The excellent book "On Rope" by Bruce Smith and Allen Padgett (about North American vertical rope techniques, published by the National Speleological Society) has a section on measuring rope strength, and here are a few points it mentions:
  • There is a U.S. test standard: 191A, Method 6016
  • It involves wrapping rope around two cylinders that are pulled apart at a fixed rate. All the parameters/dimensions are spelled out.
  • Lots of factors affect strength, and lots of different things can be measured as "strength"
  • Practical, real-world values may be significantly different from what is measured in a lab
From my own brief experience measuring Nomex thread strength in a university Textile Engineering lab, ideally what you might like to get is not a single number, but a graph of the relationship between elongation and pulling force.

Steve
_________________________
"After I had solaced my mind with the comfortable part of my condition, I
began to look round me, to see what kind of place I was in, and what was
next to be done"