Why does plastic film, like packing tape, have such great tensile strength but such poor shear strength?

Question

Why does plastic film, like packing tape, have such great tensile strength but such poor shear strength?

In: 3

Answer 1

It’s how the molecules are arranged. Most plastics have molecules that are long, often very long, chains of the building blocks. If they are all tangled up you will have a material that can stretch a long way in all directions before the chains become taut. If they are aligned straight and parallel ( think of a bundle of spaghetti) the molecules can’t be pulled out any further giving high tensile strength, but they can be torn apart sideways. Sometimes the molecules can be “crosslinked”, one chain joined to another like rungs on a ladder to give strength in 2 or 3 dimensions.