On June 28, 2013, about 1100 central daylight time, an experimental, amateur-built Martin Zenith STOL CH 750 airplane, N699TX, sustained minor damage during a forced landing after the propeller assembly separated from the engine crankshaft near Paragould, Arkansas. The pilot was not injured. The airplane was registered to a private individual and operated by the pilot under the provisions of 14 Code of Federal Regulations Part 91 as a personal flight. Visual meteorological conditions prevailed for the flight, which was not operated on a flight plan. The flight originated from the Kirk Field Airport (PGR), Paragould, Arkansas, about 1050. The ultimate destination was the Grosse Ile Municipal Airport (ONZ), Detroit/Grosse Ile, Michigan. The pilot reported that during the climb out after takeoff, about 5 miles northeast of the airport, the engine "developed an immediate and heavy vibration." He altered course to return to the airport. The intensity of the vibration increased to the level that the pilot considered shutting the engine down, when the vibration abruptly ceased. Looking out the windshield, he observed that the propeller had separated from the airplane. The pilot subsequently executed a forced landing to a field with no additional damage to the airplane. A postaccident examination of the airplane revealed that the propeller flange-to-engine crankshaft flange attachment bolts had failed. The mating surfaces of the propeller and crankshaft flanges exhibited fretting damage. Portions of five of the six bolts remained with the crankshaft flange; one bolt had separated and was not recovered. Metallurgical examination revealed that two bolt fragments exhibited features consistent with fatigue fracture. The remaining three recovered bolt fragments exhibited features consistent with overstress fracture. Damage was observed to the threads on three of the crankshaft flange bolt holes, including the bolt that had separated completely. Deposits consistent with thread lock were observed on the threads of the bolt fragments and the bolt holes in the crankshaft flange. The thread lock material appeared red in color; in addition, a substance that appeared to be yellow in color was observed between the red thread lock and the thread flank. Washers recovered with the propeller exhibited 12 radially-oriented step features on one face and 40 radially-oriented step features on the opposite face consistent with a disk-type lock washer. Impressions corresponding to the washer steps were observed adjacent to the attachment holes on the propeller flange and the clamping face of two bolt heads. The propeller flange also exhibited contact marks consistent with the edge of a larger spring or cone-shaped washer. The engine manufacturer's documentation noted that the standard washer for propeller flange installation was to be a Belleville (cone or spring) washer. The accident airplane was equipped with a 120-horsepower, six-cylinder, Jabiru model 3300 reciprocating engine. The operator reported that the airframe and engine had accumulated about 32 hours at the time of the accident, with 6 hours since the most recent inspection. Copies of the airframe and engine maintenance logbooks were provided to the National Transportation Safety Board (NTSB). The engine logbook copies included a single maintenance entry dated June 17, 2013. It noted the completion of a 100-hour inspection at an airframe/engine time of 15.2 hours. There were no subsequent entries. In July 2008, the engine manufacturer issued a service bulletin informing pilots and mechanics of several incidents in which the propeller and engine crankshaft flanges had separated. The bulletin noted that the cases had involved the improper installation of the propeller. The bulletin advised operators that if the propeller was not installed properly, it should be refitted to the correct procedure. It noted that the bolt and crankshaft threads should be cleaned to remove any remaining thread lock compound. In addition, the use of Loctite 620 was required because of its high temperature tolerance. Loctite 620 was green in color.