Fabrication and compressive strength of porous hydroxyapatite scaffolds with a functionally graded core/shell structure

A novel type of porous hydroxyapatite (HA) scaffolds with a functionally graded core/shell structure was fabricated by freeze casting HA/camphene slurries with various HA contents into fugitive molds containing a graphite template with three-dimensionally interconnected pores for the creation of a highly porous core. All the fabricated samples had functionally graded core/shell structures with 3-D periodic pore networks in a core surrounded by a relatively dense shell. The overall porosity of the sample decreased from 60 to 38 vol% with increasing HA content in the HA/camphene slurry from 20 to 36 vol% due to a decrease in porosity in both the core and shell regions. In addition, the compressive strength was improved remarkably from 12 ± 1.1 to 32 ± 3.0 MPa. The in vitro cell test using a pre-osteoblast cell line showed that the samples had good biocompatibility.     

Fractography of 3YPSZ/316L functionally graded composite subjected to indentation and flexural bending tests

In this paper, stainless-steel (AISI316L) and 3 mol% Y₂O₃-partially stabilized ZrO₂ were used for fabrication of functionally graded material by spark plasma sintering. To make a temperature gradient in FGMs, a specific die configuration was utilized, which led to higher densification. In order to study the crack initiation and propagation behavior in the layered composites during bending and indentation tests, the section morphology of the layers was analyzed by optical microscopy and scanning electron microscopy. The results showed that cracks propagate through the PSZ matrix when there is no trace of steel. However, when cracks reach steel particles they grow around the particles and choose the interface of steel and matrix as a preferable route for propagation. Furthermore, the presence of small amounts of steel particles remained on the fracture surface, demonstrated that the steel particles can improve the fracture toughness of the composite.     

Percolation effects in functionally graded SOFC electrodes

A solid oxide fuel cell (SOFC) composite electrode exhibits a superior performance compared to a single phase electrode since the electrochemically active zone is spread into its volume. A functionally graded composite electrode consisting of monosized spherical electrocatalyst and electrolyte particles is sintered numerically by the discrete element method (DEM). The electrochemical performance is evaluated by a resistance network approach using Kirchhoff's current law. In the network discretization each contact between two particles is substituted by a bond resistance defined by contact size and the type of materials in contact.The graded electrode is optimized by varying its composition at the electrolyte/electrode interface and the degree to which the composition decreases linearly towards the current collector/electrode interface. Regarding its electrochemical activity, the graded electrode does not perform significantly better than an optimized uniformly randomly mixed composite electrode but percolation of the graded electrode is improved. In order to demonstrate the importance of percolation effects, a novel better performing electrode is developed which contains electronically conducting particle chains arranged within a random packing of ionically conducting particles.     

Simulation of compositional profile in functionally graded materials

The development of a polymer based functionally graded material (FGM) of desired composition profile by the centrifugation technique requires control on centrifugation, size, shape, and concentration of suspended particles, time, viscosity variation of polymerizing fluid, etc. A simulation was conducted to observe the compositional variations with time at different places of FGM, using a modified terminal velocity equation for particle movement in polymerizing fluid. It was further modified for the particles having different sizes. The simulation demonstrated two graded‐composition profiles each one in low concentration region from where particles were moved to the other part of sample and second high concentration profile in which particles entered to increase the concentration. The third region situated between the two composition profiles was observed as that of uniform distribution of particles and the length of this region can be controlled by adjusting the size of the centrifuged sample. The simulation was compared with the experimental results of FGM having SiC particles in polysulphide epoxy resin. POLYM. ENG. SCI. 46:1660–1666, 2006. © 2006 Society of Plastics Engineers     

Peel strength improvement of foam core sandwich beams by epoxy resin impregnation on the foam surface

The interfacial adhesion characteristics between foam cores and faces affect much the structural integrity of foam core sandwich structures. The peel strength between the face plate and the foam core is one of the appropriate parameters for the interfacial characteristics of sandwich structures and its peel energy is also measured for the interfacial characterization. The peel strength is the first peak force per unit width of bondline required to produce progressive separation, and the peel energy is the amount of energy per unit bonding area associated with a crack opening. In this study, to improve the peel strength between the foam core and the face plate of foam core sandwich beams, the surfaces of foam core sandwich beams were resin-impregnated. Then the peel strength as well as peel energy of resin impregnated polyurethane foam core sandwich beams were measured by the cleavage peel test and compared with those of the same sandwich beams without surface resin impregnation on the foam surface.     

Extrusion-based additive manufacturing of functionally graded ceramics

The Ceramic On-Demand Extrusion (CODE) process has been recently proposed for additive manufacturing of dense, strong ceramic components via extrusion with uniform layered drying. This study focuses on enabling CODE to fabricate functionally graded ceramics. A controlled volumetric flowrate for each ceramic paste was used to achieve a gradient between alumina and zirconia. A dynamic mixer was built to mix constituent ceramic pastes homogeneously. Functionally graded alumina/zirconia samples were printed, sintered, and tested to examine the capability of CODE in fabricating functionally graded components. The desired and actual material compositions were compared using energy dispersive spectroscopy. Dimensions of sintered samples were evaluated to study the deformation of functionally graded components during drying and sintering. Vickers hardness was also measured at different locations, corresponding to different material compositions. Finally, a case study was conducted to demonstrate the capability of the proposed method to build functionally graded ceramics with complex geometries.     

Attaining a controlled graded distribution of particles in polymerizing fluid for functionally graded materials

The precise control on concentration profile of dispersion in functionally graded material (FGM) is essential for obtaining a desired material. A suitable simulation of parameters and an appropriate model that describes the motion of particles in the fluid can predict various aspects those are needed to produce FGM, by gravity sedimentation or centrifugation technique. Simulation was conducted to observe the changes in concentration profile, while using the following equations applicable to polymerizing fluid, and to determine the terminal velocities (V_m) of particles; V_m = {D²(ρ_s ? ρ_l)g*(1 ? ?_s)^4.65}/(18μ₀e) for gravity sedimentation and V_m = {D²(ρ_s ? ρ_l)rω²(1 ? ?_s)^4.65}/(18μ₀ e) for centrifugation, where D is the diameter of the spherical particle, ρ_s the density of solid particles, ρ_l the density of fluid, μ the viscosity of fluid, g* the acceleration due to gravity, ?_s is the volume fraction of particles, and t_c is the elapsed time of curing of thermosetting resin. b is a constant, r is the radius, and ω is the angular velocity. This simulation demonstrates that the time of centrifugation/sedimentation, particle size, distribution of particle size, and centrifugal/gravitational forces can be effectively utilized to attain a desired concentration profile in graded materials. Simulation also revealed that there exist the possibility of two graded profiles, namely low concentration profile and high concentration profile, in one sample of graded material, made either by centrifugation or sedimentation. Low concentration profile is more sensitive to particle size distribution as compared to high concentration profile. The present simulation method is also sensitive to concentration‐measuring methods. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci, 2006     

Cu基功能梯度复合材料的发展及研究现状

Cu具有优良的性能,仅次于银的导电和导热性,容易塑性变形等,但铜的强度、硬度较低,耐腐蚀、耐氧化、耐磨性差等大大限制了其应用,要求既具有高导电、导热低温延展性,同时具有高强度、耐腐蚀、氧化等性能,Cu基功能梯度材料的出现满足了材料这种高导电率、高强度以及更高的性能要求,目前有Cu-Mo,Cu-WC等功能梯度材料,在导电导热性、等离子体材料等有广泛的应用。重点综述了Cu基功能梯度复合材料的各种制备方法、应用,并对其未来发展及前景进行了展望。     

Understanding centrifugal casting in the manufacture of functionally graded materials

Centrifugal casting is a shaping technique intended for ceramic tubular structure manufacture, where the particle size and density are exploited to produce asymmetrical membranes and functionally graded materials. However, some well-established insights about particle segregation are debatable and remain unclear. For instance, small particles do not necessarily stay in the inner region, and the bigger particles do not accumulate in the outer. Herein several manufacturing parameters were studied through Taguchi’s robust design, using discrete element method-based simulations to generate the data. Alumina powder was used as model material and water as the liquid phase, to assess the green cast formation time, the cast thickness, the roundness, the changes in relative density, and particle size distribution along the cast’s cross-section. The mean particle diameter and the rotation speed are the most influential parameters for the casting time. The volume fraction of solids on the precursor slurry is decisive regarding the cast structural properties, and particle segregation is negligible, except for size differences above one order of magnitude. When a fraction of denser nickel powder was added, density segregation was also observed, but the size differences can overshadow its effect. In addition, alumina and nickel particles were cast in a lab-scale centrifuge and experimentally validated the segregation of particles. The centrifugal casting method was successfully applied for producing the Al₂O₃-Ni composites with a gradient distribution of the Ni phase.     

Fracture toughness of epoxy-based stepped functionally graded materials reinforced with carbon nanotubes

The aim of this work is to investigate the fracture characteristics of the epoxy-based stepped functionally graded materials (FGM) reinforced with carbon nanotubes (CNTs). The effects regarding fracture toughness in mode I were also studied. The specimens were fabricated with three different mass percentages of 0.1, 0.2 and 0.3%. An ultrasonic device was used to disperse the carbon nanotubes to have a uniform mixture without agglomeration of the CNT particles. Using the ASTM standard D-5045, the fracture toughness was obtained in the experiments. Some compact tension specimens were tested in a tensile machine in mode I. Two different notches were investigated to calculate the fracture toughness. For each notch, there were different fracture toughness and fracture forces values. The experiments showed that there is an improvement in the fracture resistance of FGMs and non-graded composite materials by increase in the CNTs content. The materials with the same content of carbon nanotubes do not have the same properties. It is seen that high fracture toughness can be obtained from different CNT content materials in each notch. In fact, the size of the notch affects the results. Comparing the fracture toughness values and fracture forces results showed that there is no specified rule to predict the increase in the fracture characteristics by increasing carbon nanotubes content. Fracture characteristics depend on the important parameters such as the size of the notch, CNTs content and dispersion of the carbon nanotubes.     

Failure behaviour of the single lap joints of angle-plied composites under three point bending tests

Abstract

In this paper, the response of adhesively-bonded single lap joints (SLJs) with angle-plied composite adherends subjected to flexural loading was investigated. The experiments were carried out for the adherends, glass reinforced polymer matrix, with three kinds of stacking sequence. A three-dimensional finite element (FE) model was developed using ABAQUS/Explicit. The three dimensional Hashin failure criterion with an appropriate damage evolution law was used to characterize the damage inside a ply. Cohesive zone elements were used to model the damage in the adhesive layer (AF163-2K) and the interply failure, that is, the delamination. The developed numerical model was verified with the performed experiments. The SLJs of [±20]_5s and [±45]_5s failed due to failure in the adhesive layer and the delamination between the plies, whereas that of [±10]_5s failed mainly due to the former failure. The intralaminar damage was not noticed for any case. The influence of the fiber angle of plies in the adherends, adherend thickness, overlap length, and the thickness of adhesive layer on the damage in the adhesive layer and the delamination were investigated in terms of the competition between these two failures and activation of different failure modes in each thoroughly.     

Electrical properties of barium titanate stannate functionally graded materials

Barium titanate stannate (BTS) functionally graded materials (FGMs) with different tin/titanium concentration gradient were prepared by the powder-stacking method and uniaxially pressing process, followed by sintering. Impedance spectroscopy (IS) was used to determine the electrical characteristics of FGMs and ingredient BTS ceramics, as well as to distinguish the grain-interior and grain boundary resistivity of the ceramics. Activation energies of FGMs and ingredients were calculated. It has been established that for BTS ceramics the activation energy deduced from grain-interior conductivity (0.73–0.75 eV) is defined by chemical composition, while activation energy for grain boundary conductivity (1.07–1.25 eV) is influenced by microstructural development (density and average grain size). Furthermore, for FGMs, activation energy for grain-interior conductivity kept the intrinsic properties (0.74–0.78 eV) and did not depend on tin/titanium concentration gradient, while activation energy (1.03–1.29 eV) for grain boundary was determined by the microstructural gradient. No point dissipation was observed by IS, accordingly, no insulator interfaces (cracks and/or delamination) between graded layers were detected.     

Electrical transport properties in zirconia/alumina functionally graded materials

Functionally graded materials (FGMs) are promising candidates for the fabrication of technological components, not only as structural devices, but also in electrochemical ones, such as solid oxide fuel cells (SOFC), or high-efficiency hybrid direct energy conversion systems. In the present work FGMs were prepared by the sequential slip casting technique, starting with an yttria tetragonal zirconia polycrystalline layer and increasing subsequently the amount of Al₂O₃ in the following layers. Electrochemical impedance spectroscopy (EIS) analysis was used to evaluate the electrical characteristics of these materials and to compare with those of the monolithic compacts. In general, it was observed that the FGM conductivity is ruled by the conductivity of the layer which contains the highest amount of alumina blocking particles. By EIS no electrical interfaces between adjoining layers were detected and, accordingly, no specific electric ohmic losses were observed. The conductivity of the FGMs is close to that calculated using the normalized thicknesses and the alumina volume fractions of the layers after measuring the conductivity of the monolithic materials with the same composition to what correspond to that of the final layer in the FGM. These results suggest that the gradient structure can be used to control the oxygen vacancy motion, and then applied in electrochemical devices.     

Thermal residual stresses in an adhesively-bonded functionally graded single-lap joint

This study investigates three-dimensional thermal residual stresses occurring in an adhesively-bonded functionally graded single-lap joint subjected to a uniform cooling. The adherends are composed of a through-the-thickness functionally graded region between Al₂O₃ ceramic and Ni metal layers. Their mechanical properties were calculated using a power law for the volume fraction of the metal phase and a 3D layered finite element was implemented. In a free single-lap joint the normal stress σ_xx was dominant through the overlap region of the upper and lower adherends and along the adhesive free edges, whereas the transverse shear stress σ_xy concentrations appeared only along the free edges. The peel stress σ_yy and the transverse shear stress σ_xy became dominant along the free edges of the adhesive layer. In addition, the von Mises stress decreased uniformly through the adherend thickness from compressive in the top ceramic-rich layer to tensile in the bottom metal-rich layer. In addition, the layer number had only a minor effect on the through-the-thickness stress profiles after a layer number of 50, except for the peak stress values in the ceramic layer. In a single-lap joint fixed at two edges both adherends underwent considerable normal stress σ_xx concentrations varying from compressive in the top ceramic-rich layer to tensile in the bottom metal-rich layer along the free edges of both adherend–adhesive interfaces, whereas the peel stress σ_yy and transverse shear stress σ_xy reached peak levels along the left and right free edges of the adhesive layer. The layer number and the compositional gradient exponent had only minor effects on the through-the-thickness von Mises stress profiles but considerably affected the peak stress levels. The free edges of adhesive–adherend interfaces and the corresponding adherend regions are the most critical regions, and the adherend edge conditions play more important role in the critical adherend and adhesive stresses. Therefore, the first initiation of the joint failure can be expected along the left and right free edges of the upper and lower adherend–adhesive interfaces.     

泡桐木夹层结构材料的力学性能

选用泡桐木为原料,制备出夹层结构用泡桐木绿色夹芯材料,其木质纤维具有天然蜂窝形状,结构类似于目前航空航天领域常用的蜂窝芯材;泡桐木芯材除密度略高于Balsa轻木外,其他力学性能测试指标均优于轻木,同时在价格上占有绝对的优势。采用真空导入成型工艺,成功制备出轻质高强的泡桐木夹层复合材料,通过不同跨高比试件的三点与四点弯试验,研究其典型受力破坏形态与机制;利用经典夹层梁理论预估试件抗弯刚度和受弯极限承载力,理论值与实测值符合较好,并以此为基础,提出了基于强度的优化设计方法。     

Stress wave propagation in a through-thickness functionally graded adhesive layer

In this study, an improved mathematical model is presented to investigate the stress wave propagation in two circular cylinders bonded with a functionally graded adhesive layer. In the proposed model, the spatial derivatives of mechanical properties are included in the governing equations of the wave propagation. Also, the finite-difference method was used for the solution of the governing equations and boundary conditions. The Mori-Tanaka homogenization scheme was employed to evaluate the through-thickness mechanical properties of the adhesive layer. The effects of the spatial derivatives of the local mechanical properties and the through-thickness material composition variation in the adhesive layer were examined in detail. The presence of the material spatial derivatives in the governing equations mitigated the stress and displacement levels as well as axial and radial wave speeds.     

One-step Sinter-HIP method for preparation of functionally graded cemented carbide with ultrafine grains

Ultrafine crystalline functionally graded cemented carbides (FGCCs) with a surface zone enriched in binder phase were prepared by a one-step Sinter-HIP method. The influence of sintering pressure and cubic carbide composition on the formation of gradient layer was examined. The results show that the ultrafine FGCC with surface zone enriched in binder phase can be formed by the one-step Sinter-HIP method. The process of the gradient layer formation is accelerated under higher sintering pressure; the gradient layer thickness increases with the sintering pressure increasing. The gradient layer thickness is controlled by diffusion distance of cubic carbide formers, such as Ti, Ta and Nb. The addition of (Ta,Nb)C leads to decrease the thickness of gradient layer.     

Prediction of the elastic properties profile in glass-alumina functionally graded materials

Glass-alumina functionally graded materials were obtained by percolation and alternatively by plasma spraying. The paper develops a reliable model to predict the functional gradient of the analysed systems. A finite element code, which was able to handle microstructural images, was employed to estimate the effective elastic properties along the gradient direction. The calculated values were compared with experimental data acquired by means of systematic microindentation tests. The computational approach was compared with analytical tools such as the rule of mixture. The results revealed that the elastic properties were significantly influenced by microstructural features such as the shape of the ingredient materials domains and the presence of pores at the grain boundaries. This was particularly evident in the sprayed FGMs, due to their peculiar lamellar microstructure. Even if the coating–substrate interface properties were difficult to include in the model, the numerical simulations fitted fairly well the experimental data.     

钢丝绳芯输送带粘合芯胶的研制

介绍了钢丝绳芯输送带粘合芯胶配方的研制。NR/SBR-1500/BR-9000为60/20/20时,粘合胶的性能较好;高硫体系有助于提高粘合芯胶与镀锌钢丝绳的粘合力;在一定范围内,松焦油用量增加有利于粘合力的提高;防老剂BLE(c)对胶料与镀锌钢丝绳的粘合有帮助;间甲白钴粘合体系的粘合胶对镀锌钢丝绳具有较高的粘合力。