[HN Gopher] Weaker bonds can make polymers stronger ___________________________________________________________________ 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. ___________________________________________________________________ (page generated 2023-06-26 23:00 UTC)