In physics this is called elastic vs plastic deformation.
Elastic is temporary deformation where the structure of the material is stressed, but not enough to pull it apart and rearrange the molecules/granules/etc. Like a spring or rubber band, it will go back to the same shape it was in before.
Plastic deformation is when the material has been stressed past the point of no return. The molecules have been permanently rearranged where they can't go back to their previous state without some outside help.
Different materials have different limits to how much they can be bent ( or pulled, stretched, crushed, etc ) and still retain their shape. Like the above-mentioned springs and rubber bands, you can stretch them past their limits where they will stay stretched out or torn. Mild steel is usually easy to bend in a plastic manner, but with just a little heat treating, it will be much springier.
Disappointed "elastic vs plastic deformation" isn't the top comment here.
it is mentioned in the top two comments?
It's because this is a sub for explaining like you are 5 years old. No 5-year-old is going to understand that.
Look to your right:
LI5 means friendly, simplified and layperson-accessible explanations - not responses aimed at literal five-year-olds.
Looking to the right is grass at the moment, I'm on my phone 😂
Atoms are connected together by bonds, like rubber bands. So they can stretch a little bit, then bounce back.
But if you bend them too far, the rubber bands snap and the atoms can move around some. Now they quickly regenerate new rubber bands, but they've already moved. So the rubber bands hold them in their new positions.
That's about as ELI5 as I think this gets.
Less ELI5
When you deform a material enough, you add enough energy to physically move atoms around to new positions in the crystal lattice. And once they move, you'd have to apply more energy to move them back. This doesn't always work because this process alters the lattice structure in a way that doesn't get reversed by bending it back.
past a certain strain you end up shifting the internal metallic/crystaline structure of all those atoms, once you get into the nonlinear part things start shifting. before that they are just stretching
Upon "stretching", wouldn't the atoms also move? So what determines whether something is "stretching" their structur or "moving" the structure in a manner that it won't go back on?
Imagine two egg cartons stacked so the bottoms are touching and the egg compartments are interlocking. You can wiggle the top container back and forth a little and all of the compartments will still be interlocked with the same counterparts on the other container. But, if you move top carton enough, the compartments will click into a new configuration, and they will be interlocked with the compartments one row over from the original position.
The compartments are like molecules, and they can move a little and settle back into the same configuration, or they can move a lot and settle into a new configuration.
It has to do with the structure of the atoms in the material. Some lattice or grain structures will allow a little variability at the junction points which allows for bending that will snap back upon being released.
Imagine if you made a model of the metal's crystal structure, and you connected all the atoms together with magnets. If you push or pull on the model a little, the magnets will give a bit, but then snap back into their original places. If you move them too far, though, they'll shift and the magnets will suddenly jump into new connections in a new shape.
That's a lot like how the atoms of the metal work. They're bound together by various forces, and those forces have some give before they break. Once they do give, they form new bonds and are permanently changed into a new arrangement.
Say you have a bunch of dancers or soldiers or something in a bunch of lines forming a grid. You can ask them to spread out or stand closer in every direction, only some directions, bend the formation, etc. if you stretch/twist/bend them too far though, for some reason they get confused and line next to the wrong neighbor, and then their other neighbors get confused too. They have broken their structure if they are lining up wrong
If I pull your arms apart they stretch a little bit. But if I pull really hard they pop out of place and won't go back together again. Imagine the spring is lots of little arms holding onto each other.
It is called ductility - the amount of plastic deformation (bending) it can sustain before it fails. It is one of the core performance properties of metal, and the amount of bend it can sustain will vary by its intended use.
I think this would be more about the amount of elastic deformation it can sustain before it starts deforming plastically.
You're probably right. It's been a long time since my course in material science.
ductility is for tensile stress, so being stretched out.
Imagine the stick made up of a lot of tiny balls, each of which is connected to its neighbors by tiny rubber bands. If you bend the stick a little, the rubber bands on the outside of the bend stretch, and when you let go, they pull the stick back to where it was. If you bend the stick a lot, the rubber bands on the outside of the bend snap, and there's nothing pulling the stick back to the original shape anymore.
This is a crude approximation to what's happening at a microscopic level, where the balls are the atoms and the rubber bands are the bonds between them.
elastic deformation vs plastic deformation.
every material has a maximum elastiscity, an amount they can be bent and still spring back.
If you bend it past that point it won't spring back, that's plastic deformation.
but even plastic deformation has its limits, where the material becomes damaged or breaks.
So, you know how if you pull a rubber band, it will stretch and spring back, but if you pull it too far, it snaps?.
Imagine a stack of rubber sheets, if you fold the stack, the ones in the center of the fold are being stretched a little bit, but the ones on top at the outside of the fold are being stretched a lot, and some of them might get stretched enough to break.
Every material is (at a microscopic level) built like that stack of rubber sheets. Some of them are just stretchier than others.
Wood sticks do the same thing, it can bend but reaches a point where it breaks instead of bending.
Other people have better explanations about atomic structure but that isn’t very eli5. Atoms can shift, but eventually the pattern at the atomic level has to shift and causes a permanent break.
Imagine the material is made of lots of little bits of brittle straw.
They can flex a bit, but if you bend too far some of them will snap, and there won't be tension to pull them back anymore.
This is also why if you flex a material enough it starts to weaken and break.
Many metals are full of crystalline lattice structures that behave a lot like this.
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Hey I just saw something about this, and this guy is great at explanations. It's a short. PhysicsDuck Video
Uggg ok I'm going to try to keep this simple, sorry if I fail.
Metal undergoes elastic deformation and plastic deformation. The elastic part is first, and will always return back to its initial shape, if go past the elastic limits, then you enter plastic deformation . The plastic deformation will not return on its own, but the plastic part still will.
Let's say for a given metal rod you can bend it 10° in the elastic range, if you bend it a total of 20° and then release it will spring back to 10°. Meaning the initial 10° of elastic will still spring back, but the second 10° is plastic and it's bent for good (unless you bend it back). Also you can not take this rod bent 10° and bent it to 20° and it will still spring back to its new normal which is 10°.
Pretty much all materials act like this to one degree or another, the only real difference is the degree to which they can deform in the elastic and plastic ranges. Something like glass will bend to a barely perceptible degree and shatter because it's elastic and plastic limits are nearly identical. Something like a rubber band will stretch and stretch and then snap, which is ironically for similar reasons to glass just that it's elastic range is very large, and the plastic range is only slightly larger.
Why do you think that happens?
If you bend the metal rod a little bit, all the atoms that make up that rod don’t move and stay in the same order that they were in before you bent the rod. It’s like if you have cars on a highway, and even though the spacing between the cars changes they still stay in their lane.
Now if you bend the metal rod enough so it stays bent, the order of the atoms in the rod have rearranged slightly, and they won’t pop back to their original position. Going back to the car analogy, this is the equivalent to a car from one lane merging into an adjacent lane. The order of the cars (atoms) has changed, and so it will not go back to its original shape.
This is called elastic (nonpermanent) deformation and plastic (permanent) deformation in material science and engineering.
Another interesting aspect is that for many materials, we've developed graphs showing this behavior. Therefore, for a given material, we can know how much a material can take to a reasonable degree of certainty. Of course this is only one aspect of failure analysis, but it's a good place to start.
Stress-Strain Curve
Yo that was cool, thanks for sharing :)
Check this out. Steel cable testing.
https://www.youtube.com/watch?v=RMZW1SX_rbk&pp=ygUXc3RlZWwgd2lyZSByb3BlIHRlc3Rpbmc%3D
Audio on that makes me think of the Titan sub.
It's amazing how many individual wires were able to break before complete failure. Also, that boom...
Sounds like popcorn.
Those sounds weren't actually the wires snapping except for the big boom when the whole cable broke at once. Someone was off camera making popcorn in a pot on a stovetop.
The way the steel cable goes like an explosion at the end is a good illustration of why tug of war can be so dangerous, particularly with nylon ropes.
Nylon's elasticity allows a lot of energy to be stored in the rope, then when it breaks, all that energy has to go somewhere. It can rip off fingers or even arms.
On this chart, what's the difference between Stress and Strain? How should I read the chart?
Stress is force, strain is the measured effect of that force. Increasing strain indicates increased deformation.
But what is that effect? Displacement? Why is there a dip in the Stress/Strain graph, how can more stress lead to less effect?
After the yield stress is reached, there is a period where the material can be displaced with less force, where it noticeably thins out (called necking) and stretches, at least in a lab environment.
This video of a tensile test shows the material displacement and the corresponding stress-strain curve generated in real time.
Oh, so it actually is displacement! Thanks for the video, it made the image much clearer.
Correct. Stress is force/area, and strain is a measure of the change in dimension, in this case length, vs the original dimension.
It's how much it stretches per unit of length, with units typically in micrometres per metre, which cancels out to a dimensionless 10-6.
Deformation is the effect, this is kind of an “ideal” stress-strain curve, they don’t all look like that. That “dip” is where the material starts to really “give”. If you’ve ever bent metal, you may inherently know what this point is. It’s where you finally feel the metal bend and it gets easier, but after that it takes increased force to actually break it. If that makes sense.
Ok yep, that makes sense, thanks!
These kinds of graphs are usually achieved with tensile testing. In this you basically pull on a rod with constant speed until it breaks. The small dent comes from the fact, that after the atoms (more accurate the dislocations) start to move it is easier for them to travel through the matrix. This only works until for a short distance, because there are a lot of things in a metal that hinder the travel of atoms (dislocations)
Yes. Strain is displacement, the distance of stretch basically. https://en.wikipedia.org/wiki/Strain_(mechanics)
This chat comes from measurements taken with materials, so the dip is an observed fact.
I think, though I don't know exactly, that it represents the point where the material is work-hardened. That is, metal gets harder when the molecules are moved by bending, hammering, or stretching. Try bending a paperclip back and forth and it'll get harder before breaking.
And after following the curve up the elastic region and into the plastic region to achieve permanent deformation, when you unload the material it springs back or relaxes by following the elastic portion of the curve’s slope to the final strain value.
The non-ELI5 “scientific word” for when materials don’t “return to their shape” is they reached their yield point: which is determined by displacement of the thing’s geometry’s lattice, and obviously how much force is applied.
Knowing a material’s physical properties (like the elasticity of the material) can help understand when something is going to break. Still gotta consider the geometry of the thing and the forces being applied though.
Like someone else said; once you know the material properties, the geometry of the system, and the forces applied: you can use an stress-strain chart to predict the yield point and even where it will break. Aka when an object begins to experience elastic or plastic deformation.
Man, I always wanted to study material science, but I just never made it happen. Ya know, adult life kinda got in the way of me finishing college. This stuff is so interesting to me.
Oh well, maybe someday.
If you start with understanding tension or compression forces on a particular material, it’s a great start!
Start with seeing what happens when a metal bar is pulled apart, or squished together (where the forces are only on one axis).
Bending forces and geometric inertia are some of the later steps, but you’ll start to understand better if you keep at it!
The more ya get into it too the more neat things and concepts come up. Tensegrity for example.
A great starter text that you can find old PDFs of is Callister's Materials Science and Engineering: An Introduction. It goes through your 3+1 basic material classes (metals, ceramics, polymers, plus composites) and their properties.
Shockingly digestible as far as engineering textbooks go.
Much appreciated! I'll take note of this
what is fun to note is that the plasticity (stress-strain curve) changes due changes in temperature, and this is the reason why you can freeze a lock and shatter it with a hammer. The US learned this the hard way during WW2 when the Steel ships they sent to the arctic circle to avoid German U-boats literally sheared in half and sunk.
Ductile to Brittle Transition
This is one of two actual ELI5 explanations.
Veritasium did a really cool video on a metal that revert to its original form when heated and they explained this whole concept really well (as usual)
Shape Memory Alloys!
Great explanation. I was trying to figure out how to explain plastic vs elastic deformation in ELI5 and you did it well.