Showing papers in "Composites Part B-engineering in 2020"

A review of recent research on bio-inspired structures and materials for energy absorption applications

Abstract: It is widely known that the availability of lightweight structures with excellent energy absorption capacity is essential for numerous engineering applications. Inspired by many biological structures in nature, bio-inspired structures have been proved to exhibit a significant improvement over conventional structures in energy absorption capacity. Therefore, use of the biomimetic approach for designing novel lightweight structures with excellent energy absorption capacity has been increasing in engineering fields in recent years. This paper provides a comprehensive overview of recent advances in the development of bio-inspired structures for energy absorption applications. In particular, we describe the unique features and remarkable mechanical properties of biological structures such as plants and animals, which can be mimicked to design efficient energy absorbers. Next, we review and discuss the structural designs as well as the energy absorption characteristics of current bio-inspired structures with different configurations and structures, including multi-cell tubes, frusta, sandwich panels, composite plates, honeycombs, foams, building structures and lattices. These materials have been used for bio-inspired structures, including but not limited to metals, polymers, fibre-reinforced composites, concrete and glass. We also discussed the manufacturing techniques of bio-inspired structures based on conventional methods, and adaptive manufacturing (3D printing). Finally, contemporary challenges and future directions for bio-inspired structures are presented. This synopsis provides a useful platform for researchers and engineers to create novel designs of bio-inspired structures for energy absorption applications.

Current status of carbon fibre and carbon fibre composites recycling

Abstract: The rapid rise in the applications of carbon fibre reinforced polymer matrix composites (CFRPs) is creating a waste recycling challenge. The use of high-performance thermoset polymers as the matrix makes the recovery of the fibres and the resins extremely difficult. Implementation of a circular economy that can eliminate waste and re-use resources warrants the use of efficient processes to recycle end-of-life CFRP components and manufacturing wastes. To this end, herein we present a critical review of the current technologies for recovering carbon fibres and/or the polymers and re-manufacturing CFRPs. New research opportunities in developing new biodegradable thermosets and thermoplastic matrices are also outlined together with more radical recycling strategies for the future.

30 Years of functionally graded materials: An overview of manufacturing methods, Applications and Future Challenges

Abstract: Functionally graded materials (FGMs) are a broad research area and attract considerable tremendous attention today in the materials science and engineering society. In recent years, FGMs have experienced remarkable developments in manufacturing methods. FGMs can be produced using several well-known processing techniques from conventional to advanced. This research provides an overview of manufacturing methods for FGMs, thus describing the fundamental difficulties and strengths of these methods based on the available literature over 30 years. Besides, this critical review gives a comprehensive summary of the various applications and the future trends for research required for the design and production of these materials properly with a smooth graded. The anticipated findings of the current paper can be considered as a milestone for future production and analysis investigations for FGMs and are beneficial for researchers, designers, and manufacturers in this scope.

Recent advancements of plant-based natural fiber–reinforced composites and their applications

Abstract: Demands for reducing energy consumption and environmental impacts are the major driving factors for the development of natural fiber–reinforced composites (NFRCs) in many sectors. Compared with synthesized fiber, natural fiber provides several advantages in terms of biodegradability, light weight, low price, life-cycle superiority, and satisfactory mechanical properties. However, the inherent features of plant-based natural fibers have presented challenges to the development and application of NFRCs, such as variable fiber quality, limited mechanical properties, water absorption, low thermal stability, incompatibility with hydrophobic matrices, and propensity to agglomeration. Substantial research has recently been conducted to address these challenges for improved performance of NFRCs and their applications. This article reviews the recent advancements of plant-based NFRCs, focusing on strategies and breakthroughs in enhancing the NFRCs’ performance, including fiber modification, fiber hybridization, lignocellulosic fillers incorporation, conventional processing techniques, additive manufacturing (3D printing), and new fiber source exploration. The sustainability of plant-based NFRCs using life-cycle assessment and the burgeoning applications of NFRCs with emphasis on the automotive industry are also discussed.

A critical review on the fused deposition modeling of thermoplastic polymer composites

Abstract: Fused Deposition Modeling (FDM) is a widely used additive manufacturing technology for fabrication of complex geometric parts using thermoplastic polymers. The quality issues and inferior properties of fabricated parts limited this process to manufacture parts for industrial level applications. Reinforcing the polymer with nanoparticles, short fibers or continuous fibers improve mechanical, thermal and electrical properties compared to the neat polymer. Several works have been carried out since last two decades to print quality products through FDM by using composite materials. The success of expanding this technique to industrial applications depends on the preparation of printable composite feedstock filament and printing without defects. This article reviews the challenges involved in the preparation of composite feedstock filaments and printing issues during the printing of nano composites, short and continuous fiber composites. The printing process of various thermoplastic composites ranging from amorphous to crystalline polymers is discussed. Also, detailed explanation is given about the analytical and numerical models used for simulating the FDM printing process and for estimating the mechanical properties of the printed parts. This critical review mainly helps the young researchers working in the area of processing of composite materials via 3D printing.

Self-healing cement concrete composites for resilient infrastructures: A review

Abstract: Cracks in cement concrete composites, whether autogenous or loading-initiated, are almost inevitable and often difficult to detect and repair, posing a threat to safety and durability of concrete infrastructures, especially for those with strict sealing requirements. The sustainable development of infrastructures calls for the birth of self-healing concrete composites, which has the built-in ability to autonomously repair narrow cracks. This paper reviews the fabrication, characterization, mechanisms and performances of autogenous and autonomous healing concretes. Autogenous healing materials such as mineral admixtures, fibers, nanofillers and curing agents, as well as autonomous healing methods such as electrodeposition, shape memory alloys, capsules, vascular and microbial technologies, have been proven to be effective to partially or even fully repair small cracks. As a result, the mechanical properties and durability of concrete infrastructure can be restored to some extent. However, autonomous healing techniques have shown a better performance in healing cracks than most of autogenous healing methods that are limited to healing of cracks having a width narrower than 150 μm. Self-healing concrete with biomimetic features, such as self-healing concrete based on shape memory alloys, capsules, vascular networks or bacteria, is a frontier subject in the field of material science. Self-healing technology provides concrete infrastructures with the ability to adapt and respond to the environment, exhibiting a great potential to facilitate the creation of a wide variety of resilient materials and infrastructures.

Large deformation and energy absorption of additively manufactured auxetic materials and structures: A review

Abstract: Different from conventional materials, materials with negative Poisson's ratios expand laterally when stretched longitudinally. Known as ‘auxetic’ materials, the effect means they possess particularly fascinating properties, which have recently attracted considerable attention in the literature. A range of auxetic materials has been discovered, theoretically designed and fabricated. Developments in additive manufacturing (AM) techniques enable fabrication of materials with intricate cellular architectures. This paper outlines recent progress in the development of auxetic materials and structures, and their mechanical properties under quasi-static and dynamic loading are analysed and summarised. Limited experimental studies on 3D printed auxetic materials and structures are given more attention, ahead of extensively finite element (FE) simulations. A special focus is dedicated to their large, plastic deformation behaviour and energy absorption performance, which should be stressed in their engineering applications; no review paper has yet been found regarding this. Finally, this paper provides an overview of current study limitations, and some future research is envisaged in terms of auxetic materials and structures, nano-auxetics and additive manufacturing.

Current applications of poly(lactic acid) composites in tissue engineering and drug delivery

Abstract: Biodegradable poly(lactic acid) (PLA) presents suitable physicochemical properties and biocompatibility for biomedical engineering. However, PLA has some drawbacks, such as low cell adhesion, biological inertness, low degradation rate, and acid degradation by-products. In this review, recent progress on strategies to address these problems is summarized, including novel fabrication techniques, high-performance PLA composites, and their applications for tissue engineering and drug delivery. The scaffolds, especially for bone regeneration, blood vessels, organs, and skin regeneration are evaluated, in terms of in vivo and in vitro biocompatibility and biodegradability. The enhanced mechanical, thermal, and rheological properties of PLA biocomposites are analyzed in detail. PLA biocomposites for drug encapsulation, sustained release, and tumor-targeting are also reviewed. Furthermore, the challenges and future perspectives on PLA-based biocomposites are discussed.

Preparation of two-dimensional titanium carbide (Ti3C2Tx) and NiCo2O4 composites to achieve excellent microwave absorption properties

Abstract: The appearance and development of two-dimensional titanium carbide materials provide a new idea for our research on microwave absorption materials. Its excellent electrical conductivity and surface functional groups allow it to be used as a microwave absorber. In this study, Ti3C2Tx@NiCo2O4 composites were prepared by simple hydrothermal method and subsequent annealing process. With the change of annealing temperature, the state of composites is changed, so the structure and properties of samples are further adjusted. When the annealing temperature is 350 °C, an optimal reflection loss value of −50.96 dB can be obtained at 2.18 mm. The excellent microwave absorption performance is not only caused by polarization behavior, but also related to multiple reflections and multiple scattering produced by unique structures. Therefore, the prepared Ti3C2Tx@NiCo2O4 is expected to be a promising microwave absorber with thin thickness and high absorption intensity.

Damage characterization of laminated composites using acoustic emission: A review

Abstract: Damage characterization of laminated composites has been thoroughly studied the last decades where researchers developed several damage models, and in combination with experimental evidence, contributed to better understanding of the structural behavior of these structures. Experimental techniques played an essential role on this progress and among the techniques that were utilized, acoustic emission (AE) was extensively used due to its advantages for in-situ damage monitoring with high sensitivity and its capability to inspect continuously a relatively large area. This paper presents a comprehensive review on the use of AE for damage characterization in laminated composites. The review is divided into two sections; the first section discusses the literature for damage diagnostics and it is presented in three subsections: damage initiation detection, damage type identification and damage localization, while the second section is devoted to damage prognostics and it focuses on the remaining useful life (RUL) and residual strength prediction of composite structures using AE data. In every section, efforts have been made to analyze the most relevant literature, discuss in a critical manner the results and conclusions, and identify possibilities for future work.

A highly stretchable, super-hydrophobic strain sensor based on polydopamine and graphene reinforced nanofiber composite for human motion monitoring

Abstract: Much attention has been given to flexible electronic devices in recent years. Conductive polymer composites (CPCs) have been utilized to fabricate strain sensors owing to their lightweight and high flexibility. It is a great challenge to develop flexible and wearable strain sensors with light weight, good skin affinity and gas permeability, high sensitivity and excellent corrosion resistance. In this work, electrospun thermoplastic polyurethane (TPU) nanofibers were first decorated by graphene through ultra-sonication, followed by polydopamine (PDA) modification and then hydrophobic treatment with 1H, 1H, 2H, 2H-perfluorodecanethiol (PFDT). The obtained electrical conductive polymer nanofiber composites (CPNCs) have a hierarchical polymer core/graphene shell structure and exhibit super-hydrophobicity even under harsh environments. The introduction of PDA not only improves the interfacial interaction between individual graphene sheets but also the interaction between graphene and the TPU nanofibers. Their mechanical properties including Young's modulus, tensile strength and elongation at break are significantly improved, compared to those of TPU nano-fibrous membranes. When CPNC is used as a strain sensor, it displays high stretchability, controllable sensitivity, excellent cyclical stability and durability. Hence, the nanofiber composite based strain sensor can be attached on the skin for precise monitoring of different human motions, such as tiny and large body movements and thus has promising applications in wearable devices.

A comprehensive review on mechanical properties of pultruded FRP composites subjected to long-term environmental effects

Abstract: Pultruded fiber-reinforced polymer (FRP) composite is expected to be widely applied for civil infrastructures due to its advantages of lightweight, anti-corrosion, and industrial manufacturing. Its long-term performance has attracted increasing attentions from academia and industry. This paper presents a comprehensive review on experimental studies investigating the mechanical performance of pultruded FRP composites subjected to long-term environmental effects, including water/moisture, alkaline solutions, acidic solutions, low/high temperatures, ultraviolet radiation, freeze-thaw cycles, wet-dry cycles, and in situ environments. Over 1900 experimentally determined mechanical properties of FRP materials were collected, including tensile, compressive, flexural and shear strength and moduli. The reported test data were highly dispersed, and no uniform conclusions could be drawn from these data. Exposure to water and water-based solutions (alkaline and acidic solutions) had detrimental impacts on the mechanical properties of pultruded FRP materials, whereas other environmental effects induced various levels of degradation. The degradation mechanisms for each environmental effect were discussed, and the existing design approaches were presented. Based on the findings from this review, recommendations were proposed for future works. The database presented herein, which is the largest in the available literature, enables a comprehensive understanding of the degradation behavior of pultruded FRP composites. Moreover, this work can serve as a foundation for deriving predictive models for pultruded FRP materials exposed to long-term environmental effects.

Water-based hybrid coatings toward mechanically flexible, super-hydrophobic and flame-retardant polyurethane foam nanocomposites with high-efficiency and reliable fire alarm response

Abstract: Flammability feature of combustible polymer foam materials often causes massive casualties and property loss, and it is therefore urgent to develop a green and high-efficiency strategy that can reduce or avoid the fire blaze disasters. Here, an extremely simple water-based coating approach is proposed to prepare mechanically flexible, super-hydrophobic and flame-retardant polyurethane (PU) foam nanocomposites with high-efficiency fire warning response. The hybrid ammonium polyphosphate (APP)/graphene oxide (GO) is decorated onto the PU foam surface via electrostatic interactions followed by surface silane functionalization. Interestingly, the silane and APP molecules present selective distributions on the GO and thus form micro-/nano- rough surface with low water affinity to achieve super-hydrophobicity (e.g. water contact angle of ~158.4°). Meanwhile, such hybrid APP/GO/silane coatings produce synergistic flame resistance for the PU foam materials, which is attributed to the formation of compact and uniform P-Si elements co-covered rGO layer on the foam surface. Further, the hybrid coatings can provide high-efficiency fire warning response under complicated conditions, e.g. flame detection response time of only ~2.0 s and excellent fire early warning time in pre-combustion (e.g. 11.2 s at 300 °C). Therefore, this work provides new perspectives to design and develop multi-functional coatings for fire safety and prevention applications.

A visible light driven AgBr/g-C3N4 photocatalyst composite in methyl orange photodegradation: Focus on photoluminescence, mole ratio, synthesis method of g-C3N4 and scavengers

Abstract: A visible light driven nanocomposite containing AgBr and sheet-like g-C3N4 was prepared by precipitation of AgBr nanoparticles (NPs) onto the surface of g-C3N4 nano-sheets. The composite system showed a significant enhancement in photocatalytic activity with respect to the mono-component systems in the degradation of methyl orange (MO) under visible light irradiation. Different methods including, bulky, thermal oxidation, protonation, and ultra-sonication were used for synthesis of g-C3N4 nanosheets. The resulted g-C3N4 samples were then modified by AgBr NPs via deposition-precipitation (DP) method. Among them, the catalyst containing the ‘protonated g-C3N4’ showed the best photocatalytic activity. The composite with AgBr:g-C3N4 mole ratio of 2:1 containing the ‘protonated g-C3N4’ showed the best activity. Different coupling techniques including: ‘grinding’, ‘grinding-ultra-sonication’, ‘ultra-sonication/deposition-precipitation’ and ‘ultra-sonication/deposition-precipitation/ultra-sonication’ were used for coupling of the protonated g-C3N4 with AgBr NPs. Among these catalysts, two later cases showed the best photocatalytic activities. Among the type II- heterojunction and the direct Z-scheme mechanisms, later case well described the boosted photocatalytic activity of the composite with respect to the mono-component systems.

A review on magneto-mechanical characterizations of magnetorheological elastomers

Abstract: Magnetorheological elastomers (MREs) are a class of recently emerged smart materials whose moduli are largely influenced when exposed to an external magnetic field. The MREs are particulate composites, where micro-sized magnetic particles are dispersed inside a non-magnetic polymeric matrix. These elastomers are known for changing their mechanical and rheological properties in the presence of a magnetic field. This change in properties is widely known as the magnetorheological (MR) effect. The MR effect depends on a number of factors such as type of matrix materials, type, concentration and distribution of magnetic particles, use of additives, working modes, and magnetic field strength. The investigation of MREs’ mechanical properties in both off-field and on-field (i.e. absence and presence of a magnetic field) is crucial to deploy them in real engineering applications. The common magneto-mechanical characterization experiments of MREs include static and dynamic compression, tensile, and shear tests in both off-field and on-field. This review article aims to provide a comprehensive overview of the magneto-mechanical characterizations of MREs along with brief coverage of the MRE materials and their fabrication methods.

Rational design of core-shell Co@C nanotubes towards lightweight and high-efficiency microwave absorption

Abstract: Rationally engineering on nanostructure of electromagnetic absorbers provides massive potential for eliminating the pollution caused by electromagnetic radiation. In this work, a series of core-shell Co@C nanotubes were produced via facile pyrolysis of cobalt carbonate hydroxide hydrate/dopamine precursor. Thanks to the well introduction of core-shell and nanotube structure, Co@C composite shows more superior microwave absorption than pristine Co nanoparticles. The unique microstructure provides abundant heterostructures and conductive paths to induce interfacial polarization and conductive loss for boosting dielectric loss. Through regulating the graphitization layer of C shell, the dielectric property could be further enhanced. Coupled with the favorable magnetic loss from the embedded Co nanoparticles, the products exhibit an optimal absorption intensity of −48 dB under low filler content of 30%. And effective absorption frequency bandwidth is up to 5.2 GHz at small thickness of 1.8 mm. Such excellent achievements demonstrate that the core-shell Co@C nanotube composites can be applied as a promising candidate for lightweight and thin electromagnetic absorption.

In situ deposition of pitaya-like Fe3O4@C magnetic microspheres on reduced graphene oxide nanosheets for electromagnetic wave absorber

Abstract: Reasonable nanostructure design and composition are conducive to the electromagnetic wave (EM) absorption behavior of absorbers. Herein, Fe3O4@C/reduced graphene oxide (rGO) nanocomposites with layered structure were fabricated by a feasible solvothermal method. By adjusting the amount of graphene oxide (GO), different dielectric characteristics and impedance matching conditions of Fe3O4@C/rGO nanocomposites could be obtained. Moreover, with amorphous carbon as the matching layer, Fe3O4 and rGO provide strong magnetic loss and dielectric loss, respectively. The results exhibit that Fe3O4@C/rGO nanocomposites own salient EM absorption properties under the combined action of various components and three-level layered structure. In addition, the Fe3O4@C/rGO-20 nanocomposites (the addition of GO is 20 mg) are endowed with the best EM absorption performance, which demonstrates a minimum reflection loss (RLmin) value of −59.23 dB at 6.24 GHz with a sample thickness of 3.57 mm and effective absorption bandwidth (EAB) is 6.72 GHz. Moreover, the widest EAB is 8.24 GHz at a thinner thickness of 2.6 mm with the RL value of −25.80 dB at 14 GHz. This work could be a reference for lightweight, broadband, strong absorption composite absorber.

Discontinuous micro-fibers as intrinsic reinforcement for ductile Engineered Cementitious Composites (ECC)

Abstract: Engineered Cementitious Composites (ECC) have demonstrated superior mechanical and durability performance than conventional concrete. In the micromechanical reinforcing system of ECC, fibers play a pivotal role in establishing the ultrahigh tensile ductility and autogenous crack width control. This article reviews the state-of-the-art of discontinuous micro-fibers as intrinsic reinforcement of ECC regarding technical performance as well as environmental and economic impacts. Mechanical properties of ECC made with different micro-fibers, man-made or natural, and their embodied energy, emissions and material cost, are comprehensively surveyed. Further, studies on fiber hybridization are discussed regarding the combination of different types of fibers to form synergetic reinforcements that mitigate total material cost, and potentially enhance the composite performance. Recommendations on fiber selections are highlighted and directions for future research are suggested.

Free vibration and buckling analyses of magneto-electro-elastic FGM nanoplates based on nonlocal modified higher-order sinusoidal shear deformation theory

Abstract: In this study, the free vibration and buckling responses of functionally graded nanoplates with magneto-electro-elastic coupling are studied for the first time using a nonlocal modified sinusoidal shear deformation plate theory including the thickness stretching effect. The constitutive relations for these kind of structures are defined. The equations of motion for rectangular sandwich plates in macro and nano scale are derived using a modified dynamic version of Hamilton's principle including a contribution of the electric and magnetic fields. The closed-form analytical solution to simply supported plates is obtained using Navier solution technique. A power-law distribution and a half cosine variation are used to model the variation of materials properties and electric/magnetic potentials, respectively. The analytical solutions are verified with well-known solutions in the literature. A parametric study was conducted to show the effect of nonlocal parameter, power-law index, predefined electric and magnetic fields, axial compressive and tensile forces, the aspect ratio of plates, and volume ratio of functionally graded and piezomagnetic layers on mechanical behaviors of nanoplates. Obtained numerical results can be used as benchmark values for validation of correctness of diverse analytical and numerical methods applied for design and analysis of composite nanoelectromechanical systems.

Tensile failure strength and separation angle of FDM 3D printing PLA material: Experimental and theoretical analyses

Abstract: It is discovered in this investigation that there exist two different failure modes and a special separation angle which is the demarcation point of the two different failure modes when FDM (Fused Deposition Modelling) 3D printing materials fail under a tensile load. In order to further understand the mechanical properties of FDM 3D printing materials and promote the use of FDM 3D printing materials, their tensile failure strengths at different printing angles and separation angles are measured and analysed theoretically. A new separate-modes of transversely isotropic theoretical failure model is established to predict the tensile failure strength and separation angle of FDM 3D printing PLA (polylactic acid) material based on the hypothesis of transverse isotropy and the classical separate-modes failure criterion. During this research, the tensile specimens designed according to the current test standard ISO (527-2-2012) for plastic-multi-purpose specimens are fabricated in 7 different printing angles ( 0 ∘ , 15 ∘ , 30 ∘ , 45 ∘ , 60 ∘ , 75 ∘ , 90 ∘ ) and three levels of printing layer thickness (0.1 mm, 0.2 mm, 0.3 mm). Experimental results show that the tensile failure strength increases with the increase of the printing angle or the decrease of the layer thickness. Meanwhile, inter-layer failure tends to occur when the printing angle is small and in-layer failure tends to occur when the printing angle is big. In comparison with the results predicted by the established theoretical model, all values of the Generalized-Relative-Root-Mean-Square Error are close to zero and the experimental separation angles are also between 45 ∘ and 60 ∘ . So the predictive capacity of the theoretical model is affirmed by experimental results.

Recent advances in basalt-fiber-reinforced composites: Tailoring the fiber-matrix interface

Abstract: Basalt fibers (BFs) have attracted much attention in the composites industry because they are chemically stable and have excellent mechanical and thermal properties. Due to their high commercial value, BFs have many applications in the polymer and construction industries. Although BF dosage is a key factor in basalt-fiber-reinforced composites (BFRCs), and mechanical performance improves significantly as the dosage increases, the fiber–matrix interface is another important parameter that determines the performance of BFRCs during their service life. The adhesion between the matrix and the BF is crucial for transferring stress and enhancing the mechanical properties of the composite. This paper reviews polymer and cement composites reinforced with BF and discusses strategies for improving the fiber–matrix interface, such as surface modification of the fibers and the addition of micro- and nanofillers to the matrix.

Creating MXene/reduced graphene oxide hybrid towards highly fire safe thermoplastic polyurethane nanocomposites

Abstract: Developing highly effective flame retardant polymeric materials with the release of low toxic fumes during burning still keep a huge challenge. In this work, we demonstrated successful synthesis of titanium carbide-reduced graphene oxide (Ti3C2Tx-rGO) hybrid via hydrogen bonding induced assembly of Ti3C2Tx and rGO, which was utilized to improve the thermal and fire safe performances of thermoplastic polyurethane elastomer (TPU). The results indicated that the Ti3C2Tx-rGO hybrid showed a strong adhesion and good compatibility with TPU host. Furthermore, as-prepared Ti3C2Tx-rGO hybrid was homogeneously dispersed in the TPU matrix due to the mutual intercalation of rGO and Ti3C2Tx preventing the re-aggregation. The thermal stability of TPU was dramatically improved after the introduction of Ti3C2Tx-rGO. With addition of 2.0 wt% Ti3C2Tx-rGO, the peak of smoke production rate and total smoke release of TPU nanocomposite were remarkably decreased by 81.2% and 54.0%, respectively. In addition, the TPU/Ti3C2Tx-rGO-2.0 showed distinct reductions in the peak of carbon monoxide production rate (54.1%) and total carbon monoxide yield (46.2%). The physical barrier effect, the catalytic charring of Ti3C2Tx-rGO hybrid and the chemical transformation of Ti3C2Tx were responsible for the excellent fire resistance of TPU/Ti3C2Tx-rGO systems. This work provides a novel strategy to significantly reduce the fire hazards of TPU, thus broadening its industrial applications.

3d printed continuous fiber reinforced composite auxetic honeycomb structures

Abstract: Continuous fiber reinforced thermoplastic composite (CFRTPC) auxetic honeycomb structures were fabricated using the 3D printing technology with a specific printing path planning. For comparison, auxetic honeycombs were also fabricated with pure polylactic acid (PLA). In-plane compression tests were conducted, with corresponding damage types explored using Scanning Electron Microscopy (SEM) images. A printing path-based finite element (FE) method was developed to mimic both small and large deformations of CFRTPC auxetic honeycombs, while analytical model was proposed to predict their effective stiffness and Poisson ratio. Good agreement was achieved among analytical predictions, FE simulation results and experimental measurements. A systematic parametric study was subsequently carried out to quantify the dependence of in-plane mechanical properties on geometrical parameters. Compared with pure PLA structures, the presence of continuous fibers efficiently prohibited crack propagation in the matrix for each ligament of CFRTPC auxetic honeycombs. Adding continuous fibers increased the mass only by 6%, but led to dramatic increase in compressive stiffness and energy absorption by 86.3% and 100% respectively and smaller Poisson ratios. The proposed 3D printing technology has great potential in integrated fabrication of continuous fiber reinforced composite lightweight structures having complex shapes, attractive mechanical properties, and multifunctional attributes.

Preparation of short CF/GF reinforced PEEK composite filaments and their comprehensive properties evaluation for FDM-3D printing

Abstract: Fused deposition modeling (FDM) has been successfully applied to fabricating short fiber reinforced polymer composites parts. However, due to the intrinsically limited mechanical properties of matrix polymers, there is critical need to develop fiber reinforced high-performance thermoplastic composites for FDM-3D printing to expand engineering applications. In this work, the potential of FDM-3D printing short carbon fiber (CF) and glass fiber (GF) reinforced high-performance PEEK composites has been investigated. Composite filaments with fiber contents of 5 wt%, 10 wt% and 15 wt% were prepared using extrusion process and characterized by micromorphology observation; and thermal properties testing demonstrated its better thermal stability than pure PEEK. The performance evaluation of printed CF/GF-PEEK parts was focused, including mechanical properties, microstructure, surface quality and porosity. The results indicate that the addition of CF/GF to PEEK can significantly enhance the tensile and flexural strength at the cost of ductility. Lower fiber content of 5 wt% is conducive to increasing mechanical properties, improving surface quality and reducing porosity of printed CF/GF-PEEK. GF/PEEK has better interfacial bonding than CF/PEEK due to the different surface treatments on fibers. Furthermore, microstructure observation suggests that fibers aligned along the printing orientation can strengthen the properties, while pores lead to performance degradation of 3D printed CF/GF-PEEK.

Metal-organic polymer coordination materials derived Co/N-doped porous carbon composites for frequency-selective microwave absorption

Abstract: Structure design and composition control are two crucial factors in determining the microwave absorption performance for absorbers, however, it still possess formidable challenges to balance the dual coordination between them. Herein, a coordination assembly strategy is used to synthesize Co2+-organic polymer coordination materials, then, Co/N-doped porous carbon (Co/NPC) composites, in which Co particles encapsulated in the interior framework of N-doped porous carbon, are obtained by a high-temperature treatment. The increase of the pyrolysis temperature promotes the enhancement of specific surface area while leads to lower N heteroatoms and the agglomeration of Co particles, resulting in adjusted microwave absorption intensity and bandwidth with frequency-selectivity. Balancing the promoted impedance matching, the synergetic effects between dielectric/magnetic components and multiple reflection/scattering, the Co/NPC-800 composites display the strongest microwave absorption intensity with a minimum reflection loss of −65.1 dB at 2.5 mm while an effective bandwidth (

Electrical conductivity of CNT/polymer composites: 3D printing, measurements and modeling

Abstract: We present electrical conductivity measurements and modeling aspects of carbon nanotube (CNT)/polymer composites enabled via fused filament fabrication (FFF) additive manufacturing (AM). CNT/polylactic acid (PLA) and CNT/high density polyethylene (HDPE) filament feedstocks were synthesized through melt blending with controlled CNT loading to realize 3D printed polymer nanocomposites. Electrical conductivity of 3D printed CNT/PLA and CNT/HDPE composites was measured for various CNT loadings. Low percolation thresholds were obtained from measured data as 0.23 vol. % and 0.18 vol. % of CNTs for CNT/PLA and CNT/HDPE nanocomposites, respectively. Moreover, a micromechanics-based two-parameter agglomeration model was developed to predict the electrical conductivity of CNT/polymer composites. We further show that the two agglomeration parameters can also be used to describe segregated structures, wherein nanofillers are constrained to certain locations within the matrix. To the best of our knowledge, this is the first ever electrical conductivity model to account for segregation of CNTs in the matrix. A good agreement between measured conductivity and predictions demonstrates the adequacy of the proposed model. We further evince the robustness of the model by accurately capturing the conductivity measurements reported in the literature for both elastomeric and thermoplastic nanocomposites. The findings of the study would provide guidelines for the design of electro-conductive polymer nanocomposites.

High-efficiency improvement of thermal conductivities for epoxy composites from synthesized liquid crystal epoxy followed by doping BN fillers

Abstract: Liquid crystal epoxy resin presents high intrinsic thermal conductivity coefficient (λ). However, the complex molecular structure design and tedious synthesis process severely limit its rapid development and further industrial application. In this work, a kind of liquid crystal epoxy (LCE) based on biphenyl mesomorphic unit is synthesized from 4,4′-biphenol, triethylene glycol, and epichlorohydrin. Curing agent of 4,4′-diaminodiphenyl methane (DDM) and boron nitride (BN) fillers are both performed to prepare the intrinsic highly thermally conductive liquid crystal epoxy resin (LCER) and BN/LCER thermally conductive composites via casting method. LCE has been successfully synthesized with expected structure, presenting nematic liquid crystal with range of 135–165 °C. LCER shows liquid crystal property with intrinsic λ up to 0.51 W/mK, about 3 times higher than that of general bisphenol A epoxy resin (E−51, 0.19 W/mK). Simultaneously, LCER has good thermal stability with heat resistance index (THRI) being 183.9 °C. In addition, the λ values of the BN/LCER thermally conductive composites increase with the increasing loading of BN fillers. When the content of BN fillers is 30 wt%, the λ value of BN/LCER thermally conductive composites is 1.02 W/mK, twice as much as that of pure LCER, also much higher than that of 30 wt% BN/E−51 composites (0.52 W/mK).

NiCo2O4 constructed by different dimensions of building blocks with superior electromagnetic wave absorption performance

Abstract: A series of NiCo2O4 absorbers constructed by different building cornerstones were successfully fabricated through precipitation-hydrothermal method. By adjusting the precipitants from NaHCO3, urea, and NaOH to Na2CO3, the NiCo2O4 absorbers assembled through zero-dimensional (0D) nanoparticles, one-dimensional (1D) nanorods and two-dimensional (2D) micro/nanoplates could be obtained. We found that NiCo2O4 absorbers formed by two-dimensional building blocks displayed high dielectric loss capacity but rather poor magnetic loss, resulting in inferior electromagnetic (EM) wave absorption performance. On the contrary, the sphere-like and urchin-like NiCo2O4 EM wave absorbing materials assembled by zero-dimensional nanoparticles and one-dimensional nanorods possess multiple magnetic loss mechanisms, which can achieve a balance with dielectric loss, leading to remarkably promoted EM wave attenuation performance. The effective absorption bandwidth for urchin-like and sphere-like NiCo2O4 is up to 5.84 GHz and 6.08 GHz at thickness of 1.88 mm and 2.06 mm, respectively. Moreover, the minimum reflection loss (RLmin) of sphere-like NiCo2O4 also reaches to −42.8 dB as well. The thin thickness, strong absorption capacity and wide effective absorption bandwidth (fe), which is the widest among the previously reported NiCo2O4-based absorbers so far, is prone to be a competitive candidate as the materials for EM wave absorption devices.

Synthesis, characterization, and properties of graphene reinforced metal-matrix nanocomposites

Abstract: Although significant effort has been made on using graphene as a reinforcement in metal matrix nanocomposites due to its extraordinary high strength and high modulus, metal-matrix graphene nanocomposites still exhibit large scatters in experimentally measured physical and mechanical properties. In this paper, recent progress in research on the synthesis of metal-matrix graphene nanocomposites using powder metallurgy technique involving milling, compaction, and extrusion or rolling with special emphasis on the agglomeration of graphene, interfacial bonding, and reaction between metal-matrix and graphene has been critically reviewed. Strengthening mechanisms such as grain refinement, oxide dispersions, strengthening, impeding of dislocations by reinforcement, load transfer between the matrix and graphene, and CTE mismatch in the metal-matrix graphene nanocomposites has been discussed. Existing theoretical models on the effects of graphene on mechanical properties including tribological behavior will also be discussed and compared with experimental observations. Potential future research directions in the area of graphene-reinforced MMNC will be outlined.