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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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  3. Some configurations of 3D woven composites are known to be susceptible to processing induced damage in the form of microcracks that develop in the polymer matrix during curing. The microcracking is believed to originate from high residual stresses that develop due to a significant mismatch in the coefficients of thermal expansion between the constituent materials. In this paper, we investigate the applicability of several commonly used stress-based failure criteria for glassy polymers – the von Mises, the Bauwens (Drucker-Prager), the parabolic stress, and the dilatational strain energy density. We study the microcracking phenomenon on the example of the one-to-one orthogonal configuration of the epoxy matrix/carbon fiber 3D woven composites. This configuration is characterized by the high level of the throughthickness reinforcement which appears to exacerbate the matrix damage. The investigation is based on a high-fidelity mesoscale finite element model of an orthogonally reinforced 3D woven composite. We simulate the material’s response to the uniform temperature drop from the curing to room temperature and compare the results of the simulation with the X-ray computed microtomography. We conclude that the curing induced matrix failure is well predicted by the parabolic stress criterion with a proper choice of the material constants. Initiation and propagation of this failure are simulated via sequential deactivation of the elements exceeding the allowable equivalent stress. 
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  4. There are several possible mechanisms of failure of glassy polymers that can be activated by different states of stress in the material. They are reflected in the various failure criteria used to predict initiation of damage in the polymer based on the components of stress tensor. We investigated the applicability of several popular failure criteria (the von Mises, the Drucker-Prager, the parabolic stress, and the dilatational strain energy density) to predict processing-induced damage due to cooling after curing observed in 3D woven composites with high level of through-thickness reinforcement. We developed high-fidelity mesoscale finite element models of orthogonally reinforced carbon/epoxy composites and predicted their response to the uniform temperature drop from the curing to room temperature. Comparison of the simulation results with the X-ray computed microtomography indicates that matrix failure caused by the difference in thermal expansion coefficients of carbon fiber and epoxy resin is well predicted by the dilatational strain energy criterion. Initiation and propagation of this failure was numerically investigated using sequential deactivation of elements exceeding the allowable equivalent stress. 
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