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Molecular dynamics simulations of polymers can help in understanding the dependence of molecular structure, cross-linking and the chemistry of the polymer chain and resulting thermo-mechanical properties of the polymer. Apart from these thermo-mechanical properties, molecular dynamics is also a powerful tool to examine the durability of polymers with potential aerospace applications under harsh environmental conditions. Ultraviolent radiation from the sun results in dissociation of molecular oxygen into atomic oxygen (AO) which is abundant in lower earth orbit. Testing composites under AO impact requires an extensive experimental setup to simulate low earth orbit (LEO) conditions and is therefore expensive. Using a newly developed accelerated cross-linking methodology in the framework of ReaxFF, bisphenol F and diethyltoluenediamine epoxy polymer chains are manufactured virtually. This simulated polymer is virtually tested for modulus, glass transition temperature, density and is impacted by atomic oxygen at 8 km/s. Thermomechanical properties show good agreement between experiments and simulations. Simulations the polymer during AO impact using ReaxFF provides useful insight to the degradation mechanism in terms of polymer chemistry and thermal profile.

The addition of functionalized nanosilica (NS) particles to epoxy resins is known to improve certain mechanical properties such as modulus of elasticity and fracture toughness. In the current investigation, epoxies with and without NS reinforcement were investigated. Four NS concentrations were evaluated: 0, 15, 25 and a maximum wt% NS dependent on which of the two curing agents was used. The tensile modulus of elasticity and quasi-static Mode I fracture toughness were measured and the Mode I fracture surfaces were examined using a field emission scanning electron microscope for general imaging and a scanning laser confocal microscope for quantitative information on surface morphology. Fracture toughness, as measured by critical strain energy release rate (GIc), and fracture surface area increased monotonically with increased NS content in the epoxy cured with diethyltoluenediamine (DETDA). However, for the material cured at a higher temperature with 4-4’ diamino diphenyl sulfone (DDS), GIc and surface area reach their respective peaks at NS concentrations less than the maximum value. The primary morphological toughing mechanisms observed were particle pullout and crack deflection. The DDS cured system had higher surface area than DETDA system for any non-zero NS content, but less GIc. Analysis of the experimental results led to the conclusion that GIc of the DETDA was mostly explainable in the context of NS particle pullout, as both fracture surface area and GIc varied in rough proportion to NS content. In the DDS system, however, such proportional behavior was not observed and it is believed that competing mechanisms influence GIc at NS concentrations above 15 wt%.

The development of next-generation rotorcraft requires advancement in certain key areas such as cruise speed, payload, range, along with cost considerations. Coaxial and tiltrotors are two promising candidates for future of high speed vertical flight. Traditional lag dampers are unable to produce adequate levels of damping for the rigid fiber composite blades of coaxial rotorcraft. A promising approach for achieving lightweight damping in composite blades is to add carbon nanotubes (CNTs) to the matrix material. The current investigation aims to predict the damping behavior of a rotating beam that represents a simplified fiber reinforced polymer/ CNT nanocomposite blade. The developed model takes in account different physical variables such as the critical shear stress ( ) for slip onset of the CNTs and CNT aspect ratio (l/d), volume fraction ( ), and orientation. The study first looks at a case where the nano-inclusions are aligned with the direction of loading for the rotor blade. The model predicts maximum damping for low in combination with relatively low l/d. The damping trend increases with increase in of the nanoinclusions. A damping ratio of 7.3% and 8% was observed with optimally configured parameters for aligned and randomly oriented CNTs respectively. Overall, the results suggest significant damping augmentation from the CNTs.

Carbon nanotubes (CNTs) are promising materials for increasing the damping of fiber reinforced composites on account of a hypothesized stick-slip dissipation mechanism. The current investigation aims to increase the damping of a carbon/epoxy composites by adding different types and amounts of CNTs to the interlayer regions. [90] , [0] and [0/±45] carbon/epoxy laminates were manufactured using various types and concentrations of CNTs and surfactant. Dynamic behavior was characterized in terms of the storage and loss moduli and loss factor under tensile cyclic loading. For [0] and [0/±45] laminates, the maximum increases in loss factor and loss modulus of roughly 400-600% were obtained with 10 volume percent CNT yarns aligned in the 0-deg. loading direction. The storage modulus of these laminates was not appreciably affected by the CNT yarns. Aligned CNT yarns provided higher damping than equivalent amounts of unaligned CNT buckypapers. In a series of tests on 0-deg. laminates with and without aligned CNT yarns, damping increased with strain amplitude but not with mean strain, while damping in the baseline laminate was nearly invariant with strain. This series of tests also detected no lower threshold of strain for the onset of slip by CNT and a 10x reduction in the rate of increase of damping with strain amplitude when the amplitude exceeded 190 μ.