Polycryo Strength Analysis

[Dan Johnson]

So after making my tarp out of this stuff I figure I better do some testing on it since not very many people have tried to use it as a tarp. Maybe they're smarter then me :)

The things I wanted to find out were mainly were:

1) What is the best type of tie-out?
2) How much Tape is needed on the tie-outs?
3) What is the tensil strength of this material?
4) How does this material act in stress loading?
5) How strong is the double sided tape provided in the box?

Please note that I am not a scientist but rather a very curious frugal person. Nothing I did is very scientific but it is better to have "some" data then no data at all. And of course I took lots of pictures!

All test smaples were about 3" wide and 20 something inches long. I first started out using a tie-out that I have on my tarp that I made. It's basically the tape folded over itself with a hole thru it. The tape used was Gorilla Glue Tape.

This tie-out ripped with pretty minimal effort. It still took a good pull but I thought it would have done better. This is what it looked like after.

I then tried a plastic washer sandwiched between the tape. This was an idea given to me by another member here and I'm sure happy I tired it out. Sure enough the plastic washer that measures 3/4" diameter worked like a charm. So well in fact that I used it exclusively on every test here after simply because it wouldn't rip out.

Next I needed to know how much tape to use on the tie-outs. This is very important to know since the tape is the heaviest part of the tarp. I first started out with a 4" piece of tape folded over with a contact length of 1" on the tarp body. This allowed the other inch to be used to sandwich the washer. You can see this on the photo above. I then did the same thing but used a 5" length piece of tape folded over. I kept the same washer distance but now I had a 1.5" contact length on the tarp. Then I strung up my super scientific measuring tool to see which side would pull off first.

Sure the test area looks a little "elementary" but hey, it worked. In the photo it looks like the bag is touching the ground but it is actually about 2" from it. What I found out next surprised me. I started loading up the weights. First 5lbs then 10lbs then 15lbs then 20lbs etc… it finally failed at 35 lbs. And it was the material itself that broke. Both tie-outs held beautifuly. The material upon failure made a loud snapping noise that was pretty loud. The polycryo had sheared directly in a straight line across the middle of the sample. It should be noted that on every sample conducted the polycryo sheared off in the very middle. Not near the tie-out, but in the very middle. I found this pretty interesting. Upon further review I can tell you that when loaded the polycryo does stretch. Each sample stretched about 6" before failure. It should also be noted that I measured the distance each sample of polycryo slipped from the tape. And interesting also was that there was not any slipping of the material from the tape on any test.

So after that test I could conclude that both tie-outs were overkill for the material. I then tried the test with a different tie-out. This one is a 1" piece of tape that is 4" long with the washer with a contact length of 1" on the polycryo.

The test on this tie-out was only about 20lbs. I'm not a scientist (remember), but I think that it failed sooner due the concentrated stress it applied to the very center of the sample. Versus the previous samples that were done with 2" wide tape and were able to distibute the force thru alomst the entire sample width of 3".

Here is a photo of what the polycryo looks like under 30lbs of force.

After the foroce had been applied I removed the load to examine the material. Sure enough it stretched a little but it didn't show too much signs of deformation.

Next I wanted to see how the material did when I applied a single line of it's double sided tape that is included in the packaging on the sample under stress. I figured if it improves the strength then it could be useful to help out critical areas.

The test sample held about 40lbs and just like all the other tests it failed in the very center. Here's an after photo of the stressed piece.

The next test was to figure out which is stronger: The polycryo or the doubled sided tape. I made the same 4" tie-outs on the sample and applied force till failure. I cut the sample in half and then rejoined them with the tape. I then applied force till failure. I found out that the double sided tape has a stronger hold then the material. The polycryo sheared in half on one of the sections and the taped connection looked just like new without any slipping of the material on the tape.

I hope this series of tests shines some light on polycryo for everyone. I know it did for me. I guess what I can conclude is this. My original tarp used WAY TOO MUCH tape and the tie-outs aren't nearly as strong as they could be if I used washers. For all further polycryo shelters or anything I make with the material I think I'll "hem" the edges with the double sided tape as it showed it is stronger then without it. Also, I'm sure someone could use another type of tape to reduce weight but Gorialla Glue Tape is what I had laying around and from everyone at Home Depot they said it was the strongest they sell.

Now, Lets discuss!

[Nathan Lare]

That is really good information. Do you plan to perform another test to simulate your setup (an entire section of the film with a tieout on each end…then suspend it over a line the simulate your setup and fixing one tieout and applying weight to the other tieout)? It would be interesting to know if it would fail at the ridgeline, or half way down one of the panels.

[a b]

Really nice testing.
I was wondering something simliar to the above.
How would polycro hold up to being supported by a pole such as a pyramid.
I am thinking you would just reinforce the area where the pole contacts the polycro with the tape.
Or perhaps a tape pull out at the peak with a washer that fits over a trekking pole tip.
At any rate, this is really interesting stuff.
35 lbs of force is quite a lot of shear strength for such an inexpensive material.

[John Donewar]

I'm curious as to what a piece of polycryo set up as say an A frame 6' x 8' with hemmed edges but an unreinforced ridge line would fare out. In particular I wonder what pressure the material would have to be under at the ridge line in order to achieve a tight pitch.

Since the material does stretch when under pressure getting a tight pitch may not be that difficult.

If memory serves, sunlight a.k.a. ultraviolet light will shrink this material to a point. I'm wondering how it will affect its strength and elasticity after being exposed to sunlight.

What kind of plastic washers are those? Are they available at Home Depot? Did you source your polycryo from Home Depot? When I ask for it by name at my local "Depot" all I get is blank stares. If I ask for the clear window insulating film the stares continue. Realize that I live in Southeast Louisiana and we're more familiar with trying to stay cool. ;-)

Party On,

Newton

[Steven Evans]

I'm actually glad you're not a scientist :) You have done some nice work here.

35 lbs really is quite a bit. Didn't BPL just post an article saying something along the lines that a tieout should withstand 15-20 lbs. I could be totally wrong, but I thought I read that on here.

Anyway, of particular interest to me was how much it had to stretch before breaking. It seems that would work in its favour during sudden wind gusts. I also wonder how much you could stretch it and have it return to it's original size (ie. how far can it stretch before being permanently deformed)

Again, nice work.

[Dan Johnson]

John
Yes, I got all my supplies from Home Depot. The washers are just the cheap ones they sell in packs of 4 on aisle #24 and they cost a whopping $.64. When trying to find the Polycryo ask for the Frost King (the brand name) window insulation kit. It's around $7 for the 7×9 kit.

[Bob Gross]

"Did you source your polycryo from Home Depot? When I ask for it by name at my local "Depot" all I get is blank stares. If I ask for the clear window insulating film the stares continue."

Newton, my local Home Depot store doesn't stock those items either. It has to do with mild weather localities. However, you can order a package from Home Depot online. I think the plastic is 62"x210". Mine was delivered just last week.

–B.G.–

[John Donewar]

Thanks D.

I see some experimentation of my own in the near future. ;-)

Party On,

Newton

[Daryl and Daryl]

D Johnson,

Thanks for all your testing and taking the time to explain what you did and why.

I will definitely be using that imbedded washer idea for tie outs.

Perhaps the double sided tape could be used as tie-out material instead of the, as you say, overly strong gorilla tape? Hmmmmm, but how to deal with the double sided stickyness of it?

How about rolling the edge of the polycro into a hem with the double sided tape and an imbedded washer?

Daryl

[John Donewar]

Thanks Bob

I appreciate the info.

Party On,

Newton

[Dan Johnson]

Daryl

I'm not sure if hemming the edges enough to sandwich the washer would work as well. The tape I used was 2" wide and allows for a greater contact area to dispurse the load. The polycryo may not be strong enough to have the small washer pulled against it for very long. But then again, I didn't test for that. Hmmm….

[Bob Gross]

Would it work better if the plastic washer was larger and thinner, and still held by tape?

–B.G.–

[Dan Johnson]

Steven

Unfortunately I didn't measure the before and after lengths to determine how much it stretched. I just measured how much it stretched during loading. From my findings it seems that the Polycryo does show signs of post stress but that it was pretty minimal. I also can't say how much it returned to it's normal state after loading since all my samples failed.

[Dan Johnson]

Bob

The washer was pretty thin. It could have been larger but this one had a hole in it that looked like the best fit for my guy lines. But if I could have found a thinner one I probably would have used it. I was thinking of making my own washers out of a plastic milk jug but I found these to be real convenient.

[Bob Gross]

"The washer was pretty thin."

It's time to get out your micrometer for measurements.

Yes, it is surprising how many things you can make out of milk jug plastic.

You can start by comparing the thickness of your washer to the plastic of a milk jug.

We might make a scientist out of you, but the chances are you will end up as an engineer.

–B.G.–

[Ty Ty]

Bolts and washers, basically everything in the hardware aisle at Hime Depot and Lowes is really overpriced. I bet for a couple bucks at Fastenall you could get a pack of several hundred if not a thousand of those plastic washers.

[Michael Ray]

> I guess what I can conclude is this. My original tarp used WAY TOO MUCH tape and the tie-outs aren't nearly as strong as they could be if I used washers. For all further polycryo shelters or anything I make with the material I think I'll "hem" the edges with the double sided tape as it showed it is stronger then without it.

Exactly as I figured. :) You can go with smaller washers, too, if you can find them. As I mentioned elsewhere, I don't use washers at all but make a loop of the tape only 7/16" wide to attach the guyline.

What I did not know yet was how the polycro would deform under tension so these tests were great for that. Thank you for taking the time. I'm surprised it stretched so much though I'm sure it's far less than the Heatsheets do.

I'm curious about the breaking point. Interesting that it always broke in the middle. Is it a clean or ragged break? I wonder if polycro has a bias. Could you repeat a test in a direction perpendicular to those you did?

I believe I read that polycro tears fairly easily. Have you tried tearing it on an edge?

FYI, the Duck brand is available at Walmart for $9 (last year) and is 7×10. I got them on clearance last spring.

[Dan Johnson]

Bob

I can't seem to get my hotmail account working to upload photos (I always take photos from my phone and have to email them to myself and then upload them to my computer and upload them to BPL) but I can tell you that the washers are exactly the same size and thickness of a standard US penny.

[Michael Ray]

> How about rolling the edge of the polycro into a hem with the double sided tape and an imbedded washer?

That might actually work. One of the original versions George Geist did used brass grommets along the reinforced edges.

[Tom Clark]

I am a materials scientist and got my Ph.D. in "how things break," so I will tell you that you have done a very nice job in testing the materials and different configurations. Some additional tips…
– As someone else suggested, test in different directions. Film can have very different properties in the two directions. The diagonal (i.e., corners) can give averaged performance.
– Wider is better (tape, washers). Speading out the load will provide a lower local stress.
– Can you share some more photos of where/how the material failed? Understanding that helps suggest ways to make it stronger/lighter.
– Have you thought about doubling up the Polycro somehow (maybe some adhesive between layers). If there is a directional nature to the film, you could cross laminate the reinforcement.

That cross-linked polyolefin (polyethylene or polyprolylene???) is amazing.

Tom

[Dan Johnson]

Michael:

Each sample test had a very clean break. And even more suprising was that each break was extremely perpendicular to the ground…

You asked for another test in the other direction. Would that mean that you would want a test to be performed with the tie-outs on the long side? Rather than the short side?

I have found recently that Heatsheets stretch far more easily then Polycryo. I haven't done testing on the Heatsheets but from how they feel when I try to pull them I would say Polycryo is stronger. On the other hand Heatsheets would give a lot more if a gust of wind came which could save your shelter. Polycryo would either hold strong or either rip violently. Kind of like carbon fiber.

@Bob

I went to school to be a Structural Engineer butI changed majors half way thru and always disliked science. I'll stick with the engineering side of things :)

[Bob Gross]

"Structural Engineer"

Remember what they say. Aerospace engineers make flying weapons systems. Civil engineers make targets.

–B.G.–

[Richard Nisley]

D. Johnson,

Really interesting analysis… thank you!

Just using engineering specs, the following appear to be the theoretical (smile) limits for the materials you are using:

Polycryo – (Standard Shrink Film) 12 microns thick yielding an average tensile strength of ~25.70 lbs. for a 3.00" strip and 28.40 lbs. for the 3.32" width that your photos indicate.

Gorilla Tape – Sold as 2" but, it averages 1.88 inches wide, yielding an average tensile strength of 94 lbs. for the linear inch of adhered surface on your shortest test sample.

Polycryo comparison to nylon – For the same thickness as your Polycryo (12 micron), nylon would be ~2.63x stronger.

1.1 oz. Nylon 6,6 + .25 oz. silicone coating (~480 micron typically used for UL backpacking shelters) – This conventional shelter material would be in the general ball park of 400x stronger.

[Daryl and Daryl]

D Johnson

You and others (e.g. Michael Ray)contribute a lot of good info with these low cost but time consuming experiments. At $30 a yard for Cuben it is hard for most of us to be loose and creative with that fabric. Many of the lessons learned with the cheapo stuff apply well to working with the more expensive but stronger materials, however.

Thanks.

Daryl

[Dan Johnson]

Thanks so much Richard for jumping on this thread to give your expertise! I was wondering what the tensile strength was for Polycryo and now I know thanks to you.

Since my test findings, I think I'll stick to making Polycryo tarps until I can either get the funds for some "real" material or I just happen to stumble upon someone looking to ditch their existing tarp.

It will be interesting to start seeing peoples experiments using polycryo in the field and not just for ground cloths. I can totally see this being a cheap way to add an awning or beak to a shelter. Or just close off one side of an existing tarp to protect from spray.

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