SBIR-STTR Award

Fasteners are used extensively on both military and civilian aircraft structures. Stress concentrations introduced by fastener holes, the intimate contact between these materials dissimilar in nature and/or stiffness and the inherent load transfer demands placed on the fasteners themselves can all lead to problems in service. One of the critical parameters which determines the longevity of a joint's integrity is the preload with which the parts are fastened. A tight joint insures that the two parts move as one under subjected flight loads and vibrations. Fighter aircraft are particularly susceptible to problems due to the extreme flight, vibration and environmental load conditions experienced in service. Loss of preload initially results in fretting of the joint. This leads to wear of the fastener and/or the substrate material and if undetected to crack initiation and ultimate material failure. The USAF F-15 fighter will be chosen as a typical fighter aircraft for this investigation. With the assistance of McDonnell Douglas Aircraft (MDA), information will be gathered from the F-15 Depot Maintenance organization to identify which locations on service aircraft have experienced grip-loss problems in fastened components. For each problem area, the specific parameters defining the geometry and materials will be determined. Once this data collection is complete, a typical problem area will be selected for the Phase I case study. A selection criteria process will be used to determine which problem area is best suited for more detailed analysis and testing. In the Phase I case study, the service environment of the target area on the F-15 will be estimated from MDA's extensive F-15 database. An empirical case study will be performed to gain a more thorough understanding of the physical phenomenon which causes the fastened joints to lose their preload. Twenty four representative test specimens will be assembled per the existing installation specifications. These specimen will be tested under flight load, vibrational and harsh environmental conditions. Nut torque, joint preload and microstructual analysis will be performed at various time intervals during these tests.