Why is it easier to tear paper along a crease?
To word it differently: why does a "tear" progress along a crease, if one is present?
[Physics] Tearing a piece paper along a crease
everyday-lifestress-strain
Related Solutions
Paper is a mesh of fibres usually mixed with a binder and some clay. The fibres will in turn have some microstructure depending on their origin (cloth, wood, etc). Ultimately the paper is composed mostly of cellulose molecules.
When you tear paper you are mostly pulling the mesh of fibres apart. If you look at the torn edge closely you'll be able to see the fibres sticking out of it. You will probably also fracture some fibres, and that fracture process will depend on the microstructure within the fibres. The clay particles are small and rigid, and they are likely to just separate from each other rather than fracturing. Some of them will spring off the surface as a fine dust.
Given all this complexity, the chances of you reassembling the torn pieces into anything approaching their original configuration are essentially zero.
All the force will be concentrated at the tear (the tip of the tear) if there is one. With no tear, all the material helps to hold itself together — but with a tear, all the force is localized at this tiny point and only a tiny amount of material is responsible for holding on.
The stress at this tear tip (the force per area) becomes huge because the area is very small and it overcomes the strength (the maximum or ultimate stress) of the material. On the illustration below you can clearly see that only a few particles are helping to hold the material together — they carry the whole burden. The colours show the stress distribution.
Were the material flexible and less rigid, the stress would have been spread more out, since the bonds between atoms/molecules would allow some stretching for them to "lean back" on their neighbor atoms/molecules.
This is why a crack propagates very fast in rigid or brittle materials like concrete or glass — a material might be strong and can withstand large forces for a long time, but as soon as a single tiny crack appears, the whole thing breaks in a split second because all the force is suddenly on just one point, which breaks and lets the next point take over, which breaks etc.
The more rigid and brittle the material is, the less flexible and deformable it is, meaning that the material will not absorb as much of the energy but instead have it all concentrated at the crack. This is why glass looks like it almost explodes when cracking — the cracks are propagating at huge speed. Plastic sheets on the other hand will only tear in two if, along the way, you help and localize your force correctly, and you can control the speed. It will also feel warmer at the tearing edges, because energy was absorbed.
Best Answer
Paper is made from wood, and wood is made from long fibers.
Typically the manufacturing process leaves the fibers are more or less parallel. So it is easier to tear in the direction that separates fibers from neighboring fibers than in the direction that breaks fibers. Wood is the same. It is easier to split a log than chop it.
Creasing paper breaks and/or separates some of the fibers, making it easier to break/separate the rest.
Once the tear is started, pulling the paper apart a little propagates the tip of the tear a little farther. The crease makes the paper fail more easily along the crease. So the tear follows the crease.