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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.

Principal component analysis (PCA) methods have been widely applied to damage identification in the long-term structural health monitoring (SHM) of infrastructure. Usually, the first few eigenvecto...

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.

This paper aims to propose machine-learning-based damage identification methods with features derived from moving principal component analysis (MPCA) to improve the damage identification performanc...

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.

Path independent strategy is introduced into stereo digital image correlation, in which high accuracy is attained through the inverse compositional Gauss-Newton (IC-GN) algorithm with the second and the first order shape functions for stereo matching and temporal matching, respectively. To gain strong adaptibility to large and complex deformation or considerable parallax betweent the two views, two robust methods are developped to estimate the initial guess for the iterative IC-GN algorithm. One is aided by scale invariant feature transform (SIFT) features and the other is aided by epipolar constraint. Powered by parallel computing on GPU, the proposed stereo DIC demonstrates high computation efficiency. Real-time stereo DIC is realized and used to measure the large deformation at common video recording rate (∼24 fps) with high ananlysis resolution (∼6300 points of interest in region of interest).

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.

Parallel computing techniques have been introduced into digital image correlation (DIC) in recent years and leads to a surge in computation speed. The graphics processing unit (GPU)-based parallel computing demonstrated a surprising effect on accelerating the iterative subpixel DIC, compared with CPU-based parallel computing. In this paper, the performances of the two kinds of parallel computing techniques are compared for the previously proposed path-independent DIC method, in which the initial guess for the inverse compositional Gauss-Newton (IC-GN) algorithm at each point of interest (POI) is estimated through the fast Fourier transform-based cross-correlation (FFT-CC) algorithm. Based on the performance evaluation, a heterogeneous parallel computing (HPC) model is proposed with hybrid mode of parallelisms in order to combine the computing power of GPU and multicore CPU. A scheme of trial computation test is developed to optimize the configuration of the HPC model on a specific computer. The proposed HPC model shows excellent performance on a middle-end desktop computer for real-time subpixel DIC with high resolution of more than 10000 POIs per frame.

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

Long-term structural health monitoring (SHM) has become an important tool to ensure the safety of infrastructures. However, determining methods to extract valuable information from large amounts of data from SHM systems for effective identification of damage still remains a major challenge. This paper provides a novel effective method for structural damage detection by introduction of space and time windows in the traditional principal component analysis (PCA) technique. Numerical results with a planar beam model demonstrate that, due to the presence of space and time windows, the proposed double-window PCA method (DWPCA) has a higher sensitivity for damage identification than the previous method moving PCA (MPCA), which combines only time windows with PCA. Further studies indicate that the developed approach, as compared to the MPCA method, has a higher resolution in localizing damage by space windows and also in quantitative evaluation of damage severity. Finally, a finite-element model of a practical bridge is used to prove that the proposed DWPCA method has greater sensitivity for damage detection than traditional methods and potential for applications in practical engineering.

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.

A novel path independent digital volume correlation (DVC) method is proposed, in which the internal deformation field is determined with the aid of the features extracted from the volume images. The deformation vector at each point of interest is first estimated by tracking the motion of the nearby keypoints obtained through 3D scale-invariant feature transform (SIFT), and then fed as the initial guess into the 3D inverse compositional Gauss-Newton algorithm to achieve high accuracy result. Benefiting from the robustness of SIFT to various deformation and distortion of image, the proposed DVC method demonstrates outstanding performance to automatically process the volume images containing large and complex deformation, which keep challenging to current DVC methods for decades. The proposed DVC method is further powered by the parallel computing on GPU. An ultrafast computation speed (about 10,000 POI/s) can be reached on a personal computer when dealing with the volume images of normal sizes. The 3D SIFT aided DVC method, which achieves unprecedented balance between accuracy, adaptability, and efficiency, shows great potential in the quantitative analysis of internal deformation.