Originally Posted by
joel1952
As explained earlier, the seat post was too small for the seat tube, so it was able to wobble and cause fatigue crack.
That's not exactly the mechanism of a fatigue crack. If the seatpost was too small for the seat tube, the tube would flex too much when clamping and cause excessive stress there, exacerbating fatigue loading due to combined stresses. A loose "wobbling" seat post, if it banged on the end of wobble limits, could introduce additional shock loads, which may also be reversing, also possibly exacerbating fatigue loads. Lastly, one part sliding against another can introduce "fretting" wear, which can introduce a stress concentration or stress riser.