[HN Gopher] Weaker bonds can make polymers stronger
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Weaker bonds can make polymers stronger
Author : gumby
Score : 25 points
Date : 2023-06-26 13:43 UTC (9 hours ago)
(HTM) web link (news.mit.edu)
(TXT) w3m dump (news.mit.edu)
| alexmolas wrote:
| Can this be interpreted as an example of the Braess's paradox [1]
| where removing weak links makes the structure/network less
| resistant?
|
| [1] https://en.m.wikipedia.org/wiki/Braess%27s_paradox
| nerpderp82 wrote:
| > This occurs, the researchers believe, because the weaker
| bonds are randomly distributed as junctions between otherwise
| strong strands throughout the material, instead of being part
| of the ultimate strands themselves. When this material is
| stretched to the breaking point, any cracks propagating through
| the material try to avoid the stronger bonds and go through the
| weaker bonds instead. This means the crack has to break more
| bonds than it would if all of the bonds were the same strength.
|
| > "Even though those bonds are weaker, more of them end up
| needing to be broken, because the crack takes a path through
| the weakest bonds, which ends up being a longer path," Johnson
| says.
|
| I believe it is similar. Braess's Paradox, Antifragility,
| Crystalline vs Plastic materials and smooth response profiles
| to stressors can all be related.
|
| The weaker material here is a sacrificial substance that
| spreads the load over a larger amount of material. Extremely
| strong substances that are not also plastic fail
| catastrophically, counter intuitively strong things can
| increase the local force and cause cracks to propagate through
| a material. Everything fails via crack propagation.
|
| If you over optimize your system, you do so by removing the
| portions that potentially make it resilient to stressors.
| Strength in one dimension is often at the detriment to strength
| and resilience in others.
| Arrath wrote:
| This discovery does make sense. I wonder how well it will
| scale to production, is it as simple as ensuring that this
| ~2% differing binder material is homogenized throughout the
| mix?
| Terr_ wrote:
| Just playing around with metaphors and analogies, I'm
| imagining a sheet of paper hanging vertically, its bottom
| edge held down with a weight. When nicked on one side, that
| becomes a tear that causes it to separate in two.
|
| In contrast, there's another sheet of paper which has a bunch
| of small holes punched into it. Despite being "weaker" from
| having less material, it's more resistant to a particular
| failure mode, since when the advancing tear hits one of the
| holes there's a chance that it will stop.
|
| Eh, never mind, that's probably its own differently-named
| principle in materials-science durability.
| nerpderp82 wrote:
| That is exactly how material scientists study the grain
| structure and crack propagation. If the crack joint is too
| small, it acts as a lever. This is why you fillet and
| chamfer a joint in a designed part, but the concept works
| down to the molecular level. The holes you are talking
| about are points of stress relief, without them, the crack
| can continue to travel through the material, amplifying the
| forces.
|
| Fiberglass isn't strong because the material is special, it
| is stronger because the imperfections in it have been
| removed due to surface tension self organizing the
| impurities away (that would form cracks) during the remelt.
|
| The polymer structure in the linked article is not unlike a
| fiberglass+epoxy composite.
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