On October 22, 1994, at 1308 central daylight time, a Piper PA- 18, N587PA, registered to the University of North Dakota, sustained substantial damage following a loss of engine power and subsequent forced landing near Grand Forks, North Dakota. The flight instructor and student pilot were not injured. The 14 CFR Part 91 local instructional flight operated in visual meteorological conditions without a flight plan. The flight departed Inkster Airport, Inkster, North Dakota, at 1255 with an intended destination of Grand Forks International Airport. After practicing several takeoffs and landings at Inkster Airport, the flight instructor and student began their return flight to Grand Forks. While approximately eight miles from Grand Forks, the instructor reported the engine began to run rough for a one to two second period at 2400 RPM. Immediately after the engine roughness was experienced, the propeller became unattached and the engine lost all power. A forced landing was made on a field without further incident. Post accident examination revealed the crankshaft had completely fractured. The failed crankshaft was sent to the NTSB Materials Laboratory, Washington, D.C., for examination. The laboratory reported that the separation of the crankshaft occurred at an oil transfer tube hole in the main bearing journal. This fracture was caused by fatigue. The fatigue cracking propagated through the journal on a 45 degree plane to the longitudinal direction of the crankshaft. The initial direction of the cracking is consistent with torsional loading condition. Further examination of the origin hole disclosed the depth of the enlarged diameter portions were significantly above the specified value. Normal depth is between 0.078 inch to 0.088 inch. Measurement of the oil hole was found to be 0.139 inch. See attached Metallurgist Report. The airplane logbook shows the last inspection was an annual inspection performed on November 16, 1994. The airplane had logged 27 hours since the inspection. Total time on the engine at the time of the accident was 1809. The logbook also revealed that N587PA experienced a propeller strike in May, 1991, at 589 total time. As a result of this strike the propeller was replaced. Compliance with Airworthiness Directive for post propeller strike inspection was not accomplished because this incident was classified as minor. The engine manufacturer was present during the examination of the crankshaft. The manufacturer reported that there are approximately 40,000 of this type of engine in service today with no known incidents of similar failure without a previous incident of a propeller strike.