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daithi
Oct 23, 2006, 2:21 PM
Post #51 of 53
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Registered: Jul 6, 2005
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In reply to: To show that fatigue is not an issue, take a 25Kn aluminum biner. A conservative average for fatigue life of aluminum loaded to half its ultimate tensile strength is above 10^4 cycles. So fatigue is not going to be an issue unless you are applying 12.5Kn loads to a biner for multi thousands of cycles. The approximation is rough, but that's a lot of huge wippers. How many load cycles do you reckon a belay biner is subject to over its lifetime? This is not to say fatigue is a problem for belay biners but I reckon it is subjected to a lot of cycles thankfully at very small stresses!
In reply to: As for micro-fractures (crack nucleation occuring) from dropping a biner, it doesn't happen either. The key is that Aluminum has a relatively low sensitivity to strain rate. It does not display a clear ductile to brittle transition with regard to how fast it is loaded (unlike steal). This means that unless dropping the biner results in its deformation, then nothing really happened. In reply to: MICROFRACTURES-don't exist from dropping a biner as aluminum has low sensitivity to strain rate. This seems quite an extraordinary assumption that because Aluminum isn't sensitive to strain rate it can never develop cracks in the micro-structure due to high local regions of strain. Do you have a reference for this being a known property of aluminum. I don't know about the exact alloys used for the manufacture of carabiners but I assume they are all heat treated. The crystal lattice contains local stress concentrations. Is it not possible for the local high strain caused by dropping a biner to cause a fracture in the micro-structure at such a stress concentration? A definitive statement that it never does seems counterintuitive.
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crimp2bfree
Oct 23, 2006, 4:40 PM
Post #52 of 53
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Registered: Jan 5, 2005
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In reply to: This seems quite an extraordinary assumption that because Aluminum isn't sensitive to strain rate it can never develop cracks in the micro-structure due to high local regions of strain. Do you have a reference for this being a known property of aluminum. I don't know about the exact alloys used for the manufacture of carabiners but I assume they are all heat treated. The crystal lattice contains local stress concentrations. Is it not possible for the local high strain caused by dropping a biner to cause a fracture in the micro-structure at such a stress concentration? A definitive statement that it never does seems counterintuitive. FCC metals (aluminum) are genrally highly ductile and consequently HIGHLY localized (as in so local you can see it) strain does not occur. The ductility allows the strain to spread throughout the stucture. This is the proccess of stress propegation, where localized stress concentrion become disributed. This means that any strain resulting in a fracture in a biner could not be isolated to a small micro scale location. The degree of strain nessesary to cause a fracture would result in gross macroscale deformation of the biner. Everything I said above refers to pseudo-static material mechanics. This constrasts with dynamic failure modes, which of most concern include impact and fatigue behavior. Aluminum having a low strain rate sensitivity shows that impact (ie high strain rate) analysis is not a concern. Many materials, become brittle at high strain rates, resulting in cracks with little deformation. Dynamic testing of materials is very difficult and confusing but these people are working with aluminum: http://www.me.uwaterloo.ca/...wick/smf/HSR/hsr.htm Fatigue, the other dynamic failure mode, was shown before not to be a concern under normal use. Pseudo-static analysis is therefore appropriate. As for a belay biner, the decrease in load results in a highly exponetial increase in cycles before failure-no need to worry. Even if somehow a "microfracture" did occur, it would not be too much of a concern. It would result in a local stress concentration that would then be distributed through yielding. In fact, in static alalysis of ductile materials, stress concentrations are often simply ignored. This assumes that it is a microfracture and not a macro one that would actually reduce the cross section of the biner. The concern has been that something would happen to a biner that you couldn't see and it would result in failure of the device far below its specifications. Aluminum's ductile characteristics mean that any damage to a biner will be visible due to the strain distribution. With all this being said, still be reasonable and do what you are comfortable with. If that dropped biner gives you the willies, put your keys on it. If it fell fifty feet onto leaves and dirt, it should be fine. 500 feet onto rock, I'd be concerned. The gates and where they attatch are the most prone to damage. I saw a biner explode as it went bouncing by on White Horse Slabs in NH once. If the gate seems funky, doesn't allign, probably no good.
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daithi
Oct 26, 2006, 11:16 AM
Post #53 of 53
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Registered: Jul 6, 2005
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In reply to: FCC metals (aluminum) are genrally highly ductile and consequently HIGHLY localized (as in so local you can see it) strain does not occur. The ductility allows the strain to spread throughout the stucture. This is the proccess of stress propegation, where localized stress concentrion become disributed. This means that any strain resulting in a fracture in a biner could not be isolated to a small micro scale location. The degree of strain nessesary to cause a fracture would result in gross macroscale deformation of the biner. In reply to: Even if somehow a "microfracture" did occur, it would not be too much of a concern. It would result in a local stress concentration that would then be distributed through yielding. In fact, in static alalysis of ductile materials, stress concentrations are often simply ignored. This assumes that it is a microfracture and not a macro one that would actually reduce the cross section of the biner. This all flies in the face of well known fatigue crack propagation in aluminum alloys. Obviously some micro-scale cracks do propagate when subjected to sufficient stress and enough cycles, otherwise fatigue would never be a problem in aluminum. If indeed the local stress concentration was always distributed through yielding, cracks (regardless of how they originated) would never propagate. It seems the aluminum alloy used in carabiners is bestowed with almost magical properties that makes it impervious to either develop cracks or to propagate those cracks. This makes it unique amongst any metal I am aware of. Actually a crack will propagate in any metal if the stress intensity factor is high enough, even aluminum [1]. [1] Wei, R. P., "Fatigue-crack propagation in a high-strength aluminum alloy," International Journal of Fracture, Vol. 4, No. 2, 1968, pp. 159 -- 168.
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