In addition, I'd check the balancing of the fan blade. It is possible that excessive vibration due to an unbalanced blade is what caused the fatigue crack in the first place.
Although, it could be simply due to poor materials and/or manufacturing.
Stop using that right away and I 2nd vibration issues. I would be looking up "International Comfort Products, LLC" and giving them a call to ask about repair/replacement. »en.wikipedia.org/wiki/In ··· poration