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UBC Theses and Dissertations

Towards a multiscale viscoelastic flow-stress model for composite processing Fahimi, Shayan


This work extends the Integrated Flow-Stress (IFS) model framework to new frontiers, including capturing the complex two-way interactions between the deformation of solid fiber-bed and the flow of fluid resin, as well as predicting the effect of an additional compressible phase (gas or porosity) on resin distribution and deformation of the part. The first generation of IFS models presented a sequential coupling between the flow model, based on Darcy’s law, and the stress model, based on Terzaghi’s principle, to find the distortion of composite part and its fiber volume fraction during prepreg processing. As the sequential coupling failed to account for the effect of solid deformation on resin flow, the 2nd generation 3-Phase IFS (3PIFS) model introduced a state variable called the solidification factor, which implicitly accounts for pressure sharing between the fluid and solid phases and controls the effective shear and bulk moduli of the composite system. The current Multiscale Viscoelastic 3PIFS model benefits from newly developed computational modules and material characterization procedures. Firstly, the set of governing equations in the original 3PIFS are reformulated in mixed form involving displacements and pressure as elemental degrees of freedom in order to facilitate their implementation in commercial finite-element codes. Secondly, this work uses a differential form viscoelastic material model as the constitutive equation of the resin to account for resin hardening and stress relaxation in complex multi hold cure cycles. Moreover, to find permissible values of the solidification factor during consolidation, the effect of this parameter on solid-fluid pressure-sharing is studied. This parameter is also characterized as a function of both the temperature and degree of cure. Finally, a multiscale porosity model is developed to capture the effects of gas entrapment and capillary pressure on the porosity distribution at micro- and macroscale in the prepreg. This model is verified and validated by comparing the predicted transverse strain, pressure, and porosity distribution to the results of several numerical and experimental tests in the literature involving prepreg’s cure and consolidation under one- and two-hold cure cycles.

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