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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Because in the second case, you’re tearing in the direction of the weakness in the material (the nick), while in the first case, there is no weakness in the material and so tearing it becomes very difficult.
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.
As I remember from college…
If you have a piece of paper with a tear in it, and you pull it on both sides of the tear, the pulling force around the endpoint of the tear can be up to 3 times as high as the average pulling force across the whole paper. (It depends on the length of the tear.)
So yes it is much easier to rip apart, as it’s now easier to get over the threshold, and it will tear starting from the end point of the tear.
The polymer molecules that make up the tape are shaped like long chains. If you pull the tape, these chains align in the direction of pulling, and pulling more just packs them together as they become parallel to each other. On the other hand, if there already is a cut, there is alread one side of the bulk of polymers where there are no other molecules to bond to. Thus, pulling from the cut’s size is easier, especially if you pull off-plane, where there is no inherent resistence of the material (the forces among molecules are all applied within the thin tape).
Stress concentration, if you cut a slit in a paper and applied pressure to it, all the stress that used to go through the material at the slit line can’t anymore and needs to find another path. It will flow throw the nearest material connection it can find, and since slits usually end at a single sharp point, all that stress that used to pass a whole line now passes throw a point, creating a spike of high stress that passes the failure point of the material, and so the paper will tear.
Its extremely easy to tear by hand. You just use the tip of your thumb.
Its very easy if you know how to use it.
I saw a boy meets world episode about this a long time ago, Mr. Feeny explained it really well, but I can’t remember what he said. I think he was using a different material as well, but same principal should apply.
I imagine it like this…
It primarily has to do with the force that’s tearing the tape apart and the area that this force it applied to. Let’s say you unroll some inches of tape and now your grip that tape where you want it to separate and tear at it. You will probably have an inch of tape between your hands or fingers. Imagine you tried to cut that same tape with a knife with a 1-inch-wide blade. It would be more than blunt and even if you really hacked at the tape you wouldn’t get very far. The amount of force per inch wouldn’t be very high since it gets distributed evenly across the inch of tape.
Now, if you have a stall nick in your tape it is the equivalent of having a very, very sharp knife with which you have a go at the tape. It is such a pinpoint of stress that the amount of force per inch is off the charts. And that’s why you can separate it from there.
The back of the tape is plastic, which is a polymer. Polymers are long chains of molecules. They are woven together to make a tape and an adhesive is put on one side. It’s like a flat rope, but with millions more strands that are each a lot shorter.
If a rope has a nick, in it, it will eventually fray and snap. The same thing happens with tape. Every part near the nick is put under more stress because there is less rope to support it. As the nick grows, the stress of each strand increases making it easier and easier to break.
Finally, questions the world asks that my materials engineering education will one day be able to answer
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