On August 28, 2011, about 1345 Pacific daylight time, a Wheeler Vans RV-6A, N642DW, experienced the in-flight separation of a propeller blade during climb from the Albany Municipal Airport (S12), Albany, Oregon. All engine power was lost, and the pilot made a forced landing on a grass field north of the departure airport. The airplane was substantially damaged during the propeller blade separation event. The experimental airplane was built by the pilot, and it was operated under the provisions of 14 Code of Federal Regulations Part 91. Neither the private pilot nor passenger were injured during the personal flight. Visual meteorological conditions prevailed, and no flight plan was filed. The flight originated from S12 about 1341. The pilot reported that a few minutes after takeoff he heard a loud noise and felt a vibration when a propeller blade separated from the airplane. The pilot stated that the propeller assembly had undergone maintenance by its manufacturer on three occasions. These occasions were prompted by the pilot's observation of hub grease seeping onto the shank portion of the blades. Having observed this leakage, the pilot removed the propeller and hub assembly and returned it to the manufacturer for inspection and repair. The manufacturer reportedly performed maintenance and returned the assembly to the pilot who then reinstalled it on his airplane. The propeller logbook reported that the hub was repaired and inspected by a repair station and reinstalled by the pilot/owner 3 days prior to the accident flight. The pilot reported that the propeller had 309.1 hours and the replacement hub had 114.2 hours total time. The examination of the airplane revealed wrinkles in the upper surface of both elevators, and sections of the firewall and engine cowling were damaged. A propeller blade and a section of the propeller hub separated from the engine. The separated blade and hub section was recovered to the grass field where the aircraft subsequently landed. The MT-Propeller two bladed propeller hub serial no. 100854, and the separated propeller blade model 183-402, serial no. D-030, were sent to the National Transportation Safety Board’s (NTSB) Materials Laboratory for examination. According to the NTSB Materials Laboratory Report, the inspection revealed that the propeller hub fractured near the root of one of the propeller blades. Radial lines and chevron marks in the overstress region were used to trace the crack back to its origin, located approximately 45° from the forward face of the hub toward the direction of rotation. Fatigue crack propagation was found near the origin. The fracture surface was examined using a scanning electron microscope (SEM). Finely spaced striations consistent with high-cycle fatigue were found near the origin. SEM examination along the hub wall indicated that the fatigue cracks were initiating at shallow oxidized pits on the inner surface. The hub blade bore had a golden color and exhibited circumferential sanding lines. The area around the bore, such as the seat for the preload ring had a gray appearance and exhibited a line pattern typical of machining marks. The difference in color and the sanding lines in the bore were consistent with a repair of the hub blade bore. (Refer to the attached NTSB Materials Lab Factual Report.)