On September 7, 2009, a Cessna 172S, N568ND, operated by the University of North Dakota, sustained substantial damage during a suspected hard landing. The private pilot, the sole occupant, was not injured. The 14 Code of Federal Regulations Part 91 flight departed Grand Forks International Airport (GFK), Grand Forks, North Dakota, at 1340 central daylight time. The airplane landed at St. Cloud Regional Airport (STC), St. Cloud, Minnesota, about 1600. After refueling, the airplane departed at 1640 for Duluth International Airport (DLH), Duluth, Minnesota, and arrived at 1750. The airplane departed DLH at 1830 and arrived back at GFK at 2045. Damage to the propeller was discovered during the maintenance turnaround of the airplane. The exact location when the damage to the airplane occurred was not determined. The flight was conducted in visual meteorological conditions, and a flight plan was filed. The pilot reported that he performed a normal landing at STC. He stated that the airplane ballooned during the flare so he released back pressure and flared again. After landing, he taxied to the ramp and refueled the airplane. The pilot reported that when he arrived at DLH he performed a normal cross-wind landing. After landing he taxied to the ramp and shut the engine off. No fuel was added and the pilot taxied out for departure about 40 minutes after arriving at DLH. He reported that while taxiing to runway 09, he taxied over a rough area. He stated, “I taxied over the rough spot and heard to my best knowledge as a loud bing!” He checked his instruments and nothing appeared abnormal so he continued to taxi. He departed DLH and climbed to 8,500 feet. He reported that the airplane was shaking slightly, but the engine and flight instruments were normal so he continued the flight to GFK. The pilot reported that the landing at GFK was normal. His post flight inspection of the airplane was brief and he did not inspect the propeller during his walk around. An instructor pilot, who had provided flight instruction in the airplane prior to the accident flight, stated that the flight was an instrument training flight. Only two landings were made and he stated that he was confident that no hard landing damage had occurred during the instructional flight. The lineman at GFK reported that while he was fueling the airplane he noticed that the propeller appeared to have struck the ground. He stated that about 1-2 inches of the propeller was bent backwards. He notified dispatch immediately about the discrepancy, and dispatch informed him that it had not been reported by a flight crew. An inspection of the airplane revealed damage to the propeller and the engine firewall.