Polycryo Strength Analysis

> Would that mean that you would want a test to be performed with the tie-outs on the long side?

That would be one way, but I would prefer if you were able to cut a piece of similar dimensions but in a perpendicular direction to those you used in your initial testing. That would eliminate any other variables except for a potential bias within the material.

Heatsheets do give quite a bit. Not an issue for most things except snow loading. I've determined to test that if we ever get any decent snow this year, but I suspect it would be significant.

I think polycro would work much better in that regard. It does give but not anywhere near as easily. I would not be concerned about the polycro ripping in the wind. Keep in mind you're loading only a thin piece of it vs an entire panel. It would eventually get to the point of failure but that will be very strong winds. It should easily take 50 mph if you have it fastened down well.

I'm curious now how mylar space blankets would fare in your tests. If Richard checks back, maybe he knows the tensile strength for that, too. I know those will tear catastrophically as well. I avoided using that material since it seems easier to poke than either LDPE or polycro.

> 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.

2.63 * (480/12) = 105.2 <> 400. Not being a materials scientist, is this discrepancy because tensile strength isn't linear with thickness of the material or that silicone somehow adds about 4x the strength to the nylon?

If I understand what you're saying here, Richard, if we assume 30 pounds to break the polycro sample, it would take around 12,000 pounds (30 * 400) to break a similarly sized piece of silnylon. I really find that hard to believe. I'd be amazed if it could even handle 3,000 pounds of force. If so I'd agree with that other thread the says our tarp materials are WAY too strong!

Michael,

You said, "I'm curious now how mylar space blankets would fare in your tests?" – Theorectically they will provide ~6 lbs of tensile strength per in2 surface area.

You said, "…it would take around 12,000 pounds (30 * 400) to break a similarly sized piece of silnylon". If the nlyon was used in a monolithic sheet, the same thickness as the Polycryo, it would be ~2.63x stronger.

A conventional UL silnylon shelter is not comprised of a monolithic sheet but woven threads with a significant interstitial space and surface films comprised of silicone. In my original post I quickly ball-park estimated silnylon's woven tensile strength. I computed its relative thickness compared to the Polycryo (400x). That seemed plausible because high tenacity woven nylon strapping’s tensile strength reduced to the thickness of silnylon yielded (350x).

After your above question, I looked up the US government specs for type 1 and type 2 parachute cloth tensile minimum strength; they were 85 lbs. and 135 lbs. respectively. If we use these values as the low range estimate, then the 30D silnylon would have (9.9x to 15.8x) more tensile strength than Polycryo.

Thanks, Richard. That seems much more reasonable. However, I'm sorry to say I'm confused by your mylar figure now. Only 6 PSI?

Polycro is 0.5 mil apparently just like some mylars while the Heasheets are normally 1 mil. So if they are the same thickness, it seems that your figures suggest polycro has far greater tensile strength than mylar. Indeed it appears polycro is over 15000 psi while you said mylar was about 6 psi. A quick search seems that 27000 psi may be in the ballpark for a 50 gauge thickness, which I think is about 12 microns. Of course, there are a wide variety of mylars and cross-linked polyolefin so I have no idea which ones we are using.

Michael,

The Space Brand Emergency Blanket from MPI Outdoors is .00048" thick. The material is spec’ed, by the manufacturer, as having a tensile strength of 12,500 lbs./sq. in. So, (0.00048/1) * 12500 = 6 lbs. tensile strength per sq. inch surface area.

This is in contrast to the 8.75 lbs. tensile strength per sq. inch surface area for the Polycryo. Note that the standard Polycryo thickness is 12 microns = 0.000472440945 inches. The two materials are for all practical purposes, the same thickness with only a small difference in tensile strengths.

Not forget about tie outs , I'll be making a cuben tarp with none . The tension will be taken by a dyneema thread sandwiched to the tarp and acting as a rope under the tarp. It may be necessary to reinforce the attachment of the ' rope' at the edges but most of the load should be taken by the 'rope'. This will be a simple tarp used as a cook shelter , but I'm sure more elaborations are possible.

Thanks for being patient with me, Richard. It seems an issue with units. I would have said that was 6 pounds per linear inch of the material, not per square inch since you multiplied by the area to arrive at that figure. I had assumed that's what you meant but you're known for being precise. :) I am surprised polycro is a little stouter in this regard. Seems an optimal shelter material for me except being transparent, which some might see as an advantage. Not an issue unless the moon is out.

Another experiment I'd like to do at some point is see how its properties change once it's been shrunk. Not sure what temp that takes but can't be too high since hairdryers work. I wonder if setting it in an oven at an appropriate temp would work or if leaving it folded would cause issues.

Michael

That's my next experiment with Polycryo. I want to shrink a known size (maybe 1'x1') and see how much it shrinks if I just leave the heat on it. Then I want to see what it's strength is under loading. I'm curious to see if it's more elastic then before. I guess there's only one way to know!

Alrighty guys and gals. I had some time tonight so I thought I would do another test before I head to bed. I thought I would see what happens if a pre-shrunk a piece of polycryo and then stressed it.

Just to be super scientific I wanted to see how much this stuff shrinks under the heat of my wifes cheap-o hair dryer. So I set up a 1'x1' section and put it on the ground to shrink it.

Then I shrunk it (I removed the tape prior to shrinking FYI)

After about 4 minutes this is what it looked like. It shrank almost exactly 2" in both length and width. Making it 10"x10".

Then I took a test strip and put it to the test. You can see from the photo that the polycryo really doesn't shrink evenly. I had my doubts testing it cause it looked like one side of the sample seemed to shrink more then the other. But the test most go on :)

As I loaded the sample I noticed it stretching pretty quickly. More so then the orignal non-shrunk samples. This stress test showed that the shrunk sample failed at about 22 lbs. Far less then the original non-shrunk 30lbs.

Here is a close up of the sample where it failed. This is what every sample looked like after it failed.

Also, just in case anyone was wondering. These are the plastic washers that I am using.

Edited for spelling

Cool. So it's not be as robust after being shrunk. Good to know, but can't say that I'm surprised.
It's possible it may weaken over time (exposed to hot sun), but doubt that would be significant. I think I'll devise a test for that once summer hits.

It does look kind of funny after it's shrunk without being held down, doesn't it?

That is a nearly perfect tear. Have you had the chance to test a perpendicular sample yet?

D. Johnson,

Thank you for the continued analysis… really interesting!

This is soooooooo much more interesting than watching tv.

Thanks again for the show.

D Johnson, since you're in the testing groove, I'm wondering if you would want to test a more complex tieout, say, a star shape of tape, say, 2 or 3 pieces, at angles to each other.

The failures you are getting are almost identical to the way cuben failed in a test of catastrophic failure:
http://www.suluk46.com/documents/Suluk%2046%20-%20A1%20-%20Testing%20Cuben%20Tieouts.pdf

ie, it rips right at the flat edge where the force of the stronger material meets thee weaker.

I'm curious what happens if those flat edges of force get changed to angled edges, if you know what I mean.

Polypro like stuff I think can be used for some applications, and it's a lot cheaper than cuben.

By the way, Ace hardware has rolls of this stuff, Bulk Shrink Film, item number 5069190, 25 feet by 62 inches. Cost about 12 dollars.

Not sure how many mils it is, probably the same as the other window insulation films.

ziff house, I'm intrigued by your dyneema tie out idea, can you give more details, or pictures? I got some cuben from lawson at his cuben sale with the very same thing in mind, a kitchen/ rain cover tarp, but I have been waiting to make it due to the cost of the material, want to get it right the first time. Since it's an addition to the winter setup, not a sleeping tarp, I want it as light as possible, and as simple.

WOW I was just thinking of this, and came here to post about it.. glad I saw this first!! So have you done the most important test yet.. using it on a rainy night in the woods? I'd be super interested to know the results. I've heard this material can really take a beating, and at $6 for a 6×8" piece of it, if I get two-three months out of it (of everyday use that is), I'm happy. I read a review from someone who used a single sheet as a groundcloth for an entire PCT thru hike, that really says it all!

Gregory:

I haven't been able to find the time to take it out in the woods and play with it but I've had it setup at my place for a while now and it's seen its fair share of wind (more like breezes) and rain (lots of rain). It's held up really well. I'm not sure if I said it earlier but I only "need" this tarp to last about 10 nights worth. That is the most I will do in two years so I don't need something that will last longer than that per say. Who knows, maybe I'd want to go back to tents and this way I'm not out a bunch of money. But, so far I think I've found something that really works for me and could work for probably a bunch more. The key is just to know the limits.

@D Johnson: It should in all fairness last a lot longer than ten nights on the trail, right? As tough as this material seems to be, I'd be surprised if I couldnt get months of continuous use of it. I have plans to make a custom shelter out of polycryo, will post pics when I do. Thanks!

"The key is just to know the limits"

D,

I agree with your comment. I want to have a good idea when something is going to break, tear or come apart. I can then add a margin of error or prepare for the eventuality of the failure in the most weight/cost effective way. Your tarp is perfect for this because we all know it is at the very edge of failure now or in the eventual future. But when will this failure occur and what will be the weakest link that gives first? Inquiring minds want to know. For me it is like watching a serial movie.

The area where I've gone the farthest with this "testing the limits" strategy is my myog backpack. I keep going lighter and lighter on my way to the eventual breaking point. The fabric, for example, has gone from 6 ounce pack cloth to 1.9 ounce nylon to 1.1 ounce nylon with no failures yet. Next version will be out of M50 nylon which weighs about 3/4 ounce per square yard. Regardless of the outcome I've already concluded that most packs are way overbuilt fabric wise. Most of us aren't scrambling over abrasive granite or running through thorn bushes.

Keep up the good work. I've purchased a season ticket for your adventure.

Daryl

Just as an example of how expensive Lowes and HD are on stuff like those washers…

This is probably not the same washer exactly, I can't tell what the ID of that washer is but here is something not too far off at Fastenal…

http://www.fastenal.com/web/products/detail.ex?sku=11107626

They are $0.042 cents each so 100 of them would cost $4.20 cents. Lowe's and HD probably charge $1.99 or more for 4 of them. My brother and I build 4×4's and we use a lot of grade 8 bolts. The first time we discovered Fastenal we realized that for the cost of 1 grade 8 bolt at Lowes and HD we could typically get 10-25 of that same bolt at Fastenall. Some of the smaller stuff at HD and Lowes that came in bags of 2 with the nut and/or washer we could get 100 of the same thing at Fastenall for the cost of 2 bolt/nut/washer combos at Fastenal. It is ridiculous what they charge you in the hardware department at big box home improvement stores. Heck most everything is way too high. I have found the same thing with a lot of their chemicals like insect stuff where you pay X for one small can with a couple ounces of X spray and you can get a 20 gallen drum of the same stuff at a do it yourself pest control store for the same price.

(rant over)

Regarding length of time until failure. I recall another thread or article saying the tie out tape will slip from the tarp fairly quickly (ie less than 7 days). The solution was to put some staples through the tape. This could also be a difference when using Gorilla tape.

DJ you are an inspiration. I'm a tent sleeper and this thread has given me a method to experiment with a tarp.

> Regarding length of time until failure. I recall another thread or article saying the tie out tape will slip from the tarp fairly quickly (ie less than 7 days). The solution was to put some staples through the tape.

Staples? I'd never do that. Tape should be fine if you use something that is UV resistant. Gorilla Glue tape seems to be. 3M TRANSPARENT Duct tape is. Normal duct tape is not.

You could sew the perimeter of the duck tape then seam seal it maybe. Might be overkill.

I really liked the idea of sandwiching the washers between a folded hem on the edge of the tarp. I would be curious to know how the strength would compare. Possibly with a glued hem.

NOT a fun experience. :) Really gums up the needle so you need to clean it about every foot. I had tried to do my shelter totally without sewing. That is easy without netting, of course. The tape wouldn't hold the netting though so I sewed it on. My next test with netting will use silicone or an adhesive to bond it to the LDPE or CLP.

The taped washer inspired me to try this as an option.

The photos above show a 2" wide piece of clear Frost King tape folded over at each end 2". One end has no reinforcement. The other end has a small .3gram piece of vynal from a notebook enclosed within the tape. A hole was then punched in each end with a paper punch. When I hung weights from the item the unreinforced end tore with 14 lbs of load.

I then made a new piece of tape with reinforcements at each end and tested it. The reinforcements held but the tape started elongating at about 19 lbs. Ended test. Photos below.

Conclusions

Washer or washer like reinforcement is very effective. Same conclusion as D. The technique I used could be used with any piece of stiff plastic scrap that one has. No need to buy washers.

Just wanted to make a note on the weight of the Gorilla Tape. According to the manufacturer, the 2" wide roll weighs about 0.66 oz a yard, and being such a tough tape, I'd take the ever so slightly heavier weight knowing that your tape ain't going anywhere.

Thanks for getting the weight for the tape! Now I can calculate even more…

I did the math with those numbers for the gorilla glue for my tarp and it seems that I used about 84" of tape @ .66oz a yard equals about 1.539oz. When you put it that way it doesn't seem like very much weight at all, but it does seem like a heck out a lot of tape!!!. I mainly notice it when trying to pack it up. The tape doesn't bend very well once folded over and it makes the tarp much bigger when trying to store it after use.

After my strength analysis I now can determine that I'll only need 52" of tape. That makes the total tape weight drop to .953oz which is a 38% reduction in weight if my math is correct.

Wow, I've read other threads here recently on the polycro tarp you've made. Love the idea! I have a couple ideas on what to do with the polycro I got form home depot a few weeks ago.

I'm thinking I'd like to make either a bathtub floor groundsheet for use with y poncho tarp so I can pitch it higher during rain. Or I'd like to make a beak for use with half pyramid pitching. Maybe it would be possible to combine the two idea to create a bathtub floor groundsheet with a long flap on one of the sides that could be tied up for rain protection. That probably makes no sense to read… but I guess I'll just have to try to make it and post pictures in a new thread one day.

Thanks everyone who contributed to this thread. Very useful stuff!