Wont these tests only be valid if your fuse has the same elasticity of whatever you plan to test? Look at the drop tests on spectra and nylon slings. It was the same test with very different peak forces.

It´s easier and very accurate to use a lever, either add weight (water is handy) or move the weight outwards.

To get things like your fuses calibrated go to a lifting gear company, they´ll usually bust them for a beer or two in my experience and they have calibrated pullers, slightly less accurate but still good is get a friendly crane driver to bust them while watching his load indicator.

Jim

Oh cool, thanks for the suggestion! Now I just need to find a lifting company.

G

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.

The tensile strength of tape is tested using large diameter clamping rollers, from memory something like minimum 50mm. From testing done for slacklines it is clear that with diameters under 10 times the thickness of the tape the strength drops off considerably. I´ve got a table somewhere from when I worked on this and normal nylon tube tape needs to be de-rated to about 2/3 of the original for a 10mm dia which you commonly find on karabiners and is the pin diameter used for sling testing.

Rope and cord strengths are tested in the same way but the pin diameters vary depending on the standard (ASTM D4268, EN919 etc) and other factors such as construction. Termination strength reduction is then applied for finished products.

It is also our experience that tapes fail at the karabiner not the knot, rope seem more prone to fail at the knot.

Jim

gabe's using cord, not webbing. completely different animal.