On September 8, 2018, about 1045 eastern daylight time, a Cessna 152, N6469P, was substantially damaged after it impacted terrain during a forced landing in St. Augustine, Florida. The private pilot was not injured. The airplane was owned and operated by Sunshine Aviation. Visual meteorological conditions prevailed, and no flight plan was filed for the flight that originated from the Palatka Municipal Airport (28J), Palatka, Florida, about 1030, destined for Ormond Beach Municipal Airport (OMN), Ormond Beach, Florida. The personal flight was conducted under the provisions of Title 14 Code of Federal Regulations Part 91.According to the pilot, he departed OMN at 1000 and flew to 28J. He practiced some landings and started his return back to OMN. About 12 miles east of 28J, at 2,000 ft., the engine started running rough and the airplane was "violently shaking." He also heard "clanking" that "sounded like something was loose in the engine." He saw a field to the right side of the airplane and set up for an off-airport landing. At 1,000 ft. he tried to apply power to the engine, but the engine started violently shaking again so he reduced the power. At 800 ft. he secured the airplane by turning off the magnetos, shutting the fuel off, and turning the avionics master switch off. When the airplane was 200 ft. above the field, the pilot could see crops that were higher than he thought, but he could not turn away at that point. The airplane touched down on the main landing gear and when the nose landing gear touched down in the soft ground, it dug into the ground and the airplane flipped over. The pilot subsequently egressed through the door. Examination of the wreckage by a Federal Aviation Administration (FAA) inspector revealed that the airplane sustained damage to the firewall and cockpit floor. The nose landing gear was fractured off, the right-wing leading edge was crushed, and the outboard main spar was wrinkled. The oil filter was removed and cut open. Metallic pieces were noted in the filter element. Examination of the engine by a National Transportation Safety Board investigator revealed that the No. 4 cylinder exhaust valve was fractured, and the fracture features observed on the valve stem were consistent with fatigue. The piston, cylinder head, and cylinder walls were gouged consistent with repeated contact with the exhaust valve; however, no portions of the valve head remained in the cylinder. The two-seat, single-engine, high-wing airplane was built in 1981, and equipped with a Lycoming O-235-series engine. The most recent annual inspection was completed on June 11, 2018. Review of maintenance records revealed that at the time of the accident, the airframe total time was 8,139.7 hours. The airplane had flown 95.4 hours since its last annual inspection and the engine had accumulated 1,608.7 hours since major overhaul. The last engine overhaul was performed in 1995; however, the parts replaced did not include the exhaust valves. The exhaust valves were replaced on June 4, 1990 during an engine overhaul with a total engine time of 5,186.4 hours. The engine manufacturer recommended that the engine be overhauled every 12 years or 2,400 hours, whichever comes first. The pilot held a private pilot certificate, with a rating for airplane single-engine land. His most recent FAA second class medical certificate was issued on March 5, 2018. He reported 228 total hours of flight experience at the time of the accident.