Why “Bounce Testing” should be avoided

tom donnelly wrote:

Past bounce test results - it works, if done correctly.

The other problem - if not the primary one - is that you’re comparing an impulse load to a static one - for soft goods that can likely absorb very high and short duration impulses just by internally unraveling a few strands and getting nowhere near yielding a strand - ie, like dynamic rope.   A 2mm piece of cord or whatever “calibrated” nonsense is not a 4k lb piece of webbing that’s been sitting in the sun and would require an unknown number of impulses to compare to another unknown static load to completely tear and fail.

TLDR - it’s a rathole mind-fuck of unsound logic, unclear thinking and naive engineering to believe this is something actually useful in the context of rap anchors.

Man when I said “come at me”, I didn’t mean literally, lol.    Like tossing a steak in a pool of piranha.

it’s hard to tell who James is arguing with, seems both Tom AND me?  And we are on different sides seemingly.  As far as James’ last response to me..

James W wrote: (in italics)

Mil spec webbing is ~4k lbs breaking - WTF is this bounce test you’re doing on the side of a cliff that is coming even remotely close to yielding that?  

Nobody is worried about new webbing/cordage. Point went over your head.  It’s good to go, bounce or no bounce, but no bounce tests are needed -so apparently you agree with me.

James wrote: For the premise of unwanted yielding of highly degraded webbing - that is flawed as well - there is no plastic region remaining - it’s brittle.

The point is that aged, fatigued, and degraded webbing exhibits much different mechanical characteristics than new webbing.  Plasticity is degrading over time and use so it depends where you specify.  You’re nitpicking but It appears you again agree with me. 

James wrote:  Overall - C+ for your ability to produce a diagram and notes, D- for your engineering, F for the overall recommendation.

+1 - there is no excuse to not take your time and construct an adequate anchor.  The premise of needing to do some sort of engineering test on the side of the cliff and that it would have any value what-so-ever is SO many levels of wrong-headed.

You contradict yourself above - you say F for the recommendation not to engage in bounce tests, yet again you agree with me and +1 my recommendation -  to construct adequate anchors and to not do any uncontrollable testing on the side of a cliff.  

Which is it ? -like many here, you are showing either no logic or total lack of reading comprehension - or both to post an apparent argument that agrees with me. 

To clarify again the scope of this thread is unknown and unpredictable materials like old nylon or ice anchors as raised in the accident thread.  NOT new equipment or material, nor aid or pro placement applications.  

Mikey S and rgold as usual had the best takes.  Andrew P made a good point - my mistake was in posting a graph that had confusing curves in the effort to save time (even though it’s the end points that matter and the curves themselves are not part of the point or pertinent to the discussion, takeaways,  or conclusions). 

I changed it just focus on the end point deltas. 

Interesting that Kyle posted a scatter shot of nonsense then deleted it all…Instead of just calmly suggesting clarification on a point.  Since you focused on ice, I wonder why if it’s best practice, Petzl does not recommend a 2x force test (bounce) and only recommends similar to rgold - verify by actual use.  They  go on to say…

“Beware of existing abalakovs, as they may have hidden damage and turn out to be dangerous if reused.”  

And sudden higher loads than intended, by doing a bounce, can do similar damage. Have you discussed this with Petzl engineers at the factory when you were on the UIAA safety commission ?  I have.  (Custer’s understudy for a bit bitd)

Jason K and Andrew R raised reasonable questions.  And the answer is above by Petzl and rgold — If absolutely necessary to rely on sketch tat or ice anchors, your margin is best maximized by backing up and using as intended (carefully) to verify fitness vs doing an uncontrolled higher load force test that may be counterproductive. 

Still challenge anyone to make a coherent argument why a higher load bounce on already sketchy material is the best practice. 

We need to get some canyoneers involved here since they are dedicated to the craft of rapping off sketchy sh!t.

I'm a rock climber. If things go haywire and I have to pioneer some wild rap route full of sun bleached tat I'm leaving my whole damn rack on the cliff!

Bounce testing is sheer lunacy considering the consequence of failure, and the inadequacy of the test as Mark has pointed out. My only gripe is that your graph is missing axis labels! :)

Your partner climbs up to the belay and finds it constructed of a single point of gear/webbing/whatever of unknown origin.

"All good bro, I bounce tested it."

What do you think your partner's reaction is?

Mark Greg and James are you a mechanical, system engineer or physicist ? What if you bounce on a number one stopper or 00 cam and it holds? OK?

Mark Pilate wrote:

A recent accident discussion and analysis has revealed a widespread practice and myth of the “Bounce Test” that should be widely called out and discontinued to avoid discussing the same situation in future accidents.  

This is a classic case of confirmation bias due to the low statistical nature of the outcomes, but these accidents are real and they keep happening.   The goal here is OPTIMAL decision making and practices, to give people the widest margin for error in a sport that has drastic consequences for sub-optimal decision making and practices.

Situational definition and context

• This does not apply to testing an anchor piece (pro)  that can be re-adjusted or re-placed to a better orientation or location
• The purported reason for a “Bounce Test” here is that you have a sketchy anchor that you don’t trust as in the contest of in situ “tat” similar to the recent rappel anchor failures in the Accidents thread.
• You have no way of knowing its real ultimate strength
• You think conducting a “test” will give you useful information on the strength of that anchor for when you intend to use it.
• Many erroneously assume, without evidence, the victims obviously didn’t test their anchor, because they erroneously assume that a test is definitive.

Logical Argument 

• To “test” it, you have a safe back up available and you employ it to safeguard your test
• If your test “fails”,  meaning your bounce test results in original anchor failure, then you need to construct and leave your back up anyway.
• So you had a back-up alternative to a solid anchor but chose to go with the sketchy one because you now think it’s “safe” to do so. 
• A bounce test means you have a choice.  So this test becomes essentially an economic test – to save leaving a bit of gear.
• Any arguments against these assertions?

Economic Argument

• Your life is worth more than the cost of some climbing gear.
• Any arguments against this assertion?

Technical Argument (aka  facts)

• Fatigue is a process that has a degree of randomness often showing considerable scatter even in seemingly identical samples in well controlled environments.
• The greater the applied stress range, the shorter the life.
• Fatigue life variability tends to increase for longer fatigue processes continue 
• Damage is irreversible. Materials do not recover when rested.  Load effects are cumulative 
• Fatigue life is influenced by a variety of “aging” factors,

•               temperature cycling

•               surface finish/ damage (rap ropes being pulled)

•               oxidizing (UV damage)

•               scuffing contact (fretting), etc. (normal wear)

•               Knot stresses, and load angles

•               Moisture content (wet nylon is weaker than dry)

• Macroscopic and microscopic discontinuities are common locations at which the fatigue process begins and the longer a material ages in it’s “lifetime” the greater the delta and unpredictability between the results of “test N” and “test N+1”.
• Figure 1 below is just representative example plot of generic possible “Tat” test results deviation between test A and test B.   As a material degrades and reaches the end of its life (fatigue and aging), results can vary wildly
• Any arguments against these assertions? 

Takeaways

• Bounce testing can only mislead you, and provide no real meaningful information about the state of the sketchy anchor you are about to use
• Regardless of the results, you DID just make the anchor weaker, reduced its energy absorption capability, and reduced its actual life for yourself and others.
• DO NOT bounce test ice anchors!  For example, a v-thread is bomber as is.  Ice is a brittle medium that can change rapidly with subsequent loads…way worse than old aging nylon.  All you did was weaken the anchor for no effective reason.   Even though it’s still likely to be fine, you needlessly weakened it for no benefit.
• If you absolutely need to use a sketch anchor for whatever reason, go as light and smooth as possible from the get go.  Treat it like old Nitroglycerin
• If you have a chance to back it up DO IT!

 
come at me bro

edit: only change was to clarify the diagram to emphasize the importance is the difference of the final end points of subsequent tests, not the curve itself based on feedback below -to avoid meaningless arguments in the weeds.  Many spurious arguments below in the “green” part where it doesn’t matter (new materials).

Mark Pilate wrote:

Man when I said “come at me”, I didn’t mean literally, lol.    Like tossing a steak in a pool of piranha.

it’s hard to tell who James is arguing with, seems both Tom AND me?  And we are on different sides seemingly.  As far as James’ last response to me..

James W wrote: (in italics)

Mil spec webbing is ~4k lbs breaking - WTF is this bounce test you’re doing on the side of a cliff that is coming even remotely close to yielding that?  

Nobody is worried about new webbing/cordage. Point went over your head.  It’s good to go, bounce or no bounce, but no bounce tests are needed -so apparently you agree with me.

James wrote: For the premise of unwanted yielding of highly degraded webbing - that is flawed as well - there is no plastic region remaining - it’s brittle.

The point is that aged, fatigued, and degraded webbing exhibits much different mechanical characteristics than new webbing.  Plasticity is degrading over time and use so it depends where you specify.  You’re nitpicking but It appears you again agree with me. 

James wrote:  Overall - C+ for your ability to produce a diagram and notes, D- for your engineering, F for the overall recommendation.

+1 - there is no excuse to not take your time and construct an adequate anchor.  The premise of needing to do some sort of engineering test on the side of the cliff and that it would have any value what-so-ever is SO many levels of wrong-headed.

You contradict yourself above - you say F for the recommendation not to engage in bounce tests, yet again you agree with me and +1 my recommendation -  to construct adequate anchors and to not do any uncontrollable testing on the side of a cliff.  

Which is it ? -like many here, you are showing either no logic or total lack of reading comprehension - or both to post an apparent argument that agrees with me. 

To clarify again the scope of this thread is unknown and unpredictable materials like old nylon or ice anchors as raised in the accident thread.  NOT new equipment or material, nor aid or pro placement applications.  

Mikey S and rgold as usual had the best takes.  Andrew P made a good point - my mistake was in posting a graph that had confusing curves in the effort to save time (even though it’s the end points that matter and the curves themselves are not part of the point or pertinent to the discussion, takeaways,  or conclusions). 

I changed it just focus on the end point deltas. 

Interesting that Kyle posted a scatter shot of nonsense then deleted it all…Instead of just calmly suggesting clarification on a point.  Since you focused on ice, I wonder why if it’s best practice, Petzl does not recommend a 2x force test (bounce) and only recommends similar to rgold - verify by actual use.  They  go on to say…

“Beware of existing abalakovs, as they may have hidden damage and turn out to be dangerous if reused.”  

And sudden higher loads than intended, by doing a bounce, can do similar damage. Have you discussed this with Petzl engineers at the factory when you were on the UIAA safety commission ?  I have.  (Custer’s understudy for a bit bitd)

Jason K and Andrew R raised reasonable questions.  And the answer is above by Petzl and rgold — If absolutely necessary to rely on sketch tat or ice anchors, your margin is best maximized by backing up and using as intended (carefully) to verify fitness vs doing an uncontrolled higher load force test that may be counterproductive. 

Still challenge anyone to make a coherent argument why a higher load bounce on already sketchy material is the best practice. 

Mark, I couldn’t be more proud of you.

Ok, I guess you're not trolling?  The issue is that your posts are so full of technical holes it's difficult to understand what your claims are based on.  Of course I agree that you should back up questionable answers, and not use sketchy v-threads, and your life is worth a couple bucks.  I doubt anyone disagrees with that.  The issue is with the attempts to base this in the fatigue properties of nylon, which you don't appear to really understand.

Mark Pilate wrote:

Nylon does have similar curve and varies only trivially.

Unverified.  You have not provided any evidence of this claim.  Materials, especially polymers, can display a wide variety of failure modes when tensile testing.  You have not provided a single bit of evidence that the materials in soft goods have the same behavior (as steel), nor have you provided any evidence for the single-cycle strength reduction your sketched on that plot.

https://polymerdatabase.com/polymer%20physics/Stress-Strain%20Behavior.html

Example is Ropes continuously lose stretch upon subsequent loading (falls) which is essentially same as strain hardening to an extent

False.  Strain hardening does not change the elasticity of a material.  You seem to be confusing hardness and modulus, which are completely separate properties.  That is a really big red flag about your claims in general, if this is indicative of your understanding of materials.

https://physics.stackexchange.com/questions/395105/why-doesnt-the-youngs-modulus-change-when-an-alloy-is-strengthened

Other aspects of this claim are dubious.  For strain hardening to occur, you would need to exceed the yield strength of the material in a rope on a fall and plastically deform it; this seems unlikely and you have provided no evidence for it.  Also, strain hardening occurs the first time a sample is loaded beyond the yield strength, not slowly over time.

This UIAA paper makes no mention of elasticity loss due to strain hardening (plastic elongation) specifically.  They also mention that ropes may get shorter over time, which conflicts with the idea of plastic elongation: https://theuiaa.org/documents/safety/About_Ageing_of_Climbing_Ropes.pdf

Perhaps even more importantly, you seem to think that a dynamic rope is stretchy because of the material properties of the nylon.  This isn't really true, because static ropes are made of the same material and yet they are not stretchy.  Dynamic ropes get their properties primarily from the geometry of the construction (the weave/twist), not from stretching the material itself, and as such the dynamicism is a function of that and not changes in the material.

https://www.maximropes.com/home/tech_info/rope_constructions/

"The core is comprised of straight bundles of fiber that are loosely twisted. These bundles help to keep the rope firm yet flexible.  The amount of twist in the core fibers in one of the main characteristics that dictate whether the rope is static or dynamic."

Are you saying that a material or specimen has infinite life if loads are kept below the yield point?

Yes.  This is material science 101.  Some materials have a stress where, if load is kept below that limit, they will have infinite life; other materials do not display this behavior.  Again, you apparently not being aware of really basic and fundamental material properties is difficult to reconcile with the confidence with which you're making these claims.

https://en.wikipedia.org/wiki/Fatigue_limit

Here's some data that shows how one type of Nylon will exceed 10 million cycles while retaining ~50% of its original strength.  No rappel anchor sees that many cycles, nor is a 50% strength reduction sufficient to cause anchor failure. https://www.toray.eu/eu/plastics/amilan/technical/tec_006.html#:~:text=Technical%20Information%EF%BD%9CProperties%EF%BD%9CFatigue%20endurance&text=As%20you%20can%20see%2C%20after,nylon%206)%20is%2019%20MPa

While 10^7 cycles may not be "infinity" per se, it is effectively infinity in the context of a rappel anchor.  Even an anchor that saw 100 rappels in a day (which is highly improbable) would need to be in place for 100,000 days (274 years) to reach this many cycles, and even then it's still strong enough.  This shows for repeated cycle fatigue is not the failure more of rappel anchors like you claim, as other posters have already pointed out.

Damage is irreversible. Materials do not recover when rested.  Load effects are cumulative

False.  See above.  This is the fundamental definition of elastic deformation, the material fully recovers.

Moisture content (wet nylon is weaker than dry)

The reduction in strength of wet nylon is not significant enough to cause anchor failure.  Here is some data:

http://jur.byu.edu/?p=21529

"We can conclude from the results of our testing that if used in wet conditions tubular nylon webbing will experience a small reduction in strength. This reduction is of little or no consequence to the men and women who rely on webbing for their safety so long as the manufacturer’s instructions and safe anchoring techniques are followed."

Slight tangent, but DMM found that wet dyneema/nylon blend slings with knots were actually stronger than dry ones: 

https://dmmwales.com/knowledge/march-2012/knotting-dyneema%C2%AE

 Go do your own tests and report back before spouting off.

You have not provided a single piece of test data to support all of the claims you are making.  It's not reasonable to make unsupported arguments, and then ask all counter-arguments to have data.

my mistake was in posting a graph that had confusing curves in the effort to save time

Your graph showed fundamental misunderstandings about material behavior, including:

-You showed a change in a elasticity (modulus) due to a load cycle.  Materials don't behave like this, Young's modulus is effectively constant over the life of a material.

-There is no data supporting the claim you made in that graph, which was that a sample will degrade from a strength of 4 kN to 3 kN after a single cycle of applied load.  That large a reduction in strength is not supported by any of the normal fatigue theories or curves you keep trying to reference.  

https://material-properties.org/what-is-fatigue-life-s-n-curve-woehler-curve-definition/

What is this graph even showing?  Can you post a link to the source data that this graph is based on?  What are the unlabeled axes, is it strength vs. time, strength vs. cycles, something else?  Are A and B two different pieces of material?  Does sample A magically get stronger over time and B gets weaker?  What part of this graph shows a sample that holds 3 kN in a bounce test, and then fails to hold body weight?  If you're going to try to use this to support your point, you need to do a better job explaining it...

 Petzl does not recommend a 2x force test (bounce)

This is a red herring.  Just because they don't say something, doesn't mean they support your claim.  Your specific claim is that bounce testing a v-thread will make it weaker than the force you bounce tested to.  You have not provided any evidence to support that, nor have you provided any source (e.g. Petzl) that speaks on this specific matter.

They  go on to say… “Beware of existing abalakovs, as they may have hidden damage and turn out to be dangerous if reused.”  

This is a straw man.  Of course old v-threads can be dangerous, nobody is arguing that.  We're specifically talking about your claim that loading an anchor to X force will make it less likely to withstand X force on the next cycle.

Have you discussed this with Petzl engineers at the factory when you were on the UIAA safety commission ?  I have.

This is an appeal to authority.  A vague mention of something you maybe did once doesn't support your argument.  Post a link to test data, or a quote from someone whose qualifications we can check.

Still challenge anyone to make a coherent argument why a higher load bounce on already sketchy material is the best practice.

If an anchor holds X kN when bounce tested, it is likely that it will hold that same force when used to rappel shortly thereafter.  It's not guaranteed, of course, but it is likely.

If this isn't correct, why don't you post the data proving it.  You keep arguing that this is untrue and use some vaguely applied technical-sounding terms, but you haven't posted a single shred of data to support it.  Show us tested behavior where an anchor or piece of material holds a force in the range of bounce testing, and then fails under bodyweight.

Again, I am not suggesting that people trust old materials, or that materials don't become weak over time.  Soft goods take a beating and get very sketchy, and people should not trust them.  However, the mechanism for that strength reduction is completely different than the mechanisms you suggest, and your theory that bounce testing doesn't provide useful feedback isn't supported by these mechanisms.

Mark Pilate wrote:

it’s hard to tell who James is arguing with, seems both Tom AND me?  And we are on different sides seemingly.  

You both present flawed thinking - is my disagreement.  

You’re posting a bunch of college textbook crap on material properties - it’s irrelevant.  The manufacturer has done the design and testing, your only job at the anchor is to assess whether there is damage or degradation such that the material would no longer support the load it was designed for.  This is going to be a visual inspection.  

Suggesting a decision space exists in the “fatigue” or “plastic” region of material is irrational - because in this case the anchor looks like shit and should not be trusted at all.  Your premise and supporting reasoning are a useless Red Herring fallacy.

I’m actually a fan of a few pulls and bounces - to check equalization, direction of load, settling and tensioning of soft material, settling into a crack, tightening knots, looking for possible cutting and abrading issues, etc.  

I am NOT thinking about fatigue, plastic deformation or tensile failure - because that would imply the anchor is constructed of rubbish.

Enough blather. I used to (~25 years ago) collect tat and tie them together with ring bends into long strands and bounce tested them to breakage. Nasty, crusty, desert bleached webbing. Never once did they break under static conditions(body weight between bounces). Only under 'peak' forces. Partial tears occured, occasionally, but were always easy to identify. 

So, bounce test away, is what I say(and do).

Tradiban wrote:

Mark, I couldn’t be more proud of you.

Are you two sharing medication?

Not Sure wrote:

Mark Greg and James are you a mechanical, system engineer or physicist ? What if you bounce on a number one stopper or 00 cam and it holds? OK?

I’m a Sagittarius 

Also not sure what you're point is, I don't see the need to bounce test an anchor - that should be visually inspected. Aid climbing I bounce test quite often. I wouldn't rappell off an anchor that isn't redundant or solid, regardless of bouncing.

Do I need a degree in STEM to come to these conclusions, or is 22 years of climbing and 7 years of professional instruction and mentoring enough lol

One more than just enough.

MattB wrote:

Enough blather. I used to (~25 years ago) collect tat and tie them together with ring bends into long strands and bounce tested them to breakage. Nasty, crusty, desert bleached webbing. Never once did they break under static conditions(body weight between bounces). Only under 'peak' forces. Partial tears occured, occasionally, but were always easy to identify. 

So, bounce test away, is what I say(and do).

This is a good way to crystallize the blather and total lack of basis for bouncing on sketch.   You essentially have presented data that the bounce testing was unnecessary —“Never once did they break under static conditions(body weight between bounces).” —-  agreed.  

I’ve done similar, and pull material samples regularly on an Instron. The statistical  spread on identical aged/fatigued/worn samples of dang near any material, but especially the materials in discussion here, of Pull N to N+1 is wider and more unpredictable the higher the forces used for N.       continue to argue

Example from real world.  You come to an unknown piece of tat of dubious strength and for whatever reason, you need to rely on it (let’s say it’s at the edge of useful life and if pull tested would fail at 3.5 kN like the accident example)  What is the best algorithm to follow to provide you the widest margin for the rap?  (For arguments sake you just can’t leave your back up here)

Choice A-  Bounce it with close to failure load, then rap, or Choice B- back it up while first heavier person raps carefully, then second does the same  

answer is choice B

Any of you pro bounce people, Show me any analysis or data from engineering, physics, mathematics etc that says you are statistically safer with the 2-3 x load bounce test preceding your rap, vs the careful confirmation rap of your buddy.  

If it’s not near failing, it doesn’t matter, the bounce was superfluous.  If it’s near failing, the bounce leaves you more exposed statistically than before your “test”

I’ll send the first person here, $100 worth of slings of your choice if you can prove the validity of bounce testing unknown sketch.  

The point is it doesn’t matter until it matters and it matters most at the edge of the envelope.  If you’re not at the edge you just get confirmation bias that what you are doing is smart.  

Kyle, post justification for what you are doing.  Don’t get in the weeds of metals like titanium and curves that are outside the discussion.  I have always been discussing old nylon and ice as examples of materials that have a large delta of results from one test to the next. 

Let’s cut to the chase and keep it simple—You evidently like to bounce on ice.  There must be a reason you are needlessly shocking your ice anchor to 2-3 times the intended load before committing to it.   You obviously do not understand 

1.  How strong normal ice anchors are - thus they need no bounce test

2.  the variables involved and nature of ice to think this is useful or smart if you were in fact on the edge but didn’t know it.  you must have some engineering models, test data, etc leading to your reasoning that you are therefore statistically safer with a higher load shock test that a “normal use” verification test.  Please show it  

Edit that got cut off:  I did reasonably extensive (couple dozen) firsthand tests on marginal v-threads over Covid.  (Not scientific or statistically significant) but still more validating than your intuitive gut feel with so far no engineering backing to justify your bounce procedure on fragile ice. 

Chiming in to mention that I don't think bounce testing is common practice in canyoneering which, as someone upthread mentioned, is a sport somewhat devoted to rapping off the sketchiest, sandiest, safety-absent-est anchors imaginable.

Here is a report of a webbing anchor that failed after the 3rd bounce when testing. http://canyoncollective.com/threads/webbing-failure-during-test-bouncing.25665/

I sure wouldn't have wanted to rap down after bounce test #2 thinking it was *good enough*!!

I personally think we as climbers should be a little more humble in listening and learning to the craft techniques of other rope related sports.

The reality is that you don't know the breaking strength of a particular piece of old tat, and a good bounce tells you something - namely, that the breaking strength is higher than the forces you may apply while rapping. 

To use your if/then example:

1. If the sling MBS is >> bounce and rap forces, then bouncing is superfluous - true, but you've lost a few seconds of time at most.

2. If the sling MBS is ~ bounce forces and > rap forces, then you may or may not rip the sling by bouncing it. You're arguing that there's a chance it's so close that you actually lower the MBS of the sling on your last bounce to where it's ~ or < rap forces, which takes us to cases 3 and 4.

3. If the sling MBS is ~ rap forces and < bounce forces, then you'll rip it by bouncing. If you don't bounce, you might rip it while rapping. Either you shit your pants as your backup catches you, or your last person to rap rips it and dies.

4. If the sling MBS is < rap and bounce forces, then you'll rip it by bouncing or by rapping first. Again, your pants are soiled.

Bouncing the tat doesn't matter in case 1, saves a life in case 3, saves you stress in case 4. You're arguing that case 2 is a significant risk and the pro-bounce folks don't think it is. I think case 3 is a much higher risk, especially since even the most careful rap by the last person may cause a peak load.

The last person might not die if the ends are secured at the next rap station. But idk, I’ve never taken at 60 meter factor 2 whip. Or would it be factor 1 if the rope is doubled?

Big Red wrote:

The reality is that you don't know the breaking strength of a particular piece of old tat, and a good bounce tells you something… 

Thank you Big Red for a reasoned, logical, and spot on reply.  (It’s not that hard).  You should get a free sling regardless, lol  when I said “come at me” at the opening, your responses are what I meant   

Yes, the cases of interest or discussion are in the 2-3 range of your scenarios.  (Not sure about the soiled pants part as I’ve done similar many times testing sketchy rap scenarios and a few inches of jarring when expecting the possibility is pretty trivial)

And yes I’m saying that statistically, extrapolating from aggregated meta data of similar related testing statistics, your odds/margin is better for potential “victim guy” when encountering unknown qty X , potentially somewhere in the 2-3 range, if you follow the protocol of 1. Do heavier guy first as test, then do potential victim guy.  

Adding in a 2-3x load force pre-test, ahead of it all adds an unnecessary wild card that adds little value.  
ESPECIALLY in marginal ice. That’s a whole ‘nother animal even than old tat. Again, it’s all Russian roulette at this point, but if you played it out a 1000 times, “potential victim guy” on the final rappel survives more often without the higher shock load pretest just immediately before.

I’m totally open to data or convincing to the contrary

We're talking about bounce testing tat on rap anchors? Is that the context here?

Nathan Doyle wrote:

We're talking about bounce testing tat on rap anchors? Is that the context here?

Basically.

 Old tat, or ice as well.  Basically any material  either by age, conditions, or history (tat) or it’s very nature (ice) that is very unpredictable in its response to loads and successive loading 

It's certainly a resilient topic.