This large ventilation fan has failed during its breaking-in period, resulting in blades detaching from the flange and breaking to bits due to an unknown cause.

The investigation has found the failure to be the result of defficient design - namely, insufficient strength (hardness) of the flange, relative to the high strength (hardness) of the blade's fastening screws. This allowed for a plastic deformation of the flange threading, after screw fastenning and\or during use, this in turn allowed for a weakening of the screws' pre-stressing. As a result, cyclic bending stresses developed in the screws which eventually led to a fatigue fracture, and failure of the system.

In addition, manufacturing defects were found in the flange threading , which together with the high friction factor of the galvanization contributed to insufficient fastening.