On July 16, 2012, about 1855 eastern daylight time, a Flight Design Gmbh model CTLS, N566FD, sustained minor damage during a runway excursion on takeoff at Union County Airport (MRT), Marysville, Ohio. The pilot was not injured. The aircraft was registered to B S Aviation LLC and operated by New Flyers Association under the provisions of 14 Code of Federal Regulations Part 91 as a solo instructional flight. Visual meteorological conditions prevailed for the flight, which was operated without a flight plan. The flight originated from the Ohio State University Airport (OSU) about 1805. The pilot reported that after departing OSU, he proceeded to the local practice area and completed several training maneuvers. He then flew to MRT in order to practice takeoffs and landings. After entering the traffic pattern, he landed on runway 27 (4,218 feet by 75 feet, asphalt) without incident and taxied back for departure. During the subsequent takeoff roll, the pilot reported feeling a "jolt" and hearing a "thumping" sound from the left main wheel. The airplane immediately veered to the left and departed the runway pavement. The airplane came to rest about 20 yards from the edge of the runway, after striking a Precision Approach Path Indicator light. The left main landing gear collapsed. A postincident examination revealed that outer half of the left main wheel assembly had failed. Airplane damage was limited to the left main landing gear, engine cowling, and propeller assembly. The operator reported that the right main wheel had failed on this airplane in a similar manner on December 6, 2011. Both the left and right wheel components were submitted for further examination. Metallurgical examination of the left and right outboard wheel halves revealed a crack adjacent to the rim, extending about three-quarters of the circumference in the case of the left wheel half, and about two-thirds of the circumference in the case of the right wheel half. Fracture surface features were consistent with fatigue cracking extending through the thickness of the component, with multiple origins along the outer surface. No unusual features such as deep machining marks were observed at the fatigue origin areas. The material thickness varied over the cross-section of both wheel halves, measuring about 0.083 inch at the formed radius adjacent to the rim. The specified material thickness was 0.118 inch. The measured material properties, such as conductivity, harness, and composition, were consistent with the specified material. The operator reported that at the time of the incident, the airframe had accumulated 1,396 hours total airframe time (980.3 hours tachometer time). Maintenance records revealed that the most recent annual inspection was completed on May 30, 2012, at 907.6 hours tachometer time. Records indicated that the left landing gear leg had been replaced after repair on November 4, 2011, at 702.1 hours tachometer time, subsequent to a hard landing inspection. The right main wheel outer half was replaced on December 7, 2011. An airframe maintenance time was not recorded with that logbook entry. Weather conditions recorded by the MRT Automated Weather Observing System (AWOS), at 1853, were: calm wind, 10 miles visibility, clear sky, temperature 33 degrees Celsius, dew point 12 degrees Celsius, altimeter 29.93 inches of mercury.