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Direct Loading of Dyneema Slings

patto

14-Apr-2010
11:28:41 AM

davepalethorpe your answer is incorrect and accepts the same incorrect assumptions that we were previously discussing.

On 14/04/2010 aarond wrote:
>from the listed article:
>
>"A fall factor 1 using an unknotted 120cm x 11mm dyneema sling registered
>25kN (surprisingly the sling survived, but whether ther climber or belay
>would have is another matter), a fall factor 1 with an unknotted 11mm x
>60cm dyneema slings generated 14kN."
>
>
>so falling a lesser distance does reduce the impact force even if they
>are both fall factors of 1??

As was mentioned earlier in reality for very small falls there is a difference. This difference comes about due to stretch of the other components in the system. In the case of a climbing fall it is the stretch of the falling human body. In the case of this test it is the stretch of the caribiners, and the metal weights.


aarond

14-Apr-2010
11:45:23 AM

so in the case discussed in this topic fall factors are good on paper but in reality there is more variables.

so in regards to the 2nd and 3rd post on this topic,
dieselcam you are right and do have a fundamental of understanding of the basic physics of fall arresting because of what was proven in the dmm test and from what patto has said and kind of contracted himself in accusing you.

;-)

davepalethorpe

14-Apr-2010
11:58:36 AM

On 14/04/2010 patto wrote:
>davepalethorpe your answer is incorrect and accepts the same incorrect
>assumptions that we were previously discussing.

Incorrect in what way??

patto

14-Apr-2010
12:15:57 PM

On 14/04/2010 aarond wrote:
>so in the case discussed in this topic fall factors are good on paper but
>in reality there is more variables.
>
>so in regards to the 2nd and 3rd post on this topic,
>dieselcam you are right and do have a fundamental of understanding of
>the basic physics of fall arresting because of what was proven in the dmm
>test and from what patto has said and kind of contracted himself in accusing
>you.
>
>;-)

From the very beginning I said for very small falls the impact forces are reduced due to the elasticity of our bodies. I have stated this exception quite plainly, I don't think there has been a contradiction.

I have explained that these tests do not represent the real world because of the elasticities of our bodies. I have also insisted that people shouldn't fall into the trap of thinking longer falls equal higher forces.

On 14/04/2010 davepalethorpe wrote:
>Incorrect in what way??

On 14/04/2010 davepalethorpe wrote:
>With a direct fall onto
>a dynema sling that has near zero elasticity no matter what length it is(?)
>the main factor on peak force is how quickly you are travelling (ie height
>you fell from).
The stretch dynamics of dynema is identical to climbing rope. In both cases the peak force depends directly the height and the length of rope. The only difference is the 'stretchiness' of the material.


aarond

14-Apr-2010
12:44:42 PM

yeh that's cool.

I'm just trying to make people actually think about it.
i just feel people are very quick to jump on someone with accusations of "if you really understand whats going" and "you shouldn't be climbing outdoors".
when really this guy knew what was going on.

davepalethorpe

14-Apr-2010
1:06:03 PM

>The stretch dynamics of dynema is identical to climbing rope. In both cases the peak force
>depends directly the height and the length of rope. The only difference is the 'stretchiness' of
>the material.

>"The significance of the fall factor is that, when a fall is arrested by a belayer, if the tension in
>the rope is assumed to develop according to Hooke's Law, then the maximum impact force of
>the fall depends only the climber's mass, the elasticity of the rope, and the fall factor. In
>particular, the maximum impact force does not depend on the length of the fall."

I thought that hookes law only applied providing the elastic limit of a material is not exceeded. Ok with a nice stretchy thing like a climbing rope, but I would have thought it would be an entirely different situation with a not so stretchy thing like dyneema. Then you would imagine that impact forces would increase with distance once you exceeded the elastic potential of the sling.... Happy to be proved wrong though...?!

egosan

14-Apr-2010
1:13:01 PM

I suspect that when you reach the elastic limit of the sling or the rope you start accelerating at 9.8m/s/s.

davepalethorpe

14-Apr-2010
1:21:37 PM

On 14/04/2010 egosan wrote:
>I suspect that when you reach the elastic limit of the sling or the rope
>you start accelerating at 9.8m/s/s.

Good point! Suppose I shouldn't try and be a physicist/engineer... Just seems strange to me that for example a 6m fall onto a 6m "static" dyneema sling would generate the same force as a 60cm fall on a 60cm sling. Guess that is due to other factors being involved in "real life" situations like the elasticity of your body etc (as patto has mentioned previously) reducing impact forces in small falls.

Edited - to clarify last sentence.


Hendo

14-Apr-2010
2:20:53 PM

I think the point trying to be put across is that the effect of fall distance on force is small compared to the effect of the damping factors associated with having a deformable floppy body, deforming the harness etc. Eg a big change in fall height can be negated by a small change in the rigidity of your body. In theory this is likely to be true but I haven't tested it in any scientific way and couldn't put exact or really even close ball park figures on it for real situations.

A simple rough demonstration would be to get a set of scales, a stiff ball (perhaps a ball bearing or maybe a golf ball) and a non-stiff ball (like a superball) and bounce them onto the scales at different heights and see what peak values you get. You should find the stiff ball generates high forces even from low heights. If you have a suitably less stiff ball you can drop it from much higher and still have same peak force.

I remember doing this a long time ago with a basket ball dropped from roughly body height and being surprised to get a peak force roughly about my weight.

Hooke's law is ok for many non stretchy materials eg. steel.

patto

14-Apr-2010
3:46:20 PM

Sigh. Bathroom scales aren't going to read anything close to peak forces.

The fact of the matter is VERY high forces can be easily generated by short falls if there isn't much cusioning. This isn't just in climbing this is EVERYWHERE around us. Most people don't recognise this.

I could have my big mate stand on my barefoot big toe with 1kN of force without too much concern. I could drop a 0.02kN (2 kg) hammer on my toe and break my toe. That hammer probably produces a force on my two of 100x the weight of the hammer! Drop the hammer onto my flabby stomach and I might get a bruise but I wont break anything because my stomach is soft.

Jump barefoot off a height of half a metre and land straight legged onto concrete and you'll likely end up with a serious broken bone. Do the same of 6x higher at 3m but allow your legs to bend at your knees and you'll be fine.

One Day Hero

14-Apr-2010
3:55:05 PM

On 14/04/2010 patto wrote:
>Sigh. Bathroom scales aren't going to read anything close to peak forces.
>
Ummm, I think they'll be fine for measuring the peak force of a basketball dropped from 1m. Do you even read what was said before assuming superiority in understanding physicsy stuff?


wallwombat

14-Apr-2010
3:57:54 PM

If anyone wants to get rid of their incredibly dangerous Dyneema slings, I am willing to take them off your hands. I will even pay the postage.

Be warned, they are ticking time bombs and the longer you hang on to them, the more danger you are in.

I have already replaced all my slings with bicycle inner tubes. Much safer.


Hendo

14-Apr-2010
4:05:57 PM

I only meant that as some way of getting an intuitive experimental idea of what is being talked about and I do believe it is good for that. Don't worry, it wouldn't be inaccurate to describe me as a physicist, I might not be an expert on this exactly but I have at least some idea, just trying to help explain.

When you describe dropping things onto a surface (toe, stomach etc) and the resulting pain/damage it is probably better to talk about pressure (stress) as area of contact is important. But again not something most people are fluent in.

When I was about 15 I was at Wet'nWild World (or whatever it is called) at the Gold Coast. I jumped off a bridge which went over the tyre tube floaty creek thing. For some reason I thought the water was fairly deep. Turned out I had jumped from feet about 2 or 3m above the water with straight legs into something about knee deep. I remember standing there frozen stiff with burning pain in my hip sockets unable to move for some time. Walked it off though...heheh

patto

14-Apr-2010
4:15:42 PM

On 14/04/2010 One Day Hero wrote:
>On 14/04/2010 patto wrote:
>>Sigh. Bathroom scales aren't going to read anything close to peak forces.
>>
>Ummm, I think they'll be fine for measuring the peak force of a basketball
>dropped from 1m. Do you even read what was said before assuming superiority
>in understanding physicsy stuff?

To accurately measure peak forces you need to have a very fine resolution measuring devices. I'm talking about measurements every thousandth of a second. Though for a basketball every hundredth might be ok. Either way my bathroom scale doesn't do that!

On 14/04/2010 Hendo wrote:
>When you describe dropping things onto a surface (toe, stomach etc) and
>the resulting pain/damage it is probably better to talk about pressure
>(stress) as area of contact is important. But again not something most
>people are fluent in.

Very true. I tried not to bring pressure arguments into this considering that a hammerhead is similar size to a big toe.

Incidently when you run the peak force on your foot is around 2.4x your body weight.

davepalethorpe

14-Apr-2010
4:29:39 PM

On 14/04/2010 patto wrote:

>Incidently when you run the peak force on your foot is around 2.4x your
>body weight.

ahh...now you're into my territory! Peak forces on your achilles during running are between 6-10 times body weight!

Sorry, off topic...back to the scales etc...


IdratherbeclimbingM9

16-Apr-2010
7:27:41 PM

On 14/04/2010 wallwombat wrote:
>I have already replaced all my slings with bicycle inner tubes. Much safer.

I replaced mine with the large 'O' rings that are found in dunny cisterns...
;-)

gfdonc

16-Apr-2010
9:08:24 PM

On 14/04/2010 patto wrote:
>Jump barefoot off a height of half a metre and land straight legged onto
>concrete and you'll likely end up with a serious broken bone.

Actually I once read it was possible to suicide that way as the spinal column gets driven up into the skull. Never been keen to try it though.

patto

16-Apr-2010
10:18:56 PM

On 14/04/2010 wallwombat wrote:
>I have already replaced all my slings with bicycle inner tubes. Much safer.

The bizarre thing is I have actually seen some website where somebody did this as a cheap DIY screamer. (Backup of course by a sling)

On 16/04/2010 gfdonc wrote:
>On 14/04/2010 patto wrote:
>>Jump barefoot off a height of half a metre and land straight legged onto
>>concrete and you'll likely end up with a serious broken bone.
>
>Actually I once read it was possible to suicide that way as the spinal
>column gets driven up into the skull. Never been keen to try it though.
>

Sounds like an urban myth. Sure it might be possible to damage you spine but your head isn't heavy enough for the above to happen.


Sarah Gara

18-Apr-2010
7:10:53 AM

On 16/04/2010 gfdonc wrote:
>
>Actually I once read it was possible to suicide that way as the spinal
>column gets driven up into the skull. Never been keen to try it though.
>
grr grr grr commit suicide. eek! it's not quite as bad as suicided (it's committed suicide) -really really annoys me.

I wouldn't try it either - worse than pencils up the nose... or is it? I don't know which would be worse. Both are pretty grim. x

I'll also have those slings if anyone is getting rid. x


ajfclark

21-May-2010
5:25:38 PM

On 12/04/2010 ajfclark wrote:
>If you doubt that this even happens, in one of Sharma's videos there's a snippet of him hanging off a bolt with a quickdraw clipped directly to his harness. He forgets that he's clipped in and goes to pull a move and falls double the length of the draw. It looks really painful. I'll see if I can dig out the title of the video...

It's about 28 minutes into King Lines.

There are 42 messages in this topic.