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  1. 3D woven composites, in particular carbon/epoxy, are being increasingly adopted in aerospace, wind energy, transportation and other industries due to their high strength, lightweight, good dimensional stability and delamination resistance. They are often produced by resin transfer molding with epoxy cured at elevated temperature. This process can result in high level of residual stresses due to the mismatch in thermal expansion coefficients of carbon and epoxy. In this paper, a numerical modeling in combination with blind hole drilling experiments is utilized to determine processinginduced residual stresses in 3D woven composites using the example of orthogonal reinforcement. In particular, the individual contributions of residual stress in the weft and binder tows as well as resin-rich pockets to the entire residual stress distribution are evaluated. Our studies show that these contributions are determined by both arrangement and orientation of the tows. The developed numerical modeling tool can be used in the design of reinforcement architectures with reduced levels of residual stresses. 
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  2. In this paper, the effect of matrix viscoelasticity on the development of residual stresses in 3D woven composites is investigated using Finite Element Analysis. Based on experimental observations, it is hypothesized, that the stresses develop mainly due to the difference in the coefficients of thermal expansion between the fiber reinforcement and the matrix. The model considered is a “1x1 orthogonal” 3D woven composite unit cell that is generated using x-ray computed microtomography data. In this study, cooling after curing is considered under the assumption of zero stress at the beginning of the cooling. In addition to the full time- and temperature-dependent viscoelastic formulation, the applicability of two simplified constitutive methods, elastic and variable time pseudoviscoelastic, is investigated. It is observed that the pseudo-viscoelastic method predicts similar cumulative stress distribution (Von Mises and Hydrostatic) compared to the full viscoelastic results. The elastic model presented the highest stress values while the full viscoelastic model presented the lowest stress values. 
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