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978-3-8439-4131-0, Reihe Luftfahrt
Daniel Fernández Eguibar
Selective Stitching on Composite Skin-Stiffener Bonding for Improved Damage Tolerant Structures
205 Seiten, Dissertation Universität Stuttgart (2019), Hardcover, A5
The past decades have seen a widespread use of CFRP structures, especially in the transport industry, due to their exceptional weight-specific mechanical properties. The comparatively high prices and the elevated proportion of manual labour involved in the use of pre-impregnated (prepreg) materials has pushed for the introduction and adaptation of textile processes into the composite industry.
Current composite laminates generally present a layered structure where a stack of preforms is embedded in a plastic resin matrix. Such laminates present comparatively poor out-of-plane mechanical properties in that they are susceptible to BVID, which can dramatically reduce their mechanical properties. In addition, micro-cracks present in the matrix can, under loading, grow and propagate as interlaminar cracks, thus further degrading the integrity of a structure.
Structural stitching of composite materials has shown potential in terms of delamination control, crack-arresting capabilities and damage tolerance improvements. It remains, however, an additional step in the production chain that needs to show noticeable improvements to justify the cost increase.
In order to tackle some of the disadvantages mentioned, an improved understanding of the processes involved in the Through-The-Thickness Reinforcement (TTTR) of a laminate is necessary. Only then is selective stitching a viable design approach.
This dissertation pushes said understanding further and identifies which elements and parameters of structural stitching are most effective in increasing damage tolerance of structures in order to maximise the gain of stitching while reducing the extra costs and production times to a minimum.
This work advances the current state-of-the-art understanding of structural stitching as a method to introduce TTTR in a composite laminate. It develops a selective stitching approach through which the elements and parameters of structural stitching are empirically evaluated in terms of their efficiency to increase damage tolerance and contribute crack-arresting capabilities. The systematic approach of this study is built around a pyramidal hierarchy, that first considers a wide range of stitching configurations at low complexity levels (CAI and DCB testing). Then, as the structural and geometrical complexity is raised (CAISE and 7PB), only the most successful selective stitching configurations are carried on and the underlying mechanisms to their success are examined.