After reading this (interesting!) thread, I've made some other tests on tie-outs strength.
Using 1" gorilla tape, I've had little success making washer-like reinforcements: I tryed using bottle-PET for building the reinforcement but they broke at nearly 7Kg; then I tryed a tie-out made only with gorilla tape (a simple tape-loop, with double tape inside the loop to make it non-sticking) and resisted more than 20Kg! The nice thing is that it is the simplest tie-out to make... So no washers in my future tarp!
I've then tested two different tie-outs types to discover which tie-out <-> film configuration was the strongest one. The idea I wanted to prove is that making "V" shaped tie-out would discharge the stress on a wider film area than a usual linear tie-out using same tape length. These are some photo of my tests:
(on the left a "V" tie-out, a little more "opened" than desired, on the right a usual linear tie-out)
(this is the desired "V" tie-out shape, even if a little short)
Unfortunatly I had no spare polycryo pieces to test it out, so I used some packaging film I've found in my trash and common tape. BTW it seems that "V" shaped tie-out are 1.5x TIMES MORE RESISTENT than linear tie-out without any weight penalty. I think they would also resist better than linear tie-outs when stress is not perfectly perpendicular (but no test have been done on this).
"V" tie-out idea can be applied on both tape-loop and washers-type tie-out.
I'd be very happy if someone in the forum want to verify the result of my test with polycryo. To do the test please use a square piece of polycryo at least 10"x10". Any volunteer? ;-)