Originally Posted by
guy153
It's true that steel has a "fatigue limit" so if kept within a certain range it will last theoretically forever. Carbon fibre and aluminium won't. But it's less clear how relevant this is in practice. Your steel frame might be operating outside that range and fail early (especially if it has stress risers due to defects) and your al or CF frame might last longer than you use it for.
You have misunderstood this. Aluminum has no fatigue limit because all stress causes fatigue. But CF has no fatigue limit because no stress cycling causes it to accumulate fatigue damage. They are opposites.
In the real world neither statements are predictive. Aluminum can last effectively forever if it is stiff enough, like an old Klein.
That's true of steel as well, where one way to stay below the fatigue limit is to make the structure stiffer via large diameter tubing to limit stress amplitude.
The downside to stiffness is that a person is sitting on this structure and doesn't enjoy perfect transference of vibration from road to rider. So accepting that certain kinds of frames, like traditional steel, can wear out some day is considered a reasonable trade off.