脆性材料动态破坏过程的数值模拟

根据连续介质力学基本原理和离散元法基本思想，针对脆性材料动态破坏过程问题，推导出基于圆盘单元的正交各向异性离散元二维计算模型，并利用此模型计算了各向异性薄板在冲击载荷下的应力波传播问题。通过将计算结果与LS－DYNA程序相比较，表明了本文提出的离散元模型的精确性。另外，对钢弹侵彻下滑凝土圆板破坏过程这个典型的发生连续介质向非连续介质转化的动力学问题进行了数值模拟和动画显示，并同侵彻实验中出现的几种破坏形式进行了分析和对比，从而证明了本计算方法是计算和模拟脆性材料出现从连续介质向非连续介质转变的动态破坏问题的有力工具。     

Numerical analysis of large elastic-plastic deformation of beams due to dynamic loading

Numerical calculations were performed for two examples of the response of elastic-plastic beams subjected to dynamic loads. These were a simply supported, axially restrained beam under suddenly applied uniform pressure, and an axially restrained, clamped beam with a central mass that is impacted by a projectile. Large elastic-plastic deflections were considered, and the method of finite differences was used. Two different constitutive equations were assumed: the elástic-perfectly plastic relation, and a special elastic-viscoplastic, strain hardening model. Analysis of the results included examining the interaction between the bending moment and the axial force, the variation of the axial force, bending moment and deflection with time, and the propagation velocities of the various phenomena during motion. Experiments were carried out in which a rifle projectile hit a central mass which had been fastened to a clamped beam. Comparison between the theoretical and experimental dynamic deflections shows good agreement for relatively short response times.     

爆炸荷载作用下岩石动态裂纹扩展的数值模拟

运用岩石破裂过程分析软件RFPA-dynamic,就爆炸荷载的加载速率、炮孔到自由边界的距离以及两炮孔中间空孔的大小对动态裂纹扩展方式的影响进行了研究。结果表明:随着加载速率的减小,炮孔周围的破碎区逐渐减少;裂纹开始萌生的位置逐渐由破碎区外边缘向炮孔孔壁转移;萌生的分支小裂纹逐渐减少,主裂纹扩展长度逐渐增大。由于自由边界的影响,炮孔向下扩展的裂纹逐渐弯向水平方向,且炮孔到自由边界的距离越小,这种趋势越明显。由于空孔的导向作用,使靠近空孔的裂纹逐渐弯曲向空孔处扩展,同时在空孔孔壁两端产生一条向炮孔扩展的裂纹;空孔半径大小对裂纹的导向作用,并无明显的影响;材料的非均匀性,对裂纹的扩展方式有显著的影响。     

岩石试样弹塑性破裂过程的数值模拟分析

依据岩土类材料的非匀质性特性,并采用对材料参数进行随机赋值的方法研制了弹塑性破裂过程数值分析程序,并用实例证明了程序的可靠性。应用该程序对岩石试样弹塑性破裂过程进行数值模拟研究．分析表明：数值模拟结果与试验研究结果是吻合的。从而为工程岩体断裂分析提出了一种可能的方法与途径。     

Grip pressure distribution under static and dynamic loading

The dynamic response of a vibrating handarm system is strongly related to the grip force. While the relationship between total grip force and vibration characteristics of the hand-arm system has been extensively studied, no attempts have been made to investigate the distribution of grip pressure at the hand-handle interface. The local grip-pressure distribution may be more closely related to the finger blood flow, fatigue and loss of productivity than total grip force. In the present study, distribution of static and dynamic forces at a hand-handle interface is investigated using a grid of pressure sensors mounted on the handle. The pressure distribution is acquired for different values of static and dynamic grip forces in the range of 25–150 N. The dynamic measurements were conducted at various discrete frequencies in the 20–1000 Hz range with peak acceleration levels of 0.5 g, 1.0 g, 2.0 g and 3.0 g. The grip-pressure distribution under static loads revealed a concentration of high pressures near the tips of the index and middle fingers, and the base of the thumb. This concentration of high pressures shifted towards the middle of the fingers under dynamic loads, irrespective of grip force, excitation frequency and acceleration levels. These local pressure peaks may be related to impairment of blood flow to finger tips and the possible causation of vibration white finger. Paper was presented at the 1992 SEM Spring Conference on Experimental Mechanics held in Las Vegas, NV on June 8–11.     

Compressive failure strengths and modes of woven S2-glass reinforced polyester due to quasi-static and dynamic loading

The failure strengths and modes of the woven S2-glass reinforced polyester thick laminate are characterized through a comprehensive set of experiments. Compression strength tests in different directions were performed, and optical and scanning electron microscopy studies of the fractured specimen surfaces were conducted. The failure modes are complex involving a combination of failure mechanisms. The failure stresses and strains in the ply lay-up direction were higher than those in the plane of the lamina. The material's dynamic response was determined using both the split Hopkinson pressure bar and the direct disk impact techniques. The material was found to be significantly strain rate sensitive.     

双向荷载条件下八边形蜂窝结构因胞壁缺失所致应力集中分析

因胞壁缺失引起的应力集中问题是蜂窝结构增材制造过程中亟待解决的问题.通过将胞壁缺失所致的缺陷等效成椭圆,基于复变函数方法得到了预测拉伸应力的解析公式和预测弯矩应力的分析方法,进而得到八边形蜂窝结构在双向荷载条件下缺失单行和多行胞壁产生的应力组合值.与有限元结果的对比分析验证了所提理论解析公式的有效性;同时得出距离缺陷最近胞壁上的拉伸应力理论结果与有限元结果吻合良好,弯矩应力的有限元结果与胞壁上的应力梯度呈明显的线性关系.与规则六边形蜂窝在多行缺陷条件下的应力集中问题进行对比,得出八边形蜂窝结构在荷载系数λ=0和λ=1时应力集中程度小于规则六边形蜂窝结构,从而为蜂窝结构增材制造设计提供理论指导.     

聚氨酯/钢夹芯结构爆炸载荷下动力学响应的数值模拟

聚氨酯泡沫具有良好的缓冲和吸能性能,以硬质聚氨酯泡沫作为芯体,钢作为面板的夹芯结构在工程中用途广泛。为了研究聚氨酯/钢夹芯结构在爆炸载荷作用下的抗爆性能,利用Ansys/Autodyn非线性有限元程序对聚氨酯/钢夹芯结构在爆炸载荷作用下的动力学响应进行数值模拟分析,并与相同面密度的钢板进行对比。结果表明,加入聚氨酯芯体后,结构的吸收能量为相同面密度钢板的1.49倍,整体结构的抗爆性能得到很大的提高。     

受拉钢筋混凝土构件破坏过程的数值模拟

采用三维材料破坏过程分析MFPA3D系统,对钢筋混凝土构件轴心受拉条件下的受力、变形与内部裂纹萌生、扩展及最终破坏全过程进行了数值试验研究。数值模型中引入统计分布函数反映了混凝土的非均匀性影响,并采用具有残余强度的弹性损伤本构模型及其破坏单元材料性质退化方法,利用位移加载方式对钢筋混凝土构件实施拉伸加载。通过对钢筋、素混凝土方形体以及钢筋混凝土方形柱体构件在拉伸作用下破坏过程的数值试验,分析了钢筋与混凝土两种材料之间的相互作用、约束机理和破坏机理。数值试验成果对于深入了解钢筋和混凝土的联合受力规律和钢筋在开裂前后对整体钢筋混凝土结构的作用机制有参考价值。     

Temperature evolution associated with phase transition from quasi static to dynamic loading

Thermo-mechanical coupling is an intrinsic property of first order martensitic transformation. In this paper, we study the temperature evolution during phase transition at a wider strain rates from quasi static to impact loading to reveal the thermodynamic nature of the strain rate effect of phase transition materials. Based on the laws of thermodynamics and the principle of maximum dissipated energy, a thermal-mechanically coupled model was proposed. The model shows that, in the quasi static case, the temperature profile grades around the moving phase boundary, while for the dynamic case, thermal response of the specimen can be reached homogeneously due to random nucleation. The predicted results of the model are in good agreement with the experimental results, suggesting that the interaction between the self-heating effect and the temperature dependence of phase transition behavior plays a leading role in the process of the transformation deformation mechanism associated with the loading rate.

     

On the static and dynamic equilibrium of concentrated loads in linear acoustics and linear elasticity

The displacement field in an unbounded linear elastic fluid subjected to a time-dependent point force is obtained by using integral transform techniques. Differentiation of the displacement field yields the pressure field. It is shown that the pressure on the surface of a spherical ball B _r of radius r centered at the point where the force is applied is statically equivalent in the limit as r0 to only one-third of the force. The remaining two-thirds are carried by the inertia terms. It is also shown, by an independent reasoning, that a point force cannot be carried in static equilibrium by a linear elastic fluid.The displacement field corresponding to an unbounded isotropic linear-elastic solid subjected to a time-dependent point force (the Stokes solution) is also obtained by using integral transform techniques. As is well-known, the tractions of the Stokes solution on the surface of a spherical ball B _r are statically equivalent in the limit as r0 to the force itself; consequently, the inertia terms do not contribute to the dynamic equilibrium of B _r. The contrast between the response of a fluid and that of an isotropic solid under the action of a point force is discussed.     

Numerical simulation of the galvanizing process during GA to GI transition

This paper presents the application of a three‐dimensional finite element solution algorithm for the prediction of the velocity, temperature and species concentration fields in an industrial continuous galvanizing bath. Simulations were carried out using a parallel CFD software developed at IMI‐NRC. The turbulent flow, heat and mass transfer has been solved using a high Reynolds number k–ε model. Simulations were carried out for the case when the density of the molten metal depends only on the temperature and also for the case when both temperature and Al concentration affect the density. When considering the buoyancy effect of the Al concentration, differences are especially apparent during the melting of ingots with high Al content. Otherwise, thermal effects are dominant. The continuous monitoring of the temperature and the Al and Fe content in an industrial bath was used to validate the flow, temperature and compositional variations. A period of three hours, corresponding to three different ingot additions, was simulated successfully, resulting in a good agreement of the temperature and compositional variations. Copyright © 2006 Crown in the right of Canada. Published by John Wiley & Sons, Ltd.     

Stability and limit states of elastoplastic spherical shells under static and dynamic loading

The problem of elastoplastic deformation, buckling, and postcritical behavior of spherical shells is solved using a finite element method and a cross-type explicit scheme of time integration. Stability problems for hemispherical shells under external pressure and compression between rigid plates are considered. The influence of holes and boundary conditions on shell deformation is investigated. It is shown that the calculation results are in good agreement with experimental data.     

Numerical simulation of formation of a concentrated brine lens subject to magma chamber degassing

The mathematical model of flow of a binary salt-water mixture through a porous medium in a wide range of pressure and temperature is developed taking different multiphase thermodynamic equilibria of the mixture into account. Formation of concentrated brine lenses above a degassing magma chamber is investigated within the framework of the model. The lenses are assumed to be coupled with generation of ore deposits. It is shown that the lens formation is caused by phase transitions of two different types undergoing at different depths in the magmatic fluid rising towards the surface. In the shallow zones salt precipitation on the skeleton of the porous medium in the form of a solid phase leads to clogging of pore space and reduction of the permeability. As a result, the magmatic fluid flow towards the surface is blocked and this facilitates the concentrated brine accumulation in a local zone.     

Mechanics of energy transfer and failure of ductile microscale beams subjected to dynamic loading

The mechanical response of microelectromechanical systems (MEMS) under impulse loading conditions has not been thoroughly studied to date, partially because of the lack of means to provide such extreme loading rates to miniature devices. However, the increasing use of MEMS-based sensors and actuators in adverse environments, which include extreme strain rate loading, has motivated the investigation of the response of MEMS components under these conditions. In this work, basic and mostly commonly employed Au MEMS components were subjected to impulse loads of 40 ns in duration, which were generated by a high power pulsed laser in order to achieve acceleration levels on the order of 10⁹g. This allowed for the microdevice mechanical/structural response to be investigated at time scales that were of the order of wave transit times in the substrate and the devices. Basic microscale structures, such as cantilevers and fixed-fixed beams of uniform cross-section, were employed to facilitate comparisons with companion finite element simulations in order to gain insight into the mechanisms responsible for impulsive deformation at the microscale. The simulations investigated the effect of loading rate, boundary conditions, beam length, material constitutive response, and damping on the final deformed shapes of the beams. It was found that contact and momentum transfer mechanisms were responsible for the large permanent beam deflections which were measured postmortem. Additionally, the effects of both damping and material property rate dependence were found to be dominant in determining the final deformed shape of the beams. In fact, our observations suggest that the contributions of material rate dependence and damping are not simply additive, but rather involve a coupling between them that affects the final structure response.     

入水后爆炸问题的数值模拟技术

应用各种数值计算方法及计算处理技术,编制程序实现了对弹体入水后爆炸问题全过程的数值模拟.其中弹体与水,爆轰产物气体与弹壳之间的相互作用通过流固耦合技术来描述;水面与空气、爆轰产物与水、空气之间的相互作用采用VOF方法(Volume-of-Fluid)来描述;采用了刚体-柔体转换、单元失效删除等计算技术以更高效、更好地模...     

Static and dynamic responses of suspended arch bridges due to failure of cables

Archive of Applied Mechanics - A mathematical model is proposed to investigate the behavior of a suspended arch bridge, subjected to sudden failure of cables. The main aim of this study is to...     

Numerical solution of the dynamic problem of elasticity for bodies with concentrated masses

On the basis of the virtual-displacement and the DAlembert principles, a new initial-boundary-value problem is formulated for the system of dynamic equilibrium equations of two bodies contacting with friction over a constant area. Their masses are assumed to be distributed with certain densities over the contact surfaces. An equivalent generalized problem is formulated. Classes of discontinuous functions of the finite-element method are used to construct computational schemes of high order of accuracy.__________Translated from Prikladnaya Mekhanika, Vol. 40, No. 12, pp. 65–75, December 2004.