Results from a research program to investigate the long term effects of through-the-thickness fastener clamp-up force (preload) relaxation on the strength of mechanically fastened joints for two graphite/thermoplastic composite materials (Dupont's IM6/KIII and ICI-Fiberite's IM8/APC(HTA)) are summarized and compared with analytical methods. An experimental program was conducted in which 56 mechanically fastened single-shear joints were tested. Phase I static tests established joint bearing strength as a function of clamp-up force for two types of fasteners (protruding head and countersink) with no relaxation of preload. Phase II testing monitored short-term fastener preload relaxation (up to 1 ,000 hours), with special bolt force sensor washers. Inservice parameters included were temperature, in-plane loads, and torque. The jOints were tested to failure at the end of the relaxation time period to determine any subsequent effect on joint strength. Phase I test results indicated that joint bearing strength increased by as much as twenty-eight percent over the clamp-up force range of a Ibs (fingertight) to 3,500 Ibs for both materials. Fastener head type, material, and temperature also affected the resultant bearing strength. For Phase II, fastener clamp-up force at room temperature (78°F) relaxed an average of six percent from the initial value during the short-term test period. The relaxation was projected to be as high as fourteen and sixteen percent at 100,000 hours for HTA and Kill, respectively. The elevated temperature condition (250°F) significantly increased the relaxation rate with the projected 100,000 hour relaxation amount being as high as thirty-seven percent for HTA and sixty percent for Kill. Comparison of the Phase II bearing strengths to the Phase I results indicated that portions of the data correlated well, while others did not. It was concluded that relaxation of the clamp-up force over the short-term time period did not significantly lower the bearing strength of either material, however an extended exposure to 250°F could affect the bearing strength. / Thesis (M.S.)--Wichita State University, College of Engineering, Dept. of Aerospace Engineering.