[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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