The root of the blade, the line perpendicular to the length of the blade is carrying two major forces. The first is tension, each blade is trying to break at the hub and fly off in a separate direction. Added to this the back of the blade, which is now scratched from being forced into the thrust flange on the engine is under further surface tension from the air loads trying to bend the blades forward. The scratches on the back face are the flaws that become the origin of a fatigue crack. These two forces combined with the very high loads at the ends of the scratches add up to a tension load that the metal cannot handle and a little crack forms in the end of the scratch. Each time the propeller is accelerated and loaded that crack grows another little bit. At some point we reach critical crack length and the crack runs completely through the root of the hub between the mounting hole and one edge of the blade. We just doubled the loads on the remaining half of the hub, which will succumb to the same failures in even less time.

Fatigue in highly loaded components is very very unpredictable due to all the variations we put propellers through.

Second factor is that the resilient woods and plastics we use for propellers are a little weaker than the strength of human flesh and bone. The wood and plastic blades usually break on contact with enough human parts. That new aluminum blade will proceed to meat grind your body part to nothing.

Thirdly once flung off the hub wood and plastic have pretty low mass and slow down relatively quickly. Metal on the other hand doesn't. I once investigated an Air Force mishap in which a CH-53 taxied into a parked helicopter, and they grazed rotor tips. A FOUR OUNCE blade counterbalance left the end of the slow spinning (ground idle) rotor blade, and flew horizontally for 1/8 th of a mile and went clean through the fuselage of a KC-135. It then went over base operations and broke the front windscreen of a C-130 that was another 400 yards away!

If the aluminum propeller did come off of your model it would go through you in less than a heartbeat.

HTH
Former Chief Structural Engineer for the C/KC-135 and Presidental aircraft fleets.