On July 8, 1995, at 1330 mountain daylight time, a Bellanca 8GCBC, N57669, sustained substantial damage when the left main landing gear failed during landing roll at Owl Canyon Glider Park, Wellington, Colorado. The commercial pilot was not injured. Visual meteorological conditions prevailed for this local area glider tow flight operating under Title 14 CFR Part 91. The flight originated at 1320 and a flight plan was not filed. According to the pilot, his landing sequence following the glider release contained "a mild bounce." The pilot said he added power for the second touch down and when touch down occurred the left main landing gear separated from the aircraft. Examination of the aircraft by the investigator-in-charge provided evidence that the landing gear was of the spring steel type. Separation occurred just outboard of the fuselage attach point and during the ground excursion following the separation, the left wing, wing strut, left elevator, wind screen, left side window and propeller sustained damage. Examination of the fracture surface by the NTSB Metallurgical Laboratory provided evidence of a fatigue failure and evidence that the landing gear spring steel had been ground at some point in time which caused overheating of the steel and subsequent changes in the microstructure. It was the metallurgist's evaluation, that the grinding initiated the fatigue. The metallurgist's report is attached.