Polymers and their composites have undergone massive development over the years through intensive research studies. The plethora of opportunities offered by these materials is greatly complimented by the advent of additive manufacturing. The present work analyses the mechanical properties of glycol-modified polyethylene glycol (PETG) reinforced with organically modified montmorillonite (OMMT) nanoclay and short carbon fibers (SCF). This work is the first of its kind to offer a complete overview of the mechanical properties of the composites prepared by these materials through 3D printing without the application of any post-processing techniques. These materials are initially compounded and followed by extrusion using a single-screw extruder to obtain fine filaments of 1.75 mm diameter. The specimens of PETG composite filaments were 3D printed as per the ASTM standards, using fused deposition modeling technique without any post-processing. The fabricated PETG/OMMT/ SCF specimens are tested to study its tensile, compression, flexural, impact, and hardness properties. The fractured specimens from the tensile tests are analyzed using a scanning electron microscope. It is seen that the addition of OMMT nanoclay improves the properties of the composites by a significant extent for most of the tests. However, the addition of SCF has a negligible effect on the properties of the composites due to the presence of interstitial voids and poor matrix-fiber bonding. This calls for additional process parameter variations and post-processing techniques like pre-stressing, annealing, and others. These composites can be used in a wide variety of applications ranging from secondary structures in aerospace, automotive applications to minor orthotic and prosthetic applications.

This study investigates the effect of uncertainty in composite material properties on the cross-sectional stiffness properties, natural frequencies, and aeroelastic responses of a composite helicopter rotor blade. The elastic moduli and Poisson's ratio of the composite material are considered as random variables with a coefficient of variation of around 4%, which was taken from published experimental work. An analytical box beam model is used for evaluating blade cross-sectional properties. Aeroelastic analysis based on finite elements in space and time is used to evaluate the helicopter rotor blade response in forward flight. The stochastic cross-sectional and aeroelastic analyses are carried out with Monte Carlo simulations. It is found that the blade cross-sectional stiffness matrix elements show a coefficient of variation of about 6%. The nonrotating rotor blade natural frequencies show a coefficient of variation of around 3%. The impact of material uncertainty on rotating natural frequencies varies from that on nonrotating blade frequencies because of centrifugal stiffening. The propagation of material uncertainty into aeroelastic response causes large deviations, particularly in the higher-harmonic components that are critical for the accurate prediction of helicopter blade loads and vibration. The numerical results clearly show the need to consider randomness of composite material properties in the helicopter aeroelastic analysis.

Auxetic structures are a special class of structural components that exhibit a negative Poisson's ratio (NPR) because of their constituent materials, internal microstructure, or structural geometry. To realize such structures, specialized manufacturing processes are required to achieve a dimensional accuracy, reduction of material wastage, and a quicker fabrication. Hence, additive manufacturing (AM) techniques play a pivotal role in this context. AM is a layer-wise manufacturing process and builds the structure as per the designed geometry with appreciable precision and accuracy. Hence, it is extremely beneficial to fabricate auxetic structures using AM, which is otherwise a tedious and expensive task. In this study, a detailed discussion of the various AM techniques used in the fabrication of auxetic structures is presented. The advancements and advantages put forward by the AM domain have offered a plethora of opportunities for the fabrication and development of unconventional structures. Therefore, the authors have attempted to provide a meaningful encapsulation and a detailed discussion of the most recent of such advancements pertaining to auxetic structures. The article opens with a brief history of the growth of auxetic materials and later auxetic structures. Subsequently, discussions centering on the different AM techniques employed for the realization of auxetic structures are conducted. The basic principle, advantages, and disadvantages of these processes are discussed to provide an in-depth understanding of the current level of research. Furthermore, the performance of some of the prominent auxetic structures realized through these methods is discussed to compare their benefits and shortcomings. In addition, the influences of geometric and process parameters on such structures are evaluated through a comprehensive review to assess their feasibility for the latermentioned applications. Finally, valuable insights into the applications, limitations, and prospects of AM for auxetic structures are provided to enable the readers to gauge the vitality of such manufacturing as a production method.

The effect of uncertainty in composite material properties on the nonlinear aeroelastic response and vibratory loads of a four-bladed composite helicopter rotor is studied. The aeroelastic analysis is done using a finite element method in space and time, and the composite cross section is analyzed using a variational asymptotic approach. The effective material properties of composite laminas are first considered as random variables with a coefficient of variation of 5%. The material uncertainty is propagated to cross-sectional stiffness, rotating natural frequencies, aeroelastic response, and vibratory loads of the composite helicopter rotor. The stochastic cross-sectional and aeroelastic analyses are carried out with Monte Carlo simulations. The stochastic stiffness values are scattered up to 15% around the baseline stiffness values and show a Gaussian distribution with a coefficient of variation of about 4%. The uncertainty impact on rotating natural frequencies depends on the level of centrifugal stiffening for different modes. The stochastic rotating natural frequencies indicate a possibility of their coincidence with the integer multiples of rotor speed. The propagation of material uncertainty into aeroelastic response causes large deviations from the baseline predictions and affects the crucial higher harmonics content, which is critical for vibration predictions. The magnitudes of 4=rev vibratory loads show a scattering up to 300% from the baseline value, and their probability density functions show non-Gaussian-type distributions. Further, the uncertainty results for a coefficient of variation of 10% in the material properties are obtained. The uncertainty impact on the aeroelastic response is found to be proportional to the coefficient of variation of the composite material properties.

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The focus on the combined effects of fibers and nanoparticles in the realization of novel, high-performance polymer composites has been increasing progressively. In addition to this, the intervention of additive manufacturing in this pursuit has further enhanced this interest among researchers to experimentally quantify the properties of these composites for various applications. Therefore, this study focuses on experimentally evaluating the thermal behavior of extruded glycol-modified poly(ethylene terephthalate) (PETG) comonomers reinforced with short carbon fibers (SCFs) and organically modified montmorillonite (OMMT) nanoclay that are apt for 3D printing. Different weight compositions of the aforementioned materials are prepared, compounded, and extruded using a twin-screw extruder into 16 variants of 3D printable filaments. These filaments are subjected to thermogravimetric analysis, differential scanning calorimetry, and Fourier transform-infrared (FTIR) spectroscopy as per their respective American society for testing and materials (ASTM) standards. The results show improvements in the thermal behavior of the composites for various concentrations of OMMT and SCFs. The FTIR analyses complement the capability of OMMT particles and SCFs for microvibrational damping and infrared absorption. The study also demonstrates the influence of chemical interactions between the SCFs, OMMT, and PETG on the overall performance of the composites. It is believed that this study paves way for the induction of such composites in relevant applications including secondary aerospace structures, automobile interiors, and other engineering structural needs.

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