On April 2, 1997, at 2230 eastern standard time, a Piper PA-28- 161, N6148H, flown by a certified flight instructor (CFI) and a student pilot experienced a right main landing gear collapse during the landing roll on runway 28 (6,600' x 150') at the Lafayette-Purdue University Airport, Lafayette, Indiana. The airplane was substantially damaged. Neither pilot was injured. Visual meteorological conditions prevailed and no flight plan was filed for the local 14 CFR Part 91 instructional flight. The flight originated from the Aretz Airport, Lafayette, Indiana, at 2200 est. The CFI reported that the accident occurred on the third landing of the flight. He reported that after touching down, "We slowed the aircraft by full aft elevator to turn off the runway at C1. At approximately 30 knots indicated, without ever touching the brakes, the right strut began to shimmy and then collapsed, bringing the wheel flush with the bottom of the wing. We skidded about ten yards to a stop and immediately shut sown the engine." The airplane came to rest on the runway. A post accident inspection of the airplane was conducted by Inspectors from the Federal Aviation Administration (FAA) Indianapolis Flight Standards District Office (FSDO). One of the Inspectors reported that the airplane was jacked up under the right wing at which time it could be seen that the right landing gear was "pulled away from the wing spar and positioned up and aft and slightly outboard." He continued to report three of the four screw holes on the gear strut top and bottom attachment flanges were elongated. The Inspector collected four portions of the strut/spar attachment screws which he forwarded to the NTSB for inspection. (See attached FAA Inspector's Statement) The screw portions were inspected at Engineering Systems, Inc., on April 22, 1997. The metallurgical examination of the screws revealed that two of the screws contained fatigue cracks. One of these contained "...nearly 100% fatigue type cracking with numerous ratchets present." The rim of the other screw contained "...numerous small ratchets, indicating that the fatigue cracking was associated with multiple crack initiation locations. About 90% of the fracture area was instantaneous fracture." The remaining two screw sections did not reveal any fatigue and were deemed as secondary failures. (See attached Metallurgical Report)