I used this way to test some HMPE slings I had made, just wrapping them over the jack. None of them broke, neither did they slip, and when I struck them with a metal object they gave a high-pitched "ping" telling me, that the tension was indeed very high.
The jack i used had a safety valve, so that at about 3.5 "tonnes" the fluid recirculated.
I was unable to break polyester or polyamide this way, as it elongated under tension more than the jack could move. However I succeeded in jamming a few knots severely.
Back to the subject of the sheet bends, I too have found that properly used, the single is often less prone to fail than the double. However, for a long time I haven't used any of them, as very seldom a bend is needed, and at those occasions, the Carrick Bend is faster to tie, more secure, and easier to open after heavy load. This is also the main reason why I wouldn't use the Zeppelin Bend, as it is not so swiftly tied. It is however amply secure. So my solution was an alternative, not trying to improve the sheet bends. Otoh, as a becket hitch, I use any of them, single or double, depending on thickness difference.
One important advantage of the Carrick Bend is that it is as easily tied in ropes of large diameter, and that it needs not, or rather should not, be worked tight. It takes the correct form inevitably under load if left to its own devices. So any "proper dressing" can just be forgotten. It does dress properly all by itself. A boon is that it never jams.
I use my own method of tying the Carrick Bend, reminiscent of the Weaver's method of tying the Sheet Bend:
http://web.comhem.se/~u77479609/Carrick_Bend.pdfhttp://www.youtube.com/watch?v=wTmwqYQI93YLike any choreography, it takes a bit of training to get the movements into the memory of the muscles, but once learned, just like cycling or swimming, it isn't forgotten.