Strain gauge for measuring strain in fabric

[Speakerguy]

Hey guys,

I need to measure dynamic strain in a piece of fabric. Essentially I have a fabric sample that will be fixed at the top and bottom, and the bottom will have a weight attached. The fabric will be untensioned and the weight will be dropped. I need a transducer that won't affect the way the fabric behaves but at the same time accurately measures the strain as a function of time. Most of the strain gauges I have found are for solids (steel, aluminum, concrete). Maybe I am looking at the wrong type of measurement transducer, so I thought I'd ask for ideas here.

Thanks for any help!

 

[dknguyen]

Strain is deformatnion and stress is force right? Maybe you could measure the length somehow (is that what you're after). Laser comes to mind but that's completely unfeasible for the homebuilder. Maybe you could use a really long fabric sample so that the change would be noticeable and just measure the length with simpler means, somehow...

I gots it! How about have a roller at the edge of a table and the cloth is clamped at one end and the cloth goes over the roller, bends 90 degrees towards the ground with the weight. Then you could measure how much the roller rolled to figure out the change? Or wait...does strain work that way? It might not...or a motorized roller that can measure the force it's putting out...

 

[jpanhalt]

What type of fabric and what color (type is more important)? I have never done such measurements on fabric, but I can think of three approaches. 1) The reflective properties of a fabric like polyester might change under strain as the surface changes. So, measuring the relective angle with a laser might give an indication of stretch. Unfortunately, the fabric would have to remain still (not wobble) during the test. 2) As you stretch a polymer like polyester, the internal molecular alignment changes, which affects the dielectric constant. Also, the distribution of a charge on the surface, such as static, would change. Thus, a capacitor with one plate on each side might show a change in capacitance. 3) If the fabric strands are fairly large and clear, look at them through cross-polarized lenses and a magnifying glass. Strain (and stress actually) will show as colored bands. This method is used for examining stresses in glass.

My off-hand guess is that approaches #2 and #3 would be more likely to work, and #3 would be most likely, if the strands are big enough to see.

It's an interesting problem. Please let us know what you find out. John

 

[ecerfoglio]

Hey guys,

I need to measure dynamic strain in a piece of fabric. Essentially I have a fabric sample that will be fixed at the top and bottom, and the bottom will have a weight attached. The fabric will be untensioned and the weight will be dropped. I need a transducer that won't affect the way the fabric behaves but at the same time accurately measures the strain as a function of time. Most of the strain gauges I have found are for solids (steel, aluminum, concrete). Maybe I am looking at the wrong type of measurement transducer, so I thought I'd ask for ideas here.

Thanks for any help!

if the fabric is firmly attached to the frame (top) and weight (bottom); it should be easier to measure the weight's position instead of the fabric's strain.

The sensor type deppends on the desired resolution, it may be a sonar, a potentiometer or an optical encoder driven by a wheel with a string arround it, etc

you may even adapt an electronic "vernier" caliper to get up to 5/100 of a milimeter or better - but I don't know how many readings per minute it will give.

 

[Speakerguy]

Man jpanhalt, you sure are close to what I am doing!

It is parachute fabric (synthetic). As strain (deformation) increases, the refraction of light through it changes, and there are actually two indexes of refraction due to the polymer structure. Short story is, we get a hyperspectral absorption plot for a given amount of deformation. We just need to correlate a given amount of strain to a particular absorption spectrum, hence the need to measure strain while we take the absorption measurements.

We're only doing this for a very small section of fabric right now, but the idea is to be able to do it for an entire parachute at once to analyze the forces involved when someone jumps out of a plane and deploys it. And hopefully make better parachutes for the boys in the service and whatnot. That's what the government is paying us to do, anyway.

In any case, my boss directed me towards the type of thing he wants to use, but I haven't found a particular part that is suitable. So this turned out to be a moot post since I posted before I had today's meeting with him. I just have to poke around a little harder.

 

[jpanhalt]

Man jpanhalt, you sure are close to what I am doing!

It is parachute fabric (synthetic). As strain (deformation) increases, the refraction of light through it changes, and there are actually two indexes of refraction due to the polymer structure.

The polarized light effect is called birefringence (different refractive index depending on direction). It's very useful for identifying certain crystals and ordered polymers.

Since the sky is about 5% polarized, as I remember, you could wear polarized glasses and test your concepts in the field under a real canopy John

 

[Speakerguy]

Yep, birefringence is what we're doing. My boss is the physics/optics guy who got this funded, I am a mere electrical & electronics engineer that attempts to get him the data I found a standard strain-relief measurement device that would do 15-20% max deformation, but it was meant for metals. Boss says we need about 10% minimum. I'm worried that the measurement device itself will affect the measurement of the fabric since it was intended for other substrates.

I also though of deriving position of the weight that will be attached to the fabric using an accelerometer and then deriving position, but the boss shot that one down due to his experience with accelerometers not liking 'jolts' such as would be experienced with this experiment. Any thoughts on that? I know the double integration will amplify errors but I hadn't heard concerns about accelerometers like my boss voiced before (but I've only seen them used in audio).

 

[ericgibbs]

hi spkrguy,
While looking on the web for a question set by an OP today, found this device. GP2d12
http://www.oopic.com/gp2d12.htm

Consider it say floor mounted, looking up towards the base of the 'test weight'.
Release the weight, feed the analog voltage out from this device into a PIC's ADC input. The voltage out from the GP2d12 device is almost prop to distance of the weights base to the sensor?

Whatcha think.

 

[bobledoux]

It sounds like you are trying to replicate "Federal Test Method Standard 191b, Method 5100" for grab method test for textile strength and elongation. That method employs a one inch wide set of jaws placed three inches apart.

The Vishay 500BH or 500BW are 0.7 inches wide in the gauge area. The 20CBW is two inches wide. The challenge would be to mount the gauge effectively on a fabric surface. If the test fabric is silicone coated, like Soarcoat, finding an effective adhesive might be a challenge. The gauge is also not intended to measure materials with high elongation in the stress-strain curve.

The gauge might be mounted on a second material, with known stress strain characteristics. This material would be adhered to the test sample. The second material would be intended to allow the test fabric to elongate while transferring the load to the gauge that has little elongation. This would reduce the problem of the gauge being a stress riser on the fabric and it would provide a stress averaging function on the test sample.

Making such a test unit might also allow field installation for real world testing.

 

[JimB]

Several posts "on hold" pending moderator discussion.
Meanwhile the thread is locked.

JimB

Contentious posts deleted.
Old thread remains locked.

JimB