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Regional seismic damage analysis of building clusters based on time history analysis enables damage evolution and distribution to be obtained for structural performance evaluation, seismic loss estimation, and post-earthquake emergency response decision support. Soil–structure cluster interaction (SSCI) effects have been shown to affect significantly seismic damage to buildings due to the influence of neighboring buildings through underlying soils. However, SSCI effects in current regional seismic damage simulations are either ignored or obtained with exceptionally high computation cost due to the common discretization of large scale soil domains using finite elements. To this end, lumped parameter models (LPMs) are presented in this work as a simple effective tool for regional seismic damage simulation of SSCI systems, which is particularly useful for parametric analysis and also enables fast regional damage assessments. The LPMs combine story-based multiple-degree-of-freedom buildings with ‘foundation springs’ that are used to represent soil–foundation interaction and foundation–soil–foundation interaction. Seismic damage analysis of 320 buildings on Wangjiang campus of Sichuan University in China is used as a case study. The results of the specific case study show, that in most cases, SSCI effects result in lower damage to buildings, especially on soft soils. However, the SSCI effects may lead to higher structural damage to buildings in some cases, e.g. those on stiff soils. The results also show that using nominal inter-story drift ratio which includes foundation rocking rotation as the damage index may underestimate story seismic damage to lower stories of tall buildings.

In order to simulate elastic wave propagation in a complex structure with inhomogeneous media, we often need to obtain the propagating eigenmodes of an elastic waveguide. As the waveguide is assumed uniform in one direction, the original 3-D problem can be converted into a so-called 2.5-D problem by using the Fourier transform in that direction. However, the introduction of elastic metamaterials (EMM) broadens the horizon of this subject, and new features are required in EMM waveguides that cannot be obtained by most traditional waveguide solvers. In this work, a spectral element method (SEM) is developed to simulate the elastic/acoustic waveguide problem in anisotropic media with anisotropic mass density and/or negative index parameters. To the best of our knowledge, the SEM has not been introduced previously for such a waveguide problem. For waveguides with anisotropic density that cannot be solved by the FEM in most of commercial software packages, we design an anisotropic density EMM waveguide with our SEM solver to demonstrate some intriguing phenomena. The spectral element results are verified by several numerical examples through comparison with the traditional finite element method (FEM) to show its significant advantages in term of accuracy and computation efficiency., 28 pages, 19 figures

Shape memory alloys (SMAs) have been widely used to fabricate soft actuators by embedding SMA wires into various soft matrices manufactured by conventional moulding methods or novel three-dimensional (3D) printing techniques. However, soft matrices of SMA based actuators are typically fabricated from only one or two different materials. Here, we exploit the great manufacturing flexibility of multimaterial 3D printing to fabricate various bending, twisting and extensional actuators by precisely controlling the spatial arrangements of different printing materials with different stiffnesses. In order to achieve a broad range of deformations, ten different printing materials were characterized and used in the actuators design. In addition, we developed a finite element model to simulate complex deformations of the printed actuators and facilitate the design process. The model incorporates a user defined material subroutine that describes the nonlinear temperature dependant behavior of SMAs. The results show the efficiency and flexibility of multimaterial 3D printing in tailoring the deformed shape of the SMA based soft actuators, which cannot be accomplished using conventional manufacturing methods such as moulding.

Shape-shifting structures have gained growing interest recently and found wide applications in areas such as soft robotics, biomedical devices and self-folding origami, attributed to their ability to construct complicated shapes directly from simple structures. However, an efficient method to design and fabricate programmable 3D shape-shifting structures from 2D polymer films still lacks. In this work, we design programmable shape-shifting 3D structures via the release of internal gradient stress using the frontal photopolymerization (FPP) method. First, the relation between the non-uniformly distributed material and loading parameters, and the geometric and fabrication parameters are established theoretically. The finite element (FE) model is then developed based on the theoretically obtained material and loading parameters. Next, the elastic instability in the shape-shifting behaviors of a cured film is captured through an elastic energy minimization. Furthermore, by using grayscale light patterns, it is shown that we can selectively manipulate the geometric and fabrication parameters to improve the design freedom of various complex 3D structures.

The shape memory alloy-shape memory polymer (SMA-SMP) composite actuators are the novel smart actuator designs which capable of large recoverable actuation deformation as well as retaining shape in the actuated state without consuming extra energy. This is particularly doable because of the recent progresses in additive manufacturing technologies. The 4D printed actuators designed in the previous study [7] are fabricated by multi-material inkjet 3D printing and is able to deposit various range of digital materials with different elastic stiffness in micro-scale. In this study, we have developed the numerical simulation of the 4D-printed composite actuator hinges to determine important actuation parameters and develop a design tool for future applications. The three-dimensional finite element models were conducted in ABAQUS and the nonlinear temperature dependence behaviour of shape memory alloy (SMA) and shape memory polymer (SMP) materials are incorporated through the user material subroutines (UMATs). The finite element results then have been compared with the experimental measurements and illustrate the accuracy of our numerical analysis. Therefore, the validated computational framework could be used for further development of actuators that could generate complex deformations.

By 3D printing lattice structure with active materials, the structures can exhibit shape and functional changes under external stimulus. However, the programmable shape changes of the 3D printed lattice structures are limited due to the complex geometries, nonlinear behaviors of the active materials, and the diverse external stimuli. In this work, we propose a design framework combining experiments, theoretical modeling, and finite element simulations for the controllable shape changes of the 3D printed horseshoe under thermal stimulus. The theoretical model is based on a phase evolution model that combines the geometrical nonlinearity and the material nonlinearity. Results show that the shapes with positive or negative Poisson's ratio and bending intermediate shapes can be programmed by tuning the geometrical parameters and the temperature distribution. This work provides a method to aid the design of 3D printed functional lattice structures and have potential applications in soft robotics, biomedicine, and energy absorbing fields.

Soft biomaterials have a wide range of applications in many areas. However, one material can only cover a specific range of mechanical performance such as the elastic modulus and stretchability. In order to improve the mechanical performance of soft biomaterials, lattice structures are embedded to reinforce the biomaterials. In this paper, rectangular and triangular lattice structures formed by modified horseshoe microstructures are used because their mechanical properties are tunable and can be tailored precisely to match the desired properties by adjusting four geometrical parameters, the length L, radius R, width w and arc angle θ0. A theoretical design framework for the modified horseshoe lattice structures is developed to predict the dependence of the mechanical behaviors on geometrical parameters. Both experiments and finite element simulations on lattice structures are conducted to validate the theoretical models. Results show that a wide range of design space for the elastic modulus (a few kPa to hundreds of MPa), stretchability (strain up to 180%) and Poisson ratio (ranging from -0.5 to 1.2) can be achieved. Experiments on lattice-hydrogel composites are also conducted to verify the reinforcement effect of lattice structures on the hydrogel. This work provides a theoretical method to predict the mechanical behaviors of the lattice structures and aid the rational design of reinforced biomaterials, which has applications in tissue engineering, drug delivery and intraocular lenses.

In this paper, we built up a desktop digital light processing (DLP) 3D printer and fabricated multiple size soft pneumatic actuators integrally with fast speed and high precision. The printing process is based on the projection microstereolithography method. The composition of the printing system and key parameters during the printing process were presented. Evaluation experiments demonstrate that our printer can print objects with as small as 87.5 μm size features. We first printed single pneumatic net-works (pneu-net) actuators integrally and conducted a series of actuation experiments and finite element method analyses to test the actuators’ deformation capability. We further designed a soft pneumatic gripper containing three micro pneu-net actuators with 0.4 mm wide square air channels as well as 0.2 mm thick chamber walls and fabricated the gripper integrally in less than 30 min using our printer. The grasping capability of the gripper was verified through experiments as well. Results presented in this work prove the performance of the DLP printer we build up and show the convenience of fabricating micro soft pneumatic actuators integrally using DLP 3D printing approach with fast speed and high precision.

KDP (KH2PO4) crystal is a kind of excellent nonlinear optical crystals, which has been widely used in nonlinear optical and Inertial Confinement Fusion (ICF) engineering. KDP crystal with the characteristics of low hardness, high brittleness, easy deliquescence and temperature-sensitive is easy to crack during the crystal growth, taken out from crystallizer, and the process of slicing. Stress concentration caused by the initial internal stress redistribution and the growth defect in KDP crystal is an important reason of KDP crystal cracking during sawing process. The numerical simulation model of the KDP crystal containing spherical cavity defect and sawing with fixed abrasive wire saw is established by finite element method in this paper. The effects of initial internal stress, spherical cavity defect on sawing stress are investigated. The maximum tensile stress near the defect during the sawing process is simulated and analyzed. The results show that sawing stress changes smoothly during sawing process, and the fixed abrasive wire saw slicing belongs to low stress cutting way. The sawing stress at sawing kerf is increased obviously. The crystal defect leads to local stress concentration during sawing process. The coupling effect of sawing stress with initial internal stress and the effect of stress concentration are enhanced when the sawing kerf approaches to the defect.

The emerging direct four dimensional (4D) printing approach is considered as an easy, fast and economical manufacturing strategy that fabricates complex 3D geometries evolving from printed flat patterns in response to external stimuli. However, its implementation to practical engineering applications is impeded by the fact that the existing direct 4D printing methods could not render both the large bending curvature and high loading capacity at the same time. Herein, we report a multimaterial direct 4D printing method to fabricate patterned laminate that consists of covalently bonded elastomer and high-water-content hydrogel. The dehydration induced large volumetric shrinkage (up to 60%) and great modulus escalation (from 100 kPa to 4 GPa) allow the printed flat patterns to evolve into complex 3D structures with large bending curvature (up to 0.7 mm−1) and high bending stiffness (up to 104 MPa m3). To facilitate the structural design, we develop a phenomenological model to describe the dehydration induced shrinkage and stiffening, and implement this model into Euler–Bernoulli beam theory and finite element simulations. Compared with other 3D printing technologies, the proposed multimaterial direct 4D printing approach demonstrates the merits in terms of less building time and high load capacity at both room temperature and high temperature.

Two sets of shaking table tests for comparative analysis were first designed and conducted to investigate the laws of dynamic interaction between soil and multiple high-rise buildings. The first set involves only a single high-rise building, and the second set includes a building group. Numerical results of the finite element models, using the Davidenkov foundation model regarding the dynamic non-linearity of soil, are presented and compared with experimental records. Experimental results show that the acceleration responses of structures in the case of multiple high-rise buildings are less than the response in the case of a single structure. The effect of the dynamic interaction between soil and multiple structures on acceleration responses is more obvious for short-period structures than those with long periods under EL Centro excitation. However, the motions of the long period structures in the two tests are almost the same. Opposite phenomena occur under the Shanghai man-made wave excitation. Moreover...

Three-dimensional (3D) printing has offered considerable convenience in fabricating functional structures constructed with shape memory polymers (SMPs). Conventionally, to meet the specific requirements for given applications, the shape memory property of a SMP is regulated by tailoring its molecular structure from the perspective of polymer chemistry. By virtue of the recent advances in multi-material 3D printing technology, new opportunities have emerged to customize the shape memory property from the perspective of composite material design. In this work, we propose a novel layout design strategy to 3D print multi-material composites incorporating heat-responsive SMP and flexible elastomer. Under a given volume ratio of SMP and elastomer, distinct shape memory performances are obtained for different material layouts. The underlying mechanisms are theoretically investigated based on the mechanics of composite materials and finite element (FE) simulations are conducted to provide quantitative predictions for the experimental results. As a demonstration, composites of different material layouts are used as building blocks to fabricate bilayer laminates where the incompatibility of shape fixity are pre-embedded to trigger the programmed active bending.

Because of the complexity of shale reservoirs and multiple physical coupling processes, numerical implementation remains a challenge in engineering applications. Unlike conventional reservoirs, shale reservoirs always contain two main discontinuities: bedding interface and natural fractures. The existence of these discontinuities is the key reason a fracture network is formed. Therefore, the numerical model should contain both bedding interface and natural fractures. In this paper, the fracturing propagation process is described based on continuum theory. In contrast to other research, an anisotropic strength criterion is adopted to consider the anisotropic properties of shale, including the failure of both shale matrix and discontinuities. The criterion is described by the Mohr-Coulomb model combined with tensile failure. When fractures are generated in the shale rock mass, the stiffness and permeability of the fractured rock are homogenized in an element, and volume fracturing is represented by the deformation of three orthogonal fractures in the rock element. Simultaneously, the anisotropic permeability is also enhanced. Hence, based on this continuum approach and finite element method, a three-dimensional model for shale hydraulic fracturing simulation is presented. This model fully considers the direct coupling of fluid flow and rock deformation during hydraulic fracturing. To verify the anisotropic strength criterion, a shale compression test is conducted. Through a comparison of the numerical and experimental results, the applied criterion can describe the shale compressive strength anisotropy. In addition, the KGD analytical model is also adopted to verify the performance of the developed model for hydraulic fracturing. Finally, several cases of shale hydraulic fracturing are simulated, and the results show the geometry of the fracture region under different conditions. Furthermore, the impacts of three factors on hydraulic fracturing geometry are considered, including natural fractures, spacing of perforations and differential in situ stress. The influence of these factors on the complexity and scale of the fracturing region is also discussed.

The linear heat source (LHS) method is an effective way to realize large-range and distributed monitoring of seepage. Research on this method, however, fails to consider the influence of seepage direction. Based on the seepage mechanics and heat transfer theory, this article sets up a mathematical model to describe the heat dissipation process of LHS in porous media. On such basis, the finite element method was adopted to study the influence of seepage velocity and seepage direction on the law of heat dissipation of LHS. The results showed that the heat dissipation process of LHS meet well with Newton’s Law of Cooling. Its cooling velocity was in positive correlation with the seepage velocity and infiltration angle. Hence, when the seepage direction is unknown, adopting this method for seepage identification is uncertain, may leads to misjudgment. At the end of the paper, the authors discuss a solution to this problem.

In this paper, we studied the current carrying capacity of solder joints in the Copper Column Grid Array (CuCGA) by means of finite element method (FEM). The current carrying capacity of solder joints can be reflected by the peak value of the temperature and current density. And the solder fillets were the critical region in the entire CuCGA. Therefore, the paper mainly studied the influence of the shape-parameters for solder joints and wirings to the peak value of the temperature and current density. The results were as follows, the peak value of the current density of solder fillets occurred in the geometric discontinuity region connected the solder fillets and wirings. And the peak value of the temperature of soldering fillets occurred in the geometric discontinuity region connected the solder fillets and the copper columns. With the increase of height for solder columns, the peak value of current density was constant, while the peak value of temperature decreased slightly. With the increase of diameter for solder columns, both the peak value of current density and the temperature decreased. With the increase of climbing height for soldering, the current density in the solder joints decreased. With the increased of minimum width of wiring, both the peak value of temperature and the peak value of current density appeared the trend of decreased.

Despite great progress in four-dimensional (4D) printing, i.e. three-dimensional (3D) printing of active (stimuli-responsive) materials, the relatively low actuation force of the 4D printed structures often impedes their engineering applications. In this study, we use multimaterial inkjet 3D printing technology to fabricate shape memory structures, including a morphing wing flap and a deployable structure, which consist of active and flexible hinges joining rigid (non-active) parts. The active hinges, printed from a shape memory polymer (SMP), lock the structure into a second temporary shape during a thermomechanical programming process, while the flexible hinges, printed from an elastomer, effectively increase the actuation force and the load-bearing capacity of the printed structure as reflected in the recovery ratio. A broad range of mechanical properties such as modulus and failure strain can be achieved for both active and flexible hinges by varying the composition of the two base materials, i.e. the SMP and the elastomer, to accommodate large deformation induced during programming step, and enhance the recovery in the actuating step. To find the important design parameters, including local deformation, shape fixity and recovery ratio, we conduct high fidelity finite element simulations, which are able to accurately predict the nonlinear deformation of the printed structures. In addition, a coupled thermal-electrical finite element analysis was performed to model the heat transfer within the active hinges during the localized Joule heating process. The model predictions showed good agreement with the measured temperature data and were used to find the major parameters affecting temperature distribution including the applied voltage and the convection rate.

Nanoindentation experiment together with the finite element simulation was used to study the mechanical behavior of Sn3.0Ag0.5Cu (SAC305) lead-free solder joint under cyclic loading-unloading. The effect of different maximum loads and holding time on the micromechanics behavior was investigated. The results indicated that the increase of the maximum load caused the increase of the depth of residual indentation and the area of hysteresis loop. The creep displacement of solder joint was increased with the prolonging of holding time. It increased sharply at first, and then went stabilized. In addition, a good agreement between finite element model and experimental results was reached.