On April 25, 2001, approximately 1000 mountain daylight time, a Cessna 140A, N666TC, was substantially damaged when the right main landing gear failed during landing at the Greeley-Weld County Airport, Greeley, Colorado. The recreational pilot, the sole occupant aboard, was not injured. Visual meteorological conditions prevailed, and no flight plan had been filed for the personal flight being conducted under Title 14 CFR Part 91. The flight originated at Hudson, Colorado, approximately 0915. The following is based on a telephone interview with the pilot and the accident report he later submitted. The purpose of the flight was to practice pattern work accuracy and takeoffs and landings. The pilot made two low approaches to the runway at Greeley. On the third approach, he decided to make a landing. A full stall conventional landing was made. The right main landing gear broke on touch down and the airplane skidded to a halt. The fuselage and landing gear box were buckled, and the wing spar was damaged. Both fractured ends of the main landing gear were cut away and sent to the Cessna Aircraft Company's Material and Process (M&P) Engineering Department, where they were metallurgically examined. The following is based on Cessna's M&P report: The landing gear was made of 6150M steel, and microstructure examination disclosed it had a hardness and tensile strength that met Cessna's engineering specifications. Decarburization, approximately 0.015 inches in depth, was noted near the top surface of the landing gear. There was also deformation and roughening on the inside surface, consistent with shot-peening, also required by engineering specifications. Two holes, 3/16" in diameter, and two bolts and nuts were used to attach the step assembly to the landing gear spring. The bolts were not damaged. Florescent magnetic particle inspection revealed cracks had initiated at the top end of the lower hole. No cracking was observed at the inside end of the hole. The M&P report concluded that the landing gear spring cracked through the upper hole. Chevron (or beach) marks on the fractured surface pointed back to the crack initiation site, and were consistent with fatigue cracking. The fracture features were similar to quasi-cleavage, and faint crack arrest lines were observed. The fatigue region was very small, approximately 0.075 inches in depth. The adjacent area was characterized by dimples, indicating the final breakage was due to a ductile fracture under overload.