The improvement of the hygrothermal and mechanical properties of bismaleimide and K3B/IM7 carbon-fiber composites through a systematic study of the interphase

Advanced aircraft travel at speeds in excess of mach 2 result in external skin temperatures ranging from {dollar}{lcub}-{rcub}60spcirc{dollar}C to +190{dollar}spcirc{dollar}C with extreme heating and cooling rates in the presence of moisture. This combination of environments presents a challenge to current materials. IM7 carbon fiber composites made with either bismaleimide (a thermoset) or K3B (a thermoplastic polyimide) resin are being considered for this use. The overall objective of this research was to develop a greater understanding of both the macroscopic response of these materials to hygrothermal and thermal spike environments, as well as the molecular level interactions which affect the ability of the resins to bond to the fiber.; Hygrothermal testing of BMI/IM7 composites showed that resin properties are reduced by the presence of moisture while composite properties are virtually unaffected. Exposure of composites to a relatively small number of thermal spikes between room temperature and 250{dollar}spcirc{dollar}C was shown to have little effect on BMI/IM7 composites, but causes excessive blistering of wet K3B/IM7 composites.; A macroscopic microcracking phenomenon is observed in cross-ply laminates of the BMI/IM7 system. This was shown to be due to the generation of thermal residual stresses formed during cool down from a "stress-free temperature." Both finite element and analytical models were utilized to analyze the causes of this microcracking with a focus on methods for minimizing the resultant stresses. Several methods with potential were identified and suggestions for future work are made.; Microscopically, tests of the bond strength between both the BMI and K3B resins with the unsized IM7 fiber indicated that the adhesion was low. Experimentation on the BMI/IM7 system was able to identify that the only active adhesion promoting mechanism was mechanical interlocking and not chemical bonds between functionalities on the fiber surface and the resin. Various methods for improving the adhesion in the BMI/IM7 system were attempted and some encouraging results were found.; Finally, a set of methodologies for designing an interphase material which will provide an optimum bond between a fiber and resin is presented. These methodologies have been incorporated into an artificial intelligence framework and can be used to tailor composite properties through the addition of an interphase with specific properties.