Why is packing tape nearly impossible to tear when intact, but easily shreds if you cut the slightest nick into it?
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It’s the mechanics of crack propagation. It’s applicable to most materials actually.
Basically, it’s pretty difficult to create a crack, but once a crack exists, it’s much easier to make it bigger.
You can think of the cut as a small crack, and all you’re doing is making the crack bigger until the tape fully tears.
To go into a bit more detail, when you pull on the initial crack, there is a tiny little inside corner at the end of the cut, and this tiny little radius at the end of the cut is a weak point that will allow the crack to continue. You could actually punch a bigger hole at the tip of the crack to prevent it from becoming bigger. Basically when there is a tiny radius at the end of the crack, it’s easy to make the crack propagate. When the radius at the tip of the crack gets bigger, it gets more difficult to make the crack propagate.
Pro tip: if you have a crack in something, drill a hole at the tip of the crack to prevent it from getting bigger
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