Polycryo Strength Analysis
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Dan Johnson
(Seattle)

Locale: PNW
Re: Re: Polycryo Strength Analysis on 02/12/2012 20:44:13 MST Print View

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
(--B.G.--) - F

Locale: Silicon Valley
Re: Re: Re: Polycryo Strength Analysis on 02/12/2012 20:49:10 MST Print View

"Structural Engineer"

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

--B.G.--

Richard Nisley
(richard295) - M

Locale: San Francisco Bay Area
Polycryo Strength Analysis on 02/13/2012 01:03:47 MST Print View

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 Daryl
(lyrad1) - MLife

Locale: Pacific Northwest, USA, Earth
Making Things 101 on 02/13/2012 08:17:23 MST Print View

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
(Seattle)

Locale: PNW
Re: Polycryo Strength Analysis on 02/13/2012 08:25:27 MST Print View

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.

Michael Ray
(topshot) - MLife

Locale: Midwest
Re: Re: Re: Polycryo Strength Analysis on 02/13/2012 13:31:55 MST Print View

> 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!

Richard Nisley
(richard295) - M

Locale: San Francisco Bay Area
Re: Re: Re: Re: Polycryo Strength Analysis on 02/13/2012 19:37:15 MST Print View

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.

Michael Ray
(topshot) - MLife

Locale: Midwest
Re: Re: Re: Re: Re: Polycryo Strength Analysis on 02/13/2012 20:23:47 MST Print View

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.

Richard Nisley
(richard295) - M

Locale: San Francisco Bay Area
Re: Re: Re: Re: Re: Re: Polycryo Strength Analysis on 02/13/2012 20:40:08 MST Print View

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.

Edited by richard295 on 02/13/2012 20:49:38 MST.

ziff house
(mrultralite) - F
Why on 02/14/2012 00:19:32 MST Print View

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.

Michael Ray
(topshot) - MLife

Locale: Midwest
Re: Re: Re: Re: Re: Re: Re: Polycryo Strength Analysis on 02/14/2012 04:21:44 MST Print View

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.

Dan Johnson
(Seattle)

Locale: PNW
Re: Re: Re: Re: Re: Re: Re: Re: Polycryo Strength Analysis on 02/14/2012 13:48:04 MST Print View

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!

Dan Johnson
(Seattle)

Locale: PNW
Post Shrink Stress Test on 02/14/2012 23:45:02 MST Print View

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.

test

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

test

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 :)

test

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.

test

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

test

Edited for spelling

Edited by Seattle on 02/14/2012 23:47:48 MST.

Michael Ray
(topshot) - MLife

Locale: Midwest
Re: Post Shrink Stress Test on 02/15/2012 07:47:54 MST Print View

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?

Edited by topshot on 02/15/2012 10:47:11 MST.

Richard Nisley
(richard295) - M

Locale: San Francisco Bay Area
Re: Post Shrink Stress Test on 02/15/2012 09:09:19 MST Print View

D. Johnson,

Thank you for the continued analysis... really interesting!

Edited by richard295 on 02/15/2012 11:01:20 MST.

Daryl Daryl
(lyrad1) - MLife

Locale: Pacific Northwest, USA, Earth
Re: Post Shrink Stress Test on 02/15/2012 10:52:09 MST Print View

This is soooooooo much more interesting than watching tv.

Thanks again for the show.

Harald Hope
(hhope) - M

Locale: East Bay
more complex load spread? on 02/15/2012 11:41:17 MST Print View

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.

Edited by hhope on 02/15/2012 11:47:25 MST.

Gregory Petliski
(gregpphoto) - F
re on 02/15/2012 12:08:42 MST Print View

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 6x8" 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!

Dan Johnson
(Seattle)

Locale: PNW
Re: re on 02/16/2012 23:05:04 MST Print View

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.

Gregory Petliski
(gregpphoto) - F
re on 02/17/2012 09:24:45 MST Print View

@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!