On February 16, 2018, about 1320 mountain standard time, a Scheineman-Van Buren T-18 airplane, N89ER, was substantially damaged while landing at Vance Brand Airport (LMO), Longmont, Colorado. The pilot and flight instructor were seriously injured. The airplane was registered to and operated by the pilot under the provisions of Title 14 Code of Federal Regulations Part 91 as a training flight. Day visual meteorological conditions prevailed for the local flight, which departed without a flight plan about 1220. The purpose of the flight was to orient the pilot to the airplane, which he had recently purchased. The pilot had flown about 200 hours in other tailwheel airplanes. After the pilot and flight instructor departed LMO, the pilot flew several local flight maneuvers, including slow flight, stalls, and steep turns. Returning to LMO, the pilot executed about uneventful seven landings. During the last landing, planned as a full stop, the airplane turned to the right as it decelerated below about 40 knots. The pilot attempted to correct the turn by applying left rudder control and with braking, but the airplane ground looped, departed the runway surface, and nosed over, damaging the left wing and vertical stabilizer. The pilot was unsure if he had applied any braking during the landing prior to having directional control issues. The flight instructor did not recall the last landing due to his injuries. Examination of the airplane by a Federal Aviation Administration inspector revealed no anomalies, except for a fractured rudder cable. No wear or chaffing marks were observed on the rudder cable, and no anomalies were noted with the installation. The rudder cable specifications matched the airplane build instructions. The National Transportation Safety Board Materials Laboratory examined the rudder cable with a 5x to 50x magnification stereo microscope and determined that fracture surfaces were consistent with an overstress separation.