Author Topic: Strap Testing  (Read 4094 times)

DerekSmith

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Strap Testing
« on: May 07, 2021, 02:31:30 PM »
An interesting review of strap performance, flat, twisted and knotted.

https://www.youtube.com/watch?v=ifyJjQXOttE

Putting a single twist into my tiedown straps seems to be an excellent amendment, especially as I never get anywhere near MBS, but flogging and vibration can do serious damage to the strap and the load.

Derek
« Last Edit: May 08, 2021, 12:44:32 AM by DerekSmith »

agent_smith

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Re: Strap Testing
« Reply #1 on: May 07, 2021, 11:23:12 PM »
Thanks Derek - I saw this video earlier this week.
Some interesting observations.

As with all 'enthusiast/home brew' testing, there isn't a huge sample size with each round of testing (eg 3 Sigma).
However, the results seem to follow a consistant pattern.
And I would also comment that these types of tests are always welcome!

Key points:
[ ] Breaks normally occurred at the ratchet end of the test rig - specifically at the rotating spindle - and always with evidence of heat build up causing melting - he did not advance a theory as to why the breaks always occurred at this point (with the exception of the knotted test)
[ ] The web strap suffered compression at the jack head (but no breaks at this location)
[ ] Weakest configuration was a knotted web strap - as we would expect
[ ] A few twists causes no harm - ie no significant reduction to MBS yield
[ ] wet webbing suffers some strength loss (as we would expect) - approx 10% reduction in MBS yield

...

With failure always propagating from the spindle of the ratchet, someone should be able to advance a theory to explain this...
# Likely cause could be due to tight radius that the webbing belt has to bend around - and with multiple turns around the spindle - the webbing is progressively compressed/squeezed. There will be both compression and stretching over the cross-section thickness of the webbing material - which will cause frictional heating. All of this results in a stress concentration at the spindle - which eventually leads to rupture.
« Last Edit: May 08, 2021, 12:14:44 AM by agent_smith »

DerekSmith

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Re: Strap Testing
« Reply #2 on: May 08, 2021, 01:04:07 AM »
Hi Mark,

I concur with your rationale for the consistent failure at the entry to the ratchet spindle, it being the sharpest radius in the whole system.

I am interested in you suggestion that we would expect a 10% reduction in MBS for a wet braid.  A counter argument to this would be that water would prevent the braid from exceeding 100C and therefore prevent melting and subsequent weakening.  Taking this single result at face value, do you have a perspective as to why wetting the braid should weaken it?

Of course, with synthetic fibres it is possible that the braid did not melt, but suffered from pressure flow deformation and the heating happened as the work stored in the elongated fibres was released at rupture, leaving the braid momentarily hot.  In my own trials, although at a much smaller scale, I was never able to show heating during the loading phase.

Derek

Dan_Lehman

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Re: Strap Testing
« Reply #3 on: May 08, 2021, 04:38:12 PM »
I've only glanced --sans sound-- at the video.
WOW, some kind of workshop!!

> ... 10% loss when wet

I presume that this presumes --is it anywhere stated?--
that the material is nylon, which weakens when wet
(and in towing, apparently has internal heating/friction
issues, too (!)).

Even at a some-guessed 50% weakening, note that the
WLL is 1/3, so there'd yet be margin.  But I don't think
that that much weakening was shown, was it?

Thanks!
 :)

SS369

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Re: Strap Testing
« Reply #4 on: May 08, 2021, 05:20:51 PM »
Here?s a link to read. http://jur.byu.edu/?p=21529
Basically if reads that wet nylon tubular webbing does not see a significant strength loss.

Regarding the tie down straps in the video, imo it is a slight angular difference at the ratchet that allows one side of the strap to exceed the maximum strength where it initiates failure. It looked to me as though all the breaks occurred away from the spindle. Unless the strap was moved for clarity (?)

Owning a few of the same straps, they are nylon.

SS

DerekSmith

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Re: Strap Testing
« Reply #5 on: May 08, 2021, 09:07:18 PM »
Interesting study, more analytical than the previous video.  Worth capturing a copy here.

Although only 8% reduction, it begs the question of why?  While water changes the structure and composition of vegetable fibres, by comparison, Nylon is virtually impervious to water.  Could it perhaps be a lubrication effect, allowing a lower cf between strands to exist and so reducing the effective transfer of forces from strand to strand, which in turn reduces the load sharing effect ?

Derek

Oh dear, once again the frugal limit on uploadable file size (100k limit, the doc is 280k) prevents the capture of useful information.  And so crazy in today's data storage environment where I can run a 1 terabyte storage drive on my Raspberry Pi computer for less than ?100 - that is enough to store this little file 3 thousand million times over...

SS369

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Re: Strap Testing
« Reply #6 on: May 08, 2021, 09:39:09 PM »
Hi Derek.

Nylon absorbs water and changes the molecular bonds.

Here, read this [/url] https://www.plasticstoday.com/materials/materials-analyst-part-85-fixing-brittle-nylon-product-water[/url]

SS

Dan_Lehman

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Re: Strap Testing
« Reply #7 on: May 10, 2021, 10:13:48 PM »
Here?s a link to read. http://jur.byu.edu/?p=21529
Basically if reads that wet nylon tubular webbing does not see a significant strength loss.
??!  That's an odd summary to what I'll simply quote
--one's idea of "significant" can be a YMMVarible ::

Quote
Our testing shows that webbing exposed to water experiences
a statistically significant decrease in its ultimate breaking strength ...
[and their testing gives that as "7.9%", ~=8%;
 more surprising should be this wet->dry cycle]
Webbing soaked for 20minutes and allowed to dry for 24 hours;
this cycle repeated 5 times.  --> 5.7% (!)

--dl*
====

KC

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Re: Strap Testing
« Reply #8 on: June 05, 2021, 11:15:43 PM »
Very good info; ty!
.
Most precise would take repetitive experiments.
>>But all the same would have lost bet on 1 twist as this stands.
But in real world, with more contacts to load, perhaps linear and not radial faces
>>think best overall bet would be no twist, and never a twist on a contact area if any.
Flat rope goes over bends will, as the deformed axis is the very minimalist flat one
>>round rope stands higher for more leverage against rope device
>>but in making a HH etc. can scrunch, deform across largest cross axis (to linear load down length of webbing)
.
This is most expressed in flat but wide webbing
>>much less so in 1" that is thicker than pure flat and offers less leveraged distance to deform across.
Much less i think in skinny dyneema slings therefore, and cross profile is even thicker as resistance to scrunching
>> so much so that 2 layers (dyneema folded) is about as thick as dyneema is across
>>so squarer to round to maintain more consistent strengths i think in 2 legs thru as Cow, even more so if both loaded/Girth, or with self balancing pulls /Round Sling, choker.
i see this squareness to profile when doubled also in 3/8" Tenex and paracord.
These devices seem to seek best of both worlds:
>> lay flat on a curve host were webbing is better
>> yet deform less with choke around 2 legs as are square as closest linear to round: where round is better!
Round is equilateral/ no leverage preference given/lost at any angle, square is closest non radial to that prescription i think
>>would not lower with webbing, only round from friction and on round host.
.
i also think/have always done: crank web, pinch off, clear spool, crank TIGHTER.
As the spool fills, leverage is lost.
BUT, at finish want at least 720degrees on spool for positive mechanical lock that way
.
In carrying trailers of brush loaded cross ways, we would use 2 straps lengthwise
CROSSED, so if started to slip off to one side, would tighten not loosen...
.
Set straps can be tightened more by bending together some or to side etc.

Radial vs. Linear matters in force as have tried to show;
But is also matters as a choice in the type of medium/device chosen for strength etc.
BUT, Radial vs. Linear matter perhaps most in the host, the shape the material/device then adopts as a structure along with how the knot is tied.
.
Linear rope parts are just connectors, 180 arcs especially lend controls of friction, better nips and in more than 1x180 grips
>>this is something that can be lent in part by the device around self if round but mostly inherit from the traced host.
.

These all lay flatter around host mount where round rope weakens as is not flat, but then if Half Hitch around Self deforms/weakens harsher in flat than round rope. But the Paracord, Tenex and Dyneema Sling when folded are about same size as profile, so deform less scrunched in Half Hitch around self or choke etc. IN this way perhaps the best of both worlds: flatter on host like webbing and about square /almost round when 2 legs scrunched around by another to least deformity/almost round.
« Last Edit: June 06, 2021, 12:28:24 AM by KC »
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