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Civil-Comp Proceedings
ISSN 1759-3433
CCP: 93
Edited by: B.H.V. Topping, J.M. Adam, F.J. Pallarés, R. Bru and M.L. Romero
Paper 76

Analytical Approaches to Model the Stiffness Reduction in Multi-Layered Carbon Fiber Composites with Process-Induced Defects

D. Chrupalla, D. Hartung and I. Khattab

Institute of Composite Structures and Adaptive Systems, German Aerospace Center, Braunschweig, Germany

Full Bibliographic Reference for this paper
D. Chrupalla, D. Hartung, I. Khattab, "Analytical Approaches to Model the Stiffness Reduction in Multi-Layered Carbon Fiber Composites with Process-Induced Defects", in B.H.V. Topping, J.M. Adam, F.J. Pallarés, R. Bru, M.L. Romero, (Editors), "Proceedings of the Tenth International Conference on Computational Structures Technology", Civil-Comp Press, Stirlingshire, UK, Paper 76, 2010. doi:10.4203/ccp.93.76
Keywords: composites, defects, porosity, fiber waviness, analytical models, effective material stiffness.

Injection moulded components are a cost-efficient alternative to preimpregnated fiber materials (prepregs). The use of the injection technology reduces the cost and proves the possibility of producing large composite structures, but it creates the risk of process-induced defects. Voids and fiber waviness can be frequently found in composite structures. These defects reduce the material stiffness and strength. The main reasons for voids are entrapped air bubbles which develop during the resin infiltration process, volatile gases which occur during the curing process, or bad wettability behaviour of the fibers. Voids develop either within a ply (intralaminar) or between additional plies (interlaminar). Fiber waviness defects are often developing during the manufacturing or the draping of dry textiles or prepregs; especially in the filament winding process, the development of fiber waviness defects are a problem.

In our paper, we will present analytical models which enable the characterisation of defects in composite materials. First, a method is introduced to determine the reduction of the material stiffness with an analytical model. It is based on a theory for homogenization of materials with inhomogenities and a three-dimensional laminate theory. The two approaches are combined to predict the influence of voids on the material properties of multi-layered carbon fiber composites. According to the model, the material stiffness is reduced with increasing pore volume fraction. The percentage reduction of the out-of-plane stiffness coefficients is higher than the percentage reduction of the in-plane coefficients.

Secondly, three efficient analytical approaches are presented to account for the influence of fiber waviness on material stiffnesses. In these models, fiber waviness is described by periodic functions. In the first approach, the fiber waviness is described by a harmonic sine function where the maximum amplitude of the fibers is in the laminate midplane and is decreasing to the outer surfaces of the laminate. The second and third approaches are modifications of the first approach. In the second approach, the maximum amplitude of the fiber waviness is on the outer surface of the laminate. In the third approach, the waviness is described by a Fourier series to be able to model more complex wave geometries. The maximum amplitudes are decreasing from the midplane to the outer surfaces of the laminate. We carried out studies on laminates with multiple ply orientations. It was found that especially the global normal stiffness, which coincides with the stiffness longitudinal to the fibers in a unidirectional laminate, is influenced by out-of-plane waviness; especially in a unidirectional laminate, an increasing grade of fiber waviness leads to a detrimental reduction of the stiffness.

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