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UBC Theses and Dissertations

Towards mitigation of wrinkles during forming of woven fabric composites : an experimental characterization Rashidi Mehrabadi, Armin


Woven fiber-reinforced polymer composites have become superior materials of choice in industries such as aerospace, energy and automotive, partly due to increased conformability to complex 3D shapes. Formability of woven composite reinforcements, however, is restricted by failure mechanisms such as wrinkling, and remains as a challenging issue for thermo-stamping operations. Wrinkling is today one of the most frequent defects arising during forming processes of composite materials, adversely affecting the quality of the final product, and coping with this defect requires fundamental knowledge and understanding of the fabric’s deformation mechanisms. This MASc thesis presents an analytical and experimental study on the shear deformation behavior of woven reinforcements from a multi-level standpoint as to more closely investigate the mechanisms behind formation of wrinkles. Namely, in the first stage of the research, sources of observed conflicts in the literature regarding the trend of tension-shear coupling in woven fabrics are discussed and resolved using a new characterization framework and a custom-design combined loading fixture. It is shown that in order to correctly characterize the tension-shear coupling behaviour in woven fabrics, instead of using global measured data, local normalized forces and displacements should be driven via a non-orthogonal transformation procedure, while considering kinematic force coupling in the test setup. In the second stage, a new characterization technique, namely a multi-step biaxial bias extension (MBBE) test, is proposed to determine the amount of required transverse de-wrinkling force, to flatten the wrinkles of different sizes. The underlying deformation mechanisms and the wrinkling/de-winkling force responses of the fabric are investigated, resulting into some potential practical design considerations for future 3D forming applications. Furthermore, the influence of yarn contact forces, bending rigidity, and tow slippage is highlighted for devising the de-wrinkling strategies. Finally, hemisphere forming experiments are carried out, attempting to find correlations between the results of 2D characterization experiments and those of 3D forming. Subsequently, a practical idea on modifying the blank holder geometry is proposed and validated for effectively suppressing the wrinkles and possibly other forming-induced defects during the fabric forming processes.

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