Due to technical difficulties, I was only able to get good energy return values for 6 of the 15 strings, but it did include a couple of big tension losers: Black Widow and Big Hitter Black 7. The greatest loss in energy return over 2500-3000 120 mph serves was about 3%. That's not much.
String is viscoelastic. That means that it is part viscous and part elastic. The elastic element behaves as you expect, returning most of the energy. The response of the viscous part depends on the loading time. If you put a weight on the end of the string and leave it for a long time, it keeps stretching or flowing. This is what we call creep and it is due to the viscous nature of the string. It has plenty of time to perform its slow-motion flowing. But when you impact a string with the test impact hammer, the typical dwell time is anywhere from 25-40 ms. And when you hit the strings with a tennis ball, the dwell time is about 5 ms. This is not enough time for the viscous behavior to do much. Instead the string behaves almost totally elastic.
When you pull a string to tension before clamping it off, there is enough time for some creep to occur. When you clamp off, the string is then a fixed length, at which it stays until you cut the strings out. It does not creep. Instead, it undergoes stress relaxation (tension loss). This is just the other side of creep. In creep, you pull at a constant stress and the string keeps elongating. In stress relaxation, you pull to a certain length, and then stress declines.
Both creep and stress relaxation occur due to the molecular makeup of long-chain polymers in polyesters. Even in a mono, the composition of the material is long chains of molecules. These molecules are strong and elastic along the length of the chain, but are weakly bonded with their other chain neighbors. On top of that, the chains are entwined, folded, twisted and curled. It is one big plate of spaghetti. When a stress is applied to the string, the chains can slip and slide against each other and unfold. When you tie off, the tension decreases for the same reason, the chains slip and slide back to a lower stress equilibrium.
You can actually see some of this strange behavior in our testing. For example, sometimes near peak deflection and tension, you can see the string keep elongating while losing tension so that the string stays at a given peak tension for an extended period. Also, after the hammer leaves the strings, the tension is below the pre-impact tension, but it immediately starts to climb back. It has a memory. Every string does this a little, but especially polys. In fact, for many slow speed impacts, the tension will actually be higher after the impact than before.
Manufacturers try to make the long chain molecules as straight as possible so that the string will have less creep/relaxation. This requires multiple heating, pulling, cooling sections of the process. But there is always going to be slipping and sliding, even if the molecules were all straight. Copolys play with all these bonds between and within chains to optimize particular properties.
You are not taking string to yield points during impact. The string will almost always be on the straight line slope portion of a stress-strain curve. Yes, plastic deformation has occurred in the string. You can see this if you mark sections of your string before stringing and after you cut them out. The marked lengths will be longer and will remain longer. But this type of deformation is not occurring during impact.
Our tests are probably much more vigorous than normal play, so the fact that evidence points to very little energy loss increase over a normal number of hits indicates that energy loss probably is not a very large contributor to strings going dead. Instead, I think that friction increases such that the string can no longer slide in either direction as efficiently, if at all, and thus both stores less and returns less. In this case, whether or not the string returns less energy as tension declines, it will still be less efficient in sliding and producing the benefits of sliding. Yes, whatever decrease in ER does occur will make matters worse but not significantly so.
That is why adding lubrication to the strings may revive some of the benefits of the lateral string movement and snap back. It certainly did in
TWU's lubrication experiment .