Any metal under tension stretches a bit. It also gets stress cycled (not sure of term), where it gets loaded and unloaded. Eventually the metal's elasticity drops until it yields/breaks.

For example, airplanes that fly high enough to need pressurized cabins can only pressurize and depressurize x amount of times (100k?). The incident where it looked like someone opened the top of the plane (Hawaii area airline) happened because the plane had 8 take offs and landings a day, traveling between the islands. I think it was stressed like 200k times, and the fuselage finally gave out, literally just split open. Luckily the plane held together (one flight attendant was swept out - everyone else was buckled in) and it landed safely.

(I learned this while watching a show that was called something like "World's Greatest Air Disasters" while on a plane flying cross country - I recommend not watching shows like that while on a plane. You better believe I made sure I had my seat belt on securely.)

There are two ways to deal with metal losing their elasticity.

Prevention - the best way to slow down this tendency is to use more metal. Problem is that it makes for heavier things. You should also use an elastic material. Don't make rubber bands out of hard plastic or Styrofoam - not elastic enough.

Replacement - replace the metal when it gets close to the end of its life cycle.

Finally, if the metal has any imperfections in it, it can break prematurely.

btw if it has aluminum spokes, that's not really an elastic material. As some would point out, you don't see aluminum springs do you? But you see steel, titanium, and carbon springs.

Ultimately, ridden long enough, a wheel will break a spoke. Even if it's a 48 spoke BMX wheel ridden by a 75 lbs kid. Guaranteed.

cdr