by residual stresses created during assembly which persist over a long time, and eventually lead to cracks forming (stress relief - but not so good for the connection). Usually not an issue at room temperature, but it can happen - solder has a low melting point. In the case of the observatory, the failure happened in zinc components, which has a fairly low melting point. Turbine blades are just one of the most extreme examples, it's really hard to make a metal that resists creep in that environment.

thsksbd 3y ago

Transport properties all "happen" at any temperature. The rate is exponential with T, though, so at some point you dismiss it. Kinetics usually (always? Who remembers? ) is continuous with T. Phase changes, on the other hand can happen abruptly wrt T (first order, melting/boiling ice), or gradually (second order, curie transition, boiling mixtures). Of course, first order transitions require a pure substance that doesn't actually exist since the chemical potential is infinite at infinite dilution. But that's entirely academic. EDIT: I forgot to mention that creep has many proposed mechanisms and their relative contribution is not fully understood; however most depend on a transport phenomena, typically diffusion.

WJW 3y ago
Diffusion of atoms through solid materials is one of those things I intellectually know happens, but still don't really grok. Sintering is one of the weirdest things. Why does squishing things together AND warming them up cause the particles to stick together more than when you just squish them together or just warming them up?
- imchillyb 3y ago
  > Why does squishing things together AND warming them up cause the particles to stick together more than when you just squish them together or just warming them up? >> The equations describing these laws are special cases of the ideal gas law, PV = nRT, where P is the pressure of the gas, V is its volume, n is the number of moles of the gas, T is its kelvin temperature, and R is the ideal (universal) gas constant. >> https://pressbooks-dev.oer.hawaii.edu/chemistry/chapter/relating-pressure-volume-amount-and-temperature-the-ideal-gas-law/ https://pressbooks-dev.oer.hawaii.edu/chemistry/chapter/rela... The laws of thermodynamics describe how gases work, those laws cause the effects you’re asking about. The above is a bit much for a primer on the topic, but provide enough key words for a Google-dive into the subject.
  - WJW 3y ago
    Did you miss the part where I was talking about sintering and diffusion of atoms in materials in their solid phase? A quick search in the article you linked shows zero results for either "sintering", "diffusion" or related terms. The ideal gas law is very nice but hardly applicable to describe things like creep in metals under stress, let alone things like the grain boundary diffusion mechanism for sintering metal particles together.
    - salawat 3y ago
      What is a solid but a, relatively speaking, very, very cold gas? Put two solids together with no intervening medium, and heat everything up, and you'll see some mixin' and mingling. Often, the reason you don't see more of that sort of thing is because everything is immersed in an oxygen rich fluid with a tendency to create large, relatively reluctant to mix and mingle oxide coatings before anything fun can happen. Hence why another word for cold welding is vacuum welding. Materials, when you really look at em' are not as 'solid' and 'stable' as you may think. Hence why I have a mechanics of materials book I peruse from time to time.

mensetmanusman 3y ago

https://youtu.be/EXbiEopDJ_g?si=pXwPNdH8d-AHAf2b https://youtu.be/EXbiEopDJ_g?si=pXwPNdH8d-AHAf2b Dislocation motion looks cool too. Fun math behind these strings of holes in the atomic structure.

aidenn0 3y ago

Different metals behave differently. Zinc exhibits creep at a much lower temperature than most (all?) steels, for example.