On June 5, 1995, at 1557 eastern daylight time, a Piper PA-23- 250, N40540, was substantially damaged when it landed hard and exited the side of the runway at the Mettetal Canton Airport, Plymouth, Michigan. The instructor and private pilot student reported no injuries. The 14 CFR Part 91 instructional flight operated without a flight plan in visual meteorological conditions. The flight originated from Willow Run Airport, Detroit, Michigan, at 1540. The instructor reported the airplane touched down normally on its main landing gear, but when the nose gear touched down, the airplane immediately veered to the right and exited the side of the runway. The private pilot student, who was at the controls, attempted to bring the airplane back onto the runway. Despite the student pilot's use of rudder and brakes, he was not able to regain control of the airplane. The airplane veered sharply to the left, crossed the runway, and exited the runway on the left side. The nose gear collapsed and the airplane slid several feet before coming to rest. Postaccident examination revealed the nose wheel collar assembly (P/N 752 915) had separated. Only one half of the collar assembly was located during the investigation. This failed section was sent the NTSB Materials Laboratory, Washington, D.C., for examination. The laboratory reported features representative of an overstress separation on the portion of the fractured collar which was still fastened to the intact half by a bolt and nut. The other attach hole contained only slight indications of thread imprints in the hole but showed no evidence of deformation to indicate that a bolt was in the hole at the time the collar half separated. The shimmy dampener inside the collar also displayed damage. See attached Metallurgist's Factual Report and Piper Aztec Service Manual inserts. The airplane log book shows the last inspection was an annual inspection performed on February 14, 1995. The airplane has logged 61.4 hours since the inspection.