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gunther85
Dec 7, 2005, 9:20 PM
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BlueWater- Testing Request Form
Date: 11/22/2005
Test Number: 5312
QC Rep: 452
Requested by: Scott
Sample Description: Sample #0397A & B
Test Method: Tensile Strength and Elongation of wet webbing
Was the sample conditioned prior to testing? Y or N N
Product Standards: 4200 lbf.
Testing Results:
Tensile Strength (WET) (Sample #0397A)
1) 4,324 lbf.
2) 4,408 lbf.
Elongation (WET) at 250 lb. Increments (Sample #0397B)
250 lbf. 7.00%
500 lbf. 11.00%
750 lbf. 14.00%
1000 lbf. 15.00%
1250 lbf. 17.00%
1500 lbf. 18.50%
1750 lbf. 19.50%
2000 lbf. 20.50%
2250 lbf. 21.00%
2500 lbf. 22.00%
3000 lbf. 24.00%
3250 lbf. 25.00%
3500 lbf. 27.00%
this is the data that i got from the blue water man i've been in contact with...i dunno how much it really helps because it is for wet webbing, but maybe something. Sorry it took so long to get.
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gunther85
Dec 7, 2005, 9:22 PM
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'Chris, Here is some data we compiled this morning. The product standard is
4200lbf. when dry.
The crosshead speed was 5 inches per minute. Straight pull between 5 inch
split capstans to 80% of actual wet strength'
i also got this, although i'm not quite sure what it means. anyone??
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slackinjacklyn
Dec 7, 2005, 10:07 PM
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Maybe another way of getting at this - I use a heavy duty ratchet for my 70' backyard slackline. For a backyard slackline the ratchet works great, I leave it in the system and can easily tighten or loosen it for a different feel in the line (I know the ratchet thing is a different thread but anyway...). Could one measure the tension directly by using torque ratios??? My webbing is spooled to a point where it is 1.5 inches from the ratchet axis. The lever arm of the ratchet is 11.5 inches long out from the axis. This ratio is something like 7.5 to 1. I could measure precisely but I'm guessing it takes about 100 pounds of force on the end of the lever arm (along the tangent line) to tighten my ratchet a notch. That would correspond to about 750 pounds of tension on the unweighted line?? This sounds close to what others have guessed or calculated?? With that tension I sink about 30" in the middle of the (70') line, I weigh 160 pounds.
-Terry
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patto
Jan 31, 2006, 4:46 PM
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I was surprised to see so much head scratching and debating going on in this thread when the 2nd poster, posted the answer.
Basic trigometry is 100% accurate in finding the tension in a slack line.
Regardless of the line tension or the elasticity of the line, the problem still comes down to a basic triginometric problem. If the weight is known and the angles of the line is known there is only one answer that could possible be an equilibrium. (Of course dynamic moves like jumping would greatly increase the forces.)
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slacklinejoe
Jan 31, 2006, 5:08 PM
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In reply to: Basic trigometry is 100% accurate in finding the tension in a slack line. ....... Of course dynamic moves like jumping would greatly increase the forces.
Uhm, didn't you just disprove yourself?
The problem has been coming up with a reliable mathmatical formula for what happens when you've got impact on the line line bouncing (or momentum making less impact) and still get a very close number to what is seen with dynometers - that's what we're shooting for that is the complicating factor in this thread.
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eric_t83
Jan 31, 2006, 11:16 PM
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If I get a reasonable number for the modulus, and the line does indeed follow Hooke's law(or some other known function), I can do a dynamic solution of the line force and displacement, i.e. take into account bouncing, jumping, walking etc.
It's basically just Newton's second law, it's all these empirical constants that are difficult to find...
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iltripp
Jan 31, 2006, 11:24 PM
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In reply to: If I get a reasonable number for the modulus, and the line does indeed follow Hooke's law(or some other known function), I can do a dynamic solution of the line force and displacement, i.e. take into account bouncing, jumping, walking etc. It's basically just Newton's second law, it's all these empirical constants that are difficult to find...
I've thought about doing the same thing. The problem with this approach is that the modulus of the webbing will change over time.
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skinner
Feb 1, 2006, 4:39 AM
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In reply to: I was surprised to see so much head scratching..
I'm still scratching.. and wondering if some of these super long lines you see with some freak crossing a gapping chasm, were close to their breaking strength.
PS: I only refer to them as freaks because I do not possess the titanium testicles to follow them.
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slacklinejoe
Feb 1, 2006, 5:01 AM
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In reply to: and wondering if some of these super long lines you see with some freak crossing a gapping chasm, were close to their breaking strength.
Well, just using the static trig for it, a 250 foot line with only 10 feet of sag and a 185 lb slacker will be producing 1159 lbs of tension - that's what the anchors will see with them standing still in the middle.
The problem has been what the hell is going on if they take a leashed fall, bounce or jump on the line. Since the web stretches it disipates their momentum over time - which gets into the whole modulus and hooks law bit of stuff that I won't pretend I understand adequately enough to start answering that question, much less for the whole complaint that web characteristics change with humidity, age and general condition.
At this point I'd be happy if someone just came up for an answer for the calculations for new web and wet web, we'd know the truth was closer to the new web for average condition.
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patto
Feb 1, 2006, 5:12 AM
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In reply to: The problem has been coming up with a reliable mathmatical formula for what happens when you've got impact on the line line bouncing (or momentum making less impact) and still get a very close number to what is seen with dynometers - that's what we're shooting for that is the complicating factor in this thread.
Okay, sorry for my misunderstanding.
I believe that you can safely put an upper limit of the dynamic tension being no higher than twice the static tension. The reason being that:
A regular standing jump on a firm surface I estimate to exert a load of around twice body weight. I have come to this conclusion by observing jump height (body mass) and the resultant travel during landing. The knowledge that the typical max leg press is around twice somebodies body weight.
A slack line is certainly not a firm surface, the catch is alway much softer than the catch of a regular standing jump. Furthurmore due to stretch if you double the load on a slackline then tension multiplies by an amount LESS than double.
Thus I would conclude that the upper limit of line tension during dynamic moves is less than twice the static load.
Does that make sense?
(BTW check out this site too)
http://www.wanderingphotographer.com/...tatic_analysis_2.htm
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skinner
Feb 1, 2006, 5:19 AM
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OK, off topic,, but tell me.. what is the tape for on the line?
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patto
Feb 1, 2006, 5:22 AM
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In reply to: The problem has been what the hell is going on if they take a leashed fall, bounce or jump on the line.
Well my above post tried to answer a bounce or a jump. However a leashed fall is worse. Leashed fall is at least 2m because it is a factor two fall on the leash. Furthermore your feet and legs are no longer acting as cushions.
I'm not going attempt to answer this problem with a elasticity modulous. However I would expect the forces to be around 4-5 times static tension.
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slacklinejoe
Feb 1, 2006, 6:37 AM
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patto - no offense man, but it looks like your trying to make concrete conclusions from total guesses at the forces involved. I think we're looking for something a bit more concrete. In the case of jumps, the bigger the air, the longer your mass has to accelerate - and don't underestimate the air achieved by some of those big air gurus.
As for the analysis page, yes, I've studied it a lot, that addresses the static loads on the system, as a matter of fact I used that as a basis of my slackline force calculator on my site that lets you enter the appropriate values and it'll spit out the static load. Force Calc
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patto
Feb 1, 2006, 12:06 PM
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In reply to: patto - no offense man, but it looks like your trying to make concrete conclusions from total guesses at the forces involved. I think we're looking for something a bit more concrete. In the case of jumps, the bigger the air, the longer your mass has to accelerate - and don't underestimate the air achieved by some of those big air gurus. As for the analysis page, yes, I've studied it a lot, that addresses the static loads on the system, as a matter of fact I used that as a basis of my slackline force calculator on my site that lets you enter the appropriate values and it'll spit out the static load. Force Calc
No, offense taken. :)
As we have already established this problem isn't easy to approach without all the data. However it in the face of uncertainty it is often useful to conside bounds for the estimation. The lower bound is clearly the static load.
My analysis was merely setting an upper bound. If you think that upper bound is incorrect then please explain why.
For example if you want me to be even more conservative then let me pick an upper bound of 4x the load. I don't know many people can support 4x their body weight on their leg muscles.
Either way my analysis has put concrete bounds on the limits of the forces which is better than nothing.
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eric_t83
Feb 1, 2006, 4:21 PM
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In reply to: In reply to: If I get a reasonable number for the modulus, and the line does indeed follow Hooke's law(or some other known function), I can do a dynamic solution of the line force and displacement, i.e. take into account bouncing, jumping, walking etc. It's basically just Newton's second law, it's all these empirical constants that are difficult to find... I've thought about doing the same thing. The problem with this approach is that the modulus of the webbing will change over time.
There are lots of problems here... so it would only be a very coarse approximation, but could still be interesting(at least as an academic exercise if nothing else..) Of course you have to be aware that you can drown in a river that is one feet deep on average :wink:
I'd never put my life on the line based on any type of physics calculation that didn't include very generous safety margins. I do a lot of mathematical modeling in my education, and there's always a huge amount of approximation and uncertainty present... a good deal of experience is always necessary to interpet the results, and I guess that is the case here as well.
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noshoesnoshirt
Feb 1, 2006, 5:17 PM
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How 'bout modeling the line as a spring-damper system?
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eric_t83
Feb 1, 2006, 5:28 PM
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In reply to: How 'bout modeling the line as a spring-damper system?
That's exactly how I'm thinking. But that can only be done by knowing both the spring constant and dampening constant.
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slacklinejoe
Feb 1, 2006, 6:38 PM
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Ok, so I'll throw this out using where Patto was going. We've all pretty much agreed that a leashed fall would be the worst thing going on for a single person on the line.
What about taking their mass, accelerating it the distance of the fall and inserting their accellerated mass as the new static weight? I know that ignores the disipation over time, but it might serve to give us a worst case scenario.
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misanthropic_nihilist
Feb 1, 2006, 6:43 PM
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Who cares about the oscillation of the [damped] system?
We're screwed because Young's modulus is so incredibly variable, and webbing is far from linearly elastic.
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eric_t83
Feb 1, 2006, 7:04 PM
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In reply to: Ok, so I'll throw this out using where Patto was going. We've all pretty much agreed that a leashed fall would be the worst thing going on for a single person on the line. What about taking their mass, accelerating it the distance of the fall and inserting their accellerated mass as the new static weight? I know that ignores the disipation over time, but it might serve to give us a worst case scenario.
That's the simple, boring solution, yes... :wink:
One problem with this is that some materials are very strong when a static force is slowly applied on them, but break easily by a shock, which is what you'll have in a fall.
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eric_t83
Feb 1, 2006, 7:10 PM
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In reply to: Who cares about the oscillation of the [damped] system? We're screwed because Young's modulus is so incredibly variable, and webbing is far from linearly elastic.
As I mentioned, this would mainly be an academic exercise(read: us nerds, that's who!) :)
On the other hand, it could be kind of interesting for us that jump on the line, i.e. finding out how long it takes before the line is stable enough to land on after a jump and that sort of thing. It's also a way to indentify and remove unwanted vibration, should that occur.
Are you absolutely certain that nylon webbing isn't linearly elastic?
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patto
Feb 2, 2006, 1:12 AM
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In reply to: Ok, so I'll throw this out using where Patto was going. We've all pretty much agreed that a leashed fall would be the worst thing going on for a single person on the line. What about taking their mass, accelerating it the distance of the fall and inserting their accellerated mass as the new static weight? I know that ignores the disipation over time, but it might serve to give us a worst case scenario.
Nice idea, but the dissipation over time is everything when it comes to the force exerted, that is why we have dynamic ropes.
However what we can do is consider the fall distance vs the stop distance. For example if the fall distance is 2 foot and the brake distance is 2 foot then the average deceleration during braking is 1G (because 2 divided by 2=1). But you need to add 1G due to static gravity weight so you end up with a total of 2G. Therefore the load is 2x body weight. However since the braking is not linear the peak load would be higher than this. Though remember that although the load is double the actual tension on the line increase at a much lower rate.
Also remember that a two foot jump is probably an almost impossible standing jump as it is measured as movement of the centre of mass not the height of the feet. My typical standing jump on firm ground is probably only a foot.
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