Composite specimen bearing failure reduction in iosipescu shear tests

ASTM losipescu sheart test fixtures often crush the loaded edges of high shear strength specimens before shear failure occurs between the notches. To alleviate this problem, two new shear test fixtures were designed and built. The first uses a pivoting load surface to track the specimen shape as it changes under load. The second uses a rounded load surface optimized to produce a uniform bearing pressure along the loaded edge of a quasi-isotropic sheet-molding compound (SMC) specimen at shear failure. Finite element analyses and strain gage data are presented that describe the behavior of the baseline ASTM, pivoting load surface and rounded load surface fixtures. A substantial reduction in bearing stress and edge crushing was obtained with the rounded load surface fixture. Additionally, the modified surface optimized to test SMC specimens showed no tendency to change the shear strength measured in other composite specimens.     

砂土地基锚板基础抗拔承载力PFC数值分析

在模型试验的基础上,采用PFC离散单元法对条形锚板基础在中密砂土地基中的抗拔性能进行了数值分析。数值模拟采用簇颗粒单元来模拟砂粒的不规则形状,颗粒级配根据模型试验福建标准砂的级配按照相似级配法生成,细观参数根据数值双轴试验确定。水平锚板数值模拟结果与模型试验结果基本一致。与模型试验结果相比,颗粒流数值模拟能得到颗粒间接触力链的分布及其演化规律,能从细观角度来探明宏观抗拔承载力特性的演化机理。在此基础上对倾斜锚板上拔过程进行了模拟,分析了锚板前后砂粒的运动趋势以及接触力链的演化规律,并与已有承载力结果进行了对比分析。     

The 3-D boundary element method of roller bearing by plate element analogue

The mill roller bearing is made up of an internal ring, middle rolls and an external ring, the analysis of which is a multi-bodies contact problem. In this paper, based on the three-dimensional elastic contact BEM without friction, and using the structural characteristics of roller bearings, middle rolls are described by elastic plate units of different shapes, which is placed on the internal ring. The discontinuous traction problem of the contact element can be dealt with by the traction sub-element method. Therefore the contact problem can be changed from one of multi-bodies to one of two-bodies. As an elastic deformation of the plate element, it can be calculated by an elastic contact formula and is regarded as an equivalent gap to be put into the total matrix equality. The face-to-face contact condition of displacement and traction on the element will serve as the convergence judging condition. So convergence solution can be obtained before any geometry inconsistent on the contact element may take place. Therefore, the three-dimensional distribution of load of the mill roller bearing can be achieved. It has been proved that the calculating model of this method is visualized and simple, has fast convergence solution and higher accuracy, making it an effective numerical method of designing and analyzing load characteristics of the mill roller bearing. Project supported by the National Natural Science Foundation of China (50075075).     

Modelling of dynamic ductile fracture and application to the simulation of plate impact tests on tantalum

A model of dynamic damage by void nucleation and growth is proposed for elastic-viscoplastic materials sustaining intense loading. The model is dedicated to ductile materials for which fracture is caused by microvoiding. The material contains potential nucleation sites where microvoids are generated when the local pressure overcomes the nucleation pressure. A probability density function is adopted to describe the fluctuation of the nucleation pressure within the material. The void growth is described by using a hollow sphere model where micro-inertia effects are accounted for. The matrix weakening due to void growth is also included.The model has been first tested under uniaxial deformation. When the strain rate is assumed constant, the pressure inside the material has nearly a linear response up to a maximum. An analytical expression for the maximum pressure is proposed.Finite element simulations of plate impact tests have been carried out and compared to experiments on tantalum. From simulations based on the proposed model, an increase of the spall strength is observed with higher shock intensities. Therefore, the relationship between the velocity pullback and spall strength usually assumed in the literature (based on the acoustic approach) seems to be inadequate. Velocity profiles are simulated for different flyer thicknesses and different impact velocities with close agreement with experiments.     

输电铁塔四地脚螺栓塔脚板抗拉承载力试验与计算方法研究

针对架空输电线路铁塔中常用的四地脚螺栓塔脚板进行了抗拉承载力试验和理论计算方法研究。首先,选取8个试件进行塔脚板抗拉承载力试验研究,分析塔脚底板板厚及有无加劲板对塔脚板抗拉承载力的影响;其次,通过有限元模拟塔脚板的应力分布情况,并结合“塑性分析,弹性设计”的思想,提出了一种基于屈服线理论的四地脚螺栓塔脚板抗拉承载力计算公式;最后,与试验结果、有限元仿真结果、已有技术规定中的计算结果进行了对比。结果表明:建议公式与试验结果和仿真结果较为吻合,相关技术规定中的计算结果偏于保守,验证了本文建议公式的精确性;底板厚度和加劲板的作用对塔脚板承载力影响较大。研究结果可为输电铁塔四地脚螺栓塔脚板结构设计提供参考。     

A Cyclic Constitutive Model Based on Crystal Plasticity for Body-Centered Cubic Cyclic Softening Metals

Under the framework of the small deformation crystal plasticity theory,a crystal plastic cyclic constitutive model for body-centered cubic(BCC)cyclic softening polycrystalline metals is established.The constitutive model introduces the isotropic softening rule that includes two different mechanisms:namely softening under monotonic deformation and softening under cyclic deformation on each slip system.Meanwhile,a modified Armstrong-Frederick nonlinear kinematic hardening rule is adopted.The appropriate explicit scale transition rule is selected to extend the single crystal constitutive model to the polycrystalline constitutive model.Then the model is used to predict the uniaxial and multiaxial ratcheting deformation of BCC axle steel EA4T to verify the rationality of the proposed model.The simulation results indicate that the newly established crystal plasticity model can not only describe the cyclic softening characteristics of BCC axle steel EA4T well,but also reasonably describe the evolution laws of uniaxial ratcheting and nonproportional multiaxial ratcheting deformation.Moreover,the established crystal plastic cyclic constitutive model can reasonably predict the ratcheting behavior of BCC single crystal as well.     

A two variable refined plate theory for orthotropic plate analysis

A new theory, which involves only two unknown functions and yet takes into account shear deformations, is presented for orthotropic plate analysis. Unlike any other theory, the theory presented gives rise to only two governing equations, which are completely uncoupled for static analysis, and are only inertially coupled (i.e., no elastic coupling at all) for dynamic analysis. Number of unknown functions involved is only two, as against three in case of simple shear deformation theories of Mindlin and Reissner. The theory presented is variationally consistent, has strong similarity with classical plate theory in many aspects, does not require shear correction factor, gives rise to transverse shear stress variation such that the transverse shear stresses vary parabolically across the thickness satisfying shear stress free surface conditions. Well studied examples, available in literature, are solved to validate the theory. The results obtained for plate with various thickness ratios using the theory are not only substantially more accurate than those obtained using the classical plate theory, but are almost comparable to those obtained using higher order theories having more number of unknown functions.     

Vibrating triangular plate

Summary The problem solved is that of determining the small transverse vibrations of a thin elastic plate in the shape of a 30° right triangle. The plate is simply supported on all sides, and started with a prescribed initial velocity.     

High-frequency plate flutter

Earlier, using the global instability method, the stability of a strip plate in a supersonic gas flow was investigated. In addition to the classical (low-frequency) flutter developing upon the interaction between the plate oscillation modes, a novel (high-frequency) flutter type in which the oscillations are unimodal was detected. In the present study, the effect on the high-frequency flutter of the plate width (earlier only an asymptotic analysis for a width tending to infinity was performed), its damping characteristics, and the presence of a gas at rest on the side opposite the flow is investigated.     

General non-linear plate theory applied to a circular plate with large deflections

The general non-linear first approximation thin plate tensor equations in undeformed coordinates valid for large strains, rotations and displacements are developed based on the single assumption of plane stress. These equations are then reduced to the exact tensor and physical component equations for symmetrical circular plates.An order of magnitude analysis is performed on the resulting equations which shows that they reduce to the classical linear equations for very small deflections and to the von Karman equations for moderate deflections.The solution to the problem of a clamped circular plate loaded with a concentrated load on a central rigid inclusion is obtained and agrees with the solution of von Kármán's equations for moderate deflections.Perhaps the most important result is that of finding the order of magnitude of the limiting value of deflection that would be allowed under the assumption of plane stress. It is shown that when the deflection approaches the order of magnitude of the radius, the boundary layer approaches the order of magnitude of the thickness and thus a first approximation theory is no longer valid.     

On consistent plate theories

Summary  Applying the uniform-approximation technique, consistent plate theories of different orders are derived from the basic equations of the three-dimensional linear theory of elasticity. The zeroth-order approximation allows only for rigid-body motions of the plate. The first-order approximation is identical to the classical Poisson-Kirchhoff plate theory, whereas the second-order approximation leads to a Reissner-type theory. The proposed analysis does not require any a priori assumptions regarding the distribution of either displacements or stresses in thickness direction. Received 10 January 2002; accepted for publication 16 April 2002     

The steady streaming generated by a vibrating plate parallel to a fixed plate

A viscous fluid is sandwiched between two parallel plates. The top plate performs both normal and lateral small translatory oscillations. The unsteady Navier-Stokes equations are solved using perturbations. The nonlinear Reynolds stress causes a secondary steady streaming. It is found that the normal oscillations induce a steady lift while the interaction between normal and lateral oscillations causes a directional net flux. The system thus operates as a valveless pump with rigid walls.     

Cyclic variability of pulsed spray vapor fields

 A two wavelength extinction technique is used to quantitatively measure the temporal fuel vapor field produced by pulsed automotive fuel injectors. Line-of-sight measurements are spatially deconvoluted to yield a picture of the overall, cycle-averaged, radial vapor concentration profiles. Vapor measurements are also given for single, consecutive, injection pulses, showing cycle-to-cycle variations in the vapor field. Methods to characterize the cyclic variability in the vapor field of a pulsed spray are discussed. As an illustrative example, the differences between an air assist and a non-air assist injector are shown as a function of duty cycle and axial location. Received: 11 August 1997/Accepted: 10 February 1998     

Minimal Spaces with Cyclic Group of Homeomorphisms

There are two main subjects in this paper. (1) For a topological dynamical system \((X,T)\) we study the topological entropy of its “functional envelopes” (the action of \(T\) by left composition on the space of all continuous self-maps or on the space of all self-homeomorphisms of \(X\)). In particular we prove that for zero-dimensional spaces \(X\) both entropies are infinite except when \(T\) is equicontinuous (then both equal zero). (2) We call Slovak space any compact metric space whose homeomorphism group is cyclic and generated by a minimal homeomorphism. Using Slovak spaces we provide examples of (minimal) systems \((X,T)\) with positive entropy, yet, whose functional envelope on homeomorphisms has entropy zero (answering a question posed by Kolyada and Semikina). Finally, also using Slovak spaces, we resolve a long standing open problem whether the circle is a unique non-degenerate continuum admitting minimal continuous transformations but only invertible: No, some Slovak spaces are such, as well.     

论可积微分约束引入的循环积分

对循环坐标和多余坐标间存在一类线性可积微分约束的力学系统,导出了循环积分的一般表达式. 采用实例分析了产生循环积分的原因,并解释了该循环积分的物理意义.