On February 13, 2016, about 0930 Pacific standard time, a Cessna 140G airplane, N3625V, sustained substantial damage when the left main landing gear axle broke during landing and the airplane ground looped at the Independence State Airport, Independence, Oregon. The private pilot and one passenger were not injured. The airplane was owned by the pilot and operated as a personal, cross-country flight under the provisions of 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed and no flight plan had been filed for the flight. In a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge, the pilot stated that the approach and touchdown were normal. Just after touchdown, he felt something similar to a bump, and the airplane started to drift to the left. He stated that he thought that he possibly had a flat tire and tried to compensate, but the airplane continued drifting to the left and exited the left side of the runway into the dirt and ground looped, sustaining substantial damage to the right wing and fuselage. A Federal Aviation Administration inspector from the Portland Flight Standards District Office was at the airport at the time of the accident and examined the airplane at the accident site. The examination revealed that the left main landing gear axle had fractured and the wheel assembly separated from the airplane. A detailed examination of the fractured axle by the NTSB materials laboratory revealed that the axle was fractured near the inboard end just outboard of the axle attachment flange. Portions of the fracture surface at the upper and lower sides of the axle had relatively smooth features oriented perpendicular to the outer surface, features consistent with fatigue. The fatigue cracks initiated at a fillet corner at a change in outer diameter for the axle. According to a representative for Cessna contacted by telephone, inspections of the main landing gear axle should be in accordance with Section 2A of the Maintenance Manual for the 100-series airplanes. The axles should be inspected for cracks and corrosion initially after 10 years or 4,000 hours and then at subsequent intervals of 3 years or 1,000 hours. The inspection consists of removing the wheel and completing a visual inspection. If any crack is suspected, an eddy current inspection is then required. The airplane was manufactured in 1948. The owner/operator reported that the last annual inspection was completed about 7 months prior to the accident at a total airframe time of 6,314.5 hours. No airframe logbooks were located during the investigation, therefore it was not determined when the last inspection of the main landing gear axle occurred.