Hi Patriot, SilverCreek's explanation is pretty much what happens and almost all knots fail where the main line enters the knot. It will look like the line parted in the middle because there will be no curl or trace of knot left.
I had some time so I did the search for you. I probably didn't find all the instances of knot testing but these should give you an idea of how they did it and the resulting strength ratings.
and how to tie some of them can be seen here https://www.animatedknots.com/
or here https://www.netknots.com/
I use nail knot for leader to line and Double Fisherman’s Bend to join sections of leader or tippet to leader, easier for me to tie and I can step down a greater difference in diameter, I've never had these knots fail, any failures are always the tippet, it being smaller. I did use blood knots for years and tied as Silver describes they work very well. I usually use a Duncan knot or a Double Davy knot at the fly.
I'm speechless!!! I perused the first link and the information is precisely what I was hoping to find on knot strength. I will read and bookmark each link that you so kindly provided. You have provided a ton of very useful information that all of us can benefit from. I have never heard of the Duncan or Double Davy knots. I have much to learn.
Thank you very much for taking your time to find and post this invaluable knot information. With your permission I may add your entire post to my website.