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Re: Elasticity of Phenom bench shirt
Posted By: Joe Marksteiner In Response To: Re: Elasticity of Phenom bench shirt (Ken Dresden)
Date: Monday, 20 November 2006, at 7:30 p.m.
I apologize if this is a little long and jumps around but I wanted to get it out before folks read too much into the material examination posted on the IPF web page. I’ve already seen a few posts that indicate folks aren’t interpreting the report accurately.
I’m not a textile expert, but I am an engineer and was an assistant professor in the engineering department at the Air Force Academy.
Fabric properties are determined by the properties of the thread or yarn it is woven or knit from and the weaving or knitting pattern itself. The effect of the knitting or weaving is pretty evident in the test results – the properties in the longitudinal and crosswise direction were not the same for any of the tested materials.
Weight, thickness, and density measurements in the report aren’t really what most Powerlifters are concerned about so I won’t address those beyond saying those properties were determined using standardized tests.
ISO 13934 is an engineering standard that describes a very specific test to determine some specific properties of fabrics. This is a standardized fabric test that was developed to test and compare specific fabric properties they are not oriented to powerlifting and don’t duplicate the complex loading geometries, loading rate, or other significant contributing factors.
Test description. I didn’t feel like paying $50 to download a copy. Here’s what I can tell you based on the information I was able to get for free.
The ISO 13934 fabric test is a “grab test”
A piece of fabric about 6 inches square is used for the test.
The fabric test piece is gripped by mechanical jaws like giant set of pliers in the middle of opposite sides of the test piece with 75 mm between the sets of jaws and pulled apart at a rate of 30 mm per minute.
I didn’t find a reference for the width of the section gripped by the jaws but from the pictures, I’m guessing that it’s about 25mm. Basically you are pulling on a 1-inch by 3-inch section of fabric in the middle of what looks like a 6 x 6 test piece.
There are two measurements that went into the graphs in the test document:
Elongation [%]- along the graph’s horizontal axis. This is defined as the change in length divided by the original length (so 100% elongation means that the distance between the jaws is twice what it was at the start of the test).Force [N] - The force required for elongation is plotted along the graph’s vertical axis (Newtons x 0.2248 = Pounds).
The fabric is pulled until failure – where the graph stops. The area under the graph is a measure of the energy required to stretch the fabric to the failure point where the test ends.
At this point it would seem pretty easy to say that I want the strongest and stiffest fabric possible in my shirt/suit.
Let me point out a number of differences between this test and how the fabric is used in powerlifting attire.
This is a one dimensional load test. In actual use the fabric is stretched in the longitudinal and crosswise directions at the same time. Additionally, a third stress called shear stress can also be applied during actual use.
The load is applied at a single area in the middle of the test piece material instead of being more uniformly distributed across the full width of the test piece.
The strain rate (how fast the material is being stretched) may be very different than is actually used in PL. At 30 mm per minute, it takes 1 full minute to reach 40% elongation. Nobody I know wants to spend a minute (or even 30 seconds) lowering the bar to his or her chest on bench or during the descent on a squat.
Most of the data is for elongation ranges that far exceed what we see in actual use and exceed the elastic range of the material itself. A 6-inch chest plate on a bench shirt would have to stretch to 9 inches when the bar touched the chest to reach 50% elongation.
In reality, if you stretch the material out to 40-50% elongation it is permanently deformed and will not recover to its original length. For example, the elastic limit for polypropylene is 7-10% elongation. That means for any elongation greater than 10%, the fibers will not rebound to their original length. In the test as the material between the jaws stretched, more and more of the surrounding material also began to share the load resulting in continued elongation and increasing force but the test was way beyond any elastic rebound. As I said earlier this test pulls on the material until it fails.
The test only measures the force required to stretch the material (and it goes far beyond the elastic range) it doesn’t show how much energy the material returns when you stop pulling on it. This is what you really want to know in Powerlifting, because it’s what helps you move the weight back up. After you take a shirt or suit off at the end of a meet it’s bigger than when you started - that permanent deformation is an indicator of energy that went into stretching the material but was not returned. The elastic recovery of polymer material and fabrics made of them is not 100%. That means even if you don’t stretch it until it is permanently deformed, you won’t get back all you put in. If you stretch polyester 2% you get back 99% of the energy you put in. If you stretch it to 5% you only get 89% back.
The report recommends that material not be stretched past 10% (remember I mentioned the elastic limit for polypropylene is only 7-10%). This is probably because the cloth is now permanently stretched and won’t return to its original shape. If you look closely at the graphs for the woven fabrics you will see some irregularities in the lines in the 4-12% range. Those dips are probably indicators that some of the fibers in the fabric grabbed between the jaws in the test have already exceeded their elastic limit. The knit fabrics don’t seem to show that same characteristic but woven and knit fabrics don’t share load between fibers in the same way. Your knit cotton t-shirt changes shape and stretches much more readily than your woven cotton blue jeans. Cotton fibers only stretch about 5% before tearing but you can stretch a knit cotton t-shirt a lot more than that.
This was also a single test to failure so it doesn’t show how the materials will perform on the 2nd or 3rd attempt, let alone the 2nd or 3rd meet. If you exceed the elastic limit of the fibers the shirt/suit won’t perform as well the next time it’s used.
The test only determines a specific mechanical property of the tested material. The shirt/suit is only one part of a complex mechanical mechanism of which the human body is certainly the most significant component. Denim and canvas shirts are very effective but not very elastic – cotton only stretches about 5% before it breaks and it’s elastic recovery at 2% stretch is only 74%. This would lead you to believe that the elastic response of the human body when restrained in a very tight garment is a far more significant factor in the performance of supportive lifting gear. Remember one of the first uses of supportive gear was a guy wrapping bed sheets around his body under his singlet. A well-upholstered super heavy packed into his suit like an overstuffed sausage is making a lot more of the material work than an angular hardbody with some loose spots and wrinkles.
So what does all this tell you?
All the fabrics’ ultimate strength was at elongations greater than 50%, but you shouldn’t look at the max fabric strength - you probably can’t get much past 10-15% elongation without permanently stretching the suit/shirt. If you stretch the shirt past the elastic limit to get the bar to touch it’s likely to just “hover” there when it’s time to press. If you have a very short bench stroke a stiffer fabric (one with a steeper slope like the woven fabrics) looks like it might be a better choice for you. If you have a very long stroke you may need fabric that stretches easier like a knit just to get the weight to touch.
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