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Improving the resilience of 3D-printed composites through material extrusion technology (MEX) is an ongoing challenge in order to meet the rigorous requirements of critical applications. The primary objective of this research was to enhance the impact resistance of 3D-printed composites by incorporating continuous hybrid fibers. Herein, continuous virgin carbon (1k) and Kevlar (130D and 200D) fibers were used with different weight and volume fractions as reinforcing fibers to produce hybrid and non-hybrid composites for impact resistance testing to obtain energy absorption with different impact energies: 20 J, 30 J, 40 J, and 50 J. Moreover, 0°/90° fiber orientations were used. Hybrid composites with combinations of PLA + CF + 130D KF and PLA + CF + 200D KF showed higher impact resistance, less damaged areas (71.45% to 90.486%), and higher energy absorption (5.52–11.64% more) behaviors compared to PLA + CF non-hybrids. CT scan images provided strong evidence to resist the fracture and breakage patterns, because the stiffness and elongation properties of the fibers acted together in the hybrids specimens. Furthermore, positive hybrid effects of the PLA + CF + KF hybrids also showed an ideal match of toughness and flexibility in order to resist the impacts. In the future, these hybrids will have the potential to replace the single type of composites in the fields of aerospace and automobiles. [ABSTRACT FROM AUTHOR]

The necessity for more efficient manufacturing approaches of high-performance complex fiber reinforced composites parts is increasing. This study develops a framework to design and produce topologically optimized continuous carbon fiber reinforced composites (CCFRCs) with both high manufacturing efficiency and manufacturability, and experimentally validates via material extrusion (MEX) 3D printing technique in a more than 1-m wing spar for the unmanned aerial vehicle (UAV). The load-carrying structure can be topologically optimized by a solid orthotropic material with penalization method and further designed into a continuous fiber trajectory under the consideration of optimal material density, optimal fiber orientation and 3D printing process limitation. Based on this method, the complex topological CCFRCs can be realized by a single-stroke printing path with intact fiber. The feasibility and effectiveness of the proposed framework have been evaluated on the Messerschmitt-Bölkow-Blohm (MBB) beam which reveals obvious higher performance in specific stiffness and peak load compared with commonly used honeycomb structure. As a case study, a topologically optimized UAV wing spar was designed and validated experimentally. Mechanical testing shows that the designed wing spar can survive in operating loads. Furthermore, a proof-of-concept internal wing structure was presented where conventional wing spar and ribs are replaced by CCFRCs 3D printed structures. It exhibits high load-carrying characteristics, is lightweight and simple to fabricate. [ABSTRACT FROM AUTHOR]

Continuous hybrid fiber‐reinforced composites (CHFRCs) have excellent mechanical properties, and strong designability, and are widely used in aerospace and other fields. The development of 3D printing technology offers novel directions in the design and manufacture of complicated CHFRC components in aerospace. For this purpose, this study focused on the design and characterization of hybrid continuous fiber‐reinforced composites prepared by 3D printing. The rapid mold‐free integrated manufacturing of hybrid continuous Kevlar/carbon fiber‐reinforced composites was realized by coaxial hybrid 3D printing technology. The regulation and mechanism of tensile properties of 3D‐printed CHFRCs were elucidated. The tensile strengths of 3D‐printed CHFRCs were enhanced by up to 15.3% and 92.4%, respectively, compared to single continuous carbon and Kevlar fiber composites produced by the same process parameter. A new tensile modulus predicting method for 3D‐printed CHFRCs was proposed. It mitigated the influence of complex interfacial characteristics on mechanical property prediction and achieved precise estimation of the tensile modulus of 3D‐printed CHFRCs. This will provide the theoretical support for the application and development of 3D‐printed CHFRCs. Highlights: Mouldless rapid manufacturing of coaxial hybrid fiber composites achieved.The tensile strength and modulus of the fiber composites are greatly improved.A new prediction method for the tensile modulus of 3D‐printed composites is proposed. [ABSTRACT FROM AUTHOR]

Highlights • Graphene composite was used to prepared multilayer structure for microwave absorbing. • The multilayer absorber possesses a gradient index of characteristic impedance. • Characteristic impedance was tailored by graphene content and rod width of unit cell. • The seven-layer absorber achieved an absorption above 90% in 4.5–40 GHz. • 3D printing process was used to fabricate the complex structure. Abstract An ultra-broadband electromagnetic (EM) absorber composed of composite metamaterials was designed, fabricated and characterized. Polylactic-acid/reduced graphene oxide (PLA/RGO) composites with different amounts of RGO were prepared as a primary material collection. To provide good impedance matching and efficient wave attenuation, the multilayer absorbers were designed with a gradient index (GRIN) of characteristic impedance by manipulating the RGO content and the geometric parameters of the unit cell. The absorbers with different numbers of layers were fabricated by a material extrusion process, and then the reflection loss (RL) of the prepared samples was measured. Results showed that the methodology for designing a metamaterial absorber with gradient impedance and fabricating through a 3D printing process was effective in producing ultra-broadband absorbers. The seven-layer absorber achieved an absorption above 90% in a broad bandwidth of 4.5–40 GHz. This work could provide inspiration for upgrading the performance of some conventional absorbing composites through a rational design of the composite proportion and structure. [ABSTRACT FROM AUTHOR]

A novel integrated manufacturing process based on continuous fibre reinforced thermoplastic composites 3D printing of continuous fibre reinforced composite lightweight structures (CFRCLSs) was proposed. Cross lap and panel-core lap design strategies were proposed to fabricate the CFRCLSs with a complex shape. The fracture modes during compression loading were analysed. Correlations between the process parameters, structure parameters, density, fibre content, and final performance of the printed specimens were discussed. The fibre content of the printed CFRCLSs were controlled by changing the process parameters. With a fibre content of 11.5% vol, a compression strength of 17.17 MPa was obtained for the 3D printed CFRCLSs. A heterogeneous lightweight sandwich structure specimen was fabricated to demonstrate process feasibility. This new and improved process has great potential for fabricating CFRCLSs with complex shapes, high mechanical properties, and multifunctional benefits. [ABSTRACT FROM AUTHOR]

Poly-ether-ether-ketone (PEEK) is a high-performance, temperature-resistant semicrystalline polymer which is frequently used as a replacement for metals in a wide variety of high-performance end-use application. Thermal processing conditions during manufacturing process for PEEK can implement a significant impact on its crystallinity and mechanical properties directly and indirectly. In this paper, we have used a temperature-control 3D printing system to prepare all the PEEK samples for calculating crystallinity and performing tension tests, in order to investigate the relationship between various thermal processing conditions (the ambient temperature, the nozzle temperature and heat treatment methods) in FDM process and crystallinity and mechanical properties (tensile strength, elastic modulus and breaking elongation) of pure PEEK material. All experiment results show temperature-control 3D printing method has tremendous potential to design, control and realize different degrees of crystallinity and mechanical properties for different PEEK parts, even in different regions of the same PEEK part. [ABSTRACT FROM AUTHOR]

In this paper, a multidisciplinary framework to design and produce a topological structure with inhomogeneous distributions of fiber orientation and content for 3D printed continuous fiber reinforced polymer composites (CFRPCs) was proposed to maximize load and fiber utilization efficiency. The combination of optimization design and 3D printing for composites with curvilinear fiber was achieved that the fiber orientation and content were progressively designed in the two-stage procedure. The fiber orientation and topological structure were concurrently optimized firstly. Then, the fiber content was designed through finite element analysis (FEA) and realized by changing continuous fiber path with a hierarchical structure design method. Based on this, the topological beam has been additively manufactured and experimentally investigated, showing 38.55% and 25.40% in stiffness and peak load enhancement compared with the original CFRPCs design, while 264.39% and 165.84% increments compared with pure polymer design. The proposed approach has widened design freedom of CFRPCs 3D printing, possessing potential application to solve engineering problem in aerospace, automotive and other fields. [ABSTRACT FROM AUTHOR]

A cleaner production pattern for high-performance continuous carbon fiber reinforced thermoplastic composites (CFRTPCs) has been proposed on the base of recycling and remanufacturing of 3D printed continuous carbon fiber reinforced (CFR) PLA composites. Continuous carbon fiber and PLA matrix was recycled in the form of PLA impregnated carbon fiber filament from 3D printed composite components and reused as the raw material for further 3D printing process. The original printing trajectory is reversely applied, allowing for a 100% recycling of the continuous fiber without any effect on the mechanical properties. Tensile performance of recycled carbon fiber filaments was evaluated, which was higher than that of originally printed composites. Remanufactured CFRTPCs specimens also exhibited a 25% higher bending strength than that of original ones, which experimentally demonstrated the first non-downgrade recycling process for CFRTPCs. A material recovery rate of 100% for continuous carbon fiber and 73% for PLA matrix were achieved for a better environmental impact. Energy consumption of 67.7 and 66 MJ/kg respectively for recycling and remanufacturing processes was detected and compared with conventional methods. The proposed cleaner production pattern offered a potential strategy for the low-cost industrial application of fully recyclable composites. [ABSTRACT FROM AUTHOR]

Purpose Continuous fiber reinforced thermoplastic composites (CFRTPCs) are becoming more significant in industrial applications but are limited by the high cost of molds, the manufacturing boundedness of complex constructions and the inability of special fiber alignment. The purpose of this paper is to put forward a novel three-dimensional (3D) printing process for CFRTPCs to realize the low-cost rapid fabrication of complicated composite components.Design/methodology/approach For this purpose, the mechanism of the proposed process, which consists of the thermoplastic polymer melting, the continuous fiber hot-dipping and the impregnated composites extruding, was investigated. A 3D printing equipment for CFRTPCs with a novel composite extrusion head was developed, and some composite samples have been fabricated for several mechanical tests. Moreover, the interface performance was clarified with scanning electron microscopy images.Findings The results showed that the flexural strength and the tensile strength of these 10 Wt.% continuous carbon fiber (CCF)/acrylonitrile-butadiene-styrene (ABS) specimens were improved to 127 and 147 MPa, respectively, far greater than the one of ABS parts and close to the one of CCF/ABS (injection molding) with the same fiber content. Moreover, these test results also exposed the very low interlaminar shear strength (only 2.81 MPa) and the inferior interface performance. These results were explained by the weak meso/micro/nano scale interfaces in the 3D printed composite parts.Originality/value The 3D printing process for CFRTPCs with its controlled capabilities for the orientation and distribution of fiber has great potential for manufacturing of load-bearing composite parts in the industrial circle. [ABSTRACT FROM AUTHOR]

[Display omitted] • Modeling and 3D printing of variable stiffness composites with curved fibers. • Limitations of 3D printing are taken into account in the modeling. • 3D printing of the variable stiffness composites without gaps and overlaps. • 3D printing by the control of positioning a nozzle and the adaptive feed of plastic. • Comparison of numerical and experimental strain fields of the VSCs via FEM and DIC. In this paper, biomimetic continuous fiber reinforced composite structures were modeled and printed on the basis of variable distributions of fiber orientation and the fiber volume fraction, which are similar to the wood structure in the vicinity of a knot. The variable distributions of curved fibers in the variable stiffness composites (VSCs) without gaps and overlaps were realized during the 3D printing process by the control of positioning a nozzle and the adaptive feed of plastic. To demonstrate efficiency of the modeling method and the 3D printing process, a VSC plate with a hole under tensile loading was designed, printed and tested. The results of tensile tests showed that the main failure of the composite plates occurs far from the hole. Furthermore, numerical and experimental strain fields obtained by the finite element method and digital image correlation, respectively, are similar. The efficiency in fiber savings for the composite plates was improved by the change in reinforcement from unidirectional fibers to curved fibers adapted to heterogeneous stress fields. [ABSTRACT FROM AUTHOR]

Purpose Artificial electromagnetic (EM) medium and devices are designed with integrated micro- and macro-structures depending on the EM transmittance performance, which is difficult to fabricate by the conventional processes. Three-dimensional (3D) printing provides a new solution for the delicate artificial EM medium. This paper aims to first review the applications of 3D printing in the fabrication of EM medium briefly, mainly focusing on photonic crystals, metamaterials and gradient index (GRIN) devices. Then, a new design and fabrication strategy is proposed for the EM medium based on the 3D printing process, which was verified by the implementation of a 3D 90o Eaton lens based on GRIN metamaterials.Design/methodology/approach A new design and manufacturing strategy driven by the physical (EM transmittance) performance is proposed to illustrate the realization procedures of EM medium based device with controllable micro- and macro-structures. Stereolithography-based 3D printing process is used to obtain the designed EM device, an GRIN Eaton lens. The EM transmittance of the Eaton lens was validated experimentally and by simulation.Findings A 3D 90o Eaton lens was realized based on GRIN metamaterials structure according to the proposed design and manufacturing strategy, which had the broadband (12-18 GHz) and low loss characteristic. The feasibility of 3D printing for the artificial EM medium and GRIN devices has been verified for the further real applications in the industries.Originality/value The applications of 3D printing in artificial EM medium and devices were systematically reviewed. A new design strategy driven by physical performance for the EM device was proposed and validated by the firstly 3D printed 3D Eaton lens. [ABSTRACT FROM AUTHOR]

3D printing of continuous fiber reinforced thermoplastic composites (CFRTPCs) enables the fabrication of multiscale structures, whose features can simultaneously span the microscale fiber trajectory and macroscale topological structure. In this study, a multiscale design and manufacturing strategy integrating concurrent optimization of micro fiber orientation and macro structural topology was developed for CFRTPCs and realized by ingenious path planning for 3D printing process. Typical structures, such as Messerschmitt-Bölkow-Blohm (MBB) beam and cantilever beam, were verified experimentally in comparison with the monoscale structures. Structural stiffness and peak load could be improved by 36.27% and 64.43% respectively for MBB beam, 123.07% and 52.16% respectively for cantilever beam, showing the significant influence on concurrent material and structure design for CFRTPCs. Multiscale concurrent design and 3D printing could promote the potential of CFRTPCs, and even challenge traditional design and manufacturing mechanism relating material and structure scale. [ABSTRACT FROM AUTHOR]

To fully exploit the preponderance of three-dimensional (3D)-printed, continuous, fiber-reinforced, thermoplastic composites (CFRTPCs) and self-reinforced composites (which exhibit excellent interfacial affinity and are fully recyclable), an approach in which continuous fiber self-reinforced composites (CFSRCs) can be fabricated by 3D printing is proposed. The influence of 3D-printing temperature on the mechanical performance of 3D-printed CFSRCs based on homogeneous, continuous, ultra-high-molecular-weight polyethylene (UHMWPE) fibers and high-density polyethylene (HDPE) filament, utilized as a reinforcing phase and matrix, respectively, was studied. Experimental results showed a qualitative relationship between the printing temperature and the mechanical properties. The ultimate tensile strength, as well as Young's modulus, were 300.2 MPa and 8.2 GPa, respectively. Furthermore, transcrystallization that occurred in the process of 3D printing resulted in an interface between fibers and the matrix. Finally, the recyclability of 3D-printed CFSRCs has also been demonstrated in this research for potential applications of green composites. [ABSTRACT FROM AUTHOR]

Overall performance of curvilinear fiber reinforced composite structures (CFRCSs) can be designed by adjusting the local fiber orientation and content. In this paper, an optimized design method based on stress gradient distribution for CFRCSs was proposed to improve the efficiency in strength of continuous fiber reinforced composites. The mechanism of fiber content regulation in continuous fiber reinforced composites 3D printing was studied. The adaptive feed calculation method of resin was proposed, and the 3D printing of CFRCSs was realized. A 3D printed composite perforated plate with a hole under tensile loading was optimized and fabricated to verify the effectiveness of the proposed methods. After the optimized design, the maximum stress concentration factor was reduced by 36%, and the ultimate tensile strength was increased by 42%. Through the proposed optimization design method, the fiber content distribution corresponds to the stress distribution, and the fiber direction distribution corresponds to the maximum principal stress direction distribution. At the same time, the stress was redistributed to reduce the stress concentration. Therefore, the ultimate strength was improved. The optimized design method and 3D printing method of CFRCSs have potential application prospects in aerospace, automotive and other fields. Image 1 [ABSTRACT FROM AUTHOR]

A novel 3D printing route to fabricate continuous fiber reinforced metal matrix composite (CFRMMC) is proposed in this paper. It is distinguished from the 3D printing process of polymer matrix composite that utilizes the pressure inside the nozzle to combine the matrix with the fiber. This process combines the metallic matrix with the continuous fiber by utilizing the wetting and wicking performances of raw materials to form the compact internal structures and proper fiber-matrix interfaces. CF/Pb50Sn50 composites were printed with the Pb50Sn50 alloy wire and modified continuous carbon fiber. The mechanical properties of the composite specimens were studied, and the ultimate tensile strength reached 236.7 MPa, which was 7.1 times that of Pb50Sn50 alloy. The fracture and interfacial microstructure were investigated and analyzed. The relationships between mechanical properties and interfacial reactions were discussed. With the optimized process parameters, several composites parts were printed to demonstrate the advantages of low cost, short fabrication period and flexibility in fabrication of complex structures. [ABSTRACT FROM AUTHOR]

3D printing provides a new technical means for the preparation of functionally controlled continuous fiber reinforced composites (CFRCs) with variable fiber content. The constitutive model of 3D printed CFRCs with different fiber contents are the basis for establishing mechanical analysis models and design methods of functionally regulated CFRCs based on 3D printing. In this paper, stiffness and strength properties of 3D printed CFRCs with different fiber contents were systematically studied, and the mapping relationship between material properties and fiber content was obtained. The failure processes of 3D printed CFRCs under different loads were studied by microstructural morphology and interfacial performance analysis, revealing the formation mechanisms of the material properties. The constitutive model and finite element analysis method of 3D printed CFRCs were established by using the material properties of 3D printed CFRCs with different fiber contents. The mechanical analysis mode was used to predict and analyze the performance of functionally graded CFRCs with variable fiber content. The results show that the mechanical analysis mode can be used for the optimization design and mechanical analysis of functionally controlled CFRCs. [ABSTRACT FROM AUTHOR]

The local properties of continuous fiber reinforced heterogeneous composites (CFRHCs) can be tailored through fiber arrangement design to maximize load and fiber utilization efficiency. Design method and integrated manufacturing process for CFRHCs have been investigated in the current research, based on 3D printing of continuous fiber reinforced composites (CFRCs). Based on the tensile and compressive properties of 3D printed CFRCs, the optimal design method of CFRHCs under process constraints and actual working conditions was studied. The mapping relationship between fiber volume content and process parameters was studied, and a digital transition from optimized design to 3D printing solutions was implemented. The flexural strength of the optimized CFRHCs is 207 MPa, and the maximum increase ratio is 115% compared with other heterogeneous designs with the same fiber volume content. The failure process of CFRHCs was analyzed by the simulation model, and the damage mechanism of CFRHCs under bending load was revealed. The optimization design and integrated manufacturing solution of the CFRHCs have potential applications in aerospace, automotive and other fields. [ABSTRACT FROM AUTHOR]

The increasing needs for new types of computing lie in the requirements in harsh environments. In this study, the successful development of a non‐electrical neural network is presented that functions based on mechanical computing. By overcoming the challenges of low mechanical signal transmission efficiency and intricate layout design methodologies, a mechanical neural network based on bistable kirigami‐based mechanical metamaterials have designed. In preliminary tests, the system exhibits high reliability in recognizing handwritten digits and proves operable in low‐temperature environments. This work paves the way for a new, alternative computing system with broad applications in areas where electricity is not accessible. By integrating with the traditional electronic computers, the present system lays the foundation for a more diversified form of computing. [ABSTRACT FROM AUTHOR]

Fibre-reinforced composite thin-walled structures are often subjected to complex working conditions with multiple loads. By combining various fibres, continuous hybrid fibre-reinforced composites (CHFRCs) can achieve good comprehensive performances and positive hybrid effects, which will improve the ability of composite structures to withstand complicated loads. Coaxial CHFRCs were prepared quickly without the use of a mould by 3D printing. The influence of the hatch spacing on hybrid effect, dynamic and static bending properties and failure mode were investigated by the research of fibre content, hybrid ratio, and interface. The results showed that the 3D-printed coaxial CHFRCs had a positive hybrid effect for the impact strength. A high positive hybrid effect could be attained by modifying the process parameters. At the same time, the composite's comprehensive performances were enhanced. The impact strength of 3D-printed coaxial CHFRCs was 1.94 times greater than that of the continuous carbon-fibre-reinforced composites, and its flexural strength was 1.75 times greater than that of the continuous Kevlar-fibre-reinforced composites at same process parameter. With their high comprehensive performances and high positive hybrid effect, 3D-printed coaxial CHFRCs have a lot of potential use in thin-walled structures in areas such aerospace. • Coaxial continuous hybrid fibre-reinforced composites (CHFRCs) were prepared quickly without a mould by 3D printing. • The effect of the hatch spacing on hybrid effect, dynamic and static bending properties and failure mode were investigated. • The 3D-printed CHFRCs had an impact strength 1.94 times than CCFRCs and a flexural strength 1.75 times than CKFRCs. • The 3D-printed CHFRCs performed high comprehensive performance and had a positive hybrid effect on the impact strength. [ABSTRACT FROM AUTHOR]

A gradient-refractive-index radar absorbing structure(GRINRAS) for stealth aircraft is proposed in this paper. It is a layered shell composed of a gradient-refractive-index (GRIN) refractor, an absorber and a reflector. The GRIN refractor consists of the isotropic non-resonant woodpile metamaterials, which was designed according to the permittivity distribution equation of the electromagnetic(EM) black hole to bend and match the waves from the air to the radar absorbing structure. The absorber dissipates the EM waves and transforms them into the heat energy. The reflector reflects the EM waves to the absorber and prevents them from entering the inner space of the structure. A ring and a board GRINRAS were designed, and then fabricated by the three-dimensional (3D) printing process of stereolithography (SL). Simulation and experimental results show that the GRINRASes can control the wave propagation and their absorption capacity is better than −10 dB in the broadband of 12–18 GHz. The GRIN refractor and the absorber have almost equal contributions to the absorption capability. This is the first radar absorbing composite structure composed of the gradient 3D metamaterials. It demonstrates the feasibility of using the metamaterials and 3D printing technology in the innovative radar absorbing structure (RAS) of the stealth aircraft. [ABSTRACT FROM AUTHOR]

Multifunctional heat flux manipulation plays an essential role on the industrial processes. A well-designed spatial distribution of thermal conductivity enables a variety of thermal functions, including thermal cloaks and rotators. Despite the promising potential of metamaterials in achieving multifunctional heat transfer in complex nonconformal geometries, significant challenges remain due to limitations in design methods. In this work, a novel approach based on conformal discretization was proposed to address these major challenges, in which the thermal conductivity tensors of discrete functional cells were diagonalized with the local thermal conductivity tensors as inputs. To verify the effectiveness of the conformal discretization, a nonconformal thermal rotating cloak was designed and manufactured. Both simulations and experiments confirmed the effectiveness of this approach in obtaining multifunctional thermal metamaterial with remarkable thermal cloaking and rotating capabilities. Moreover, the nonconformal thermal rotating cloak still maintained thermal functionality well under the condition of omnidirectional heat flow. This work provides a feasible approach to realize multifunctional thermal manipulation in arbitrarily transformed regions, and thus shows significant application potential in the fields of aerospace engineering and civil engineering. [ABSTRACT FROM AUTHOR]

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. • The continuous fiber reinforced composite auxetic honeycomb was fabricated by 3D printing technology. • A combined experimental, analytical and numerical approach was employed to investigate the in-plane behaviors. • Adding continuous fibers lead to dramatic increase in mechanical properties and smaller Poisson ratios. [ABSTRACT FROM AUTHOR]