Here is a comment from a person called "Mapes" on <a href="http://www.physicsforums.com">www.physicsforums.com</a>: ----- Creep is quite well understood today: it is the time-dependent deformation of materials under stress, as brewnog stated. Inkling, your guess about creep deformation is exactly right. There are several ways creep can occur, and they all involve diffusion. Generally, the bonds between the iron atoms in your piano string occasionally break and re-form due to random thermal energy. Any atomic rearrangement that lets the string be longer is favored because you are pulling on it so hard. Eventually, the lengthening of the string is noticeable. Creep strictly occurs in all loaded materials above 0K. It is usually negligible in steel at room temperature, but you've found an exception, because the stress is large and the sensitivity of your detector (the human ear) is very good for detecting out-of-tune strings. One of my professors liked this trick question: which creep mechanism [there are several, including bulk diffusion, grain boundary diffusion, and dislocation climb] is active in material xx at yy°C? The answer is always all of them (they just might be negligible). ----- Full thread here: <a href="http://www.physicsforums.com/showthread.php?s=f59a52eac47cac840dbddef984c592bd&p=1699460#post1699460">http://www.physicsforums.com/showthread.php?s=f59a52eac47cac840dbddef984c592bd&p=1699460#post1699460</a>