Why You Tear Already Cut Plastic Bags More Easily?

We’ve all tried to pull plastic bags apart. If the plastic bag is new and immaculate, tearing it apart is significantly harder than when it already has a cut. Whenever the plastic bag has a cut, it just seems to smoothly come apart when you pull on it. In fact, this behaviour is used by manufacturers when they leave cuts or notches in the packaging to enable hassle-free unpacking for their customers. On a similar note, the same holds true when you try to tear paper. We’ve all known from art class that if you crease paper first by folding it, it is significantly easier to tear it along the crease. When using scissors to cut paper or similar material, we sometimes cut the paper first, and from thereon, we are able to slide the scissor without cutting, and the paper (or similar material) just comes apart as you slide along. What’s causing this? As you might have already guessed, it is the same phenomenon behind all of these situations, and this article tries to look into the science behind what’s causing already cut plastic bags to come apart more easily.

This essay is supported by Generatebg

Stress

You’ve probably heard the word stress in the usual day-to-day context, or sometimes in a psychological context, but the term ‘stress’ in this article refers to a physical phenomenon. In mechanics, stress is used to represent a physical quantity that describes the internal forces exerted by internal particles of a material on each other. It usually increases in value when the forces experienced by a material increase.

Strain

Strain is also a word that you’ve probably heard in the usually day-to-day or psychological context. But in mechanics (and in this article), ‘strain’ is used to represent a physical quantity used to represent deformation experienced by a material. Stress and strain are related. For materials such as plastic or paper, stress and strain are positively correlated. This means that as the stress increases, the strain also increases. For the materials under consideration, stress and strain are (quasi) linearly related up to a certain point, beyond which the relation becomes non-linear and irreversible. This is the point beyond which your typical plastic bag stretches like goo, or your paper stretches a bit (before it tears). From experience, you’d know that there’s no going back to the original form beyond this point. In the business, this point is known as yielding point.

Stress Concentration

Now, we arrive at the heart of the phenomenon. Why are you able to tear an already cut plastic bag more easily? This is because of a phenomenon known as stress concentration. It just happens to be the case that imperfections in a material lead to stress concentration. These imperfections might appear as holes, cuts, creases, wrinkles, or may appear in any other form.

When a cut already exists, all the force applied to the material gets concentrated on the tip of the cut (or imperfection, in general), and this concentration causes the small area of the tip to go beyond yielding point, which makes the stress to flow smoothly as the cut seems to flow further into the material. This is in extremely simple terms what actually goes on.

Conversely, if you are aiming to design or build something robust or something aimed at carrying loads or forces, imperfections are the first thing that you need to be worried about.

Now that you are aware of the basic science behind this phenomenon, do you think that you can apply this knowledge productively to anything else you do in day-to-day life? Feel free to comment below.

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Further reading that might interest you: Is Technology Dependency Really a Concern? And AI writes an Essay on Simulation.