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Experimental study and analytical approaches to avoid matrix defects during composites manufacturing Mohseni, Seyyed Mohammad

Abstract

Resin matrix defects are common during the manufacture of composite parts. These defects lead to rework or part rejection which ultimately lead to increased manufacturing cost. This study investigates different sources of such defects, the underlying physics, and proposes several analytical models for their mitigation. The goal of this work is to develop a comprehensive and practical analytical approach for process design to avoid matrix defects during processing. Prior to gelation, the resin can release volatiles which may lead to porosity formation. An experimental study was conducted to investigate the conditions under which moisture dissolved in the resin lead to bubble growth and porosity. The experimental setup uses accurate temperature readings and time-lapse micrographs of bubbles. Subsequent data analysis and image processing validated the application of existing bubble dynamics models to predict the onset of moisture-induced bubble growth. These models, together with cure kinetics models for the resin, were used to develop an analytical approach to suppress moisture-driven porosity through cure cycle design (temperature and pressure). Other sources of porosity include gas entrapment and incomplete resin infiltration. The interaction between these two mechanisms was investigated using a novel experimental design that isolates the effect of vacuum and applied pressure. Results showed that resin infiltration influences gas transport out of the laminate by reducing the available transport pathways. A fully coupled transport model was developed that explicitly includes this interaction during processing of partially-impregnated prepregs. It was shown that the model can predict the measured porosity using material parameters from the literature. The resin microstructure after gelation suppresses bubble growth and resin infiltration. However, continuing cure in a constrained geometry leads to internal stress development and potential defects. An experimental setup was used which allows direct observation of the resin behavior after gelation and the formation/propagation of defects. It was found that post-gelation defect formation is hydrostatic stress-driven and the defect morphology is cure rate-dependent. An analytical approach was proposed, and experimentally validated, to avoid post-gelation defects based on comprehensive material characterization and internal stress evolution calculations.

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