When tension is gradually increased on the same piece of test string, nylon and poly stretches very differently. I clamped a piece of string and marked out exactly 20cm before any tension is applied. Then I gradually applied a tension of 15kg, 20kg, 30kg, 40kg and then back to 30kg and finally 20kg. The initial length of 20cm was measured at each stage and recorded.
The graph below shows roughly how nylon and poly strings would stretch.
Nylon is represented by the thinner line and poly the thicker line. Nylon displays a high initial stretch of about 8-10% even at a low tension of 15kg (33lbs). With additional tension, nylon stretches very slowly, adding only about 2% length each time as the amount of weight increases.
Poly does not stretch much at low tensions. So far, the poly I've tried starts stretching at about 40+ lbs. The way poly stretches is totally different from nylon. Poly stretches more at the later stages of higher weights. And then suddenly, as denoted in the graph at point X, poly fails and either suddenly snaps or deforms to a piece of lifeless string.
When both strings are completely removed from any tension, nylons recovers from most of the stretch. Poly, however, retains most of the length stretched.
This difference in behaviour has very important implications on how both strings should be strung and at what tension. Naturally, the durability of both strings would also be drastically different.
