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 is made of a relatively inelastic polymer.
While some plastics are fairly ‘stretchy’, these are a huge pain in the ass to cut. Think about duct tape. It’s extremely strong but also quite difficult to work with.
Packing tape is strong but not particularly robust – it cannot stretch much before it breaks.
All materials experience extra stress at inside corners. This is because of the geometry of the object. A cut is just a very sharp inside corner, and so the stress at the tip of a cut is *huge* compared to elsewhere. More or less, all the force that’s no longer being held by the area that was cut is now borne by the tiny bit of material right where the cut ends.
The stretchier the material, the more this force is spread out. Packing tape isn’t very stretchy, so this force isn’t spread out much.
The end result is that cuts cause a huge amplification of the force at the edge of the cut, causing the cut to grow.
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