Dielectric properties and degradation monitoring in polymer-matrix structural composites

摘要

Polymer-matrix composites (PMCs), material systems made of two or more phases, are increasingly being utilized in aerospace applications due to their high strength-to-weight ratios and corrosion resistance. The present reasearch investigates two different types of PMCs: particle-reinforced and fiber-reinforced.To adequately describe some surprising properties that are observed in polymer-matrix composites reinforced with nanoscale filler particles, a three-phase theoretical model is proposed that is suitable for describing the effective permittivity of polymer-matrix composites containing spherical nanoparticles. The model accounts for the presence of an interphase region, which takes on a dominant role in influencing properties of nano-filled composites due to the large surface-area-to-volume ratio of nanoscale fillers.Regarding fiber-reinforced PMCs, spectroscopic techniques have been utilized to determine indicators of composite aging due to thermo-oxidation which are generally undetected by other non-destructive methods, such as ultrasonic testing. Two aerospace composite systems, bismaleimide/glass-fiber and epoxy/carbon-fiber, are thermo-oxidatively aged at a variety of temperatures. Both aged systems are analyzed via photoacoustic spectroscopy, an abosorbance-based IR spectroscopy technique, while the bismaleimide/glass-fiber system is also analyzed dielectrically. Spectroscopic properties are correlated with interlaminar shear strengths to determine the viability of particular degradation indicators and it is observed that changes in permittivity and dissipation factor of bismaleimide/glass-fiber both correlate well with reduction in interlaminar shear strength due to thermo-oxidative aging. The degradation is characterized through kinetic modeling, and the rates and activation energies of the degradation are extracted. Finally, dielectric properties as a function of temperature are measured for bismaleimide resin, and a major relaxation is modeled and characterized.