Steve Evans has done some testing of tarp corner tie out breaking points.

here

In most of the tests the parent fabric tears at the point or along the line where the reinforcement patch ends.

Is my logic correct that enlarging the size of the reinforcement patch will reduce the load where the reinforcement patch ends and the parent fabric begins? If so it seems then that the corner tie out could take more load before the parent fabric would tear.

I think you're right - bigger reinforcement

Maybe a circular reinforcement would work a little better

my composite materials consultant (son who designs/builds airplanes) has explained that in materials layered like that the stress gets concentrated along the line where it steps down from two layers to one and that is why the failure happens at that location. It would SEEM that since a larger patch would have a longer such boundary then the stress would be spread out over a longer boundary and Daryl's conjecture would be correct.

I'll ask.

Since you're glueing the reinforcement patch, maybe it would be better if it had fingers, rather than a smooth edge.

On the other hand, the bulk fabric is going to fail at some point, and maybe the simple triangle shape is fine.

The fact that it didn't fail at the grosgrain stitches means the reinforcement is successful.

At some point the reinforcement patch grows nearly to the size of your tarp.

Jim,

Thanks for the offer to ask your son.

Daryl

i would think that a circular patch roughly the same number of square inches would hold together better , since the boundry would be much longer and spread the load out over a greater area. of course-increasing the size of the patch wouldn't hurt either. interesting to me is the fact that all the catastrophic failures were in the bias direction. it would seem to sugget that if you were looking to make the most of the cuben, it would pay to re-orienate the cuben to have the strands in the same direction of pull from the lines.

Edited by the_gr8t_waldo on 12/26/2011 16:52:47 MST.

Many many flaws in this persons understanding of material engineering.

constant and impulse forces are different and materials handle them in different ways. He seems to want to test impulse but then tests for constant force?

He completely ignores HOW the material failed, only noting that it failed "catastrophically", which is a useless information point.

I'd like to see this test done again by someone that understands what they're doing, such that a better understanding about the failure modes can be made.

I'm also interested in knowing, from field use, the failures of cuben fiber tarps, again, I don't think constant force damage is a concern.

Steven,

Might you be interested in doing some testing and reporting your results? Sounds like you might have some good knowledge in the area. I know Steve Evans is very open to feedback, new ideas, etc.

Daryl

I don't have (or access to) the equipment or materials for testing like Mr Evans seems to. I would be more than willing to help him formulate better testing for what he's looking to accomplish.

It may be better for him to befriend a local mechanical engineering professor at his local state University, as well as read some books on material engineering and material chemistry.

Without going there, the best things he could do though, to really understand the failure modes of the material and designs are to:

- Obtain highest frame-rate camera he can "afford"/find to record the process by which the failures occur.
- Repeat the tests to allow for variations in material and design manufacturing to be eliminated, he admits this in his reports.
- in the case of these corner tie out tests, to differentiate between constant force failures and impulse force failures.

I've long thought corner tie outs are poorly engineered, but the only better solutions I can think of would probably be much more costly to manufacture. If one was really being a gram weenie it could be worth it, but I think with the price of cuben most would rather go for the cheapest solution that works well enough.

Hey Steven, just wondering what kind of corner tie outs you'd design. I know this is theoretical for you, but I'd certainly benefit from your knowledge.

Jim: I concur with your son that stress concentrates when you have a discontinuity. One helpful way to visualize stress is as if it flowed like water and you want to AVOID the turbulance created by sharp edges and other transitions.

For dynamic stresses, imagine waves rolling through your materials. You want the waves to disapate quietly and not crash like ocean waves do when moving suddenly from deep to shallow water.

One easy-to-cut and light-weight way to do reinforments that spread those stresses from one material to another more gradually would be in a star pattern with very long legs to the stars:



This is only a mock-up. I wouldn't use duct tape for real, but it made a nice contrasting color.

Editted to add: I'd probably do this 3-pointed style if I was sewing materials. With iron-on materials, I might go to a 5-pointed star. Note that any of these give you some substanial thickness right at the corner when you place a grommet or punch/burn a hole for a guyline. Which is a good thing because that is an even higher stress point sometimes.

Edited by DavidinKenai on 12/27/2011 11:24:14 MST.

That's actually one I've thought about, though more in the reverse direction (have the reinforcing material star out from the tie-out point. It started with the idea that an arced taping would probably be better than the typical triangle, but then I realized you can both reinforce the mother material a longer distance from the tie out and place more of that force on the tape using the same amount of tape material by going with a long-armed star pattern.

One thing Suluk notes and David makes note of, is that the tapes seem to work better than sewing a second material. The points where the thread punctures the material becomes a stress point and allows for a high failure rate. A strong tape will spread this force over a larger area, and possibly be easier to manufacture?

I would also go with ~5 "star" points for a 90deg corner, and I would probably have the points that extend along the material edge be longer as I imagine those lines of force are more important.

Again, this is probably more costly since there is more wasted tape material, but for weight concerns, and spreading out the load bearing more a better design.

If I had more understanding of how and when the tie out points are failing for people with cuben tarps I could probably make some better suggestions.

What is the weight per sqyd for the typical tapes used for cuben tarps?

All the tests are in pulling at the same angle, one which I don't believe to be the true direction of real world forces. Though probably a good "worst case" test, I don't know how much it helps solve a problem that occurs in the real world.

Darryl,
To put it simply, yes, the larger re-enforcement patch will be stronger.

Steven, thank you for the wonderful things you wrote about me and my knowledge. They were all quite offensive. I just hope the BPL community recognizes how ridiculous your advice and scrutiny of a 30 dollar/3 hour test setup is. It's a tarp tie out, not the space shuttle dude. You do realize that I post all this info online to help people build their own gear and have fun??? not teach them about "impulse and constant forces"...backpackers don't care about that stuff. Feel free to perform your own tests, and please post them if it will help us push the envelope even further, I always encourage it, but let me guess...you don't have the time, resources, etc....I love that excuse the best.

Just a heads up, the star design is a great idea and has been done. Do some searching and you will find it. I'd post a link for you but I'm in South America until end of January and want to get back to my beer and beach towel.

Edited by Steve_Evans on 12/27/2011 23:46:59 MST.

Here's a sample of a reinforcement patch where I'm trying to implement the star or finger ideas presented here.



Black fabric = parent material
White fabric = reinforcement
Fingers = double stick tape

I wouldn't extend the double stick tape beyond the reinforcement. I only extended it so you could see the five finger pattern.

I sewed the corner loop on but did not sew elsewhere.

I think the 5 pieces of double stick tape could be replaced with stitching.

I have a vague memory that this entire issue has been addressed before on BPL myog forums. Can't recall for sure.

Daryl: That looks good. You've got great thickness at the attachment point, no single seam transitioning from patch to parent, and a gradual extensions further out into the parent material. And you've used minimal material/weight to do it.

Do extend the reinforcement beyond the corner triangle. That's where you get the benefits of a more gradual transition and less weight than a very large triangle.

My tapered arms are slightly more weight efficient and probably better with dynamic stresses but are definitely more labor and at some point, you're just gilding the lily.

Time spent sewing means less time spent hiking.

Steven Adeff, your comments were rather insulting (whether intended or not) to an inspired set of tarp tie out tests and subsequent web posting done by Steven Evans. he answered so many questions that the MYOG community has. His research and explanations were done such that all but the most imbecilic of us could understand what he did and how those results can help us manufacture stronger gear. If you have better suggestions I would recommend that you contact him directly rather than publicly insult him as you did.

Steven Evans, Thank you for all your research and non-stop pushing of the envelope for what can be accomplished.

john the xcar

Look at how they make the cuban sails

Notice the reinforcing patches on the spinnaker clew in the photo.

Craig, what you see there is a type of well engineered joint in that it tapers from the joint out, and the area near the joint is the strongest. If I were to do this with cuben fiber, I would use the cuben with the thinnest mylar, something like 0.33 oz/yd and go out further from the corner than usual, and use multiple layers with the highest number of layers being closest to the corner. Kind of like:


corner
_______
______________
_______________________
_______________________________ tarp fabric ↓
==================================================================

That sail also alters the shape of the reinforcement, so they must be accounting for increased strain in certain directions. The same could be done for tie outs, but I doubt it's worth the effort to figure out the optimal shape.