DURING CRUISE FLIGHT THE PILOT REPORTED A SEVERE VIBRATION. DURING THE FORCED LANDING THE AIRPLANE COLLIDED WITH TERRAIN AND CAME TO REST UPRIGHT. EXAMINATION OF THE PROPELLER REVEALED THAT A PITCH CHANGE KNOB ON ONE OF THE PROPELLER BLADES WAS FAILED. METALLURGICAL EXAMINATION OF THE FAILED PITCH CHANGE KNOB REVEALED THAT IT FAILED DUE TO FATIGUE.