Thermal conductivity of ceramic green bodies during drying

The thermal conductivity of alumina and kaolin green bodies has been studied as a function of the water loss during drying. Experimental measurements show strong variations with 3 distinct regimes. In the first regime, thermal conductivity increases during shrinkage. When shrinkage stops, a decrease in thermal conductivity with water loss is observed which becomes even stronger during the last phase of drying. This can be explained by the variations in the volume fractions of each phase and the effective thermal contacts between grains. Using analytical relations, the thermal resistance of an equivalent plane of small area grain-grain contacts is shown to increase strongly at the end of drying due to the removal of water. Finally, in certain drying conditions, if a portion of the heat required for drying, is supplied by conduction through the green body, then the rate of water evaporation increases with higher thermal conductivity.     

Distribution of water in ceramic green bodies during drying

In order to investigate drying mechanisms at different stages, the distribution of water within the ceramic green bodies at different scales has been examined. The experimental measurements, using a simple weighing technique and Magnetic Resonance Imaging (MRI), show that during the first stage of drying involving shrinkage the material is constituted of uniquely solid and water with no gradient in water content within the sample. Then, during the second stage of drying, significant differences of water content as a function of position appear. As a complement, at the grain scale, observations using environmental scanning electron microscopy were made giving useful information on the solid–liquid–gas interfaces in the near surface part of the green body. Finally, the gradients in the water distribution were exploited to make a simple estimate of the diffusion coefficient of water with its dependence on the moisture content.     

Assessment of the initial moisture content on soybean drying kinetics and transport properties

Several researchers have developed studies to obtain a mathematical model able to describe grain drying kinetics. However, most of these studies neglect the effect of grain initial moisture content on drying curves. In this study, we assessed the dependence of drying curves and mass transfer coefficients on this initial moisture, air temperature, and its velocity by measuring grain mass losses within time on a tray dryer. Mathematical models were adjusted and results indicated that initial grain moisture content has significant influence on drying curves and mass transfer coefficients.     

The performance of adhesive-bonded thin-gauge sheet-metal structures with particular reference to box-section beams

In an effort to reduce vehicle weight, the automotive industry is developing car body structures made from light-weight materials such as composites, plastics and aluminium alloys. Fabrication of these materials using traditional welding techniques is not feasible and adhesive bonding is now being investigated as a potential assembly method. To assess performance characteristics of bonded vehicles, thin-gauge sheet-metal box-section beams have been used to simulate structural details in automotive applications such as car bodies and commercial vehicles. Beams were fabricated from flanged strips by different joining methods to form box-section structures approximately $\text{1}\text{m long}\text{× 60}\text{mm square}$. Tests were carried out to determine torsinal and flexural rigidity and ultimate torsional and flexural strengths, and in the majority of tests, bonded structures gave better characteristics than the equivalent riveted or spot-welded beams. The failures of beams under 3-point bending have been related to buckling of the side webs and further experimental tests have shown that collapse is critically dependent on flange-bends radius. Finite element techniques have veen used to analyse stress distribution in the beam section and this confirms the experimental observations of beam collapse.     

Development of a hoop-strength test for model sphero-cylindrical dental ceramic crowns: FEA and fractography

Here we describe the development of a mechanical test that simulates a circumferential “hoop-stress” at the margins of model dental ceramic crowns. A simplified sphero-cylindrical geometry was modelled in two thicknesses and machined out of two glass-ceramic materials. The specimens were loaded by a conical metallic piston to induce circumferential hoop-stresses at the margins until fracture. Broken specimens were fractographically analyzed to determine the location of the fracture origin and to measure the dimensions of the critical defect, allowing the calculation of the stress at failure from fracture mechanics relations. For thick specimens, machining defects at the outer margins barely played a role in fracture initiation. For thin specimens, outer defects started to dominate the failure mode. CAD/CAM machining showed to predispose thinner margins to early fracture. Crowns should be designed with increased thickness in order to minimize the effect of marginal damage in the circumferential stress field inducing fracture.     

Mechanical properties of hybrid multilayer graphene/whisker-reinforced ceramic composites: Simulation and experimental investigation

The trial-and-error method used in ceramics research has certain limitations such as the high blindness of material component design. Moreover, calculations of the toughness of ceramics using the extended finite element method, which is the most broadly applied technique, are complicated. To overcome these issues, in this study, multilayer graphene (MLG)/Si₃N₄ whisker (Si₃N_4w)-reinforced Si₃N₄ ceramics (MWSCs) were used as the model material, and the modeling of MWSCs was conducted using Voronoi tessellation. Additionally, a more concise novel approach was applied for the prediction of the fracture toughness of MWSCs. Furthermore, the optimal MLG and Si₃N_4w contents were predicted, and then they were verified by fabricating MWSCs using spark plasma sintering (SPS). Simulation results indicated that the optimum MLG and Si₃N_4w contents to enable the toughness and hardness to reach the maximum values (9.87 MPa·m^1/2 and 23.19 GPa) were 1 wt% and 3 wt%, which were consistent with the experimental results. Consequently, the effectiveness of the proposed method was verified. Moreover, the experimental values of the maximum fracture toughness and hardness were 11.04 MPa·m^1/2 and 20.29 GPa, which were 47.20% and 12.10% higher than those of Si₃N₄ ceramics reinforced with 1 wt% MLG, respectively. The synergistic toughening effects of MLG and Si₃N_4w were significantly reflected. The load-bearing effect, bridging, and crack deflection induced by MLG and Si₃N_4w were the key reasons for the improvement in the mechanical properties of MWSCs.     

Multiscale modelling of the thermoelastic properties of alumina-zirconia ceramics for 3D printing

Additive manufacturing appears to facilitate the accurate manufacturing of alumina-zirconia technical ceramics. Nevertheless, the fine tuning of the manufacturing of these components by 3D printing requires an analysis of the parameters that influence their final thermoelastic properties. In this context, this work presents the application of (finite element-based) numerical procedures that aim at the prediction of the effective thermoelastic properties of 3D-printed alumina-zirconia ceramics. The numerical modelling considers three different scales: micro-, meso- and macroscale. The microscale corresponds to the microstructural level of, sintered at 1500 °$\mathrm{C}$, slip-casted samples with different compositions of alumina-zirconia. On the other hand, the macroscale corresponds to the macrostructural level of porous lattice of 3D-printed ceramics, being defined at the mesoscale level by a periodic unit cell. Thus, an initial microstructural analysis (at microscale level) provides the influence of the alumina/zirconia ratio on the (macroscopically homogeneous and isotropic) material thermoelastic properties, which together with the definition of the geometry of a periodic unit cell (at mesoscale level), provides, by a second analysis (at both the meso- and macroscale levels), the coupled influence of material and geometry of the macrostructural lattice on the structural (macroscopically heterogeneous and anisotropic) thermoelastic properties. Moreover, experimental thermoelastic properties of the sintered slip-casted specimens were obtained for several alumina/zirconia ratios and analysed together with microstructure patterns. Prediction of the microstructural effective thermoelastic properties was also made using micromechanics and composite theory (analytical) models. All the numerical, experimental and analytical results for the microstructural level are presented and compared. Numerical results for the meso- and macrostructural levels are also presented.     

A mathematical model of plant growth with particular reference to winter wheat

The mathematical analysis presented in this paper of plant growth is based on the assumption that the influence of deficiencies in the major root nutrients of water, N, P and K is to reduce the fraction of the plant mass devoted to photosynthesis by increasing the root fraction, thereby diminishing the rate of growth of the total plant.A reciprocal phenomenon occurs with deficiency of light. Using literature data on the influence of environmental conditions on plant growth and in particular on the root/shoot ratio, the relevant rate expressions are derived in a simple form and appear to be generally similar for a wide range of plants during their vegetative phase of growth.     

Multiscale model to investigate the effect of graphene on the fracture characteristics of graphene/polymer nanocomposites

In this theoretical research work, the fracture characteristics of graphene-modified polymer nanocomposites were studied. A three-dimensional representative volume element-based multiscale model was developed in a finite element environment. Graphene sheets were modeled in an atomistic state, whereas the polymer matrix was modeled as a continuum. Van der Waals interactions between the matrix and graphene sheets were simulated employing truss elements. Fracture characteristics of graphene/polymer nanocomposites were investigated in conjunction with the virtual crack closure technique. The results demonstrate that fracture characteristics in terms of the strain energy release rate were affected for a crack lying in a polymer reinforced with graphene. A shielding effect from the crack driving forces is considered to be the reason for enhanced fracture resistance in graphene-modified polymer nanocomposites.     

Finite element-based machine learning approach for optimization of process parameters to produce silicon carbide ceramic complex parts

Design and fabrication of silicon carbide ceramic complex parts introduce considerable difficulties during injection molding. Due to the great importance in processing optimization, an accurate prediction on the stress and displacement is required to obtain the desired final product. In this paper, a conceptual framework on combination of finite element method (FEM) and machine learning (ML) method was developed to optimize the injection molding process, which can be used to manufacture large-aperture silicon carbide mirror. The distribution characteristics of temperature field and stress field were extracted from FEM simulation to understand the injection molding process and construct database for ML modeling. To select the most appropriate model, the predictive performance of three ML models were estimated, including generalized regression neural network (GRNN), back propagation neural network (BPNN) and extreme learning machine (ELM). The results show that the developed ELM model exhibits exceptional predictive performance and can be utilized to predict the stress and displacement of the green body. This work allows us to obtain reasonable technique parameters with particular attention to the loading speed and provides some fundamental guidance for the fabrication of lightweight SiC ceramic optical mirror.     

Macroscopic model of concrete subjected to alkali-aggregate reaction

The structural behaviour of concrete affected by alkali-aggregate reaction (AAR) is difficult to model due to the amount of random parameters that govern this chemical process. The aim of this work is to present a macroscopic approach whose main features are the consideration of uncoupling between AAR and stress and the representation of the anisotropic characteristic of chemical swelling. Experimental results concerning reactive concrete samples were simulated to verify whether the model was capable of describing the behaviour of affected structures under certain loading and boundary conditions. Loading-unloading response was also considered to simulate the effect of joints opening, which is a commonly used technique for releasing AAR generated stresses in affected structural elements. The obtained results were compared to test data and showed good agreement.     

A mathematical model of root exploration and of grain fill with particular reference to winter wheat

In the first part of this paper a model is derived for the exploration of the soil by roots in a growing crop with particular reference to cereals. The distribution of root density with depth is related to the growth of the above-ground parts and to the varying demand these place on the roots modified by the changing texture and nutrient status of the soil with depth. In the second part of the paper the growth of cereal seeds is modelled by being related to the photosynthetic capacity of the plant and the duration of its life following anthesis. The death of the plant is considered to be a consequence of the parasitic action of the seeds.     

Application of the design of experiments procedure to the behaviour of adhesively bonded joints with plastically deformable adherends to enable further understanding of strain rate sensitivity

The study presented in this paper was carried out to investigate further the effects of strain rate on the strength of adhesively bonded single lap shear joints. Tests were carried out on two different configurations of adhesively bonded joints that were designed to exhibit different behaviours. In one configuration both adherends were made from a relatively low strength grade of aluminium such that both would exhibit significant plastic deformation prior to adhesive failure. The other configuration used one adherend that was significantly stronger such that only elastic deformation was exhibited prior to failure of the adhesive. The joint specimens were tested at several different strain rates using a servo-hydraulic test machine and the results analysed using statistical methods. To further understand the results Finite Element models of the joints were created using a Cohesive Zone Model to predict damage development and failure in the adhesive. The Design of Experiments procedure was used to study the effects of material parameters relating to both the adherends and the adhesive in the Finite Element models. The results of the testing suggested that the strength of joints formed from two adherends that exhibited plastic deformation prior to failure did not show statistically significant sensitivity to strain rate. Interpretation of the results of the Finite Element analyses suggested that the adherend yield was the main factor influencing failure load in the adhesive for joints of this type.     

Numerical analysis of the stress distribution in single-lap shear tests under elastic assumption—Application to the optimisation of the mechanical behaviour

The single lap joint is the most used test in order to analyse the behaviour of an adhesive in an assembly as on one hand, the manufacturing of such specimens is quite easy, and on the other hand they require only a classic tensile testing machine. However, such specimens are associated with complex loading of the adhesive, i.e. non-uniform shear stress along the overlap length, quite large peel stress at the two ends of the overlap and significant edge effects associated with geometrical and material parameters. In addition, the stress concentrations can contribute to fracture initiation in the adhesive joints and thus can lead to an incorrect analysis of the adhesive behaviour. Therefore, understanding the stress distribution in an adhesive joint can lead to improvements in adhesively bonded assemblies. The first part of this paper presents the influence of edge effects on the stress concentrations in single lap joints under elastic assumption of the material and using a pressure-dependent elastic limit of the adhesive. In the second part, some usual geometries, proposed in the literature about stress limitation, are compared with respect to the maximum load transmitted by single lap joint. The last part presents some geometries, which significantly limit the influence of edge effects and are more appropriate for analysing the behaviour of the adhesive.     

3D models of specimens with a scarf joint to test the adhesive and cohesive multi-axial behavior of adhesives

In this paper, accurate numerical analyses of the stress distributions within the adhesive in scarf joints under elastic assumption using 3D models are developed. An elastic limit model that takes into account the hydrostatic stress and von Mises equivalent stress, permit to define the more stressed parts of the adhesive with respect to the scarf angle. It is shown that high stress concentrations are generated near the edges and the corners of the specimen at the interface between the substrate and the adhesive. The use of ronded substrates does not lead to decrease significantly these stress concentrations. It is shown that cleaning the free edge of the adhesive changes the location of edge effects without completely erasing them. A modification of the geometry, has been proposed. This modification leads to a quasi-homogeneous distribution of the stresses in the thickness of the adhesive with stresses that tend to 0 near the edges.     

Reaction Engineering Approach (REA) to model the drying kinetics of droplets with different initial sizes—experiments and analyses

Droplets with different initial sizes, which are typical in conventional liquid atomization for spray drying applications, will result in varying drying and crust formation histories. It is essential for any droplet drying model to accurately capture such fundamental phenomena. This study used a newly constructed glass-filament single droplet rig to evaluate the applicability of the Reaction Engineering Approach (REA) in describing such effect. For the three initial sizes (1, 2 and 3 μL) tested, the glass filament gravimetric method clearly distinguished the different drying kinetics and the crust formation phenomenon, delineated by the drying behavior. Analysis from the drying kinetics revealed that the main premise of the REA, which utilizes a material-specific master activation energy curve, is applicable to droplets of different initial sizes at all the three air temperatures tested. This allowed the REA to accurately predict the different temperature and moisture histories given by droplets with different initial sizes. The result supports the REA as a good modeling approach for a wide range of initial droplet conditions. A new master curve approach was proposed to predict the diameter change of droplets with different initial concentrations. Validation with the current and past experimental data revealed that this approach has strong potential to account for the different feed concentrations typically found in spray drying applications.     

Study on mechanical properties of hot pressing sintered mullite-ZrO2 composites with finite element method

In this study, mullite–zirconia (ZrO₂) composites were fabricated by hot pressing sintering method. The effects of sintering temperature and holding time on the microstructures, phase compositions and mechanical properties of the composites were investigated. The results indicated that the size of t-ZrO₂ grain varies with sintering temperature and holding time, and the maximum flexural strength of 674.05?MPa and fracture toughness of 12.08?MPam^1/2 are obtained when the sintering temperature is 1500?°C with holding times of 20 and 60?min, respectively. Finite element method was employed to analyze the relationship between grain size and mechanical properties of mullite–ZrO₂ composites for the first time. The results showed that the maximum stress on mullite–ZrO₂ interface increases with the growth of t-ZrO₂ grain size, which enhances the generation and propagation of cracks on grain boundaries significantly and degrades the flexural strength and fracture toughness of the mullite–ZrO₂ composite ceramics.     

Numerical study of lap joints with composite adhesives and composite adherends subjected to in-plane and transverse loads

In this paper, single lap joints for joining fibre composites were modeled and a three-dimensional finite element method was used to study the joint strength under in-plane tensile and out-of-plane loadings. The behaviour of all the members was assumed to be linear elastic. The adherends were considered to be orthotropic materials while the adhesive could be neat resin or reinforced one. The largest values of shear and peel stresses occurred near the ends of the adhesive region, as expected. The values and the rate of variation in peel stress was more than that of shear stress. By changing the properties and behaviour of adhesive from neat epoxy (isotropic) to fibre composite adhesive (orthotropic) and with various fibre volume fractions of glass fibre, the ultimate bond strength increased as the fibre volume fraction increased, in both tensile and transverse loadings. Also, changing the orientation of fibres in the adhesive region with respect to the global axes influenced the bond strength.     

Antagonistic effects on mechanical properties of microfibrillar composites with dual reinforcement: Explanation by FEA model of soft interface

Nanofillers (NF) in microfibrillar composites support melt‐drawing and may lead to improved performance. However, antagonistic effects have also been found. The deterioration of mechanical properties by drawing in the presence of NF, as found by other authors in analogous undrawn systems, has not yet been explained. Experiments indicating the importance of NF migration between the HDPE matrix and the PA6 fibrils in the course of drawing have led to a tentative conclusion of changed crystallinity in the interfacial area resulting in a layer with reduced modulus. This was confirmed by the finite element analysis considering the formation of a “soft” interface as a result of reduced content of HDPE spherulites at the fiber surfaces. The results show a marked impact of this phenomenon on modulus. This original concept presents a basis for explaining some antagonistic effects in multicomponent polymer systems and a tool for the more rational design of composite materials. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44712.     

Numerical study of residual stress and crack nucleation in thermal barrier coating system with plane model

The residual stresses could cause extensive damage to thermal barrier coatings and even failure. A finite element model of thermal barrier coating system had been designed to simulate the residual stresses and then to analyze the crack nucleation behavior. The distribution of normal and tangential stress components along top coat (TC) / thermally grown oxide (TGO) and TGO / bond coat (BC) interfaces are shown in this work. It is found that the maximum tensile stress along TC/TGO interface occurs in the peak region during heating-up, and that along TGO/BC interface is also located in the peak region, but during the process of cooling-down. A parameter correlating the normal stress component with corresponding tangential one was used to evaluate the interfacial cracks, indicating that cracks will initiate at the peak-off region of TC/TGO interface in the heating-up phase, but for TGO/BC interface, cracks will initiate at the peak position in the cooling-down phase.