Thermal Expansion

It would be easy to measure. Next time bring a caliper, measure the width of the rock at the nut placement, wait until it gets stuck and measure the width again.

Definitely the rock.

Total movement is going to be dependent on how the rock is constrained. Most splitters, are constrained from lateral movement by adjacent bodies which would make the crack shrink as heat was absorbed. In the case of a split boulder or monolith where lateral movement is not constrained it could go either way.

It is impossible to make any generalizations based on measurement no matter the accuracy, as the rock's movement was dependent on its constraint, its mass, its make up.

Again no generalization about the increased amount of force to remove a stopper given a displacement of 'X' as most/all of the time there is some local and permanent deformation of the stopper when placed. (Scratches, dings, etc).

Interesting phenomenon. Made my Monday a little more interesting.

I almost FEA'ed in until I realized the constraints were unknown.

20 kN wrote:It would be easy to measure. Next time bring a caliper, measure the width of the rock at the nut placement, wait until it gets stuck and measure the width again.

Jacob S wrote: I almost FEA'ed in until I realized the constraints were unknown.

Picked up a #4 BDel cam on Zodiac last year. My partner can across it fixed and couldn't get it out. By the time I got up to it, the temp had raised quite a bit (from very cold to sweaty hot) I reached for it and it came out with no difficulty.

I'm skeptical that such an effect is significant. Stoppers get stuck all the time, even when there is no temperature change. It seems too easy to blame rising temps on the inability to get a nut out. Anybody have any real data?

Gunks Jesse wrote:Easy experiment: next time you can't clean a nut and expect thermal expansion, take the exact same size and try to slot it in on top. If it is the nut expanding it should go in. More likely it is the crack contracting and it won't go in. Think expansion joints and bearings on bridge decks...

I've experienced this at least 3 times:

On Spaceshot, after cleaning and hauling the first aid pitch, I decided to hang my lead rope from a large blue DMM alloy offset in the crack next to the anchor. When After leading the next pitch (in full Sun), I rapped back to my anchor and tried to clean the nut. I couldn't get it to budge at all and I ended up leaving it there. The piece had never been weighted by me and only held my coiled 60 meter rope.

Again on Spaceshot, my last placement before Earth Orbit ledge was a tight #1 C4 placed in late afternoon, just as the Sun was setting. When I came to clean it, I couldn't move it at all. I worked on it for 10 minutes or so before deciding to haul before it got too dark. The next morning, I rapped down to it and it came out with no effort at all.

On Zodiac, I placed a small brass offset shortly before the P3 belay anchor. My partner spent several minutes trying to clean it with a nut tool, but we gave up. We were in the hot afternoon Sun and were preparing to fix to the ground. The next morning while jugging up, I came up to the nut and decided to give it a shot. It came out easily, no nut tool required.

Spaceshot seems to eat up nuts due to this effect!

There is a thread on Supertopo where Greg Stock, Yosemite park geologist, describes using instruments to measure the thermal expansion effects on rock.

Found it: supertopo.com/climbers-foru…

On the second page there are some links to abstracts of scientific talks they gave about the work.

Yes, some rocks do expand, sometimes quite quickly, when they warm up. It's well known in Yosemite, in fact NPS geologists even have devices measuring expansion to better learn about rockfall triggering:

supertopo.com/climbing/thre…

Here's another thread on expansion:

supertopo.com/climbing/thre…

Personally I've had a #1 Camalot literally squished by expansion of an immense flake (100+ feet tall, several feet thick, unknown width, basically the outer skin of a big section of a Tuolumne Dome). We couldn't budge it, came back the next morning when it was cold, and there was about 1cm extra space (almost half an inch!). Whole cam was slightly squished/deformed!

I've encountered this exact scenario. Placed a small nut in the shade, the sun hit it a short time later and it was badly stuck. After working on it for some time and making very little progress, we left it. I returned a few days later and it lifted out without even needing a nut tool. I've since used the the same nut in the same placement several times and not encountered the problem again. I too suspect that temperature changes can cause cracks to expand and contract.

Thermal Expansion is calculated by
Change in Length = Thermal Expatiation Coefficient * Change in Temp * Initial Length of Object

The expansion for high strength 7000 series aluminum alloy used in Carabiners is about .000024 in/in C
For an average sized nut the expansion due to change in temp is so small you can throw it out.

However the expansion of the rock is where thing get interesting. Doing a little research I found the average thermal expansion for sand stone is .00003 in/in C

Data found here if you want to do calculations on other types of rock pubs.usgs.gov/of/1988/0441/…

If we assume 3 feet on either side of the rock is expanding, with no cracks for 6 feet total of expanding rock and a 5 degree c change in temperature the rock expands about .011 in