what changes in the structure of an object that allows something to permanently bend (i.e folding paper)
In: Physics
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.
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