fabric tension, PES material

[mountaineerxman]

Greetings,
I want to measure fabric tension in a material. I have a cylindrical bag made from PES (polyether sulphone), PVC coated. (Mehler Texnologies, POLYMAR 8556)
The bag is partially filled with water and air. The bag is in flowing water. I am trying to find out a way to measure the tension in the fabric as it collides with various objects. Anyone know of proven methods for this type of measurement? Most of my work has been done with metals, and strain gages would be my first inclination. I am finding out that this may not be the best way to go for an anisotropic material. thanks.

 

[Compositepro]

In this case PES means polyester rather than polyethersulphone. A blend of PET and PBT is called PES.

Fabric tension will cause much higher strains than typical strain gages are designed for. You should probablly be looking at extensometers or force measuring devices.

 

[mountaineerxman]

Thanks for the material clarification. I was clueless.
Thanks for the advice.

 

[Demon3]

The strain of the polymer will change orientation and thus the refractive index. By looking at strained polymers using polarized light you will see colors due to the different refractive index in different directions.

These colors can be correlated to the strain if you calibrate them versus known strains. In this way you can quantify the strain without using for example a strain gauge, which would alter the system.

I used this method many years ago to detect stresses in tableware glaze (ceramics).

Chris DeArmitt

"Knowledge has no value except that which can be gained from its application toward some worthwhile end."
Think and Grow Rich - Napoleon Hill

 

[btrueblood]

I knew I'd seen references to this work elsewhere (maybe NASA tech briefs or similar?).

http://jim.sagepub.com/cgi/content/abstract/11/5/351

Somewhere else, years back, I read a paper or two that described putting very fine wires into the fabric, and measuring resistance across the wire, essentially a strain gage but at much larger scales than the preprinted gages we all know.

The other main diagnostic I know of from airbag systems is to use high-speed cameras, and measure the actual deflections of the bag surface (accounting for parallax and other errors while doing so), translating those measurements to a dynamic model, and backing out the membrane stresses from said model. Painful, but it can be done if the numbers are worth enough to you.

 

[mountaineerxman]

Thanks btrueblood. I was thinking in my mind that they have to do something for airbags, but I didn't kow what it could be other than destrucitve testing. that is a real possibility, although I see it being the most expensive.

 

[Demon3]

Do you have any comments on the cheap, easy, non destructive method I proposed?

Chris DeArmitt

"Knowledge has no value except that which can be gained from its application toward some worthwhile end."
Think and Grow Rich - Napoleon Hill

 

[mountaineerxman]

Yes Demon, thanks. I need to look into it further, but it could be the answer. The big kicker is going to be convincing a customer that I can get legitimate numbers from this method. It had always been discussed that some kind of sensor would be put on the bags to give us data, so I am going to have to do some learning to get up to speed on the science behind it and pitch the idea. I'll let you know if I try this method and how it works out.

 

[Demon3]

The only caveat is that, as you increase the strain (and orientation), the colors change through the rainbow. Then, with more stress, you go through all the colors again. I saw a wall chart showing this years ago. Anyway, the trick is to make sure which color cycle you are in so you know the absolute strain.

Chris DeArmitt

"Knowledge has no value except that which can be gained from its application toward some worthwhile end."
Think and Grow Rich - Napoleon Hill

 

[mountaineerxman]

One problem with the polarized light solution is that it would be nice to have data as a function of time. A little background on the experiment: this cylindrical plastic bag is being used to rapidly plug a breach in a levee. We are simulating a levee breach in a somewhat controlled environment, but it is baically a man made river. A picture of the bag as it is plugging the breach in the levee is attached for your viewing pleasure. It would be valuable to get the fabric tension of the bag at various stages of the process, from deployment and as it begins to roll up on the levee and plug the hole and as the water builds up against it.

 

[Demon3]

Wow. That is quite some device. The refractive index method will of course only work with transparent materials so that's out for a yellow bag.

Chris DeArmitt

"Knowledge has no value except that which can be gained from its application toward some worthwhile end."
Think and Grow Rich - Napoleon Hill

 

[btrueblood]

Chris, mountain -

I also was envisioning a bag made from coated fibers (the word fabric was my cue). Not sure how you could see fringe patterns in multiple fibers, unless I am missing something. I say this with all due respect to Chris, he's a damn sharp guy, and if there is a do-able method for fibers i'd like to know...

 

[Demon3]

Time to readjust to the new paradigm now that we've seen the part.

The pressure in the bag must tell you the stress on the fabric. Afterall, no matter which direction the stress is applied from, it will be exerted equally across the total surface area of the fabric. So, all you need to know is the internal water pressure (from a tite gauge or other simple pressure meter) and the surface area of fabric.

As this is not my strongest area, would someone more mechanical please correct me if I'm wrong.

Chris DeArmitt

"Knowledge has no value except that which can be gained from its application toward some worthwhile end."
Think and Grow Rich - Napoleon Hill

 

[patprimmer]

Back in the days when I did textiles testing, we would have marked the surface with a grid and measured the deflections with a vernier. Electronics had not been developed then.

We also increased load on test samples until they failed then studied the failure.

This looks more like a trial and error and experience type scenario rather than a scientific lab test to me as the conditions under use will be very variable under very poor conditions and at times desperate with an element of panic by the operators. Not a situation for consistent and predictable correct use.

A high efficiency nylon yarn will be better overall than polyester due to it's higher elongation and the retention of properties after weaving and coating.

Software for analysis of car tyre impact performance might help. I only mention this because I guess it is likely to already exist and there are some similarities in how it might be broken in some cases.

If you post in the automotive wheels and tyre section asking f software exists, a guy called capriracer might help.

DO NOT DOUBLE POST THE OP FROM HERE. Ask a specific new question on software and impact testing of coated fabrics. Include your photo as it is big help in understanding our question.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

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[mountaineerxman]

thanks Pat. Is this the ide that you are talking about:

http://www.tekscan.com/industrial/tirescan-system.html

It captures tire footprint pressure patterns. It looks like an off the shelf product that cold be adapter for other uses.
I agree that what I am looking at is more of a science project/research. The folks handing out the money seem to disagree! Good ideas and suggestions from everyone. thanks.

 

[btrueblood]

Chris,

No expert here neither, but I know that curvature of the membrane also plays a role (in addition to the pressure) in the derivation of stress in diaphragms. I'm still thinking about your idea, but extending it - perhaps a pressure-sensitive coating? Could an aluminum or similar reflective layer be applied, then a clear coat of something that provides birefringence/photoelasticity? Hm, like mtnman says, it sounds like a science project...

 

[patprimmer]

I do not know the software, I just guessed that something would be available for tyres as suspension engineers need tyre data to optimise suspension.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules