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Re: webbing strength

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  • dlfrost_1
    ... not ... outter ... unison, or ... would ... designed ... strength ... Your description of unequal loading inside/outside is correct. But there is an
    Message 1 of 4 , Oct 31, 2005
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      --- In hammockcamping@yahoogroups.com, "Dave Womble" <dpwomble@y...>
      wrote:
      > I always thought that rope strength was degraded with tight bends
      > (which knots do) that required the individual fibers to stretch at
      > significantly different rates. When that happens, the stress is
      not
      > shared equally by all the fibers that make up the rope, with the
      outter
      > fibers at the bend taking on more of the stress and causing them to
      > fail first and at a lower stress than a rope without bends... it is
      > like all the fibers that make up the rope are not working in
      unison, or
      > parallel anymore. So applying that philosophy with webbing it
      would
      > depend on how it is being reshaped. It seems like webbing is
      designed
      > to handle tight bends along its flat part but not so much along its
      > edge. But if you are just gathering webbing and rolling it into a
      > tubular position, I wouldn't think you would be degrading its
      strength
      > much, it at all.

      Your description of unequal loading inside/outside is correct. But
      there is an additional factor: The inner-radius segment of the line
      acts as a static bulk about which the outer segment is stretched in a
      levering action which amplifies the loading much further. If it
      weren't for this levering action rope would have a working load
      rating that's 33% of its maximum (as webbing does) rather than the 10-
      20% used currently. (You can also see why nylon is so superior for
      rope: The memory-stretch allows it to accommodate both the radius
      the levering problems much better than other fibers.)

      When rumpled, webbing only has the partial-loading problem to contend
      with, and so retains much of its strength. For example, a water knot
      loop in webbing retains about 75% of the webbing's total strength.
      When loops are properly sewn into webbing (i.e. X-box and oversewn)
      that figure is over 90%. (I also suggest that loops be 3 inches long
      after sewing so that any rumpling at the tie-off is smoothed out by
      the time it reaches the stitching.)

      Webbing's only weakness is wear. Because all of the fibers are
      exposed, it wears out faster than core-and-sheath ropes. On the
      other hand, you can easily see the wear, so inspection and
      replacement are not problematic.

      Doug Frost
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