Authors:J. Liu; L. L. Ke; Y. S. Wang Pages: 99 - 111 Abstract: The perturbation method is applied to investigate the frictionally excited thermoelastic dynamic instability (TEDI) of a functionally graded material (FGM) coating in half-plane sliding against a homogeneous half-plane. We assume that the thermoelastic properties of the FGM vary exponentially with thickness. We also examine the effects of the gradient index, sliding speed, and friction coefficient on the TEDI for various material combinations. The transverse normal stress for two different coating structures is calculated. Furthermore, the frictional sliding stability of two different coating structures is analyzed. The obtained results show that use of FGM coatings can improve the TEDI of this sliding system and reduce the possibility of interfacial failure by controlling the interfacial tensile stress. PubDate: 2019-02-01 DOI: 10.1007/s10409-018-0804-x Issue No:Vol. 35, No. 1 (2019)

Authors:Jifan Zhong; Yaochen Zheng; Jianqiao Chen; Zhao Jing Pages: 201 - 211 Abstract: The large design freedom of variable-stiffness (VS) composite material presupposes its potential for wide engineering application. Previous research indicates that the design of VS cylindrical structures helps to increase the buckling load as compared to quasi-isotropic (QI) cylindrical structures. This paper focuses on the anti-buckling performance of VS cylindrical structures under combined loads and the efficient optimization design method. Two kinds of conditions, bending moment and internal pressure, and bending moment and torque are considered. Influences of the geometrical defects, ovality, on the cylinder’s performances are also investigated. To increase the computational efficiency, an adaptive Kriging meta-model is proposed to approximate the structural response of the cylinders. In this improved Kriging model, a mixed updating rule is used in constructing the meta-model. A genetic algorithm (GA) is implemented in the optimization design. The optimal results show that the buckling load of VS cylinders in all cases is greatly increased as compared with a QI cylinder. PubDate: 2019-02-01 DOI: 10.1007/s10409-018-0791-y Issue No:Vol. 35, No. 1 (2019)

Abstract: Cells were suggested to sense matrix rigidity by applying fluctuating forces, but the underlying mechanism remains elusive. Here, with a generic filament-crosslinker modeling system for stress fibers, we demonstrate that high mechanical forces can be induced by specific protein–protein interactions with biased kinetics. Strikingly, we further find that there exist two patterns of force generation, a stable pattern and a fluctuated pattern, in agreement with previous experimental observations. Our analysis indicates that the fluctuated force profile is essentially due to force-induced structural instability during structural assembly. We suggest that how cells utilize or circumvent such stable forces or fluctuated forces may be important in other biological processes as well, though whether such forces should be regarded as passive or active is still tentative. PubDate: 2019-03-21

Abstract: The pseudo-excitation method combined with the integral transform method (PEM-ITM) is presented to investigate the ground vibration of a coupled track-soil system induced by moving random loads. Commonly in the track model, the rail, sleepers, rail pads, and ballast are modelled as an infinite Euler beam, discretely distributed masses, discretely distributed vertical springs, and a viscoelastic layer, respectively. The soil is regarded as a homogenous isotropic half-space coupled with the track using the boundary condition at the surface of the ground. By introducing a pseudo-excitation, the random vibration analysis of the coupled system is converted into a harmonic analysis. The analytical form of evolutionary power spectral density responses of the simplified coupled track-soil system under a random moving load is derived in the frequency/wavenumber domain by PEM-ITM. In the numerical examples, the effects of different parameters, such as the moving speed, the soil properties, and the coherence of moving loads, on the ground response are investigated. PubDate: 2019-03-15

Abstract: In this study, a three-dimensional mathematical model was used to study the contribution of clathrins during the process of cellular uptake of spherical nanoparticles under different membrane tensions. The clathrin-coated pit (CCP) that forms around the inward budding of the cell membrane was modeled as a vesicle with bending rigidity. An optimization algorithm was proposed for minimizing the total energy of the system, which comprises the deforming nanoparticle, receptor–ligand bonds, cell membrane, and CCP, in which way, the profile of the system is acquired. The results showed that the CCP enable full wrapping of the nanoparticles at various membrane tensions. When the cell membrane tension increases, the total deformation energy also increases, but the ratio of CCP bending to the minimum value of the total energy of the system decreases. The results also showed that the diameter of the endocytic vesicles determined by the competition between the stretching of the cell membrane and confinement of the coated pits are much larger than the nanoparticles, which is quit different as the results in passive endocytosis that is not facilitated by the CCPs. The present results indicate that variations of tension on cell membranes constitutes a biophysical marker for understanding the size distribution of CCPs observed in experiments. The present results also suggest that the early abortion of endocytosis is related to that the receptor–ligand bonds cannot generate adequate force to wrap the nanoparticles into the cell membrane before the clathrins respond to support the endocytic vesicles. Correspondingly, late abortion may relate to the inability of CCPs to confine the nanoparticles until the occurrence of the necking stage of endocytosis. PubDate: 2019-03-11

Abstract: A diffusive–stochastic–viscoelastic model is proposed for the specific adhesion of viscoelastic solids via stochastically formed molecular bonds. In this model, it is assumed that molecular-level behaviors, including the diffusion of mobile adhesion molecules and stochastic reaction between adhesion molecules and binding sites, are Markovian stochastic processes, while the mesoscopic deformation of the viscoelastic media is governed by continuum mechanics. Systematic Monte Carlo simulations of this model are used to investigate how competition between the time scales of molecular diffusion, reaction, and deformation creep of the solids may influence the lifetime and dynamic strength of their adhesion. The results reveal that there exists an optimal characteristic time for molecular diffusion corresponding to the longest lifetime and greatest adhesion strength, which is in good agreement with experimentally observed characteristic time scales of molecular diffusion in cell membranes. In addition, the results show that the viscosity of the media can significantly increase the lifetime and dynamic strength, since deformation creep and stress relaxation can effectively reduce the concentration of interfacial stress and increase the rebinding probability of molecular bonds. PubDate: 2019-03-07

Abstract: Direct numerical simulations (DNS) of turbulent flow over a drag-reducing and a drag-increasing riblet configuration are performed. Three-dimensional two-point statistics are presented for the first time to quantify the interaction of the riblet surfaces with the coherent, energy-bearing eddy structures in the near-wall region. Results provide statistical evidence that the averaged organization of the streamwise vortices in the drag-reducing case is lifted above the riblet tip, while in the drag-increasing case the streamwise vortices are embedded further into the riblet cove. In the spanwise direction, the cores of the streamwise vortices over the riblet surfaces are shown to be closer to each other than those for flow over the smooth wall, and wider riblet spacing leads to more reduction on their spanwise distances. In the cases with riblets the streamwise vortices have longer streamwise lengths, but their inclination angles do not change much. PubDate: 2019-03-05

Authors:Huajing Guo; Bin Sun; Zhaoxia Li Abstract: In order to better understand the fatigue mechanisms of steel structures working under high temperature, a multi-scale fatigue damage model at high temperature is developed. In the developed model, the macroscopic fatigue damage of metallic materials due to the collective behavior of micro-cracks is quantified by using the generalized self-consistent method. The influence of temperature on fatigue damage of steel structures is quantified by using the previous creep damage model. In addition, the fatigue damage at room temperature and creep damage is coupled in the multi-scale fatigue damage model. The validity of the developed multi-scale damage model is verified by comparing the predicted damage evolution curve with the experimental data. It shows that the developed model is effectiveness. Finally, the fatigue analysis on steel crane runway girders (CRGs) of industrial steel melt shop is performed based on the developed model. PubDate: 2019-02-27 DOI: 10.1007/s10409-018-00834-x

Authors:Yanzhong Wang; Jin Qian Abstract: The mechanical behavior of filamentous actin bundles plays an essential role in filopodial protrusions at the leading edge of crawling cells. These bundles consist of parallel actin filaments that are hexagonally packed and interconnected via cross-linking proteins including α-actinin, filamin, and fascin. When pushing against the plasma membrane and/or external barriers, actin bundles in filopodial protrusions inevitably encounter a compressive load. The bending stiffness and buckling stability of actin bundles are therefore important in determining the filopodial architecture and subsequent cell morphology. In this work, we employ a coarse-grained molecular dynamics model to investigate the buckling behavior of cross-linked actin bundles under compression, explicitly accounting for the properties of the constituent filaments and the mechanical description of the cross-linkers. The bending stiffness of actin bundles exhibits a generic size effect depending on the number of filaments in the bundle, explicitly depending on the degree of interfilament coupling. The distinct buckling modes are analyzed for bundles with different coupling states and crosslinker strengths. This study could clarify the stability and buckling mechanisms of parallel packed actin bundles and the structure–function relations of mechanical components in filopodial protrusions. PubDate: 2019-02-27 DOI: 10.1007/s10409-019-00838-1

Authors:Meng Wang; Shaobao Liu; Fei Li Abstract: Hydrogel microwell arrays (HMAs) have been wildly used for engineering cell microenvironments by providing well-controlled biophysical and biochemical cues (e.g., three-dimensional (3-D) physical boundary, biomolecule coating) for cells. Among these cues, the oxygen microenvironment has shown great effect on the cellular physiological processes. However, it is currently technically challenging to characterize the local oxygen microenvironment within HMAs. Here, we prepared HMAs with different cross-linking concentrations to adjust the structural and physical properties of HMAs. Then we introduced a scanning electrochemical microscopy (SECM)-based electrochemical method to map the surface topography and oxygen microenvironment around HMAs. The SECM results show both the 3-D topography and the oxygen permeability of HMAs in aqueous solution. The obtained oxygen permeability of HMAs increases with increasing the cross-linking concentration, and the microwell boundaries show the highest oxygen permeability throughout HMAs. This work demonstrates that SECM offers a high spatial resolution and in situ method for characterization of the topography and the local oxygen permeability of HMAs, which can provide useful information for better engineering cell microenvironments through optimizing HMAs design. PubDate: 2019-02-23 DOI: 10.1007/s10409-018-0832-6

Authors:Yuntao Xia; Charlotte R. Pfeifer; Dennis E. Discher Abstract: Cell migration through very narrow spaces in tissues has been seen in both physiological and pathological contexts. For example, immune cells squeeze through the vasculature and the extracellular matrix to reach wound or disease sites, and similarly, cancer cells crawl through interstices in tissues to invade tumor-free regions. The bulky and stiff nucleus of a cell is a barrier to such constricted migration—with smaller pores exponentially more difficult for passage. Cells must actively deform their nuclei to squeeze through constrictions, and this involves the stress-generating cytoskeleton. Here we review: (1) nuclear structures and morphological regulation, (2) proposed mechanisms that drive constricted migration, (3) short-term consequences such as nuclear envelope (NE) rupture and DNA damage during such process, (4) biophysical factors that facilitate NE rupture, and (5) long-term consequences such as genomic variation caused by repetitive NE rupture. Both experimental results and modeling are provided with the intention to better understand constricted migration. PubDate: 2019-02-22 DOI: 10.1007/s10409-018-00836-9

Authors:Yanyao Bao; Ling Li; Luming Shen; Chengwang Lei; Yixiang Gan Abstract: Dynamic wetting plays an important role in the physics of multiphase flow, and has a significant influence on many industrial and geotechnical applications. In this work, a modified smoothed particle hydrodynamics (SPH) model is employed to simulate surface tension, contact angle and dynamic wetting effects at meso-scale. The wetting and dewetting phenomena are simulated in a capillary tube, where the liquid particles are raised or withdrawn by a shifting substrate. The SPH model is modified by introducing a newly developed viscous force formulation at the liquid–solid interface to reproduce the rate-dependent behaviour of the moving contact line. Dynamic contact angle simulations with the interfacial viscous force are conducted to verify the effectiveness and accuracy of this new formulation. In addition, the influence of interfacial viscous forces with different magnitude on the contact angle dynamics is examined by empirical power-law correlations; the derived constants suggest that the dynamic contact angle changes monotonically with the interfacial viscous force. The simulation results are consistent with experimental observations and theoretical predictions, implying that the interfacial viscous force can be associated with the slip length of flow and the microscopic surface roughness. This work demonstrates that the modified SPH model can successfully account for the rate-dependent effects of a moving contact line, and can be used for realistic multiphase flow simulations under dynamic conditions. PubDate: 2019-02-22 DOI: 10.1007/s10409-018-00837-8

Authors:Yaojie Yu; Feng Liu; Tingbo Zhou; Chao Gao; Ya Liu Abstract: Low Mach number flows are common and typical in industrial applications. When simulating these flows, performance of traditional compressible flow solvers can deteriorate in terms of both efficiency and accuracy. In this paper, a new high-order numerical method for two-dimensional (2-D) state low Mach number flows is proposed by combining flux reconstruction (FR) and preconditioning. Firstly, a Couette flow problem is used to assess the efficiency and accuracy of preconditioned FR. It is found that the FR scheme with preconditioning is much more efficient than the original FR scheme. Meanwhile, this improvement still preserves the numerical accuracy. Using this new method and without the Boussinesq assumption, classic natural convection is directly simulated for cases of small and large temperature differences. For the small temperature difference, a p and h refinement study is conducted to verify the grid convergence and accuracy. Then, the influence of the Rayleigh number (Ra) is analyzed. By comparing with the reference results, the numerical results of preconditioned FR is very close to that calculated by incompressible solvers. Furthermore, a large temperature difference test case is calculated and analyzed, indicating this method is not limited by the Boussinesq assumption and is also applicable to heat convection with large temperature differences. PubDate: 2019-02-22 DOI: 10.1007/s10409-018-00835-w

Authors:Yao Zhang; Longqi Wang; Haisheng Zhao; Seng Tjhen Lie Abstract: This paper proposes an approach to extract the mode shapes of beam-like structures from the dynamic response of a moving mass. When a mass passes through a beam containing several artificially installed humps, its vertical acceleration can be recorded. After applying fast Fourier transformation to the dynamic response, one can extract the mode shapes of the beam. The surface roughness was neglected compared to the humps and its adverse effect on the extraction was reduced. The passing mass performs as both “exciter” and “massage receiver”; therefore, this method requires only one single accelerometer, making it more convenient and time saving in practice. Moreover, to estimate the possible error in extracting mode shapes, a wavenumber domain filtering technique is used to reconstruct the general profiles of mode shapes. Experimental validation of this approach in laboratory scale was conducted. The experimental results show that the proposed method performs well in extracting lower order mode shapes. It should also be noted that the passing mass can not have a very high velocity (e.g. 80 mm/s), otherwise the mass may jump and separate from the beam, and the proposed method may fail to identify mode shapes. PubDate: 2019-02-19 DOI: 10.1007/s10409-018-0831-7

Authors:Hong-Ling Ye; Zong-Jie Dai; Wei-Wei Wang; Yun-Kang Sui Abstract: A new topology optimization method is formulated for lightweight design of multimaterial structures, using the independent continuous mapping (ICM) method to minimize the weight with a prescribed nodal displacement constraint. Two types of independent topological variable are used to identify the presence of elements and select the material for each phase, to realize the interpolations of the element stiffness matrix and total weight. Furthermore, an explicit expression for the optimized formulation is derived, using approximations of the displacement and weight given by first- and second-order Taylor expansions. The optimization problem is thereby transformed into a standard quadratic programming problem that can be solved using a sequential quadratic programming approach. The feasibility and effectiveness of the proposed multimaterial topology optimization method are demonstrated by determining the best load transfer path for four numerical examples. The results reveal that the topologically optimized configuration of the multimaterial structure varies with the material properties, load conditions, and constraint. Firstly, the weight of the optimized multimaterial structure is found to be lower than that composed of a single material. Secondly, under the precondition of a displacement constraint, the weight of the topologically optimized multimaterial structure decreases as the displacement constraint value is increased. Finally, the topologically optimized multimaterial structures differ depending on the elastic modulus of the materials. Besides, the established optimization formulation is more reliable and suitable for use in practical engineering applications with structural performance parameters as constraint. PubDate: 2019-02-19 DOI: 10.1007/s10409-018-0827-3

Authors:Chao Yu; Guozheng Kang; Di Song; Xi Xie Abstract: Existing experimental results have shown that four types of physical mechanisms, namely, martensite transformation, martensite reorientation, magnetic domain wall motion and magnetization vector rotation, can be activated during the magneto-mechanical deformation of NiMnGa ferromagnetic shape memory alloy (FSMA) single crystals. In this work, based on irreversible thermodynamics, a three-dimensional (3D) single crystal constitutive model is constructed by considering the aforementioned four mechanisms simultaneously. Three types of internal variables, i.e., the volume fraction of each martensite variant, the volume fraction of magnetic domain in each variant and the deviation angle between the magnetization vector, and easy axis are introduced to characterize the magneto-mechanical state of the single crystals. The thermodynamic driving force of each mechanism and the thermodynamic constraints on the constitutive model are obtained from Clausius’s dissipative inequality and constructed Gibbs free energy. Then, thermodynamically consistent kinetic equations for the four mechanisms are proposed, respectively. Finally, the ability of the proposed model to describe the magneto-mechanical deformation of NiMnGa FSMA single crystals is verified by comparing the predictions with corresponding experimental results. It is shown that the proposed model can quantitatively capture the main experimental phenomena. Further, the proposed model is used to predict the deformations of the single crystals under the non-proportional mechanical loading conditions. PubDate: 2019-02-19 DOI: 10.1007/s10409-018-0816-6

Authors:Dan Wang; Yajun Yin; Zheng Zhong; Zhu Su; Zhili Hu Abstract: Based on the natural exponential pair potential \( U\left( R \right) = C{\text{e}}^{{ - {R \mathord{\left/ {\vphantom {R {\lambda_{0} }}} \right. \kern-0pt} {\lambda_{0} }}}} \) , the interaction potential between curved surface body and on surface particle is studied. Firstly, the interaction potential is written as a function of curvatures through the differential geometry. Secondly, idealized numerical experiments are designed to test the accuracy of curvature-based potential. Then, the driving forces induced by curvatures are analyzed, which confirms that micro/nano-curved surface bodies can induce driving forces; curvatures and the gradient of curvatures are the essential elements forming the driving forces. Finally, by combining with the curvature-based potential and driving forces, the movements on surface particles and the evolution of surface morphology of curved surface bodies are predicted. PubDate: 2019-02-19 DOI: 10.1007/s10409-018-0826-4

Authors:Jialiang Sun; Qiang Tian; Haiyan Hu; Niels L. Pedersen Abstract: An axially variable-length solid element with eight nodes is proposed by integrating the arbitrary Lagrangian–Eulerian (ALE) formulation and the absolute nodal coordinate formulation (ANCF). In addition to the nodal positions and slopes of eight nodes, two material coordinates in the axial direction are used as the generalized coordinates. As a consequence, the nodes in the ALE–ANCF are not associated with any specific material points and the axial length of the solid element can be varied over time. These two material coordinates give rise to a variable mass matrix and an additional inertial force vector. Computationally efficient formulae of the additional inertial forces and elastic forces, as well as their Jacobians, are also derived. The dynamic equation of a flexible multibody system (FMBS) with variable-length bodies is presented. The maximum and minimum lengths of the boundary elements of an FMBS have to be appropriately defined to ensure accuracy and non-singularity when solving the dynamic equation. Three numerical examples of static and dynamic problems are given to validate the variable-length solid elements of ALE–ANCF and show their capability. PubDate: 2019-02-04 DOI: 10.1007/s10409-018-0823-7

Authors:Yazhi Zhu; Michael D. Engelhardt; Zuanfeng Pan Abstract: Micromechanics-based models provide powerful tools to predict initiation of ductile fracture in steels. A new criterion is presented herein to study the process of ductile fracture when the effects of both stress triaxiality and shear stress on void growth and coalescence are considered. Finite-element analyses of two different kinds of steel, viz. ASTM A992 and AISI 1045, were carried out to monitor the history of stress and strain states and study the methodology for determining fracture initiation. Both the new model and void growth model (VGM) were calibrated for both kinds of steel and their accuracy for predicting fracture initiation evaluated. The results indicated that both models offer good accuracy for predicting fracture of A992 steel. However, use of the VGM leads to a significant deviation for 1045 steel, while the new model presents good performance for predicting fracture over a wide range of stress triaxiality while capturing the effect of shear stress on fracture initiation. PubDate: 2019-02-02 DOI: 10.1007/s10409-018-0825-5

Authors:Yongjun Pan; Yansong He; Aki Mikkola Abstract: In this paper, a tailored four-step Adams–Bashforth–Moulton (ABM) algorithm is applied to a semirecursive formulation to perform a real-time simulation of a semitrailer truck. In the ABM algorithm, each integration step involves two function evaluations, namely predictor and corrector. This is fundamentally different when compared to the classic fourth-order Runge–Kutta (RK) integrator approach that contains four function evaluations. A semitrailer truck under investigation is modeled in term of a semirecursive method and simulated by using the presented ABM algorithm. The results show that the four-step ABM method can reduce CPU time almost 50% for solving the truck dynamics with very similar accuracy, in comparison to the fourth-order RK method. The presented ABM algorithm could be used in the semirecursive formulation to carry out accurate real-time simulation of medium-large vehicle systems. PubDate: 2019-02-02 DOI: 10.1007/s10409-018-0829-1