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Glassy carbon (GC) materials demonstrate excellent thermal stability and mechanical response with low mass densities, which makes them excellent candidates for use in ablatives and carbon-carbon composites (C/C composites) used in aerospace and hypersonic vehicle structures. Although GC materials have been in development and use for decades, molecular simulation protocols need to be developed to accelerate the optimization of processing cycles and to drive the development of the next generation of C/C composites. The objective of this research is to establish reactive molecular dynamics (MD) simulation protocols to accurately predict the evolution of the molecular structure and properties of furan resin during polymerization and pyrolysis processes. The polymerization simulation protocols have been validated via comparison of the predicted density and Young’s modulus of furan resin with experimental values. The MD pyrolysis simulations protocols are validated by comparison of calculated density, Young’s modulus, carbon content, sp2 carbon content, the in-plane crystallite size (La), out-of-plane crystallite stacking height (Lc), and inter-planar crystallite spacing (d002) with experimental results from the literature for furan resin derived GC. Simulation parameters, such as temperature and pressure, are optimized within relatively short simulation times (2000 ps), and the predicted structures and mechanical properties are shown to agree with experimental measurements. The modeling methodology established in this work can provide guidance for the development of next-generation C/C composite precursor chemistries for thermal protection systems and other high-temperature applications.

The complex structural and chemical changes that occur during polymerization and pyrolysis critically affect material properties but are difficult to characterize in situ. This work presents a novel, experimentally validated methodology for modeling the complete polymerization and pyrolysis processes for phenolic resin using reactive molecular dynamics. The polymerization simulations produced polymerized structures with mass densities of 1.24 ± 0.01 g/cm3 and Young’s moduli of 3.50 ± 0.64 GPa, which are in good agreement with experimental values. The structural properties of the subsequently pyrolyzed structures were also found to be in good agreement with experimental X-ray data for the phenolic-derived carbon matrices, with interplanar spacings of 3.81 ± 0.06 Å and crystallite heights of 10.94 ± 0.37 Å. The mass densities of the pyrolyzed models, 2.01 ± 0.03 g/cm3, correspond to skeletal density values, where the volume of pores is excluded in density calculations for the phenolic resin-based pyrolyzed samples. Young’s moduli are underpredicted at 122.36 ± 16.48 GPa relative to experimental values of 146 – 256 GPa for nanoscale amorphous carbon samples.

During the manufacturing of composite structures, cure shrinkage of the thermoset matrix and differential thermal contraction mismatch between the matrix and fiber reinforcement cause the formation of residual stresses, which can result in a loss in structural durability. New multiscale computational process modeling is essential for linking material chemistry, processing parameters, residual stress evolution, and optimizing mechanical performance. This study establishes a new multiscale process modeling method to accurately predict residual stresses in a unidirectional carbon fiber/epoxy composite using molecular dynamics and finite element analysis simulation techniques. The results of this work demonstrate that process-induced residual stresses have a significant impact on the composite strength in transverse tension, out-of-plane shear, and in-plane shear, with a maximum reduction in strength of 35%. Moving forward, this method can be used as a design and optimization tool for future composite structures for specific engineering applications and can provide processing parameters that can maximize desirable composite properties and/or minimize composite manufacturing energy and cost.

It is well-known that all-atom molecular dynamics (MD) predictions of mechanical properties of thermoset resins suffer from multiple accuracy issues associated with their viscous nature. The nanosecond simulation times of MD simulations do not allow for the direct simulation of the molecular conformational relaxations that occur under laboratory time scales. This adversely affects the prediction of mechanical properties at realistic strain rates, intermediate degrees of cure, and elevated temperatures. An efficient method of correcting such MD predictions of elastic properties is proposed and demonstrated. The phenomenological approach is used to map the predictions of Young's modulus and Poisson's ratio for a DGEBF/DETDA epoxy system to the corresponding laboratory-based properties for intermediate degrees of cure and temperatures above and below the glass transition temperature. The approach uses characterization data from dynamical mechanical analysis temperature sweep experiments. The mathematical formulation and experimental characterization of the mapping are described, and the resulting corrections to the predicted elastic properties for various degrees of cure and temperatures are compared with the experiment. This mapping is particularly important to mitigate the strain-rate effect associated with MD predictions, as well as to accurately predict mechanical properties at elevated temperatures and intermediate degrees of cure to facilitate accurate and efficient composite material process modeling.

It is well-known that all-atom molecular dynamics (MD) predictions of mechanical properties of thermoset resins suffer from multiple accuracy issues associated with their viscous nature. The nanosecond simulation times of MD simulations do not allow for the direct simulation of the molecular conformational relaxations that occur under laboratory time scales. This adversely affects the prediction of mechanical properties at realistic strain rates, intermediate degrees of cure, and elevated temperatures. An efficient method of correcting such MD predictions of elastic properties is proposed and demonstrated. The phenomenological approach is used to correct the predictions of Young’s modulus and Poisson’s ratio for a DGEBF/DETDA epoxy system for intermediate degrees of cure and temperatures above and below the glass transition temperature. The approach uses characterization data from dynamical mechanical analysis temperature sweep experiments. The mathematical formulation and experimental characterization of the correction is described, and the resulting corrections to the predicted elastic properties for various degrees of cure and temperatures are compared with experiment. This correction is particularly important to mitigate the strain-rate effect associated with MD predictions, as well as to accurately correct predicted mechanical properties at elevated temperatures and intermediate degrees of cure to facilitate accurate and efficient composite material process modeling.

A novel approach is established for fabricating high-strength and high-stiffness composite laminates with continuous carbon nanotube (CNT) yarns for scaled-up mechanical tests and potential engineering applications. Continuous CNT yarns with up to 80% degree of nanotube alignment and a unique self-assembled graphitic CNT packing result in their specific tensile strengths up to 1.9 N/tex. Unidirectional CNT yarn reinforced composite laminates with a CNT concentration of greater than 80 wt.% and minimal microscale voids are fabricated using filament winding and aerospace-grade resin matrices. A specific tensile strength of up to 1.71 GPa/(g cm-3) and a specific modulus of 256 GPa/(g cm-3) are realized; the specific modulus exceeds current state-of-the-art IM7, T1100G and even M60J unidirectional carbon fiber composite laminates. The results demonstrate an effective approach transferring high-strength CNT yarns into composites for applications that require specific tensile modulus properties that are significantly beyond state-of-the-art carbon fiber composites, and potentially open a new performance region in the Ashby chart for composite material applications.

Boron nitride nanotubes (BNNTs) are a very promising reinforcement for future high-performance composites because of their excellent thermo-mechanical properties. To take full advantage of BNNTs in composite materials, it is necessary to have a comprehensive understanding of the wetting characteristics of various high-performance resins. Molecular dynamics (MD) simulation provides an accurate and efficient approach to establish the contact angle values of engineering polymers on reinforcement surfaces. In this research, MD simulations are used to predict the contact angle values of different epoxy systems on a BNNT surface. A reactive INTERFACE force field was parameterized and utilized in the simulations to accurately describe the interaction of the epoxy monomers with the BNNT surface. The effect of epoxy monomer type, hardener type, and temperature on the contact angle is established. The results show that contact angles decrease with increases in temperature for all the epoxy/hardener systems. The bisphenol-A based epoxy system demonstrates better wettability with the BNNT surface than the bisphenol-f based epoxy system. As wetting properties drive the resin infusion in the reinforcement materials, these results are important for the future manufacturing of high-quality BNNT/epoxy nanocomposites for high-performance applications.

The next generation of ultra-high strength composites for structural components of vehicles for manned missions to deep space will likely incorporate flattened carbon nanotubes (flCNTs). With a wide range of high-performance polymers to choose from as the matrix component, efficient and accurate computational modeling can be used to efficiently down-select compatible resins, drive the design of these composites by predicting interface behavior, and provide critical physical insight into the flCNT/polymer interface. In this study, molecular dynamics simulation is used to predict the interaction energy, frictional sliding resistance, and mechanical binding of flCNT/polymer interfaces for epoxy, bismaleimide (BMI), and benzoxazine high-performance resins. The results indicate that the BMI has stronger interfacial interaction and transverse tension binding with flCNT interfaces, while the benzoxazine demonstrates the strongest levels of interfacial friction resistance. Comparison of these results with similar results from the literature for other high-performance resins indicates that BMI demonstrates the best overall compatibility with flCNTs for use in high-performance structural composites.

There has been long-standing interest in ultra-fast Metal-Insulator Transition (MIT) in VO2 because of their possible applications in data processing systems specially field-effect transistors. The MIT in VO2 is associated with a structural phase transition, sharp resistivity and optical transparency changes by several orders of magnitudes, which happens close to room temperature (at ~ 340 K). The phase transition occurs from a monoclinic (M) structure to a tetragonal rutile (R) structure. The present work investigates the effect of W dopants in MIT of individual single-crystalline VO2 nanowires by use of aberration corrected scanning transition electron microscopy and in situ TEM. In this study different doping concentration of W were used, which corresponds to different transition temperatures around 340 K. The atomic scale Z-contrast imaging of individual single-crystalline W x V1-x O2 nanowires indicates W dopant atoms and the strain caused by W atoms in in the structure. The localized strain caused by dopants play critical role in metal-insulator phase transition and control of electrical property of doped VO2.

*† ‡ § ** A non-linear, continuum-based constitutive model is developed for carbon nanotube materials in which bundles of aligned carbon nanotubes have varying amounts of crosslinks between the nanotubes. The model accounts for the non-linear elastic constitutive behavior of the material in terms of strain, and is developed using a thermodynamic energy approach. The model is used to examine the effect of the crosslinking on the overall mechanical properties of variations of the crosslinked carbon nanotube material with varying degrees of crosslinking. It is shown that the presence of the crosslinks has significant effects on the mechanical properties of the carbon nanotube materials. An increase in the transverse shear properties is observed when the nanotubes are crosslinked. However, this increase is accompanied by a decrease in axial mechanical properties of the nanotube material upon crosslinking. arbon nanotube (CNT) composite materials have the potential to provide order-of-magnitude increases in stiffness-to-weight and strength-to-weight ratios relative to current materials used for aerospace structural applications. These properties are especially important in the design and development of ultra-light-weight aircraft such as new classes of Unmanned Aerial Vehicles (UAVs), in which the primary requirements are those associated with long-duration, high-altitude flights. To facilitate the development of CNT-based materials for this purpose, constitutive relationships must be developed that predict the bulk mechanical properties of the materials as a function of the molecular structure. Within the past few years, considerable effort has been expended to synthesize CNT/polymer composites that take advantage of CNT properties by enhancing the load transfer between the CNTs and the adjacent polymer molecules. 1-9 One approach to achieving this is to form chemical bonds between CNTs and adjacent polymer molecules (functionalization). Despite the potential increase in load transfer that follows from functionalization with respect to the load transfer that occurs without the presence of a chemical bond, it has been recently demonstrated that functionalization itself can measurably affect the structure and mechanical properties of CNTs, 10,11 which can, in turn, ultimately alter or perhaps degrade the bulk mechanical properties of CNT/polymer composite materials. As an alternative approach to increasing load transfer efficiency between the CNT and surrounding polymer, NASA Langley Research Center has recently developed a material in which single-walled CNTs are covalently bonded together (crosslinked) with the chemical linking agent 1,3-bis(4-aminophenoxy-4’benzoyl)benzene (1,3-BABB). In this material, the short, organic linker units allow for direct CNT-to-CNT load transfer, therefore, possibly allowing for improved overall mechanical properties with respect to pure CNT-based materials and CNT/polymer materials in which the CNTs are functionalized to the polymer only. Since it is expected that the bulk mechanical properties in this new class of materials are expected to be affected by intrinsic and sometimes subtle changes in the molecular structure, a multi-scale modeling approach must be developed to allow for accurate design of materials and provide the constitutive relationships necessary for macro-scale analysis methods. The objective of the present paper is to develop non-linear constitutive models for these crosslinked CNT materials and to examine the influence of crosslink density on the overall, continuum-level elastic properties. Following a brief description of the materials, the equivalent-continuum modeling approach is described in detail. * Staff Scientist, National Institute of Aerospace. Senior Member, AIAA.