On May 21, 1999, at 1400 central daylight time, a Bellanca 8KCAB tailwheel equipped airplane, N5500C, was substantially damaged following a loss of directional control while landing near Lufkin, Texas. The flight instructor and the instrument rated private pilot receiving instruction were not injured. The airplane was owned and operated by Nacogdoches Taildraggers, Inc., of Nacogdoches, Texas. Visual meteorological conditions prevailed for the Title 14 CFR Part 91 flight for which a flight plan was not filed. The local instructional flight originated from the A.L. Mangham, Jr. Regional Airport (OCH), near Nacogdoches, Texas, at approximately 1330. According to the FAA inspector who responded to the accident site, the flight arrived at the Angelina County Airport (LFK), located near Lufkin, Texas, to practice takeoffs and landings. The flight instructor reported that following their second practice landing, a normal wheel landing on runway 15, the "U-bolt" attaching the left main landing gear to the fuselage failed, resulting in the separation of the left main landing gear. The left wing contacted the ground and the airplane came to rest on the west side of the 4,550 foot runway. Examination of the 1978 model aircraft revealed structural damage to the left wing and the left main landing gear attaching point. Additionally, one propeller blade was bent and the underside of the airframe was damaged. A review of the maintenance history of the airplane by the FAA inspector revealed that the airframe had accumulated a total time of 708 hours. The failed landing gear U-bolt, Bellanca part number 1-9805, was shipped to the NTSB Materials Laboratories in Washington, D.C., for examination and evaluation. The metallurgical examination revealed that the bolt fractured as a result of fatigue. See enclosed report for details of the examination. The surface winds at the time of the occurrence were reported from 200 degrees at 5 knots.