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.

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 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.

Honeycomb structures find numerous applications in automotive, aerospace, sports, and other similar engineering fields. Such incorporation is made possible by the excellent crushing resistance and specific energy absorption capabilities. However, manufacturing such structures through conventional processes is highly laborious and expensive. Such a drawback can be largely mitigated by the adoption of additive manufacturing (AM) processes. Consequently, in this study, hexagonal honeycomb structures are subjected to experimental tests to determine their compressive strength under different loading rates. In addition to this, attempts have also been made to evaluate the effect of different materials and the unit cell dimensions on the compressive properties. The test specimens of different wall thicknesses are manufactured by fused deposition modelling (FDM) using PLA and ABS as the base materials. The samples are then subjected to compressive tests using a standard UTM to quantify the effect of the cell geometrical parameters and the loading rate on the overall compressive nature of the structures. The results show that the compression properties are primarily affected by the loading rate, material properties and the cell-wall thickness of the structures. The initial compressive yield stress and plateau stress generally increase up to a given value of loading rate, after which the strength decline. The cell-wall thickness of the structure influences the threshold loading rate. Therefore, this study provides a preliminary understanding of the compressive properties of AM hexagonal honeycomb structures to analyse the prospects for application in real-world engineering applications. It is proposed that such structures find profound applications in structural components of aerospace equipment, automotive parts, sports gear, and other similar areas of interest where high strength and energy absorption are of predominant importance.

Apart from the inherent anomalous behaviour under tensile and compressive structures, auxetic structures have shown improved energy absorption characteristics that are of prime interest to various fields of study. This is further exemplified by additive manufacturing (AM) techniques and polymer composites to tailor the shape, geometry and form of these structures. Consequently, this paper aims to characterise the in-plane compressive behaviour and negative Poisson’s ratio (NPR) of the most prominent auxetic structures fabricated additively used polymer nanocomposite materials. The study incorporates the use of glycol-modified polyethylene terephthalate (PETG) and nanocomposites of PETG filled with organically modified montmorillonite (OMMT) nanoclay particles to produce auxetic structures fabricated through fused filament fabrication (FFF). Different structures such as hexagonal re-entrant honeycomb structures, peanut-shaped honeycombs, chiral honeycomb structures and missing rib structures are characterised for their compressive performance through experimental approaches involving mechanical testing and digital image correlation (DIC). Different parameters such as the peak crushing strength, average crushing strength, NPR, specific energy absorption (SEA), and crush force efficiency (CFE) of these structures are evaluated at different strain rates/loading rates for varying concentrations of nanoclay and PETG. It is observed that higher loadings of nanoclay particles lower the compressive strength of the structures. Additionally, the NPR decreases with increasing strain rates and is also influenced by the composition and the resultant stiffness. Moreover, the geometrical parameters of the structure largely influence its strain energy absorption. The results have shown that such material-structure combinations can produce structures of high-performance capabilities suitable for aerospace applications.