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1. Influence of Temperature-Dependent Resin Behavior on Numerical Prediction of Effective CTEs of 3D Woven Composites

   https://doi.org/10.12783/asc36/35938  

   Vasylevskyi, Kostiantyn; Drach, Borys; Tsukrov, Igor (September 2021, Proceedings of the 36th ASC Technical Conference, College Station, TX, USA, 2021)

   null (Ed.)

   3D woven composites are well known for their high strength, dimensional stability, delamination, and impact resistance. They are often used in aerospace, energy, and automotive industries where material parts can experience harsh service conditions including substantial variations in temperature. This may lead to significant thermal deformations and thermally-induced stresses in the material. Additionally, 3D woven composites are often produced using resin transfer molding (RTM) technique which involves curing the epoxy resin at elevated temperatures leading to accumulation of the processing-induced residual stress. Thus, understanding of effective thermal behavior of 3D woven composites is essential for their successful design and service. In this paper, the effective thermal properties of 3D woven carbon-epoxy composite materials are estimated using mesoscale finite element models previously developed for evaluation of the manufacturing-induced residual stresses. We determine effective coefficients of thermal expansion (CTEs) of the composites in terms of the known thermal and mechanical properties of epoxy resin and carbon fibers. We investigate how temperature sensitivity of the thermal and mechanical properties of the epoxy influences the overall thermal properties of the composite. The simulations are performed for different composite reinforcement morphologies including ply-to-ply and orthogonal. It is shown that even linear dependence of epoxy’s stiffness and CTE on temperature results in a nonlinear dependence on temperature of the overall composite’s CTE. 

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2. Numerical Evaluation of Reinforcement Morphology Contribution to Process-Induced Residual Stresses in 3D Woven Composites

   Vasylevskyi, K.; Buntrock, H.; Gross, T.; Drach, B.; Tsukrov, I. (June 2019, Proceedings of NUMIFORM 2019: The 13th International Conference on Numerical Methods in Industrial Forming Processes)

   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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3. Effect of nanoscale interface modification on residual stress evolution during composite processing

   https://doi.org/10.1177/00219983231179522  

   Kumar, Deepak; Dusabimana, Marie Claire; Al-Haik, Marwan; Namilae, Sirish (June 2023, Journal of Composite Materials)

   The interface characteristics of the matrix and fibers significantly influence the evolution of residual stress in composite materials. In this study, we provide a methodology for reducing the residual stress in laminated composites by modifying the thermomechanical properties at the fiber–matrix interface. A hydrothermal chemical growth method was used to grow Zinc Oxide nanowires on the carbon fibers. We then utilized a novel digital image correlation approach to evaluate strains and residual stresses, in situ, throughout the autoclave curing of composites. We find that interface modification results in the reduction of residual stress and an increase in laminate strength and stiffness. Upon growing ZnO NWs on the carbon fibers, the maximum in situ in-plane strain components were reduced by approximately 55% and 31%, respectively, while the corresponding maximum residual stresses were decreased by 50.8% and 49.33% for the cross-play laminate [0°/90°] layup in the x and y directions, respectively. For the [45°/-45°] angle ply layup in the x-direction, the strain was decreased by 27.3%, and the maximum residual stress was reduced by 41.5%, whereas in the y-direction, the strain was decreased by 166.3%, and the maximum residual stress was reduced by 17.8%. Furthermore, mechanical testing revealed that the tensile strength for the [45°/-45°] and [0°/90°] laminates increased by 130% and 20%, respectively, with the interface modification. 

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4. Experimentally Validated Simulations of Blind Hole Drilling in 3D Woven Carbon/Epoxy Composite with Processing-Induced Residual Stresses

   https://doi.org/10.12783/asc36/35944  

   Leos, Arturo; Vasylevskyi, Kostiantyn; Tsukrov, Igor; Gross, Todd; Drach, Borys (September 2021, Proceedings of the 36th ASC Technical Conference, College Station, TX, USA, 2019)

   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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5. Implementation of Numerical Models to Evaluate Intrinsic Residual Stresses in 3D Woven Composites by Blind Hole Drilling Tests

   https://doi.org/10.12783/asc35/34925  

   Vasylevskyi, Kostiantyn; Tsukrov, Igor; Buntrock, Hilary; Gross, Todd; Drach, Borys (September 2020, Proceedings of the 35th ASC Technical Conference, Hoboken, NJ, USA, 2020)

   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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