A coupled process model for co-cure bonding of honeycomb core sandwich structures
Date
2022
Authors
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Journal ISSN
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Publisher
University of Delaware
Abstract
Honeycomb core sandwich structures are co-cured to bond partially cured thermoset prepreg facesheets with an adhesive film to both sides of the honeycomb core under a pre-defined pressure and temperature cycle inside an autoclave. The co-cure process (i) cuts down the processing time and resources, by combining two processing stages of facesheet consolidation and bonding processes into one single operation, and (ii) facilitates the production of complex-shaped parts by consolidating the compliant partially cured prepreg rather than rigid cured laminates. However, excessive dependency of the co-cure process on the materials and process parameters makes it susceptible to defect such as poorly consolidated facesheets and highly porous bond-line causing premature failure of the structure. Therefore, due to their lightweight, there is a great propensity to use honeycomb sandwich structures in the aerospace industry and to improve manufacturing sustainability, developing a physics-based model to understand and diagnose this manufacturing process is indispensable. ☐ This dissertation pursues a framework to provide simulation models for the involved phenomena in the co-cure process of honeycomb sandwich structures with an emphasis on the porosity development within the bond-line. In sandwich structures, the bond-line is the region binding the facesheet to the core structure. The pre-existing micro-scale voids (bubbles) within the materials, adhesive and prepreg, are assumed to be the main source of porosity. The growth and shrinkage of the bubbles according to the process parameters, autoclave temperature and pressure, and the pressure inside the vacuum bag, are addressed by proposing models for the transient and steady-state diffusion-induced growth. Respectively, a bubble growth stability map is constructed to facilitate designing the cure cycle for the co-cure process of sandwich structures. ☐ Coupled models for the adhesive fillet shape, bubble escape phenomena and facesheet consolidation process during the co-cure process of honeycomb sandwich structures are formulated. The adhesive fillet shape model depends on the material wetting characteristics such as surface tension and contact angle. This is used in the model to address the bubble escape phenomenon that stochastically computes the probability of escape of bubbles within the bond-line into hollow core cells. To account for the role of facesheet consolidation on the bond-line porosity development, the previously formulated model is used to calculate the amount of prepreg resin bleeding into the bond-line. The bubble growth model, adhesive fillet shape, and the porosity simulation models are experimentally investigated, and the results show that the models successfully capture the trends. This demonstrates the usefulness of the coupled model as a tool in process screening to maximize the quality of the co-cured parts. ☐ A simulation software “SANDWICH” is designed containing the models for the facesheet consolidation process, adhesive fillet shape, core pressure evolution and the bond-lie porosity development. By SANDWICH, the user can simulate the entire physical phenomena during the co-cure process of honeycomb sandwich structures by selecting the materials and defining the process parameters. Using this software, the optimization for twelve process parameters of the co-cure process is investigated. The objective function is constructed to reflect the quality of both the facesheet consolidation and bond-line porosity. Finally, a data set consists of process parameters and simulation results is constructed to build a framework for the theory-guided machine learning predictive tool to reduce the simulation time and to facilitate the optimization of the co-cure process of honeycomb sandwich structures.
Description
Keywords
Bubble growth, Consolidation process, Honeycomb sandwich structures, Polymer composite materials, Porosity, Process development