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This study focuses on a methodology based on a genetic algorithm to design variable-thickness streamlined radomes with graded dielectric multilayered walls for airborne applications. The methodology begins with a monolithic variable-thickness radome design as the initial configuration. Then, the relationship between the wall thickness and dielectric constant is revealed, which simplifies the subsequent design. Finally, the radome wall is designed as a lamination, and the dielectric constant distribution of the lamination is determined to finish the whole design. By utilizing this methodology, a variable thickness tangent ogive radome with a graded porous nine-layered wall is designed. According to a comparative study, the proposed design shows better electromagnetic (EM) performance than a constant thickness radome with a graded porous wall. In addition, radomes with diverse graded wall configurations designed by the methodology show robust EM performance, which suggests that the proposed method provides the possibility to further design high-temperature mechanical properties along with superior EM performance.

This paper studies the effects of hygrothermal environment at different temperatures and ultraviolet (UV) radiation on the bending properties of epoxy polymer mortar (EPM). The microstructure changes of EPM during aging were studied by scanning electron microscopy, and the bending properties of EPM were predicted by the Arrhenius law. The results showed that the bending properties of EPM were greatly affected by the temperature in the hygrothermal aging, but not evidently affected by ultraviolet radiation in UV aging. The prediction of Arrhenius model shows that the EPM will steadily retain 92.8%, 89.1% and 79.4% of the original flexural strength after long-term hygrothermal aging at $$40^{\circ }$$ C, $$60^{\circ }$$ C and $$80^{\circ }$$ C, respectively.

Because of the advantages of a uniform distribution of reinforcing particles and in situ preparation, in situ precipitation has become an important way to prepare magnetic and other smart hydrogels. An important step in this process is to immerse hydrogels in alkaline solution to implant magnetic particles. Previous studies generally have ignored the effect of this process on the network structure and mechanical properties of hydrogels. In this study, we immersed polyvinyl alcohol (PVA) hydrogel samples in sodium hydroxide solutions of different concentrations to study changes in mechanical properties, such as stress–strain relationship, self-recovery, and fracture failure. The results showed that after the immersion process, the hydrogel's tensile and compressive properties changed significantly, and the failure behavior changed from brittle fracture to ductile fracture. Through a microscopic mechanism, the alkaline solution caused a high degree of phase separation and crystallization within the polymer network, thereby changing the PVA hydrogel network from a single phase to a multiphase. Hence, we used a continuous multiphase network model with a certain probability distribution to describe this tensile behavior. This model well described the stress–strain relationship of the hydrogel from stretching to fracture and revealed that the macroscopic failure corresponded to the peak of fracture distribution. Studies have shown that attention should be paid to the influence of the in situ precipitation on the mechanical properties, and the probabilistic multiphase network model can be used to predict the mechanical behavior of hydrogels with multiple phase separation.

The microstructure of the extracellular matrix (ECM) plays a key role in affecting cell migration, especially nonproteolytic migration. It is difficult, however, to measure some properties of the ECM, such as stiffness and the passability for cell migration. On the basis of a network model of collagen fiber in the ECM, which has been well applied to simulate mechanical behaviors such as the stress-strain relationship, damage, and failure, we proposed a series of methods to study the microstructural properties containing pore size and pore stiffness and to search for the possible migration paths for cells. Finally, with a given criterion, we quantitatively evaluated the passability of the ECM network for cell migration. The fiber network model with a microstructure and the analysis method presented in this study further our understanding of and ability to evaluate the properties of an ECM network.

Failure surfaces of foam materials in the principal-stress/strain space are quite different from those of dense materials. They cannot be deduced directly by several simple tests such as uniaxial loading and pure shear loading. However, it is difficult to carry out arbitrary proportional triaxial loading tests on materials, resulting in the global topology of failure surfaces of metallic foams remaining unknown. To overcome the difficulty of triaxial loading tests, we carried out numerical simulations by applying triaxial loadings on 3D Voronoi models, which have been used successfully to study the global topology of yield surfaces of metallic foams. In the numerical simulations, 3D Voronoi structures were used to simulate the meso–structures of metallic foams, and only the parameters of the matrix material (aluminum) are needed. Considering the influence of multi-axial effects on the failure of metallic foam, a new failure criterion for metallic foams based on the mass of failed elements was proposed, and sufficient failure points in the principal-stress/strain space were obtained to characterize the failure surfaces well. Results indicated that the failure surface in the in the principal-stress space looks more complicated than the failure surface in the principal-strain space, so it is better to depict the failure surface in the principal-strain space, which approaches an ellipsoid. Furthermore, self-similarity exists in failure surfaces of metallic foams with different relative densities. Because a metallic foam may not fail when the compressive loadings are dominant, the failure surface appears to be a “missing area” in the permissible principal-stress/strain space. The boundaries of missing areas were determined analytically and validated numerically.

Epoxy polymer concrete (EPC) is widely applied in engineering for its excellent mechanical properties. The impact loads and severe climatic conditions such as ultraviolet radiation, temperature change and rain erosion are in general for its engineering practice, potentially degrading the performance of EPC. In this paper, a procedure of accelerated aging for EPC, imitating the aging effect of ultraviolet radiation and hygrothermal conditions based on the meteorological statistics of Guangzhou city, was designed. After various periods of accelerated aging, the dynamic behaviors of EPC were studied by using a Split Hopkinson Pressure Bar (SHPB). The verification of the experimental data was performed. The two-stage dynamic compression stress-strain curves were obtained: (a) linear growth stage following by strain hardening stage at impact velocity 12.2 m/s and 18.8 m/s, (b) linear growth stage and then a horizontal stage when impact velocity is 25.0 m/s, (c) linear growth stage following by strain softening stage at impact velocity 29.2 m/s. The experimental results show that the specimens after longer accelerated aging tend to be more easily broken, especially at impact velocity 12.2 m/s and 18.8 m/s, while the strain rate is the main factor affecting the compression strength and stiffness. Ultimately the influence of strain rate and equivalent aging time on dynamic increase factor was revealed by a fitting surface.

The bleb morphology and its changes are an important mechanism of cell’s amoeboid migration. By releasing bonds between the membrane and the cortex of a cell, the formation of bleb can be observed experimentally, but the mechanism that affects the size and shape of this kind of bleb is waiting for further study. In this paper, a two-dimensional fluid-solid coupling model is established to describe a cell with membrane, cortex and cytoplasm in a solution, and a numerical solving method for the fluid-solid coupling model is developed to simulate the behaviors of cell bleb. The effects of parameters, such as the number of broken bonds, the viscosity coefficient of the cortex, and the cell’s membrane modulus on the size and the shape of the bleb were investigated. Numerical results show that the model is effective to simulate the formation and evolution of cell’s bleb, and derive the contribution of several affecting factors to the bleb shape and size clearly.SIGNIFICANCETo understand the process of cell migration with bleb pseudopods in the amoeba cell migration, it is necessary to study the formation mechanism of cells protruding bleb. In this paper, we propose a reasonable and reliable cell numerical model. With this model we successfully simulate the bleb phenomenon consistent with the experimental phenomenon by changing the key impact factors. The method in this paper is applicable to the cell model of amoeba cell migration pattern, which helps to understand the important role of blebs in the process of cell migration.

Biological soft tissues usually perform physiological functions in solutions, and the mechanical properties of the bio-soft matters in air and solution may differ from that in solution, but the measurement technique of the dynamic mechanical properties of ultra-soft materials in solution is still an unexplored realm. In order to develop a feasible method to test ultra-soft materials in solution by the split Hopkinson pressure bar (SHPB) technique, this paper uses the liquid silicone rubber (LSR) whose property is seldom affected by water and the purified water as a testing material and a solution, respectively. The double-striker electromagnetic driving SHPB system, which has been designed for ultra-soft materials, is adopted to carry out dynamic test for LSR in solution, meanwhile the testing process was recorded by a high-speed digital camera. The tests include three cases named as: Specimen in air, No specimen in water, Specimen in water. Specimen in air was set as a reference; No specimen in water was used to learn the mechanism of load transferring of the water and to set the correction of the effect of the water; Specimen in water was the standard test in water whose results must be modified for the characterization of materials. The high-speed images show that the water may be regarded as a steady flow until the constant strain rate loading ended. Based on such phenomenon and the unchanged volume of specimen and by simplifying the description of the flow field, a data correcting method was set up on the basis of the deduction of both the specimen deformation and fluid flow field to modify the experimental result tested in water. According to the comparison, experimental result in air shows good agreement with corrected result in solution, indicating the feasibility of our recommended method for the SHPB tests in solution. In this way, our research is of great significance in laying an important foundation in the new academic field of the dynamic mechanical behaviors of bio-soft matters in solution.

Epoxy polymer concrete (EPC) has found various applications in civil engineering. To enhance the flexural performance of EPC, two kinds of short natural fibers with high specific strength (sisal fibers and ramie fibers) have been incorporated into EPC. The results of mechanical tests show that a small loading of natural fibers (0.36 vol%) can significantly increase the flexural strength of EPC by 25.3% (ramie fibers) or 10.4% (sisal fibers). This enhancement is achieved without any sacrifice of compressive strength of EPC. The reinforcing effects of short natural fibers on the flexural properties and compressive properties of EPC decrease with further increase in fiber content, due to the insufficient wetting of fibers by epoxy resin which results in poor interfacial bonding. The reinforcing mechanisms of short natural fibers are explored according to the observation of fracture surfaces and micromechanical modelling. It is found that the parallel model based on the rule of mixture can be a good approximation to describe the improvement in flexural strength of the short natural fiber reinforced EPC at low fiber volume fractions.

It has been reported that the measurement results by the method of indentation for soft viscoelastic materials are affected apparently by experimental conditions, such as strain rate and indentation depth. In order to enhance the effectiveness of the method of indentation and find optimized loading rates in tests, the Lee–Radok's solution was modified with Hay's correction to describe the indentation response for soft viscoelastic materials, further, a pseudo dissipative work was defined in a normalized indentation loop and its variation against the loading rate was obtained. It is found the pseudo dissipative work exists several maximal values related to the viscous properties of materials, and the corresponding loading rates can be regarded as the optimal loading rates in indentation tests. Based on the theoretical analysis, a new method how to determine the optimized loading rates in indentation was recommended, which was verified by tests of indentation and uniaxial compressive creep for a PVA hydrogel.

There are multiple controversies regarding the yield surface of metallic forms, and the primary cause is that the characterizations are summarized by notably scarce yield points in the plane of effective stress and mean stress. In order to solve these controversies, experiments and numerical simulations were carried out to obtain sufficient yield points for metallic foams covering the entire permissible principal-stress space and principal-strain space to study properties of yield surfaces essentially. In experiments, uniaxial and biaxial tests were implemented to learn properties of metallic foams in yield and brittle fracture. In numerical simulations, 3D Voronoi models were built to simulate closed-cell aluminum foams with meso-structures, in which only the material parameters of aluminum were needed. The Voronoi models were verified first by uniaxial and biaxial tests. With the initial yield criterion of metallic foams under multi-axial loading proposed in the view of dissipation energy, sufficient yield points were obtained for metallic foams, which directly represent the global topology of yield surfaces in principal-stress space and principal-strain space. In addition, the yield surface was characterized by the yield points in principal-stress space and in principal-strain space respectively; moreover, the yield lines determined by the yield points having the same Lode angle were presented. It was observed that yield lines with different Lode angles in the plane of the von Mises stress and the mean stress were not coincident, which is why the controversial characterizations of the yield surface of metallic foams exist. However, the yield lines were nearly identical in the plane of effective strain and mean strain, suggesting that it is better to characterize the yield surface of metallic foams in principal-strain space than that in principal-stress space. Furthermore, effects of the initial density of metallic foams on yield surfaces were investigated. It was found that the asymmetry of tension and compression increases as relative density increases, but the asymmetry was eliminated when using two parameters to normalize the yield surfaces.

The aluminum foam sandwich is a composite structure with light weight and high specific energy absorption under impact. However, penetrated behaviors of the composite structures at elevated temperatures still remain unclear. A systematic numerical model of the aluminum foam sandwich was developed with the nonlinear, finite element program ABAQUS. In this model, a constitutive relation of aluminum foam with temperature effects was put forward based on uniaxial compressive tests. Subsequently, a failure criterion including both average stress and von Mises stress is obtained through compression, tension and penetration experiments at elevated temperatures. Besides, the eroding contact ensures the touch between the punch and foam core. The numerical model well captured the penetrative process well including the failure features and force-time curves at elevated temperatures. Meanwhile, the energy and max force of aluminum foam sandwich with different mass ratios are discussed. The results disclosed strong temperature effects on the penetrated behaviors of aluminum foam sandwich plates, and reveal the changes of failure modes at elevated temperatures.

Nowadays, radomes that are employed to protect antennas inside from physical environment are required to have dual-band or even multi-band transmission performance. In this paper, a design scheme based on the theory of small reflections is proposed for the design of dual-band and multi-band A-sandwich radomes. Subsequently, two A-sandwich composite radome walls are designed and fabricated according to the design scheme. Finally, both numerical simulations and experiments are conducted to verify the electromagnetic characteristics of the radome walls. Results indicate that one of the A-sandwich radome walls has two passbands in 4.0–11.4 GHz and 25.2–40.0 GHz, while the other one has three passbands in 4.0–8.2 GHz, 18.0–20.5 GHz, and 29.1–40.0 GHz, respectively. The proposed method is experimentally demonstrated to be an effective approach for designing dual-band and multi-band dielectric radome walls for both centimeter and millimeter wave applications.

The properties of metallic foams are closely related to the average pore size, relative density and degree of the shape irregularity in the meso-structure. In this paper, gradient metallic foams with Voronoi structures based on different degrees of shape irregularity are constructed and numerically tested under different impact velocities. We combine three types of metallic foams with identical relative density but different degrees of shape irregularity in a specific sequence (from large to small or from small to large) to construct the new gradient models. A series of impact tests at different velocities are performed on the gradient metallic foams to acquire deformation characteristics and stress states. According to the results from the inertia effect and the energy absorption capacity, a gradient metallic foam with a shape irregularity that changes from large to small (negative gradient) is the optimal design. The simulation results provide a fresh perspective and methodology to design gradient metallic foams at the meso-structure scale for use as energy absorption materials.

Considering the lack of abundant experimental data on metal foams under tensile triaxial loadings, characterization of the yield surface of metallic foam is controversial. In this paper, we explore a numerical method to study the yield properties of closed-cell aluminum (Al) foam under triaxial loadings using a three-dimensional (3D) Voronoi model. The finite element model was verified by a uniaxial compressive experiment with optimized modeling and computing parameters. Three normal stresses (including tension and compression) were proportionally applied on the cubic Voronoi foam to conduct numerical simulation experiments, in which each stress proportion of the von Mises stress over the mean stress includes at least three different combinations of triaxial loadings. Results indicated that the yield surface, covering the entire spectrum of stress paths from hydrostatic compression to hydrostatic tension in the plane of the von Mises stress and the mean stress, could be approximately depicted by a parabola or ellipse, but it is not symmetric about the zero mean stress. Owing to the asymmetry of the yield surface in the compressive and tensile states, the yield surfaces should be normalized by compressive and tensile uniaxial yield stresses, respectively, which leads to the conclusion that normalized yield surfaces are almost independent of both relative density and thickness distribution of the cell wall. Moreover, the scatter in the same proportion of the von Mises stress over the mean stress, but different triaxial stress combinations, shows that it might not be sufficient to depict the yield surface only by the von Mises stress and the mean stress.

Modern aircraft have reached higher and higher flight speed, resulting in the urgent demand of design and characterization for high-temperature airborne radomes, especially for those with dual-band or multi-band properties. In this paper, an A-sandwich radome wall structure made of quartz is designed for both centimeter and millimeter wave applications at high temperatures. Its transmission performance in the 4– 40 GHz frequency range at ambient temperature to 800 °C is characterized by a broadband free-space microwave measurement system, which is mainly composed of a vector network analyzer, spot-focusing lens horn antennas, and a furnace for heating sample. A calibration method is proposed to eliminate the temperature-dependent distortion of microwave signals by the microwave-transparent walls of the furnace, and the errors caused by the wall position variations as well. Transmission measurements show that the radome wall structure is feasible for dual-band applications in the 4– 10 GHz and 24– 40 GHz frequency spans with the transmission efficiency higher than 70% at temperatures up to 800 °C, and demonstrate the effectiveness of the design method. It can be expected that the free-space microwave measurement system in conjunction with the proposed calibration method is capable of damage detection for aerospace structures under high-temperature conditions. Keywords: Radome wall structure, High-temperature, Dual-band, Transmission measurement, Calibration

Epoxy polymer concrete (EPC) has found increasing applications in infrastructure as a rising candidate among civil engineering materials. In most of its service environments, EPC is inevitably exposed to severe weather conditions, e.g., violent changes in temperature, rain, and ultraviolet (UV) radiation. In this paper, we designed an accelerated aging test for EPC, which includes periodic variation of temperature and water spray, as well as intensive UV-light irradiation, imitating the outdoor environment in South China. The experimental results show that the flexural performance of EPC is found deteriorate with the aging time. An aging process equivalent to four years (UV radiation dose) results in up to 8.4% reduction of flexural strength. To explore the mechanisms of observed performance degradation, the EPC specimen in the four-point-bending test is considered as a layered beam. The analysis indicates that the loss of flexural load-carrying capacity of an aged EPC beam is dominated by the reduction of mechanical properties of the surface layer. The mechanical properties of the surface layer are closely associated with the aging of epoxy mortar, which can be approximated as a reciprocal function of the aging time. By introducing damage to the surface layer into the layered beam, the proposed model demonstrates a good ability to predict the residual flexural strength of EPC during the aging process

The hydrogel can be regarded as a system comprising of a three-dimensional (3D) polymer network entrapping water in intermolecular space. Contributions of polymeric fibers and water to macrostresses of hydrogels are usually separated by fitting stress–strain relations from experiments. In this paper, we developed a new method to determine the two contributions by applying triaxial loads on a 3D micro-fiber network model, in which not only the uniaxial tension or uniaxial compression direction, but also the other two lateral directions are applied with external loads for keeping the unchanged volume of the fiber network model throughout the loading process. The lateral forces on the model can be regarded as the forces of water on the micro-fiber network at boundaries, since the lateral boundary conditions of hydrogels under uniaxial loading are completely free. With the calculation of the effective areas of polymeric fibers and water on the lateral surfaces, the hydrostatic pressure of water can be derived in each loading step, and further, the contributions of polymeric fibers and water to macrostresses of hydrogels in uniaxial loading direction can be determined quantitatively. The results show that in small deformation (the strain is less than about 10%), contributions of the water and polymeric fibers are of the same order of magnitude. In larger deformation, polymeric fibers are decisive in the contribution to stresses in tension, while water takes the dominant contribution in compression. Therefore, the difference in the contribution of water in tension and compression can explain the phenomenon of different initial elastic moduli of hydrogels in tension and compression. The method recommended in this paper can be used to learn the contributions of polymeric fibers and water in hydrogels with diverse fiber contents and even under multi-axial loading.

Layer structure like laminated composite beam (LCB) is widely used in engineering, and its mechanical properties always play an important role in application. Actually, its stress, strain and bending resistance will change with the variation of elastic modulus, layer thickness and damage parameter. In this paper, several significant variables, namely height, elastic modulus and damage modulus are considered comprehensively to estimate flexural damage and strength of double-layer LCB. Based on the definition and development prediction of damage, the position of the neutral axis when the beam fails is analyzed first, and then the estimation of its flexural strength is given. In addition, a reliable prediction of damage failure for LCB is carried out, and the change law of the neutral axis position and flexural strength of LCB under different variables is discussed accordingly. This will facilitate a more reasonable design of the laminated composite structure in engineering practice.

Mechanical adaptability of implants to the surrounding alveolar bone is one of the critical factors that influences its long-term service. Experimental study shows that widely adopted titanium implants are prone to induce stress shielding due to the considerable mismatch of moduli between stiff implant and alveolar bone, which is considered to induce resorption of peri-implant bone tissues and impair the retention of implants. Moreover, current commercial dental implants with fixed stiffness are unable to meet various alveolar bone conditions of the patients of different ages and physical situations. To solve the two problems, a novel design of metal-polymer structure is proposed for dental implants, which combines two kinds of biocompatible materials, i.e., titanium and polyether-ether-ketone (PEEK). The proposed composite structure allows patient-specific adjustment of equivalent modulus for dental implant in a cost-effective way. In this paper, three designs are given and compared to show how an optimized biomechanical health degree of peri-implant bone can be achieved to fit the alveolar bone condition of patients.

Epoxy polymer concrete (EPC) has high volume fraction of aggregate and extraordinary mismatch of mechanical properties between the granite aggregate and the epoxy matrix, which make the mesoscale modeling a challenging issue. This paper demonstrates how the mesoscale model of EPC is generated and employed to investigate the mechanical responses of EPC under tension and bending. By introducing rotation operation into take-and-place procedure, high aggregate volume fraction (over 75%) of irregular shaped aggregate can be achieved efficiently following various practical gradations. Based on the obtained geometrical model, a three-phase model consisting of aggregate, matrix, and interfacial transition zone (ITZ) is constructed, in which the ITZ is defined as stiff as the granite aggregate and as strong as the epoxy matrix to coordinate the deformation of the other two phases in EPC. The proposed model demonstrates good performance in finite element analysis to explore the stress–strain relation and damage development of EPC during tensile deformation.

Modeling and computing parameters in nonlinear finite element simulations significantly affect simulation accuracy and efficiency even when it is carried out using commercial software, such as ABAQUS, ANSYS, etc. Yet comprehensive effects of these parameters on simulation results have seldom been reported. In this article, we explore the effects of several important parameters, such as mass scaling type and value, element type and size, and loading velocity, on the accuracy and efficiency of nonlinear finite element simulation of metallic foams based on three-dimensional Voronoi mesostructures. Analysis indicated that these parameters did affect simulation accuracy and efficiency, and three optimized nondimensional parameters were recommended. Based on the verified model and optimized parameters, effects of cell-wall thickness distribution on the uniaxial properties of metallic foams were also investigated. Simulation results showed that the different distribution of cell-wall thickness in modelin...

The impact response of lightweight concrete filled with expanded polystyrene beads (EPS concrete) has been studied experimentally using a drop hammer system equipped with a high speed photography system. According to the impact force-time curves and the images of specimen surface obtained during the tests, the impact response of EPS concrete can be summarized as a four-stage procedure. The damage of EPS concrete is found to be a progressive process, which initiates from the upper part and develops downwards, leading to fluctuation of impact force. Based on the experiments, the energy dissipation capability of EPS concrete is evaluated, which demonstrates a dependence on the fracture mode of EPS concrete at the impact velocities in a range of 2–4 m/s. The energy dissipation capacity of EPS concrete is found insensitive to the EPS volume fraction, when it is in a high range from 70.2 to 80.3 vol%. Up to 18.6% increase in energy dissipation is observed in the concrete with 80.3 vol% EPS under impact at 4 m/s.

Considerable experimental work on carbon nanotube-reinforced composites has shown that the reinforcement efficiency of carbon nanotubes (CNTs) becomes lower than the theoretical expectation when CNT content reaches a critical value. This critical volume fraction (percolation threshold) is considered related to the formation of percolating network. In this work, a percolation model is proposed to describe the observed sharp decrease in the reinforcement efficiency of multiwalled CNTs (MWCNTs) dispersed in thermoplastics when the CNT content exceeds the percolation threshold. The percolation threshold is estimated via a numerical simulation of randomly curved CNTs according to the statistics on geometrical features of real CNTs. The percolation model, integrated into the Halpin–Tsai equations, is verified using the experimental data of various thermoplastic composites reinforced with MWCNTs. The developed mechanical model achieves a good agreement with the measured moduli of nanocomposites, and demonstrates an excellent prediction capability over a wide range of CNT content.

For high-water content hydrogels in compression, the water inside of hydrogels contributes to the response of hydrogels to external loads directly, but part of the water is expelled from hydrogels in the meantime to change the volume of the hydrogel and reduce the contribution. In order to consider the contribution of the water in the constitution equation, PVA (polyvinyl alcohol) hydrogels with high-water content were used as examples, and compressive experiments were carried out to measure both the stress-strain relation and the change of the volume in the meantime. By considering the effect of the difference of the contribution of water in different directions of the hydrogel, we deduced a new constitutive equation, which can pretty well depict the stress-strain of hydrogels with different water contents. The results showed that the contribution of water to the total stress increases with the compression strain and even exceed that of the polymer, although the expelled water reduces the contribution at the early loading stage, which well explains the difference of elastic moduli of hydrogels in compression and tension.

A new design of aluminum foam composite panels with enhanced ribs was proposed, and the energy absorption property of it was analyzed by combining dynamic testing and numerical simulation. The opti...

The anti-creep capability of metallic foams is important to structural applications at elevated temperatures. Currently, available creep models for opened-cell foam materials are usually derived by...

In the study of the mechanical properties of metallic foam, the relative density (or porosity) and the average size of cells are two key parameters of the meso-geometry, but it is also well known experimentally that the two parameters alone are not enough since the mechanical properties of metallic foams are different even with the same initial relative density and average cell size. In this paper, we have classified the irregularity of cells into two types to describe polygonal and polyhedral cells in 2D and 3D metallic foams, which are called size irregularity and shape irregularity, respectively. The former reflects the deviation of the size of a cell from the average cell size in the foam and the latter reflects the deviation of the shape of a cell from a circle with the same area (2D) or a globe with the same volume (3D). With the two kinds of definitions, effects of the irregularity of cells in aluminum foam on mechanical properties are investigated using the Voronoi tessellation technique and the finite element method. The well- designed 2D and 3D Voronoi models are constructed, of which the statistic distributions of size and shape irregularity are presented. The compression simulations of Voronoi-based models indicate that the yield stress of metallic foam is seldom affected by the size irregularity, but significantly affected by the shape irregularity. The more regular the foams, the higher will be their yield plateau at constant overall relative density and average cell size.

Incorporation of nanoparticles into polymer matrix was found to considerably improve the compressive performance of unidirectional fiber reinforced composites. In our experimental study, an increase by 62.7% in the longitudinal compressive strength of unidirectional carbon fiber reinforced composites is attained by dispersing 8.7 vol.% SiO2 nanoparticles into epoxy matrix. A compressive strength model is established to quantitatively describe the reinforcing effects of nanoparticles, which combines a modified microbuckling model for unidirectional fiber reinforced composites and a constitutive model for nanocomposite matrices under compression. In the two models, the coupling of damage and plasticity is considered to contribute to the nonlinear response of nanocomposite matrix. The proposed strength model demonstrates excellent prediction capability in experimental verification. A small relative deviation below 8.2% is achieved between the predicted compressive strength of unidirectional fiber reinforced composites and the measured values, which is at the same level of random error in experiments.

As an ultra-soft material (elastic modulus in magnitude of kPa), polyvinyl alcohol (PVA) hydrogels have the potential to substitute articular cartilage, but the measurement of the dynamic stress–strain relations of ultra-soft materials is still challenging. In this paper, a double-striker electromagnetic driving split-Hopkinson pressure bar (SHPB) system was developed, in which all the bars were made of polycarbonate, and the polycarbonate striker was pushed by a metal striker driven electromagnetically to ensure the precise control of impact velocity. With the new SHPB system, well design of the size of hydrogel specimen and rational processing of the signal data, the stress–strain relations of hydrogels with varied PVA contents at different strain rates were measured successfully. Experimental results indicate that PVA hydrogels are a positive strain rate sensitive material with different strain-rate effects at low and high strain rates. Finally, based on the latest quasi-static constitution of the PVA hydrogel, a rate-dependent constitutive equation was recommended, which may well depict the mechanical behaviors of hydrogels with different fiber contents at varied strain rates. It also derives that the contributions of strain rate and fiber content on the mechanical behaviors of the hydrogel are relatively independent.

Laminated composite beams are commonly used in engineering applications involving macro to nano structures. Based on the assumption that plain sections remain plain after deformation, this paper analyzes stress distributions in cross-ply laminated composite beams with rectangular cross-sections, and formulates the basic damage equations through Kachanov's damage definition and Janson's failure criterion. The location of the neutral axis and the ultimate bending moment are obtained for pure bending cases. The effect of the elastic modulus of the two layers on the damage evolution is analyzed; a reasonable damage composite beam model is proposed to predict the ultimate bending moment.

Split Hopkinson pressure bar (SHPB) technique is a widely adopted method to study the dynamic mechanical behaviors of materials. However, the strong localization of the deformation in aluminium foams (near the ends of the specimen) may invalidate the assumption of uniform strain within the specimen granted in the analysis of SHPB results. To appraise how critical this issue could be, the deformation process of aluminium foam under middle speed impact was investigated by combining SHPB technique with high speed photography. Based on the experimental results, finite element analysis was carried out to explore the characteristics of the localized deformation in aluminium foam. The strain distribution was quantitatively analyzed and showed the dependence on the imposed strain rate. An evaluation of the deformation localization by defining maximum loacalized strain and strain rate indicates that the maximum strain and strain rate in the specimen can be more than 100% higher than the average values.

The liquid rubber-based concrete is a new kind of concrete-like composite in which the matrix is the liquid rubber substituting the conventional cement mortar; the aggregate is also a stone with irregular shape and geometry. Such a new concrete not only has heterogeneous mesostructure, but also is a composite with two basic materials with significantly different mechanical properties. The damage and failure features under loading are keys for the further improvement of mechanical properties of the new material. In this article, the new concrete is modeled as a three-phase composite consisting of aggregate, liquid rubber matrix, and interface between the aggregate and the liquid rubber matrix. The interface is regarded as an independent material with given geometry and mechanical properties. The new material’s tensile properties were simulated dynamically in a condition of uniform displacement load on the tensile boundary and a deactivating element technique was used to deal with the failed elements. Special attention was paid to the influence of the loading rates and the properties (thickness, strength, failure strain) of the interfacial ‘material.’ Finally, the tensile properties of the new concrete were simulated successfully; the deformation and failure of the mesostructure of the new concrete were observed. The approaches will give some highlights to the optimization methods of the new concrete. Also, the numerical techniques for modeling, such kind of materials with strong heterogeneous mesostructure were recommended.

SUMMARY A mixed finite element for coupled thermo-hydro-mechanical (THM) analysis in unsaturated porous media is proposed. Displacements, strains, the net stresses for the solid phase; pressures, pressure gradients, Darcy velocities for pore water and pore air phases; temperature, temperature gradients, the total heat flux are interpolated as independent variables. The weak form of the governing equations of coupled THM problems in porous media within the element is given on the basis of the Hu–Washizu three-filed variational principle. The proposed mixed finite element formulation is derived. The non-linear version of the element formulation is further derived with particular consideration of the THM constitutive model for unsaturated porous media based on the CAP model. The return mapping algorithm for the integration of the rate constitutive equation, the consistent elasto-plastic tangent modulus matrix and the element tangent stiffness matrix are developed. For geometrical non-linearity, the co-rotational formulation approach is utilized. Numerical results demonstrate the capability and the performance of the proposed element in modelling progressive failure characterized by strain localization and the softening behaviours caused by thermal and chemical effects. Copyright 2005 John Wiley & Sons, Ltd.

Split Hopkinson pressure bar (SHPB) technique is the most important test method to characterize dynamic stress–strain relations of various materials at different strain rates, and this technique requires uniform deformation of specimen during the experiment. However, some studies in recent years have found obvious deformation localization within metal foam specimens in SHPB tests, which may significantly affect the reliability of the results. Usability of SHPB to characterize dynamic stress–strain relation of metal foam becomes doubtful. In this paper, based on experimental verification, we carried out numerical simulative SHPB tests to study the problem, in which the metal foam specimens were modeled to have 3D meso structures with properties of their matrix material. Numerical simulative SHPB tests of aluminum foam specimens with varying thickness at different strain rates were performed. Deformation distribution in each local region of the specimen was examined and a concept of “effective specimen” was presented. Appropriate specimen thickness and range of testing strain rate were suggested based on quantitative analysis. Finally, we recommended a method how to revise the nominal strain and strain rate measured by traditional SHPB method to acquire the reliable dynamic stress–strain relation.

Static uniaxial compressive experiments were conducted to study the mechanical behaviors of polyvinyl alcohol (PVA) hydrogels in ambient conditions. It was found that water is expelled during the compression of hydrogels that have high water contents. It means hydrogels may be a mass variable under the compression. In order to depict the mechanical properties intrinsically, a variable mass model with meso-scale cells was proposed to simulate PVA hydrogels. In the model, there are uniform cells with frames of polymer fibers and water, and a virtue membrane designed to wrap up the boundary of the model. The model not only depicts the behaviors of the compressive mechanics and the expelled water, but also explains the nonlinear stress–strain relation of PVA hydrogels and why the hydrogels with high water content demonstrate a modulus considerably lower than the bulk modulus of water.

Basing on the existing thermo-hydro-mechanical constitutive model presented in reference[1], and the chemo-mechanical constitutive model presented in reference[2], a coupled chemo-thermo-hydro-mechanical(CTHM) constitutive model of porous media is developed in the present paper. The chemoplastic and thermoplastic coupling behavior is considered in the present constitutive model. The present coupled CTHM constitutive model has been integrated into the coupled thermo-hydro-mechanical mathematical model including contaminant translation in porous media. The numerical results are illustrated to particularly emphasize the effects of the contaminant concentration and temperature to the coupled CTHM system in porous media.

Metallic foam is a typical porous material with heterogeneous meso structures that affect the metallic foam's mechanical properties, in which shape irregularity of meso structures (porous cells) is the key. Shape irregularity of a porous cell reflects the deviation the cell shape from a sphere having the same volume. This paper examines the effects of meso shape irregularity of metallic foam on yield features based on three-dimensional (3D) Voronoi model and the finite element (FE) method. Three cubic foams designed by 3D Voronoi technique were constructed to furnish different statistics of shape irregularity but with the same initial density. Three normal stresses were applied on cubic metallic foam proportionally to implement the triaxial proportional loading in triaxial compression and tension. The yield surface of each 3D Voronoi foam under triaxial loading was calculated, and the influence of shape irregularity on the yield surface was investigated. The results show that the yield surface in (σe, σm) space may be depicted by an ellipse for metallic foams with different shape irregularities. The meso shape irregularity does clearly affect the yield surface with larger shape irregularity producing a smaller yield surface. This means that the metallic foam with larger shape irregularity yields more easily, and this feature is also reflected in the normalized yield surface for metallic foams with different irregularities.

With the demand of lightweight structure, more and more metal foams were employed as impact protection and efficient energy absorption materials in engineering fields. But, results from different impact experiments showed that the strain rate sensitivity of metal foams were different or even controversial. In order to explore the true hiding behind the controversial experimental data about the strain rate sensitivity of metal foams, numerical simulations of split Hopkinson pressure bar (SHPB) tests of the metal foams were carried out by finite element methods. In the analysis, cell structures of metal foams were constructed by means of 3D Voronoi, and the matrix metal was assumed to be no strain rate sensitivity, which helps to learn the strain rate effects quantitatively by the foam structures. Numerical simulations showed that the deformation of the metal foam specimen is not uniform during the SHPB tests along the specimen, and the strain–stress relations of the metal foams at two ends of the specimen are different; there exists strain rate sensitivity of the metal foams even the matrix metal has no strain rate sensitivity, when the strain of the metal foams is defined by the displacement difference between the ends of the specimen; localized deformation of the metal foams and the inertia effect of matrix metal are the two main contributions to the strain rate sensitivity of the metal foams.

This paper presents the low-velocity impact tests on the sandwich plates with aluminum foam core and aluminum skins at elevated temperatures. A furnace, attached to an Instron Dynatup 9250 HV drop hammer system, was designed to accomplish the penetration tests at temperatures up to 500°C. In order to process the experimental data accurately, the numerical vibration analysis was conducted to determine the threshold frequencies of the fast Fourier transform (FFT) filter for the original impact data. The experimental results showed that the failure modes of the sandwich, peak load and absorbed energy varied obviously with temperatures. Furthermore, the results showed that the failure modes of the top skin and metal foam core showed minor changes with respect to temperatures. Whereas the failure mode of the bottom skin and peak loads changed significantly with respect to temperatures. Also, the absorbed energy revealed a three-stage variation with the change of temperature.

The split Hopkinson pressure bar (SHPB) is most widely used to measure the dynamic mechanical properties of materials. Such a testing methodology implies the assumption of uniform deformation during an impact test. However, the experimental verification of this assumption for aluminum foams has, as yet, been unreported. In this paper, a test system combining the SHPB with a high-speed digital camera was designed and constructed to study the problem. In the system, the synchronization between SHPB and the high-speed digital camera, the lighting, and the surface treatment of specimen are established. The deformation of meso-structure of aluminum foams during the SHPB impact was observed successfully; furthermore, the localized strains along the specimens were measured quantitatively. Experimental results show that the deformation is non-uniform; that means the assumption of uniform deformation for aluminum foam is not well satisfied. Therefore, a need exists for some modifications to characterize the dynamic mechanical properties of aluminum foams by SHPB.

A new type of concrete was developed using liquid rubber (polyurethane) as binder and graded stones as aggregate. The mechanical properties of the prepared liquid rubber-based concrete (LRBC) were systematically investigated through flexural and compressive tests. The dynamic response of LRBC to impact was studied using a spit Hopkinson pressure bar system, which demonstrates a non-monotonic strain rate sensitivity of LRBC. Furthermore, three typical pavement performance tests, i.e., rutting, seepage and sanding tests, were carried out to evaluate the potential of LRBC for the applications in pavement systems. According to the experimental study, the optimum rubber-aggregate ratio was found to be 17% (weight of binder by that of aggregate). The LRBC prepared with this recipe showed the superiority in most pavement performance indicators to other three types of conventional asphalt concrete.