Nanocharacterization of Composite Adhesive Bonding Systems

Abstract

University of WashingtonAbstract Nanocharacterization of Composite Adhesive Bonding Systems Rita Jeannine Taitano Johnson Olander Chair of the Supervisory Committee: Brian D. Flinn Department of Materials Science and Engineering This research was directed toward understanding the fundamental science behind polymer matrix-adhesive interactions in adhesively bonded composite aircraft materials. Nanoindentation and nanochemical techniques were used to characterize various regions of adhesively bonded carbon fiber/epoxy composite samples including the matrix resin, adhesive, and bondline mixing zones (interface/interphase). Adhesive regions in cobonded systems were found to have distinct nanomechanical properties. Regions with a higher degree of comingling between adhesive and resin materials, in general, were found to have higher nanomechanical properties unique from the bulk materials. Nanochemical techniques were also shown to be capable of identifying changes in the chemical constituents within cobond system bondline regions. Nano-dynamic mechanical analysis (nanoDMA) methodology was developed and validated through model-based controlled mixtures to directly characterize bondline regions within various bond architectures. NanoDMA measurements overall have good agreement with traditional Tg methods. NanoDMA and the model-based system evaluation indicates that the TgTan(δ) in comingled regions may be estimated for certain systems with Reuss Rule of Mixtures. However, some adhesives show a 3rd order dependence on the volume of adhesive within the comingled region and a baseline characterization curve should be generated for each system. A composite adhesive bonding system baseline with controlled high temperature exposures above and below the anticipated adhesive’s glass transition temperature were characterized to assess high temperature effects on nanomechanical properties. This was then compared to systems with long-term on-aircraft time temperature and stress exposures (scrapped parts), and systems with on-ground, outdoor environmental exposures. When baseline cobond system coupons were conditioned at higher temperatures for short durations, subtle increases in nanomechanical properties were observed, suggesting additional crosslinking at higher temperatures. Exposure below anticipated Tg show that a “post-cure” effect may occur while exposure above Tg show some indications of degradation. Despite subtle changes in the nanomechanical properties, the bonding systems showed long-term thermal exposure stability at temperatures below Tg without any indication of degradation. Overall, implementation of nanomechanical characterization provides value in identifying initial and environmental exposure compatibility of materials within adhesively bonded systems. Combined with other techniques, these methods may significantly reduce the barriers for developing new bonding systems for use in the aerospace industry. Further investigation is recommended to correlate nanomechanical properties to chemical and marco-mechanical properties traditionally used in assessment of bonding systems.