The pilot reported that he thought he struck a pothole while landing on the grass runway. During the landing sequence, the airplane experienced a postimpact fire, and the airplane was destroyed. The pilot sustained thermal injuries and was hospitalized for an extended time period. Postaccident examination of the airplane revealed the right main landing gear axle had fractured during the landing sequence, and the landing gear leg punctured the fuel tank which likely contributed to the postimpact fire. Metallurgical examination revealed a series of ratchet marks consistent with multiple fatigue crack initiation points joining into a single larger crack on the right main landing gear axle. The fatigue crack extended across about 30% of the cross section of the tube before final failure due to overstress. The locations of the fatigue initiation points were consistent with the cyclic bending forces that would have been applied to this section of the landing gear under normal landing and takeoff conditions.