Mechanical response and heat buildup in repetitively impacted elastomers

Experimental and analytical investigations were made to find the rebound characteristics and temperature rise in elastomer slabs subjected to repetitive impact. For its mechanical response, the elastomer was modeled as a linearly viscoelastic material, and its frequency and temperature dependent moduli were determined by Rheo-vibron tests. The experimental traces of impact force vs. time were studied in relation to various input parameters (e.g., impact velocity and slab thickness). An iterative method was devised to calculate the pulse from the input parameters enabling the analyst to estimate the peak force or duration of impacts occurring in a specific damper. As opposed to single (isolated) impacts, the repetitive impact process is conducive to temperature rise in the elastomer slabs resulting in a steady state temperature distribution. Under various input parameters, the temperature on the surface and inside the rubber was measured in the steady state. Both a simplified and a pulsed-heat-generative mathematical model was used to analytically estimate these temperatures.     

基于Choi-Williams分布的侵彻层数识别方法

针对侵彻多层硬目标过程中实测加速度信号的振荡叠加,以及可能引起的层数误判问题,采用时频分析法研究弹体主轴方向加速度信号的Choi-Williams能量分布特征,提出了以侵彻过程加速度信号的能量分布为依据的层数识别方法。侵彻8层等间距混凝土靶板的实测数据处理结果表明,该方法可快速准确实现层数识别,解决了低通滤波方法无法获取准确层数信息的问题,为实时控制起爆位置提供依据。

     

爆炸载荷下RC单向简支板的能量法优化设计

由钢筋混凝土(RC)板的抗力-位移(R-y)曲线特性可知,曲线下方面积代表板在爆炸载荷作用下可能吸收的能量. 对爆炸载荷作用下RC板的R-y曲线采用双折线模型,计算不同配筋率的R-y曲线下方面积,面积最大时的配筋率即可定义为相应设计的最佳配筋率.考虑材料非线性及高应变率对混凝土和钢筋强度的影响,采用分层法迭代求解抗力-挠度(R-y)曲线. 研究结果可供受爆炸载荷的RC板优化设计借鉴.     

火灾下钢筋混凝土板的热弹塑性有限元分析——基于S-R分解原理(Ⅰ:理论)

火灾试验研究表明,火灾下钢筋混凝土受弯构件的变形非常大,转动也很大,尤其是单面受火的板.基于S-R分解原理的更新拖带坐标有限元法分析这类构件,有利于跟踪变形物体中各点的变化,保证单元的质量守恒.用有限元增量法求解,还可以避免对坐标的修正,而且将转动作为一个独立的自由度,提高了求解效率,特别适合于几何非线性、材料非线性问题的求解.本文采用此方法对火灾下钢筋混凝土板进行编程分析.同时,为克服在时间步内温度路径难于确定的问题,本文给出了平面应力状态下的混凝土热弹塑性积分方案的初值表达式.通过实际编程发现,该方法求解效率高,精度也比较好.     

Thermal stresses in functionally graded materials caused by a laser thermal shock

 Mathematical simulation of a thermal shock method for reliability testing of functionally graded material (FGM) is performed with the end to determine operating parameters of the testing device (power of a laser, laser beam radius, duration of heating) and to investigate the effect of the composition of FGM on a magnitude of thermal stresses in a coating. An analytical method for solution of the thermal elasticity problem is developed whereby the approach of a multilayer plate is used for determining temperature and thermal stresses distributions in a coating. We considered the limiting case of the obtained solution when the thickness of a layer is infinitesimally small and the number of layers tends to infinity. This procedure allowed us to obtain the thermal stresses distribution in a FGM coating. The results for the FGM coating composed of WC (tungsten carbide) ceramics and HS-steel are presented. It is showed that variation of the volume content of ceramics strongly affects thermal stresses in a coating and they decrease significantly in the case of the uniform spatial distribution of ceramics. Received on 21 November 2000 / Published online: 29 November 2001     

叠层结构陶瓷强韧化设计的力学分析

基于梁的理论和裂纹在异材界面上拐折与扩展条件,对叠层结构陶瓷弯曲试件的强度与韧性进行了力学分析,获得叠层结构中陶瓷基片和界面层的材料性能、结构几何参数与叠层结构弯曲强度、韧性间的定量解析关系,给出相应的影响曲线,并对Si3N4/BN叠层结构四点弯曲试样断裂功的理论值与实验结果作对比讨论。     

Strength calculations and optimal weight design of multilayer shell-shaped composite products under a set of loads

The stress-strain state of axisymmetric multilayer shells is analyzed using kinematic and static hypotheses that allow for the transverse shear stresses satisfying the necessary equations of state, continuity conditions at the boundaries between the layers and given boundary conditions. A numerical solution of the problem of the stress-strain state for a multilayer bar is compared with the Lekhnitskii solution (for a cantilever beam loaded by a concentrated force and moment) to asses the applicability of the employed bending equations of multilayer shells. It is shown that these solutions are in good agreement. The problem of the initial fracture of the shells considered is formulated using phenomenological strength criteria for each layer. A coordinate-wise descent method in the unit interval is proposed to solve weight optimization problems for multilayer shells of composite materials under combined loading. Regions of safe operating loads and the optimal weight distribution of layer thicknesses are determined for a multilayer bar acted upon by a uniformly distributed load and concentrated force.     

A micromechanical model to describe thermal fatigue and bowing of marble

Marble slabs are frequently used as façade panels to externally cover buildings. In some cases a bowing of such façade panels after a certain time of environmental exposure is experienced. The bowing is generally accompanied by a reduction of strength which increases with increasing degree of bowing. In the present paper, a theoretical model to calculate the progressive bowing and the thermal fatigue of marble slabs submitted to temperature cycles is presented. The model, developed within the framework of fracture mechanics, takes into account the mechanical microstructural characteristics of the marble as well as the actual cyclic temperature field in the material. The slabs are subjected to a thermal gradient along their thickness (due to different values of temperature between the outer and inner sides of the slab) as well as to thermal fluctuation on the two sides of the slab due to daily and seasonal temperature excursions. This thermal action causes a stress field which can locally determine microcracks due to decohesion of calcite grains. Stress intensification near the cracks occurs and leads to crack propagation in the slab. Such crack propagation under thermal actions is evaluated and the corresponding deflection (bowing) is calculated. Some examples are presented which show the strong influence of material microstructure on the degree of bowing.     

泡沫铝防护钢筋混凝土板的抗爆性能

为研究多孔吸能材料泡沫铝板对工程结构的抗爆防护作用,开展室外爆炸破坏实验,分别对设置不同泡沫铝防护层的钢筋混凝土(reinforced concrete,RC)板在爆炸荷载下的动态响应及破坏模式进行了研究,并运用LS-DYNA软件建立了有限元模型。通过与实验对照,验证了模型的可行性,对比分析了有、无泡沫铝防护层钢筋混凝土板的损伤破坏规律,并讨论了泡沫铝密度梯度分布和纵筋配筋率的影响。结果表明:有限元模型能够有效分析含泡沫铝防护层RC板的动态响应及其破坏形态;泡沫铝防护层能够有效减小钢筋混凝土板的挠度变形,降低试件的破坏程度;泡沫铝密度由下到上递增情况对RC板的减爆效果最好;增大配筋率可以提升泡沫铝防护RC板整体抗爆性能。     

Modelling the Stationary Vibrations and Dissipative Heating of Thin-Walled Inelastic Elements with Piezoactive Layers

A coupled dynamic problem of thermoelectromechanics for thin-walled multilayer elements is formulated based on a geometrically nonlinear theory and the Kirchhoff–Love hypotheses. In the case of harmonic loading, an approximate formulation is given using the concept of complex moduli to characterize the cyclic properties of the material. The model problem on forced vibrations of sandwich beam, whose core layer is made of a passive physically nonlinear material, and face layers, of a viscoelastic piezoactive material, is considered as an example to demonstrate the possibility of damping the vibrations by applying harmonic voltage to the oppositely polarized layers of the beam. Substantiation is given for a linear control law with a complex coefficient for the electric potential, which provides damping of vibrations in the first symmetric mode at the linear and nonlinear stages of deformation. The stress–strain state and dissipative-heating temperature are studied     

Acoustic wave incidence on a multilayer medium containing a bubbly fluid layer

The problem of acoustic wave reflection and transmission through a multilayer medium containing a bubbly fluid layer is considered. For the water-water with air bubbles-water model the wave reflection and transmission coefficients are calculated and compared with the experimental data. The problem parameters, at which these coefficients take extremum values, are determined. The influence of vapor within the bubbles on the acoustic wave transmission through a layer of a fluid with the vapor-gas bubbles is shown.     

A locally conservative and energy-stable finite-element method for the Navier-Stokes problem on time-dependent domains

We present a finite-element method for the incompressible Navier-Stokes problem that is locally conservative, energy-stable, and pressure-robust on time-dependent domains. To achieve this, the space-time formulation of the Navier-Stokes problem is considered. The space-time domain is partitioned into space-time slabs, which in turn are partitioned into space-time simplices. A combined discontinuous Galerkin method across space-time slabs and space-time hybridized discontinuous Galerkin method within a space-time slab results in an approximate velocity field that is H(div)-conforming and exactly divergence-free, even on time-dependent domains. Numerical examples demonstrate the convergence properties and performance of the method.     

Four-parameter formulae for wave propagation through a periodic system of inhomogeneous slabs

The classical problem of wave propagation through a periodic system of inhomogeneous films, plates or slabs is examined afresh. Former methods are generalized into a four-parameter theory, the best four parameters being chosen, thereby extending, consolidating and systematizing these previous methods and the resulting formulae for the reflection and transmission coefficients. Various deductions are made from these results concerning perfect trabsmission and eigenvalue conditions.     

Simplified monoharmonic approach to investigation of forced vibrations of thin wall multilayer inelastic elements with piezoactive layers under cyclic loading

A coupled dynamic problem of electromechanics for thin wall multilayer elements is formulated based on the Kirchhoff–Love hypotheses. In the case of harmonic loading, a simplified formulation is given using the monoharmonic approach and the concept of complex moduli to characterize the cyclic properties of the material. The problem of forced vibrations of three-layer beam, whose outer layers are made of a viscoelastic piezoactive material, and, the inner layer of a passive physically nonlinear material, is considered as an example to demonstrate the possibility of the technique elaborated. The possibility of damping the forced vibrations of a structure with the help of harmonic voltages applied to the external piezoactive layers is studied. Results obtained for the transient response of the beam using the complete model are compared with data found using the simplified model. Limitations on the simplified model application are specified.     

应力波在多层膜状材料中衰减特性的实验研究

脉冲载荷作用下,多层膜状材料衰减应力波的特性及其规律研究在工程结构设计中具有重要意义。本文采用分离式Hopkinson压杆(以下简称SHPB)装置,从实验角度对多层石英布和多层"石英布+聚酯铝膜"两种不同结构类型膜状材料衰减应力波特性进行研究。分析了透射应力波及应力冲量随多层膜状厚度变化的关系。通过对实验数据的拟合,给出了多层膜状衰减应力波能力和材料厚度以及材料结构的数学表征。研究结果表明,两种不同结构的多层膜状材料应力波衰减随厚度的变化关系都遵循指数衰减规律,而具有"多层石英布+聚酯铝膜"叠层结构的材料对应力波的衰减能力略强于单一多层石英布材料。本研究可作为工程应用多层膜状材料衰减应力波的实验依据。     

Stable Equilibrium of the Multilayer Lining of a Vertical Mine Opening in Elastoplastic Rock Mass

Based on the three-dimensional linearized theory of stability, the stability problem is solved for the multilayer lining of a vertical circular mine opening in rock mass under inelastic conditions. The effect of the geometric and physicomechanical parameters of the lining and rock on the critical contact pressures is evaluated     

Ultimate admissible dynamic strains in closed cylindrical vessels

A problem of determining the ultimate dynamic state of multilayer closed cylindrical vessels in emergency situations, such as explosive loading by high-intensity internal pressure, is considered. Elastic strains are assumed to be negligibly small as compared to plastic strains; therefore, the problem solution is constructed on the basis of the model of a rigid-plastic material with linear hardening. It is demonstrated that the solution of the dynamic deformation problem considered reduces to integration of a system of two ordinary equations for the functions of displacements of the inner surface of the vessel and of the massive non-deformable cover of the vessel.     

基于恒等映射多层极限学习机的高速列车踏面磨耗预测模型

针对高速列车车轮踏面磨耗单一模型无法对各种复杂工况下列车车轮踏面磨耗进行定量计算的问题, 提出一种基于恒等映射多层极限学习机的高速列车车轮踏面磨耗测量方法. 首先将恒等映射引入到多层极限学习机中, 提出一种基于恒等映射的多层极限学习机模型(identity multilayer extreme learning machine, I-ML-ELM), 采用机器学习公共数据集对该模型进行性能验证, 数值结果表明I-ML-ELM模型具有较好的准确性与泛化性; 然后基于车辆-轨道耦合动力学理论建立高速列车的车辆-轨道耦合动力学模型, 模拟列车运行的不同工况, 观测和分析高速列车的车轮踏面磨耗情况, 并通过I-ML-ELM预测模型对高速列车车轮踏面磨耗量进行学习及预测; 最后应用高速列车车轮踏面磨耗的实际测量值对I-ML-ELM预测模型进行进一步的验证, 结果表明: I-ML-ELM预测模型的各项性能参数指标在整体上优于以下五种网络: ELM, FLN, ML-ELM, ML-KELM和DLSFLN, 通过高速列车线路实测数据的进一步验证表明, 本文提出的基于I-ML-ELM的高速列车车轮踏面磨耗预测模型能较好地反映不同参数对高速列车车轮踏面磨耗值的影响规律.      

Reflection and transmission in anisotropic dissipative multilayers

A planarly-stratified multilayer is considered with material properties depending on the Cartesian coordinate normal to the layers. Upon the assumption that the time dependence be harmonic, the equations of motion and the constitutive equations (of a viscoelastic solid) are given the form of a first-order system of ordinary differential equations. The propagator of the whole multilayer is determined and hence the reflection and transmission matrices are obtained for different boundary conditions. Next a new algorithm, which avoids some drawbacks of other procedures, is outlined. The reflection and transmission matrices of the multilayer are determined by recursive relations via the matrices, of reflection and transmission, associated with the single layers.     

On the Finite Motions Generated by a Mode I Propagating Crack

The motion field surrounding a rapidly propagating crack, loaded symmetrically about the plane of the crack, is investigated. The problem is formulated within the framework of finite elastodynamics for thin slabs composed of compressible hyperelastic material. Writing the motion equations, the initial and the internal boundary conditions, with respect to a coordinate system that translates with the moving crack tip, we perform an asymptotic local analysis for a traction-free straight crack that suddenly grows at constant velocity. Moreover, the asymptotic Piola–Kirchhoff and Cauchy stress fields are computed, and we discuss the order of singularity of the dynamic stresses. This revised version was published online in August 2006 with corrections to the Cover Date.