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Process-induced wrinkling and waviness in prepreg charge-forming

University of British Columbia

Carbon-epoxy prepreg C-channels were charge-formed by drape forming onto an aluminum tool. Flange lengths, forming temperatures, ply counts, ply sequences, forming methods, and curing bag types were varied and related to wrinkling, waviness, termination profiles, thickness profiles, microstructures, and mesostructures. Post-cure microscopy evaluation revealed wrinkling in parts formed at room temperature. Greater flange lengths and thinner laminates resulted in greater wrinkling severity. Fibre misalignments were more severe for the ply sequence whose 0° plies were located at the surface, taking the form of full-ply waviness after cure. A wrinkling conversion mechanism was proposed to explain the disappearance of externally visible defects after forming and the appearance of full-ply waviness after cure. The extent of conversion was attributed the curing bag tension and the post-formed wrinkled ply’s location within the charge with respect to the laminate’s outer surface. In all flanges, waviness misalignment angles were equal to or significantly greater than wrinkling misalignment angles. Missing length at the flange termination provided a good approximation of the excess length trapped in the form of wrinkling and full-ply waviness. Additionally, part thickness profiles provided further information on the occurrence of wrinkles. Furthermore, fibre misalignment types and locations were corroborated by non-destructive surface photography. Whether laminates were formed by hand lay-up or by drape forming, waviness gradients were found through the thickness of all 0° plies and attributed to the absence of sufficient intra-ply shear. The differences observed when forming the same ply sequence in alternate orientations may provide insight into the forming compound curvatures. The suggested post-form wrinkle conversion to waviness mechanism provides reason to develop waviness detection capabilities and to improve the understanding of mechanical performance knockdown effects from waviness. This study also proposes that post-forming part surface inspection for externally visible defects can be a critical step in identifying post-cure waviness sites. Defects detected in surface photographs after cure showed promising applicability for locating and identifying types of defects in mould-side surface plies. The occurrence of wrinkling and subsequent full-ply waviness is expected to be reduced if inter-ply slip characteristics are improved, though waviness gradients within individual plies will remain.

Text

2016-01-27

Attribution-NonCommercial-NoDerivs 2.5 Canada

http://hdl.handle.net/2429/56721

Materials Engineering

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/2.5/ca/