USnavy wrote:
cracklover wrote:
If the MBS is exact, it should fail at just about 3kN.
A loop of that should get me a force of around 4.5-5.5kN.
I dont know that that statement is correct. Awhile back I did some testing of 1" webbing tied in a loop with a water knot. I tested it in a hydrulic pullback ram press with steel carabiners attached between the equipment and the webbing. The 1" webbing was rated for 19 kN. With the webbing tied in a loop as I described above, the loops of webbing failed at 22 - 23 kN, which is no where near double its rated strength. Interestingly enough the webbing never failed at the knot, it always failed at the carabiner. In once case the webbing started to tear at the knot about 15% of the way through, and in the end the webbing still failed at the carabiner.
I looked at the biner and there were no sharp edges on it. I swapped them out with others but still got the same result. I came up with a theory as to why this is but I have never been able to really research it so I am just guessing here. But I theorised that the webbing is failing,
a. only slightly above its rated strength vursus double its rating.
b. at the carabiner
because although the load is being supported by two pieces of webbing between the carabiners, at the dead center of the carabiners there is only one piece of webbing. Think of it this way, if you took a picture of a loop of webbing and drew a line straight through the middle in MS Paint, each side of the line would have a piece of webbing (two pieces total) which would share the load. But right where the line intersects the webbing, there is only one piece of webbing, there is nothing else to share the load, thus the loop is limited to the breaking strength of a single piece of the material your using.
It seems like this could be a viable theory as if you look at 1" sown slings, they are normally rated to 22 kN. Well half that reason is because UIAA requires them to be rated to 22 kN, but if creating a loop of webbing would effectively double its strength, those slings would hold a lot more than 22 kN, but they don't. I have tested them, they fail around their rating.
It would be nice if someone who understood the mechanics of whats going on in my scenario chimed in.
gabe's using cord, not webbing. completely different animal.
