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  1. null (Ed.)
    Manufacturing-induced residual stresses in carbon/epoxy 3D woven composites arise during cooling after curing due to a large difference in the coefficients of thermal expansion between the carbon fibers and the epoxy matrix. The magnitudes of these stresses appear to be higher in composites with high throughthickness reinforcement and in some cases are sufficient to lead to matrix cracking. This paper presents a numerical approach to simulation of development of manufacturing-induced residual stresses in an orthogonal 3D woven composite unit cell using finite element analysis. The proposed mesoscale modeling combines viscoelastic stress relaxation of the epoxy matrix and realistic reinforcement geometry (based on microtomography and fabric mechanics simulations) and includes imaginginformed interfacial (tow/matrix) cracks. Sensitivity of the numerical predictions to reinforcement geometry and presence of defects is discussed. To validate the predictions, blind hole drilling is simulated, and the predicted resulting surface displacements are compared to the experimentally measured values. The validated model provides an insight into the volumetric distribution of residual stresses in 3D woven composites. The presented approach can be used for studies of residual stress effects on mechanical performance of composites and strategies directed at their mitigation. 
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  2. null (Ed.)
    3D woven carbon/epoxy composites are often produced using resin transfer molding technique which includes epoxy curing at elevated temperatures. The process may lead to accumulation of the intrinsic residual stresses during cooling of the material caused by the mismatch between carbon and epoxy coefficients of thermal expansion. This paper deals with implementation of mesoscale finite element models to evaluate intrinsic residual stresses in 3D woven composites. The stresses are determined by correlation of the surface displacements observed after drilling 1-mm diameter blind holes with the corresponding predictions of the models. We investigated how a numerical representation of the composite plate surface affects the correlation between the experimental measurements and numerical predictions and how it influences the evaluation of the process-induced residual stresses. It has been shown for ply-to-ply woven composites with different pick spacing that the absence of the resin layer leads to more accurate interpretation of the experimental measurements. The prediction of the average residual stress in the matrix phase of the composite was found to be sensitive to the surface representation accuracy, however, the residual stress magnitude and distribution was not affected fundamentally. 
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  3. null (Ed.)
    Three panels of 3D woven carbon fiber/RTM6 epoxy composites with a ply-to-ply weave with 12x12 (warp/weft) picks per inch (ppi), 10x12 ppi, and 10x8 ppi were fabricated by resin transfer molding. Realistic finite element models of each weave architecture were constructed using Dynamic Fabric Mechanics Analyzer. The resin properties were isotropic and linear elastic and dependent on temperature. The resin-infiltrated fiber tow properties were estimated using homogenization based on Hashin and Shapery formulas. The model was considered to be at zero stress at the 165C curing temperature. The stresses resulting from cooling the composite to 25C were estimated using the resin temperature-dependent properties and the temperature independent properties of the tows. The displacement fields resulting from holes drilled through the middle of the top warp or weft yarn were estimated by virtually drilling a hole in the finite element model and were measured on the specimens using electronic speckle pattern interferometry. In general, the measured displacements transverse to the yarn were lower than the predicted displacements. This suggests the resin in the infiltrated yarns relieves some of the stress by permanently deforming during cooling. The measured displacements along the yarn were approximately the same for the 12x12 ppi,, lower for the 10x12 ppi, and significantly higher for the 10x8 ppi. 
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  4. Intrinsic residual stresses in woven composites result from the coefficient of thermal expansion mismatch between the fibers and the matrix. Extrinsic residual stresses result from large scale thermal gradients during curing and cooling. Intrinsic residual stresses in 3D woven composites are sometimes severe enough to cause micro-cracking in the matrix. They are also expected to impact the fatigue resistance and the impact resistance. To the best of our knowledge, there have been no spatially resolved measurements of the intrinsic residual stress field as a function of position in the repeating weave pattern. We used digital image correlation (DIC) and electronic speckle pattern interferometry (ESPI) to measure the surface displacement field resulting from drilling a 1 mm diameter hole at four selected locations in two different 3D woven composite architectures that represent low and high through-the-thickness constraint. The two methods are used because the displacements sometimes on the lower end of the resolution for the DIC method and the displacement gradients are sometimes too steep to resolve the fringes for the ESPI method. Finite element models constructed with realistic fiber geometry using Dynamic Fabric Mechanic Analyzer software were utilized to estimate the residual stress field from cooling from the curing temperature. Holes were manually inserted by deactivating the elements in the hole region and the resultant displacement fields were compared to the measurements. In general, the measured displacement fields were lower in magnitude than the model predictions. In some cases, the sign of the predicted displacement field is opposite to the observed field which could be attributed to differences between the actual hole location and the hole in the model. 
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