Core shot??

[sonnylawrence]

Brian Jennings sent me a dynamic rope with possible core damage to be broken on the Instron slow pull machine. From the picture, note that the sheath was torn, revealing the core which had a few threads puffed out. First a test sample from the same rope was broken. A double fisherman’s knot was used to make a loop. It broke at 23 kN. The knot was intact. The test piece also broke at 23 kN. However it did not break at the damaged area. Instead, it broke at the point that the rope bends around the pin in the Instron machine.

[Zarka]

It broke at 23 kN. The knot was intact. The test piece also broke at 23 kN.

Please clarify: 23 kN on each strand (2 strand total=46 kN?) or 11.5 kN for a single strand of the rope?

What was the size of rope?

Did both test ropes fail at the rod?

[sonnylawrence]

Brian will have to tell us about the rope: make, model, year, mileage.

The 23 kN is a rough breaking value for the "system." This system consists of the doubled rope, knot and relatively sharp bend around the steel pins in the Instron machine. If these pins were frictionless pullies, this system would have a greater chance of breaking at the knot which is the weak point. So, very roughly the rope broke at 11.5 kN.

[SirKnotcelot]

If these pins were frictionless pullies, this system would have a greater chance of breaking at the knot which is the weak point. So, very roughly the rope broke at 11.5 kN.
I'm impressed by how short the test specimen is, which I think violates some
guidelines about making a good test. Lyon Equip. conjectured that the rupture
at pins vs. knots (also testing the Grapevine) resulted because of the movement
of rope around the pin at high pressure, as the knot, compressing under tension,
feeds material to its side of the loop. That said, someone tested the same knot
structure (i.e., a Dlb.Overhand/Strangle knot) tied as a hitch--8mm around a
standard 'biner: the break came in the main line where the end turns around it
to make the overwrap of the Strangle knot (NOT at the 'biner!).

SirK