Why is it that when you cut a small part of plastic it makes tearing much easier?
In: 17
Making a cut produces a focus point that will have to absorb all of your applied stress, rather than spreading it more evenly across the entire surface.
The plastic is strong enough in bulk to absorb your strength, but not strong enough to stop all that force applied to a single spot.
It’s kind of like how a knife is able to cut things, but in reverse.
A sharp knife takes your applied force and concentrates it down to a tiny area that is able to push through what you’re cutting. If you apply the same amount of force with, say, your thumb, it’s obviously not gonna work as well because of the bigger area.
Likewise, having a small cut means that the tension forces get focused down to the “point” of the cut. Without it, you’re just stretching the whole width between where you’re pulling.
In engineering we call it a stress riser. All of the force you’re pushing into the cut gets delivered to the end of the cut, greatly decreasing the energy needed to pull it apart. This is the same principle that causes cracks to propagate through stone, and the reason you see sharp corners filled or rounded in manufacturing.
ELI5: Let’s say you have a sheet of plastic and you begin stretching it from the sides, notice how the entire sheet stretches length wise almost evenly, now if you start again on a new sheet but with a cut on it’s side you’ve effectively decreased the length of the part you’re stretching from the size of the sheet to the size of the cut, since there is less material to resist the forces it tears across the entire thing.
Bonus: The cool thing is, you can prevent a cut from tearing by punching it with a hole-puncher because this restores some of its length and this is done to prevent tears in e.g, metal girders using drills or hole-saws (circle shaped saws for drills) to literally just drill out the cracks and refill them after.
Here’s a couple videos about metal crack repair that show how it’s done:
[Engine Block Repair](https://youtu.be/vm_I34zDGas)
[Cast Iron Crack Repair](https://youtu.be/Pq0wfU4ZaKk)
This behaviour is the same for any material, plastic, paper, metal, concrete… Whatever.
Mechanical resistance in materials is opposed to stress (the physical term not the abstact term).
By stress we mean a the quantity of force applied to unit surface.
This means that to overcome the resistance of *any* material, you need to increase the stress applied, and you may do so either increasing the strength or reducing the surface.
A tear is (in its most basic), a reduction in surface. It causes an increase of stress at the same force applied, the increase may be such to break your material.
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