Re: How can you test the strength of fabrics?

Hi Felicia!

Sorry it's taken so long to get back to you. I hope this is in time for your science project, and if not, I hope you found what you needed through other sources...

So you'd like to know how to test the strength of fabrics? Well, the bad news is that there's no universally standard method for testing that. But the good news is that there's no universally standard method for testing that! You get to make one up!

Now, I can't think of any ways to test a floppy piece of cloth under compression, bending, or fatigue testing, but that doesn't mean that you won't be able to think of a way to do it. But I think a good indicator for overall strength of a cloth is Tensile Strength. Tensile Strength is how much force it takes, pulling in opposite directions, to get an object (cloth, rope, steel beam, etc...) to fail. Different things fail in different ways. Brittle objects will crack, rubbery objects will stretch, and may eventually tear. Really soft objects will stretch for a really long time. Some substances, like Silly Putty, behave differently based on how fast or slow you apply the force. If you pull it quickly, it snaps. If you pull is slowly, it'll stretch. But I'm getting off the subject...

Here's my suggestion for a possible method for testing the Tensile Strengh of Cloth. There's many other methods, so this may or may not be the best one for your experiment.

Take a sample of the bleached and unbleached cloth, and cut (ask your parents to help if you need it.) them into five cm wide by ten cm long strips. Tape either end of the strip to the edge of your desk, very securely, and put a few heavy books on top of it. Tape the other end (very securely as well) to a basket or a pail or any other kind of container. So the tape is being pulled in one direction (down) by the pail, and in the other direction (up) by the desk. Next, you take the scissors, and cut a one centimeter deep notch in the side of the strip of cloth, about halfway between the top and the bottom. What this notch does is concentrate the force at that one spot, so that when the strip fails (I'm guessing by tearing) it will fail at that one spot.

So now that you have the pail pulling down on the strip, start adding weights (lead fishing weights, coins, small rocks, stuff like that) to the pail. At a certain weight, (and only if both taped ends are secure), the cloth will tear in two. Once that happens, record the weight of the pail and all the things you put in them. This weight value will indicate how much strength the cloth has. Repeat this for each of the bleached (test) samples, and each of the unbleached (control) samples.

Some things to remember:

1. I would suggest testing at least three bleached and three unbleached samples. If the results vary between similar samples, then there is something different about them, or something different about the way you tested it. Ideally, scientific results should be repeatable by not just you, but anyone else in the world who wants to try and repeat your experiment. So write down everything you do, and try and be specific!

2. More on repeatability: Make sure your cloth samples are identical to each other in as many ways as you can. If you cut them diagonally to the thread of the cloth, then they all have to be that way. If you bleach one strip for 5 minutes, then all the bleached strips have to be that way as well.

3. Tensile strength is normally given as a "Stress" value. That means that it is actually the load applied, divided by the cross sectional area that it is applied over. So if I hang 20 Newtons on a square bar that is 10 cm by 10 cm, then the applied load is 200,000 N/m^2, or 200 kilopascals (kPa). If each of the cloth samples have the same cross sectional area (for example, the width and length of the strip are the same, and the notches are the same depth), then the weights can be compared to show relative strength to each other. But if you want a strength value you can compare to the values you find in textbooks, you'll have to measure the thickness and width of the cloth and the notch depth, and calculate the stress on the cloth.

4. The cool thing about this, as well as any other kind of experimental design, is that as long as your test results and methods are consistent and repeatable, you can go beyond just comparing bleached and unbleached cloth, and try this on different things. Try different kinds of papers, different plastics, leather, vinyl, anything you can get your hands on. This method is limited to samples that will fail before the tape fails, but if you want to test stronger things, you can check your high school (or maybe a nearby college) physics or engineering lab for stronger equipment.

5. One last thing. Make sure you ask your parents before testing anything that might be important or valuable. When your sample is no longer in one piece when you're done, that's called destructive testing. There's lots of non-destructive tests out there, but I'll let you look those up.

Jeff Yap
Mad Scientist