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Drop test fuse breaking strengths.
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Rob.hack


May 2, 2012, 2:06 PM
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Drop test fuse breaking strengths.
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I am interested in conducting drop tests on individual gear placements.

From reading other threads on this forum I believe that the 'fuse' method would be the most suitable.

What I've been unable to find is data on the breaking strength of the knotted cord fuse, ideally in a dynamic setting.

I don't believe the method of doubling the rated strength of the cord and decreasing it by a percentage relative to the knot being used gives a sufficiently accurate figure.

Could anybody comment on this?

Have any empirical tests been conducted to ascertain the breaking strengths of different fuses? Ideally finding setups with a breaking strengths at intervals up to that generated by a 'worst case scenario' fall.

I am looking specifically for studies with suitably large sample sizes so I can determine the range of the breaking strength.

Thank you in advance.


majid_sabet


May 2, 2012, 3:24 PM
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Rob.hack wrote:
I am interested in conducting drop tests on individual gear placements.

From reading other threads on this forum I believe that the 'fuse' method would be the most suitable.

What I've been unable to find is data on the breaking strength of the knotted cord fuse, ideally in a dynamic setting.

I don't believe the method of doubling the rated strength of the cord and decreasing it by a percentage relative to the knot being used gives a sufficiently accurate figure.

Could anybody comment on this?

Have any empirical tests been conducted to ascertain the breaking strengths of different fuses? Ideally finding setups with a breaking strengths at intervals up to that generated by a 'worst case scenario' fall.

I am looking specifically for studies with suitably large sample sizes so I can determine the range of the breaking strength.

Thank you in advance.

there is been few test done by tom moyer on knots and many other people but IMO, what we lack the most is drop test on climbing gear such as nut or cams ,jumars, daisy the skinny ass runners and spectra


Rob.hack


May 2, 2012, 4:18 PM
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Re: [majid_sabet] Drop test fuse breaking strengths. [In reply to]
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Thanks.

My goal is to begin testing gear in a rock face. I believe a lot can be learned from lab tests but also that there are variables that cannot be replicated in a lab setting. To begin testing I need a way of ensuring that I am subjecting the gear to sufficiently high forces.

Specifically I would be testing the pull out strength of placements, rather than their breaking strengths.

I believe that using a shock limiting fuse made of cord would be a better choice than using a dynamometer or other similar device as the initial cost is lower and th setup is lighter and more expendable. That is providing that a fuse can provide a suitable degree of accuracy.

Eventually, when I have a appropriate rig I would like to flm the loading of the pieces in an effort to analyse the physics that they are subjected to at the point of loading.


USnavy


May 2, 2012, 7:21 PM
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Re: [Rob.hack] Drop test fuse breaking strengths. [In reply to]
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Rob.hack wrote:
From reading other threads on this forum I believe that the 'fuse' method would be the most suitable.

.
In my experience, the fuse method is not as accurate some people think. I did some drop testing on a few pieces of cordlette awhile back and my load cell registered a different breaking strength every time. The standard deviation was through the roof if I remember right. Furthermore, the actual breaking strength of the fuse and the expected strength were quite a bit different. However, I did not conduct some wide-scale testing of a hundred samples. I only tested a handful of samples. But the seemingly random and somewhat unpredictable breaking strain of the samples I did test leads me to believe that using a load cell or similar device is the only appropriate option if you want any reasonable level of accuracy.

The other issue is that a fuse works to a very limited degree to tell you if a fall exceeded a roundabout value, but it does not really tell you how much force the top piece would have actually experienced in a fall. If you want, I can tell you where you can get a portable load cell analyzer that can be powered off of a laptop and a 5,000 lbs load cell for under $600.

If you look at my thread here:

http://www.rockclimbing.com/cgi-bin/forum/gforum.cgi?post=2574668;sb=post_latest_reply;so=ASC;forum_view=forum_view_collapsed;page=unread#unread

I used the load cell analyzer I am referencing to generate the data in that post.


(This post was edited by USnavy on May 2, 2012, 7:36 PM)


JimTitt


May 2, 2012, 10:00 PM
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Fuse testing with commercially available cord is worthless, the variation in actual strength to rated strength can easily be 10 to 15%, the variation in the knots about he same and dependong on the knots and materail the difference between static (the rated strength) and dynamic tests around 25%. If you could get an accuracy of 30% that would be good IŽd think.

"I believe a lot can be learned from lab tests but also that there are variables that cannot be replicated in a lab setting"
The variables outdoors are effectively infinite which is why lab tests are done.

Introducing cord into a dynamic system will change the force felt by the piece as the dynamic characteristics of cord is usually different to that of climbing rope.

Drop testing is laborious, slow and expensive. To perform a meaningful number of tests outdoors takes a small army of helpers and a carefully chosen site. We use an powered winch for hoisting the weight for good reason and you need a rope sponsor as well!


gunkiemike


May 5, 2012, 12:32 PM
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Re: [JimTitt] Drop test fuse breaking strengths. [In reply to]
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I've done LIMITED testing on fuses made from sewn 1" webbing. My experience, backed up by others with commercial experience sewing life-critical gear, is that the tensile strength of a sewn splice is proportional to the number of stitches. I used ordinary sewing thread, which of course is ridiculously weak compared to what's used for bar tacks and the like. But I was able to demonstrate a remarkably linear relationship between stitches & strength ranging from well under body weight to approx 925 lb. nb - all were based on slow pull testing.


USnavy


May 6, 2012, 12:19 AM
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gunkiemike wrote:
I've done LIMITED testing on fuses made from sewn 1" webbing. My experience, backed up by others with commercial experience sewing life-critical gear, is that the tensile strength of a sewn splice is proportional to the number of stitches. I used ordinary sewing thread, which of course is ridiculously weak compared to what's used for bar tacks and the like. But I was able to demonstrate a remarkably linear relationship between stitches & strength ranging from well under body weight to approx 925 lb. nb - all were based on slow pull testing.
That is interesting. But the big question is how do you know how many times the thread penetrated through the material? I dont have that much experience stitching webbing, but the experience I do have says that you cannot simply count how many stitches there are in a single bartack and then just multiply it by the number of bartacks. I found that when stitching webbing, sometimes the webbing would not advance as fast on one side of the webbing, or something to that extent, and I would end up with closer spaced stitches (although barely noticeable) on one side. What do you think?


JimTitt


May 6, 2012, 12:53 AM
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gunkiemike wrote:
I've done LIMITED testing on fuses made from sewn 1" webbing. My experience, backed up by others with commercial experience sewing life-critical gear, is that the tensile strength of a sewn splice is proportional to the number of stitches. I used ordinary sewing thread, which of course is ridiculously weak compared to what's used for bar tacks and the like. But I was able to demonstrate a remarkably linear relationship between stitches & strength ranging from well under body weight to approx 925 lb. nb - all were based on slow pull testing.

Bar-tacking is undoubtedly more accurate but it is then a chicken and egg situation. To make the fuses accurately you need a commercial bar-tacker and to calibrate them you need a drop tower and strain guage. Seems a bit of an expensive and roundabout way of achieving a less accurate result than just using the strain guage in the first place.


gunkiemike


May 6, 2012, 3:15 AM
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USnavy wrote:
That is interesting. But the big question is how do you know how many times the thread penetrated through the material?

Easy, I am doing simple straight stitching on a home sewing machine. I count the stitches each time.


USnavy


May 6, 2012, 3:59 AM
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gunkiemike wrote:
USnavy wrote:
That is interesting. But the big question is how do you know how many times the thread penetrated through the material?

Easy, I am doing simple straight stitching on a home sewing machine. I count the stitches each time.
That sounds like a lot of work. When I was doing some highline testing I accidentally stitched a piece of webbing with 50% cotton 50% nylon. I had eight bar tacks that were double backed. There were probably some 750 thread penetrations. Yet the stitching failed at less than 1800 lbs. So it seems if you are using cotton, you need a hell of a lot of tacks to get any worthwhile strength from the webbing.


Partner cracklover


May 8, 2012, 7:46 AM
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JimTitt wrote:
Fuse testing with commercially available cord is worthless, the variation in actual strength to rated strength can easily be 10 to 15%, the variation in the knots about he same and dependong on the knots and materail the difference between static (the rated strength) and dynamic tests around 25%. If you could get an accuracy of 30% that would be good IŽd think.

Interesting.

Anyway, for your purposes, clearly that range is absolutely way too high to be any use. But for my purposes, and other recreational climbers, it can be an effective test.

I wanted to see if my Aliens had a bad braze and would fail at fairly low loads. I was able, unless I'm very much mistaken, to test my Aliens to 4-5 kN. My cords always failed at the same place (where they entered the double-fisherman's knot), which makes me think the variability might be a little less. Of course that spread (20% difference from greatest to least) is way more than would be acceptable for any of your purposes, but it was fine for me. Even if the force got a little higher, I wouldn't damage the piece, and lower, I would still have confidence that it probably was not a defective piece.

It also has the benefit of being cheap, and relatively easy for a recreational climber to do. However, knowing how much of a range there is between tests, I might in the future do two or three tests for a piece I was concerned about.

GO


gunkiemike


May 16, 2012, 4:00 PM
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USnavy wrote:
gunkiemike wrote:
USnavy wrote:
That is interesting. But the big question is how do you know how many times the thread penetrated through the material?

Easy, I am doing simple straight stitching on a home sewing machine. I count the stitches each time.
That sounds like a lot of work. When I was doing some highline testing I accidentally stitched a piece of webbing with 50% cotton 50% nylon. I had eight bar tacks that were double backed. There were probably some 750 thread penetrations. Yet the stitching failed at less than 1800 lbs. So it seems if you are using cotton, you need a hell of a lot of tacks to get any worthwhile strength from the webbing.

Not really that hard. I put 185 stitches in the webbing (polyester thread as I recall) in less than a minute. Took another minute to count them. Pull testing broke it at 925 pounds. Results were VERY linear from 8 to 185 stitches when I was doing this. This wasn't for "safety gear" but just to make a fuse for Alien testing. If I wanted something that wouldn't break, sewn aiders for example, I would extrapolate the results and go with 400-500 stitches, minimum.


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