Size-dependent effect on biaxial and shear nonlinear buckling analysis of nonlocal isotropic and orthotropic micro-plate based on surface stress and modified couple stress theories using differential quadrature method

The size-dependent effect on the biaxial and shear nonlinear buckling analysis of an isotropic and orthotropic micro-plate based on the surface stress, the modified couple stress theory （MCST）, and the nonlocal elasticity theories using the differential quadrature method （DQM） is presented. Main advantages of the MCST over the classical theory （CT） are the inclusion of the asymmetric couple stress tensor and the consideration of only one material length scale parameter. Based on the nonlinear von Karman assumption, the governing equations of equilibrium for the micro-classical plate consid- ering midplane displacements are derived based on the minimum principle of potential energy. Using the DQM, the biaxial and shear critical buckling loads of the micro-plate for various boundary conditions are obtained. Accuracy of the obtained results is validated by comparing the solutions with those reported in the literature. A parametric study is conducted to show the effects of the aspect ratio, the side-to-thickness ratio, Eringen＇s nonlocal parameter, the material length scale parameter, Young＇s modulus of the surface layer, the surface residual stress, the polymer matrix coefficients, and various boundary conditions on the dimensionless uniaxial, biaxial, and shear critical buckling loads. The results indicate that the critical buckling loads are strongly sensitive to Eringen＇s nonlocal parameter, the material length scale parameter, and the surface residual stress effects, while the effect of Young＇s modulus of the surface layer on the critical buckling load is negligible. Also, considering the size dependent effect causes the increase in the stiffness of the orthotropic micro-plate. The results show that the critical biaxial buckling load increases with an increase in G12/E2 and vice versa for E1/E2. It is shown that the nonlinear biaxial buckling ratio decreases as the aspect ratio increases and vice versa for the buckling amplitude. Because of the most lightweight micro-composite materials with high strength/weight and stiffness/weight ratios, it is anticipated that the results of the present work are useful in experimental characterization of the mechanical properties of micro-composite plates in the aircraft industry and other engineering applications.

Design of Measuring Instrument with Whole Direct Method for Bed Shear Stress Under Two-Dimensional Water-Flow Co-action

The present study aims at the design and making of measuring instrument of whole direct method for bed shear stress under two-dimensional water-flow co-action. The instrument combines the traditional strain gauge with a precise pressure gauge, and adopts the method directly measuring the difference between the lateral hydrodynamic pressure and different head pressures on both sides of the force plate. As a result, such an instrument solves a technical puzzle of the past strain gauge, i.e. the difficulty to set apart shear stress and lateral force. Static force test and sink test both prove that the instrument is precise, stable and applicable to the measurement of rough beds with different shear stresses.

ANALYSIS OF SHEAR STRESS DISTRIBUTION IN HONEYCOMB AIRCRAFT WING STRUCTURE SUBJECTED TO (S.T.) TORQUE

Using the method of elasticity, an analytical approach is developed to analyze the shear stress in a honeycomb wing structure with a large aspect ratio under the condition of free torsion. The formulas of shear stress, warping and angle of twist are derived. These formulas are both useful and convenient from the point of view in the structure design.

COMPUTATION OF SUPER-CONVERGENT NODAL STRESSES OF TIMOSHENKO BEAM ELEMENTS BY EEP METHOD

The newly proposed element energy projection(EEP) method has been applied to the computation of super_convergent nodal stresses of Timoshenko beam elements.General formulas based on element projection theorem were derived and illustrative numerical examples using two typical elements were given.Both the analysis and examples show that EEP method also works very well for the problems with vector function solutions.The EEP method gives super_convergent nodal stresses,which are well comparable to the nodal displacements in terms of both convergence rate and error magnitude.And in addition,it can overcome the “shear locking” difficulty for stresses even when the displacements are badly affected.This research paves the way for application of the EEP method to general one_dimensional systems of ordinary differential equations.

The Effect of Near-Wall Vortices on Wall Shear Stress in Turbulent Boundary Layers

The objective of the present study is to explore the relation between the near-wall vortices and the shear stress on the wall in two-dimensional channel flows. A direct numerical simulation of an incompressible two-dimensional turbulent channel flow is performed with spectral method and the results are used to examine the relation between wall shear stress and near-wall vortices. The two-point correlation results indicate that the wall shear stress is associated with the vortices near the wall and the maximum correlation-value location of the near-wall vortices is obtained. The analysis of the instantaneous diagrams of fluctuation velocity vectors provides a further expression for the above conclusions. The results of this research provide a useful supplement for the control of turbulent boundary layers.

On the Construction of Mohr Stress Circle and a Graphic Method in Representing the Stress Components on Different Oblique Planes

The Mohr stress circle for three-dimensional stress is usually determined by equations,which is lack of intuitionistic meanings and difficult to understand. In this paper, the construction of Mohr stress circle is illustrated directly by numerical method. The shortcoming of Mohr stress circle in representing the stress components on different oblique planes for three-dimensional stress is analyzed. A three-dimensional figure is given to describe the variation of normal and shear stress on different oblique planes at one point for the first time.

Non-destructive Evaluation of Absolute Stress in Steel Members Using Shear-Wave Spectroscopy

Non-destructive measurement of absolute stress in steel members can provide useful information to optimize the design of steel structures and allow the safety of existing structures to be evaluated.This paper investigates the non-destructive capability of ultrasonic shear-wave spectroscopy in absolute stress evaluation of steel members.The effect of steel-member stress on the shear-wave amplitude spectrum is investigated,and a method of absolute stress measurement is proposed.Specifically,the process for evaluating absolute stress using shear-wave spectroscopy is summarized.Two steel members are employed to investigate the relationship between the stress and the frequency in shear-wave echo amplitude spectrum.The H-beam loaded by the universal testing machine is evaluated by the proposed method and the traditional strain gauge method for verification.The results show that the proposed method is effective and accurate for determining absolute stress in steel members.

Three-dimensional lattice Boltzmann method for simulating blood flow in aortic arch

The three-dimensional （3D） lattice Boltzmann models, 3DQ15, 3DQ19 and 3DQ27, under different wall boundary conditions and lattice resolutions have been investigated by simulating Poiseuille flow in a circular cylinder for a wide range of Reynolds numbers. The 3DQ19 model with improved Fillippova and Hanel （FH） curved boundary condition represents a good compromise between computational efficiency and reliability. Blood flow in an aortic arch is then simulated as a typical haemodynamic application. Axial and secondary fluid velocity and effective wall shear stress profiles in a 180° bend are obtained, and the results also demonstrate that the lattice Boltzmann method is suitable for simulating the flow in 3D large-curved vessels.

Micromechanical modeling of hollow cylinder torsional shear test on sand using discrete element method

Previous studies on the hollow cylinder torsional shear test(HCTST)have mainly focused on the macroscopic behavior,while the micromechanical responses in soil specimens with shaped particles have rarely been investigated.This paper develops a numerical model of the HCTST using the discrete element method(DEM).The method of bonded spheres in a hexagonal arrangement is proposed to generate flexible boundaries that can achieve real-time adjustment of the internal and external cell pressures and capture the inhomogeneous deformation in the radial direction during shearing.Representative angular particles are selected from Toyoura sand and reproduced in this model to approximate real sand particles.The model is then validated by comparing numerical and experimental results of HCTSTs on Toyoura sand with different major principal stress directions.Next,a series of HCTSTs with different combinations of major principal stress direction(a)and intermediate principal stress ratio(b)is simulated to quantitatively characterize the sand behavior under different shear conditions.The results show that the shaped particles are horizontally distributed before shearing,and the initial anisotropic packing structure further results in different stressestrain curves in cases with different a and b values.The distribution of force chains is affected by both a and b during the shear process,together with the formation of the shear bands in different patterns.The contact normal anisotropy and contact force anisotropy show different evolution patterns when either a or b varies,resulting in the differences in the non-coaxiality and other macroscopic responses.This study improves the understanding of the macroscopic response of sand from a microscopic perspective and provides valuable insights for the constitutive modeling of sand.

CALCULATION OF THE FUNDAMENTAL SOLUTION FOR THE THEORY OF SHALLOW SHELLS CONSIDERING SHEAR DEFORMATION

In this paper, some formulas are derived for the numerical computation of the fundamental solution obtained in ref. [1] and relevant computer methods are also discussed in detail. As an application of the fundamental solution, problems of a concentrated normal force acting on infinite shallow shells having positive, zero and negative Gaussian curvatures are calculated according to the numerical methods given in the paper.

Characteristics of the Mesozoic and Cenozoic Tectonic Stress Fields of the Urumqi-Usu Region, Xinjiang

Based on the study of folds and related conjugate shear joints, the tectonic stress fields of the Urumqi-Usu region to the north of the North Tianshan Mountains have been reconstructed. Furthermore the author discussed the tectonic movements and their dynamic features. The early tectonic movement in the investigated region occurred from the end of the Late Jurassic to the initial stage of the Early Cretaceous, with the maximum (tensile) and minimum (compressional) principal stress trajectories in the tectonic stress field being in E-W and S-N directions respectively; the late tectogenesis took place from the end of the Early Pleistocene to the initial Middle Pleistocene, with the maximum and minimum principal stress trajectories in the late stress field striking in WNW and NE-NNE directions respectively. Through computer-aided simulated calculation by the finite element method and analysis of geological structure, it has been ascertained that the early tectogenesis is a nearly N-S compressive movement and the late one a NE to nearly N-S compressive movement with reverse shear. The dynamic force which caused the tectogeneses came from the movement of the southern major fault, i.e. the North Tianshan Mountains.

Experimental Confirmation on the Calibration Curves for Preston＇s Method

The Preston＇s method is considered as one of the most commonly employed methods to measure the wall shear stress. However, it is only possible to determine the wall shear stress from measured pressure differences of the Preston tube and undisturbed static pressure, combined with calibration curves, which depend on the Preston tube diameter, fluid density, and viscosity. Since its invention, no significant advancement in theory has been made, and calibration curves proposed by Preston, Patel and Bechert are still in use. In the present study, a need to measure surface shear stress over a circular cylinder prompted us to develop our original Preston tube system. The developed system has been calibrated by measuring the wall shear stress in the fully developed turbulent flow regime in a circular pipe. The present results generally confirm the previously reported calibration curves. A slight modification of the coefficients in the calibration equation shows further improvement.

Couple stress nanofluid flow through a bifurcated artery—Application of catheterization process

In this article,we are exploring the hemodynamics of nanofluid,flowing through a bifurcated artery with atherosclerosis in the presence of a catheter.For treating obstruction in the artery,one can use the catheter whose outer surface is carrying the drug coated with nano-particles.The resultant solvent is considered as blood nano-fluid.Blood being a complex fluid,is modeled by couple stress fluid.In the presence of nano-particles,the temperature and the concentration distribution are understood in a bifurcated stenotic artery.The concluded mathematical model is governed by coupled non-linear equations,and are solved by using the homotopy perturbation method.Consequently,we have explored is the effects of fluid and the embedded geometric parameters on the hemodynamics characteristics.It is also realized that high wall shear stress exists for couple stress nano-fluid when compared to Newtonian nano-fluid.which is computed at a location corresponding to maximum constriction(z=12.5)of the artery.

THE ANALYSES OF THE THREE-DIMENSIONAL STRESS STRUCTURE NEAR THE CRACK TIP OF MODE Ⅰ CT SPECIMENS IN ELASTICPLASTIC STATE(Ⅱ)--THE ANALYSES OF THE STRESS STRUCTURE

Based on[1],the stress structures of the smooth region and shear lip of the specimens have been investigated in the paper.The characteristics of the stress structure in the smooth region have been found that the variable z can separated out;the stresses in the midsection can be obtained by the plane strain FEM results or HRR structure modified by the stress triaxiality.The effects of load level and thickness on the stress structure can be reflected by the distribution of CTOD along the thickness direction.The obtained expressions of the stresses are very simple and visualized.The analyses of the stress structure in the shear lip show that the stresses can be obtained by different methods of interpolation to a certain precise degree.A new degree parameter of the plane strain state has been put forward and studied.The parameter can reflect relatively well the variation of the kind and thickness of the specimen as well as the load level.The fracture parameter has also been investigated to be sure that it can be obtained by modified CTOD with the stress triaxiality.

Cohesive Strength and Seismogenic Stress Pattern along the Active Basement Faults of the Precordillera-Sierras Pampeanas Ranges,Western Argentina:An Experimental Analysis by Means of Numerical Model

A two-dimensional finite element method （FEM） model that incorporates faults, elastic rock physical properties, topographical load due to gravity and far-field plate velocity boundary conditions was used to recognize the seismogenic stress state along the fold-and-thrust belt of the Precordillera-Sierras Pampeanas ranges of western Argentina. A plane strain model with nine experiments was presented here to examine the fault strength with two major rock phyical properties： cohesion and angle of internal friction. Mohr-Coulomb failure criterion with bulk rock properties were applied to analyse faults. The stress field at any point of the model was assumed to be comprised of gravitational and tectonic components. The analysis was focused to recognize the seismogenic shear strain concentrated in the internal-cristaline domain of the orogene shown by the modeling. Modeling results are presented in terms of four parameters, i. e., （i） distributions, orientations, and magnitudes of principal stresses （σ1 and σ3）, （ii） displacement vector1 （iii） strain distribution, and （iv） maximum shear stress （τmax） contour line within the model. The simulation results show that the compressive stress is distributed in and around the fault systems. The overall orientation of of σ1 is in horizontal directions, although reorientations do occur within some stress weaker parts, especially subsequent to the faults. A large-scale shear stress is accumulating along the active faults of Tapias-Villicum Fault （TVF）, Salinas-Berros Fault （SBF）, Ampacama-Niquizanga Fault （ANF） and Las Charas Fault （CF）, which could act as local stress and strain modulators to localize the earthquakesoccurrence.

预应力UHPC-NC组合梁结合面徐变剪应力分析

在恒载应力作用下,预应力UHPC-NC组合梁因两种材料不同的徐变特性及加载龄期的差异导致结合面产生徐变剪应力,分别采用解析法和有限元法对结合面徐变剪应力进行分析。在解析法中,用三角级数表示UHPC-NC结合面的应力分布,基于结合面上的变形协调,利用最小余能原理推导出结合面徐变剪应力计算公式。在有限元法中,将组合梁UHPC层与NC层均划分成四边形平面应力单元,对结合面则采用等效杆单元模拟。以T形截面预应力UHPC-NC组合梁为例进行计算方法的验证,两种方法的计算结果吻合良好,且研究发现徐变剪应力最大值位于梁端;在此基础上对影响结合面徐变剪应力分布的UHPC层压应力、梁高、计算龄期、环境年平均相对湿度等参数进行分析,结果表明:结合面徐变剪应力最大值随着UHPC层压应力、计算龄期、环境年平均相对湿度的增大而增大,但基本不随梁高变化;结合面徐变剪应力最大值的位置随着梁高的增大而逐步向跨中靠近,其他因素不影响应力最大值的位置。研究结果可为UHPC-NC组合梁结合面的抗剪设计提供参考。

残余应力对桥梁波形钢腹板屈曲性能的影响

以一座1200型波形钢腹板桥梁为研究对象,基于有限元软件建立了考虑残余应力的波形钢腹板数值分析模型,以此来研究残余应力对桥梁的波形钢腹板屈曲性能影响。考虑残余应力对波形钢腹板屈曲性能的影响情况,通过分析波形钢腹板的厚度、长高比以及钢材强度变化,提出了考虑残余应力时波形钢腹板极限屈曲荷载的简化计算方法。研究结果表明:残余应力降低了波形钢腹板的极限屈曲荷载,使得波形钢腹板在屈曲后破坏速度加快;随着厚度及钢材强度的增大,波形钢腹板屈曲极限荷载对残余应力的敏感程度呈增大趋势;随着长高比的增加,波形钢腹板屈曲极限荷载对残余应力敏感程度呈减小趋势;可采用折减系数法来计算焊接残余应力对波形钢腹板屈曲性能的影响。

基于数字图像技术的致密砂岩纯剪裂缝断裂过程实验

纯剪应力状态下致密砂岩断裂过程是理解致密砂岩破坏失效机理的关键,然而在纯剪应力状态下,初始裂缝是发育扩展为I型裂缝或II型裂缝却一直有所争议。本文基于新型光学测量技术数字图像相关法和双切边压缩实验理论方法,开展了双切边压缩实验并通过数字图像相关法得到了位移和应变场。实验结果表明:双边开槽压缩实验能在槽尖(预制裂缝尖端)产生纯剪应力状态;致密砂岩试件起裂过程包含2个阶段的I型裂缝,第一阶段裂缝从槽尖起裂,第二阶段裂缝从试件中心两槽尖相连区域起裂。本文研究成果为数字图像技术应用在岩石断裂力学领域提供了新思路,对分析水力压裂中致密砂岩纯剪裂缝的演化规律有一定参考价值。

高层建筑剪力墙采用置换法加固技术研究

由于施工不当等原因造成高层建筑剪力墙的混凝土强度不符合设计要求的情况时有发生,目前常用的方法为有支撑卸荷加固法、分段置换加固法。有支撑卸荷加固法存在成本高、风险大、施工工作面要求高等问题,分段置换加固法存在工期长、应力后滞等问题。以某25层剪力墙结构为例,重点介绍了新型剪力墙置换加固卸载装置对该建筑物地上1层部分剪力墙进行加固时采用的加固方法、工作原理。该方法能有效解决常规剪力墙置换法的问题,大幅降低造价、缩短工期,为类似工程提供安全、经济、可靠的加固方法。

含碎石芯软黏土复合试样大三轴试验研究

以拟建的某水电站碎石桩地基处理为背景,研发了能提高重塑软黏土固结效率、减小试样扰动的含碎石芯软黏土复合试样的室内试验制样方法,开展了不同围压和碎石芯置换率的复合试样室内大三轴试验研究.研究表明,复合试样在较小围压下表现出应变硬化特性,在较高围压下则基本呈现软化特征,且碎石置换率越小,软化特征越明显,相同围压时,复合试样的初始变形模量随试样面积置换率的增大而增大;在高围压和低置换率时,软黏土碎石芯复合试样的剪切破坏面明显,在低围压和高置换率时,复合试样中部出现较明显的鼓胀现象,碎石芯最大鼓胀量总体随着围压和置换率的提高而增大.规范方法高估了复合试样(地基)的内摩擦角,面积置换率越大,规范计算值与试验结果的差值越大.高围压下传统应力叠加法高估了软黏土振冲置换碎石桩复合地基的承载能力,低估了复合地基的沉降,置换率越高,相同轴向应变时与复合试样大三轴试验的偏应力误差越大.