A large fan assembly deformed and broke at multiple locations. The user wanted to know whether the bearing pillow block fracture caused the fan blade assembly to crack, or whether a fan blade assembly fracture caused the pillow block to crack. Close inspection of the entire length of the crack showed the crack probably grew quite a while before it was large enough to cause the final catastrophic event. No evidence of fatigue cracks was visible on the broken pillow blocks. In the absence of some other contradictory information, the usual conclusion would be to presume that the fatigue crack predated the single overload crack.

Macrofractographs of the fracture surface from a multibladed fan showed that cracks started at the corner where bending stress was concentrated and propagated through the blade by fatigue. Peak stress at the monitoring position was less than 10 MPa. To simulate crack growth, the rotor was repeatedly deformed by a hydraulic fatigue tester. Comparison of striations of the failed blade with that of the tested one revealed the failed blade was loaded with more than 30 MPa of stress. These tests confirmed that the rotor and blades had sufficient strength to withstand up to 3x the stress of normal operation. The casing of the fan was vibrated at 10 to 60 Hz. Peak stress easily overcame 30 MPa, which was enough to initiate cracking. The fracture surfaces and starting position were the same as those on the failed fan. It was concluded that the exciting force from an air compressor caused blade failure.

A blade from the engine cooling fan of a pickup truck fractured unexpectedly. The blade was made from type 301 stainless steel in the extra full hard tempered condition with a hardness of 47 HRC. Failure analysis indicated that the blade fractured in three modes: crack initiation, fatigue crack propagation, and final rapid fracture in a ductile manner The fatigue crack originated near a rivet hole.