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"blade tip speed is slower and out of the harmonics area"
Brent may have something here.
Let me preface my remarks...I have no knowledge as to why the cast [not stamped?] two blade fans fracture at the hub.
Bare in mind the the fan and water pump impeller spin 1-1/2 times the engine rpm.
The engine is good for 2600 rpm so the fan spins 3900 rpm at times.
Good point blades don't fracture at the tip. I doubt if it's harmonics but more like stress.

What causes stress? I can't hear anything 'whining', snapping like a bull whip [that's hyper sonic compression of air], or whomping air like a propeller and no thumping. Hummmmm says the boy.
~Vibration? Out of balance and shaking. Should feel the vibes by touching the water pump shaft. Or listening to the whirl with a long handled screwdriver to the ear.

~Bending? More like...

~Flexing toward radiator while pulling air? Straightening out when slowed. Back and forth till she breaks. So rpm's climbing and backing back down kill engines...why not fans? Could cause the root of the blade to flex and start to fracture.
You know...I wonder what a guy could see with a variable flash zeon strobe light.
Freeze the moving blade.
Might be able to detect flex.
The fan works hard at moving air.
Resistance through air squares as speed doubles.
Your hand out the window at 25 mph vs at 60 mph. Speed flips the hand right up.

Air has weight. 1.087 lb per one cubic foot at 70 deg F. Less weight when hot, but the same one cubic foot volume.
One cubic foot is two cubic foot at 600 deg F. But weighs the same as one cubic foot at 70 deg F. [standard air]
Engine hp used to turn the fan moving hot air decreases.
If the fan was driven by an electric motor the cold start motor amp would drop as the air through the fan heated. New York Blower manual on the four standard Fan laws explain this.
I think I read that the fan moves 220 cubic foot a minute at X? rpm.
That's about 240 lb of air a minute.
If anyone wants to play with the 'draft-through-the-radiator' divide the 220 cuft by the sq foot opening of the radiator. Just for poops and grins. That air heated to about 160 deg F gives the approximate btu cooling so 220 cfm x 1.087 weight of air x [160 deg F -70 deg F air; rise in deg F] = 21,522 btu per hour. [btu's are always in hours] Fun?

~Pulling like centrifugal forces. I don't see it. The blade will fling but that's after it fractures. That's the result. Not the cause.

~After 80 years, I think the hub/blade root just get worn out. The problem is the new replacement fans are sometimes of bad metallurgy, materials/process, and craftsmanship.

~I have heard that a cast fan that has been cleaned and vapor blasted with glass beads then degassed at a high temperature, then cooled and powder coated gloss black and cured at a lower temperature will show the crack developing. HIGH GLOSS thermo-set epoxy polyester cured powder paint is brittle and will show cracks if the blade flex's at the hub. An early warning? I don't know. Speed kills. A four blade fan doesn't move more air 'cause the blades have less pitch I hear but they make noise as Bruce says. Weigh more too.

When I wash my BMW 320iS and it's ideling away, the electric cooling fan kick's on and the car moves foward about 1/3 of an inch. Is that cool or what?

That's my 2 cents.

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