what changes in the structure of an object that allows something to permanently bend (i.e folding paper)
In: Physics
I’m not scientist but I think the structure of an object IS what allows it to bend. Try and fold a piece of paper in half 8 times.
Solid materials are made up of tightly packed molecules, which is the most energy efficient way to be in. If you bend something, this structure is changed to a less energy efficient form.
The molecules are moving within the material, so when you hold it long enough, they will eventually reach the energy efficient state again, but now in the new shape.
The time and force it takes to achieve this differs for each material.
Let’s use paper from your example to explain this.
Paper is not one solid, contiguous thing on a microscopic level. Paper is really made from layers and layers of interlocking plant fibers. Those fibers are made of layers and layers of interlocking cellulose molecules, which [look like this, more or less](https://upload.wikimedia.org/wikipedia/commons/f/f9/Cellulose-Ibeta-from-xtal-2002-3D-balls.png).
Those cellulose molecules form fibers because sometimes hydrogen (the white balls) on the outer side of one strand of cellulose will bond to an oxygen (the red balls) on a neighboring strand. The fibers form the paper due to the process in which the paper’s made leaving them physically interlocked, and some of the hydrogen bonding between fibers. It’s a very weak bond, which is why paper’s so easy to tear and bend.
~~As to why it stays bent, though. As you bend paper, it requires you put energy into the act, and that energy breaks some of the hydrogen bonds, changing the orientation of some of the fibers within the paper, which then form new hydrogen bonds.~~EDIT: It was pointed out that what I said wasn’t quite correct. Creasing *does* break some of the fibers, and that does add up over time.
So when you’re bending the paper, you’re changing its structure at a microscopic level.
Civil engineer here. Besides what’s happening on the molecular and cellulose level, there is also something called Young’s Modulus, which is a ratio of the stress exerted on a material (in terms of force, such as Newtons or lb/ft^2 or kips) vs the strain (change in L or A per original dimensions). All solid materials have this characteristic. For paper, it is very, very, very low, so that humans can rip it easily or whatever. When you bend paper slightly, it will go back into place. This is the plasticity index, and it indicates the threshold before which the material will return to its original form. Again, paper’s super weak, so it’s practically nonexistent, but steel works the exact same way just with much stronger molecular and physical bonds. Upon surpassing the plasticity index, the material can no longer return to its original form. It is therefore “deformed” in whichever position it was put into, and that’s considered a fold. This is a very tangential connection but is nonetheless a phenomena that occurs as a result of the various qualities of paper that make it the way it is, and explains from the physical perspective why creases happen
Edit: forgot about strain
And thanks for the silver!!
Interesting stuff. I love hearing the chemical explanations for things. On the physics end, we quantify the tensile limits to which a material may bend without permanent deformation with a quantity referred to as [Youngs Modulus](https://depts.washington.edu/matseed/mse_resources/Webpage/Biomaterials/young’s_modulus.htm). Similarly, an object’s ability to withstand “shearing” strain without snapping or breaking is quantified via its [Shear Modulus, aka its Modulus of Rigidity](https://en.wikipedia.org/wiki/Shear_modulus)
The answer differs depending on the material but u/Zemedelphos and u/hickeycurran mostly cover it from two different views. u/Zemedelphos is incorrect in the last 2 paragraphs. u/hickeycurran is simplifying things to a single isometric material.
For elastic materials there is a difference between elastic deformation (temporary) and plastic deformation (permanent). This model is often applied to all materials in structural design as a simplifying assumption.
Folding paper is plastic deformation. Bending paper without creasing would be elastic deformation.
Edit: “wrong” is the wrong word. u/Zemedelphos is technically correct, but the last 2 paragraphs are more misleading than helpful for a basic understanding.
I want you to imagine playing with a set of [small magnetic spheres](https://i.ebayimg.com/images/g/g5oAAOSw9ZtcpKO4/s-l640.jpg).
If you have a nicely arranged sheet of them and try to bend them, they sometimes can snap to a different ordered position. That is bending or folding them.
Now, this kind of bonding is more similar to how metals bond, rather than solids in general. So this only really gives you a decent idea of how bending metal works at the microscopic level.
Non-metals (such as paper) work a bit differently, but still in a *kinda* similar way.
Now, note that molecular bonding works with electric forces, rather than magnetic forces, so the way the individual molecules behave is different to how the individual magnets behave. However in terms of the big picture, some of the same kind of order can be seen when you look at the whole collection of molecules/atoms, vs the whole collection of magnets.
Every (most) material has a point at which after repeated bending, it will not return to its original form. Have you ever repeatedly bent a piece of plastic or metal enough and it breaks off? This is because the material where the bend is gets stressed and loses its internal structure that gives it the ability to stay straight.
Any material can change its shape. That’s called *deformation*, which literally means “getting out of shape”.
Some materials can change their shape a lot and still return to the original shape. Like rubber, or steel that’s specifically made for use on springs. The fancy word for this kind of deformation is *elastic deformation*.
Other materials, like play dough, glass, coal, or diamond can only change its shape a little bit without permanent shape change or breaking apart. When you push the material beyond a certain point, it won’t return to its original shape any more. This is called *plastic deformation* because it’s changing the shape of the object – kind of like plastic surgery. The limit is correspondingly called *plastic deformation limit*.
With very strong chemical bonds between the atoms or molecules, you usually get very rigid structures that don’t deform easily. With weaker bonds, you get materials that are more flexible, but as long as the bounds are strong enough it still takes a considerable force to make them give completely.
Then there are materials like play dough or clay, which has so weak forces keeping it together that not only is it easy to change its shape, the change is usually also permanent. This is because the play dough molecules easily forms new bonds, weak as they are. That’s why you can join together two pieces of play dough seamlessly, while trying to join two bits of rubber for example requires some chemical help (usually called glue).
When an elastic deformation happens, typically the atoms or molecules making up the material move a little relative to each other, but the bounds that keep them together are not broken. That means the material keeps its molecular structure.
When the bending, stretching, compressing or shearing load is removed, an elastic material will spring back to its original shape. But any material can only change its shape a certain amount. Beyond that, it either breaks, or deforms permanently.
When a material reaches its plastic deformation limit, the chemical bounds keeping atoms or molecules together start breaking, and the atoms and molecules start shifting relative to each other. In some materials, like the aforementioned play dough or clay, new bonds are formed immediately and the material just assumes its new shape. In other materials, like paper, wood, or most metals for example, new bonds don’t form so easily so the material can become permanently weakened. Forming new bonds usually requires some amount of energy, which can be done by heating the material, but since wood and paper are flammable, you know what tends to happen instead.
For metal, things are a bit more complicated. Each plastic deformation breaks some bonds, but some new bonds may develop so the bent piece can still have significant strength. However, in most metals a permanent shape change also always weakens the structure. So in critical applications – like the crumple zones of an automobile – you can’t just bend the structure back into its original shape, because it won’t have its original strength.
If enough deformations happen at a certain point on a metal object, the remaining bonds become too weak to hold the object together and it comes apart, like if you’re bending a piece of welding wire back and forth.
But when metalworking is done at high temperatures, the metal becomes more like very tough play-dough, since the heat allows the metal bonds to break and re-form more easily. This means that much like play-dough, heated metal can be forced into a new shape, and the metal atoms can form new bonds that become stronger when they cool down and the metal solidifies. But going into more depth would be *way* beyond ELI5 stuff, this post is borderline too detailed as it is.
in a really simplified nutshell, using paper:
one side of the paper is stretching and the other is compressing. if the object doesn’t have elastic properties, it should stay that way
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