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1. Core–Shell Rubber Nanoparticle-Modified CFRP/Steel Ambient-Cured Adhesive Joints: Curing Kinetics and Mechanical Behavior

   https://doi.org/10.3390/ma17030749  

   Okeola, Abass Abayomi; Hernandez-Limon, Jorge E; Tatar, Jovan (February 2024, Materials)

   Externally bonded wet-layup carbon fiber-reinforced polymer (CFRP) strengthening systems are extensively used in concrete structures but have not found widespread use in deficient steel structures. To address the challenges of the adhesive bonding of wet-layup CFRP to steel substrates, this study investigated the effect of core–shell rubber (CSR) nanoparticles on the curing kinetics, glass transition temperature (Tg) and mechanical properties of ambient-cured epoxy/CSR blends. The effects of silane coupling agent and CSR on the adhesive bond properties of CFRP/steel joints were also investigated. The results indicate that CSR nanoparticles have a mild catalytic effect on the curing kinetics of epoxy under ambient conditions. The effect of CSR on the Tg of epoxy was negligible. Epoxy adhesives modified with 5 to 20%wt. of CSR nanoparticles were characterized with improved ductility over brittle neat epoxy; however, the addition of CSR nanoparticles reduced tensile strength and modulus of the adhesives. An up to 250% increase in the single-lap shear strength of CFRP/steel joints was accomplished in CSR-modified joints over neat epoxy adhesive joints. 

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2. Low velocity impact behavior of auxetic CFRP composite laminates with in-plane negative Poisson’s ratio

   https://doi.org/10.1177/00219983231168698  

   Lin, Wenhua; Wang, Yeqing (April 2023, Journal of Composite Materials)

   Introducing auxeticity or negative Poisson’s ratio is one potential solution to mitigate the low velocity impact damage of fiber reinforced polymer matrix composites, which can be achieved by tailoring the layup of an anisotropic composite laminate. This study aims to investigate the effect of laminate-level in-plane negative Poisson’s ratio on the low velocity impact behavior of carbon fiber reinforced polymer (CFRP) matrix composites using numerical simulations. The layups of the auxetic composites that allow them to produce negative Poisson’s ratios are identified based on the Classical Lamination Theory and verified through fundamental coupon-level experimental tests. To ensure meaningful comparisons, the non-auxetic counterpart composites are designed by allowing them to produce positive in-plane Poisson’s ratio while closely matching the longitudinal effective modulus of the auxetic laminate. The simulation results indicate that the auxetic laminates suffer smaller (12.6% on average) delamination area in top and bottom interfaces, much smaller (38% on average) matrix compressive damage in the top and bottom plies, and smaller (14.6% on average) fiber tensile damage area in each ply of the laminate at relatively higher impact energies (5 and 8 J). 

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3. Prediction of Damage Initiation and Simulation of Damage Propagation in 3D Woven Composites during Processing

   Kuksenko, D.; Drach, B; Tsukrov, I. (October 2017, Proceedings of the American Society for Composites Technical Conference)

   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. Utilizing Stress-based Failure Criteria for Prediction of Curing Induced Damage in 3D Woven Composites

   Tsukrov, I.; Drach, B.; Drach, A.; Gross, T. (August 2017, Proceedings of 21st International Conference on Composite Materials)

   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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5. ANALYTICAL MODEL FOR COMPOSITE TRANSVERSE STRENGTH BASED ON COMPUTATIONAL MICROMECHANICS

   https://doi.org/10.1615/IntJMultCompEng.2023048428  

   Shah, Sagar P.; Maiarù, Marianna (January 2023, International Journal for Multiscale Computational Engineering)

   The transverse strength of fiber-reinforced composites is a matrix-dominated property whose accurate prediction iscrucial to designing and optimizing efficient, lightweight structures. State-of-the-art analytical models for compositestrength predictions do not account for fiber distribution, orientation, and curing-induced residual stress that greatlyinfluence damage initiation and failure propagation at the microscale. This work presents a novel methodology to develop an analytical solution for transverse composite strength based on computational micromechanics that enables the modeling of stress concentration due to representative volume elements (RVE) morphology and residual stress. Finiteelement simulations are used to model statistical samples of composite microstructures, generate stress-strain curves,and correlate statistical descriptors of the microscale to stress concentration factors to predict transverse strength as a function of fiber volume fraction. Tensile tests of thin plies validated this approach for carbon- and glass-reinforced composites showing promise to obtain a generalized analytical model for transverse composite strength prediction. 

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