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Monday lunchtime-Fatigue testing in soft rock

mikllaw

8-Nov-2010
5:05:23 PM

Due to limited time I made up something I call a Block Test:- do 5 cycles at a low load, 5 a bit higher etc until rock cracking develops, then continue cycling at that level; I think it’s a good comparative test for that anchor and rock type.

I tested 75 mm carrots, 75 mm x 12 mm dynabolts, and an 80 mm leg length Ubolt (10 mm shaft). As my pressure gauge is marked in 1000 psi divisions I started at 1000lb (3.3 kN) and went up in 500 psi (1.65 kN) increments (except for the Ubolt which went up more rapidly).

I tested the carrots first and they were loaded to 5 x 3.3 kN, 5 x 5.8 kN, 5 x 8.3 kN, then started rock cracking at the 2nd or 3rd cycle at 10.8 kN. One came out after 12 cycles at 10.8 kN, the other lasted 13 cycles at 10.8 kN, then as I raised the load again it failed at 11.8 kN, these loads were about 80% of the breaking strength of new bolt (14 kN).

The 2 dynabolts were loaded to 5 x 3.3 kN, One of the nuts loosened at this point and I re-tightened it, then took it through 5 x 5.8 kN cycles. They showed a lot of rock cracking at 8.3 kN and failed after 6 and 7 cycles respectively, about 50% of the breaking strength of 19 kN from previous testing (the two I tested failed at 19 kN and 28 kN). I was amazed at how quickly rock started cracking around these units, I suspect the multi-part design is just too flexible. Once some bending and rock crushing started, increased flexing occurred. Harder rock would have made a big difference.

I assumed the Ubolt would be good and ran it up in bigger load increments;
5 x 3.3 kN
5 x 8.3 kN
5 x 13.3 kN
5 x 18.3 kN
5 x 23.3 kN
5 x 28.3 kN
5 x 33.3 kN, at this stage some minor rock cracking occurred. I was getting tired so decided to run it up to failure, but an oil leak in a pump fitting occurred at 39.2 kN, terminating the test. There was bending of the Ubolt as well as rock and glue cracking, but it still looked very solid.

Ubolt at 39 kN

A weird fatigue graph...

So, for soft sandstone it looks like dynabolts are not useful if they are going to be fallen on regularly, ditto with carrots (though they performed better). Both looked terrible long before they came out. Bigger dynabolts (14mm or 16 mm dynabolts should be 3.8 and 6.5 times stiffer) could have much better fatigue lives but would be expensive in stainless. The Ubolts seemed bomber and I’d expect rings to have about 60% of their strength, which is sufficient. Smaller 8 mm shaft rings might be a bit more marginal in poor Blueys rock, but are probably ok in most good Blueys rock.

Fatigue is significant in a sports climbing world, the major factor is the flexibility of the shaft, place appropriate gear.


Butters81

8-Nov-2010
7:12:31 PM

Great work.
It's good to have some quantatative evidence of what most of us already suspect (ie, U's are bomber)


salty crag

8-Nov-2010
8:55:43 PM

Good stuff, feel good as I'm about to make a few u-bolts. Have you done any testing in Basalt?

hero

8-Nov-2010
10:11:46 PM

I was a lttle tired at lunch today as well.

mikllaw

9-Nov-2010
8:44:40 AM

On 8/11/2010 salty crag wrote:
>Good stuff, feel good as I'm about to make a few u-bolts. Have you done
>any testing in Basalt?

Basalt should be much much stronger, expansion bolts work well, and failuire occurs by pullout or shaft failure, rather than by rock crumbling, but at higher loads.

mikllaw

9-Nov-2010
8:57:42 AM

A confession- I did fudge the results a bit.
I placed the carrots as I would normally (i.e. pretty well; I used the best bit flat piece of rock, reamed the hole till the carrot inserted 20 mm, then pounded plenty) but placed the dynabolts as they would be placed "normally"; both were on slightly lumpy patches so the brackets sat about 2 to 4 mm off the rock surface. I expect that if the placements were reversed, that the dynabolts might be slightly better than the carrots in fatigue strength.
The poor rating of the carrots is no surpise, the old carrots in The Glen looked poxy after a few years (tho I wonder how much of that was bracket and crab wear).

In the US about 10% of the fixed anchors at places like Red and New River Gorges are loose (probably more due to leverage on brackets than the actual loads), and many of them are very rusty. Neil emailed me last week from RRG:-
"It's rusting to sh?t in Red River Gorge. Most of The Motherload has bolts almost black with rust - all flaky."

gfdonc

9-Nov-2010
10:11:43 AM

On 8/11/2010 salty crag wrote:
>Good stuff, feel good as I'm about to make a few u-bolts. Have you done
>any testing in Basalt?

Yeah! And what's he done on grit?

mikllaw

10-Nov-2010
12:06:35 PM

The only 2 fatigue failures i know of were
- a bobby pin (2 x 8mm shaft gluein P bolt) at the base of Old Blobby, The freezer, where 25 mm of (very bad) rock had crumbled away, but it was still hard to remove.
- A carrot in Bullethole wall at Berowra fatigued till failure during repeated falls while an increasingly nervous belayer kept warning his leader that the bolt was coming out, I don't know if any injury occured. Replaced by a 14 mm reo shaft x 200mm length Ubolt

One Day Hero

10-Nov-2010
5:14:57 PM

On 9/11/2010 mikllaw wrote:
>Basalt should be much much stronger, expansion bolts work well, and failuire
>occurs by pullout or shaft failure, rather than by rock crumbling, but
>at higher loads.

I think that should read "bolts in basalt fail by unplugging the block which they are in, resulting in a 'thunderdome style' ride with a 150kg block clipped to your rope" or "bolts in basalt fail when the entire pillar collapses, squashing you and your belayer into meat-jam"

mikllaw

10-Nov-2010
5:25:37 PM

On 10/11/2010 One Day Hero wrote:
>On 9/11/2010 mikllaw wrote:
>>Basalt should be much much stronger, expansion bolts work well, and failuire
>>occurs by pullout or shaft failure, rather than by rock crumbling, but
>>at higher loads.
>
>I think that should read "bolts in basalt fail by unplugging the block
>which they are in, which results in a 'thunderdome style' ride with a 150kg
>block clipped to your rope" or "bolts in basalt fail when the entire pillar
>collapses, squashing you and your belayer into meat-jam"

Those, too, are acceptable alternatives I wish to examine from a safe distance.


Pat

10-Nov-2010
6:05:48 PM

On 9/11/2010 mikllaw wrote:
>A confession- I did fudge the results a bit.
>I placed the carrots as I would normally (i.e. pretty well; I used the
>best bit flat piece of rock, reamed the hole till the carrot inserted 20
>mm, then pounded plenty)

these were not glued in carrots? What diameter were they mikl and did you prepare them with ground tapers?

mikllaw

11-Nov-2010
7:24:38 AM

On 10/11/2010 Pat wrote:
>these were not glued in carrots?
No, SGABs are strong but you still have to buy a bracket or use a keyhole which are problematic, why not put in a Ubolt if you have power and glue and time, or a carrot if you are hand drilling and have no time

>What diameter were they mikl
3/8" standard (= 9.5 mm)

>and did you prepare them with ground tapers?
yes, standard blueys grind


salty crag

11-Nov-2010
10:35:27 PM

On 10/11/2010 One Day Hero wrote:
>I think that should read "bolts in basalt fail by unplugging the block
>which they are in, which results in a 'thunderdome style' ride with a 150kg
>block clipped to your rope" or "bolts in basalt fail when the entire pillar
>collapses, squashing you and your belayer into meat-jam"
Er thanks for that ODH, It will no doubt spring to mind just as I clip my fat bod into some dodgy Basalt choss...

mikllaw

12-Nov-2010
8:23:28 AM

Just to populate the graphs better, and get some more statistics I'll fatigue test another Ubolt (too much work), another 2 carrots and 2 dynabolts.

The carrots will be in bad placements and the dynabolts in good placements. I might do one of each of these just at 3.3 kN cycles (average hard falls) till failure or till i get bored.

Has anyone seen fatigue failures on good rock?

patto

12-Nov-2010
11:43:55 AM

The relationship between fall factor and force is NOT linear.

The calculated force for a 0.3FF is around HALF the force for a 1.78FF. By my calculations the force on the climber should be around 3.8kN which means the force on the gear is 6kN.

This should be no surprise considering a top rope fall still puts around 3x body weight on the top piece. So around 2.4kN.

Impact Force:

F=mg+mg(1+2fEA/mg)^0.5
f-fall factor
E-youngs modulus of rope
A-area of rope cross-section
m-mass
g-gravity acceleration

Force on top piece is ~1.6x impact force.
Drop drag on previous pieces increases force, belay slip decreases.

EDITED FIGURES AS THERE WAS A SLIGHT ERROR IN EXCEL.

mikllaw

12-Nov-2010
12:27:33 PM

On 12/11/2010 WM wrote:
>can you plot this for me? (yes i'm being lazy)
>i'm particularly interested to see how quickly the peak force rises between
>FF 0 (0.8 kN - sitting on rope) and FF 0.3 or so.

>edit: BTW not only does that formula assume a super hard belay (eg Grigri
>anchored down with no rope slip), it doesn't account for harness squish
>and belayer lifting.
And no 'soft' option like a grigri, it assumes a rigid connection at the end, like a well bounced knot

patto

12-Nov-2010
12:40:32 PM

On 12/11/2010 WM wrote:
>can you plot this for me? (yes i'm being lazy)
>i'm particularly interested to see how quickly the peak force rises between
>FF 0 (0.8 kN - sitting on rope) and FF 0.3 or so.
As I indicated earlier FF 0, sitting on the rope is NOT body weight!

It is actually 2x. Furthermore the force on the top piece is 1.6x. Thus a sit on the rope is 3x body weight on top piece.

EDIT: To clear things up I referring to a fall when there is 0 slack in the rope. This is different from the static case of resting on the rope.

mikllaw

12-Nov-2010
1:03:04 PM

On 12/11/2010 patto wrote:
>As I indicated earlier FF 0, sitting on the rope is NOT body weight!
>It is actually 2x.
2x is force on top piece, or 1x force on rope, whichever you want.

rightarmbad

12-Nov-2010
1:23:47 PM

Hang on, when sitting on the rope, the force is never more than 2 x body weight on the top piece, if the climber is stationary and not bouncing up and down.
Any friction will only serve to reduce force on the top piece to the point where it becomes body weight if the friction through the top piece becomes infinite.

patto

12-Nov-2010
1:50:23 PM

On 12/11/2010 rightarmbad wrote:
>Hang on, when sitting on the rope, the force is never more than 2 x body
>weight on the top piece, if the climber is stationary and not bouncing
>up and down.
>Any friction will only serve to reduce force on the top piece to the point
>where it becomes body weight if the friction through the top piece becomes
>infinite.

Sorry. I should have explained it better. I was basing my comments on:
"can you plot this for me? (yes i'm being lazy)
i'm particularly interested to see how quickly the peak force rises between FF 0 (0.8 kN - sitting on rope) and FF 0.3 or so."

In this context I took 'sitting' on the rope as describing a fall when the rope has 0 slack in it. The peak on the climber here is 2x body weight.

Of course your static weight is like you said simply body weight. Top piece load is like you said up to 2x.

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