If you have cracks in the fan blades that are deeper than 1/16″ then you should consider not operating the fans and plan on repairing or replacing them depending upon the extent of the cracks and where they are located. It is good practice to have one set of replacement fan blades at a minimum this way you can shutdown and replace the affected blades and then take time to properly inspect and repair the cracked blades.
If the cracking is minimal to where it just catches your finger tip and no fiberglass is exposed the fan blade may be a candidate for cleaning, prepping and recoating in-place.
As for tripping on vibration prior to breaking? Most times no. The failure is more often than not immediate and may result in additional damage to adjacent blades.
If you have additional questions, comments or concerns please do not hesitate to contact me a firstname.lastname@example.org 202-486-5694. I have over 12 years of experience as a Commissioning Manager with SPG (SPX) Dry Cooling and B&W SPIG Dry Cooling and am presently a consultant to the Power Industry specializing in Air Cooled Condensers and Cooling Towers.