(Dr. William Johnson, advisor)

"Mechanical Behavior of Mechanically Fastened Composite Joints"

Abstract

Polymer matrix composites (PMC) offer excellent strength- and stiffness to weight ratios and as such are very attractive for aerospace applications. One such material, IM7/PETI-5, is being considered a structural component on the High Speed Civil Transport (HSCT).  Because the HSCT is designed for sub-sonic cruise, some of the structural components will be exposed to elevated temperature (177^oC, 350^oF).  The elevated temperature could be a problem for the PMC because elevated temperature is known to degrade the mechanical properties of the polymer matrix and consequently the composite itself.  Therefore, there is a need to access the long-term affects of time and temperature on the composite joint’s performance.  The effects of time and temperature were observed in the three different ways:  (1) elevated temperature (177^oC, 350^oF) aging, (2) bolt bearing creep, and (3) bolt bearing fatigue.

The goal of this project was to establish creep and fatigue design criteria for the IM7/PETI-5 composite joint with an emphasis on long term durability.  In the bearing creep and fatigue tests, IM7/PETI-6 showed little time dependant behavior and almost no hole elongation was observed prior to failure.  Therefore, the IM7/PETI-5 composite has proved to be highly creep and fatigue resistant.  The results of the creep and fatigue tests for the IM7/PETI-5 were compared with results from testing on IM7/K3B, which show that the IM7/PETI-5 has improved elevated temperature properties.  During composite testing, some fastener failures were observed.  Fracture mechanics was used to predict the crack growth in steel fasteners and the results were compared with experimental data.