Why is packing tape nearly impossible to tear when intact, but easily shreds if you cut the slightest nick into it?

Question

Why is packing tape nearly impossible to tear when intact, but easily shreds if you cut the slightest nick into it?

In: 6079

Answer 1

If you have a piece of paper in the shape of an L, and you pull on both ends, all that force is concentrated on the inside corner of the L and none of that force will be felt on the outside corner of the L (you’ll even see the paper bulge on the outside corner). The stress concentrates on that one inside corner and the paper will easily rip there first.

The same thing effectively happens with a small cut. The cut is essentially an inside corner.

This is also why hairline fractures in bones, metal beams, concrete and other load-bearing materials are so dangerous.