分区界面元-有限元-无限元混合模型

利用界面元良好的相容性,引入过渡界面元的概念．实现了界面元与有限元二种数值计算方法的结合,并提出了一种界面元-有限元-无限元混合模型。这种混合模型既可以发挥界面元计算精度高、适用于不连续变形等优点．又能够充分利用有限元的计算效率和无限元方便处理无限域介质的特点,较为和谐地解决了计算精度和计算效率的矛盾。数值算例表明,本文所建立的混合模型的有效性,揭示此类混合模型具有广阔的工程应用前景。     

缝合复合材料层间开裂的试验和数值研究

缝合复合材料层合板中贯穿厚度方向的缝线,能有效增强层合板的抗分层能力。本文对一种碳纤/环氧缝合复合材料层板进行了短梁三点弯试验,测得了压头的载荷-位移曲线,并观察了层间裂纹的扩展,证实了缝线对层间裂纹的阻滞作用。建立了三维有限元模型模拟了上述试验,模型中相邻的铺层之间布置了一层初始无厚度的界面单元,界面单元的失效自然模拟层间开裂,而缝线简化为面积等效的梁单元,数值结果与试验观测吻合。     

基于非线性弹簧界面的复合材料分层损伤预测

基于功能等效的原理,用自定义本构的非线性弹簧模拟复合材料层间界面,弹簧的刚度随着界面层的牵引力-位移曲线变化,可以表征界面材料性能的线性和非线性退化过程。对复合材料单一型分层和混合型分层损伤演化过程进行了模拟分析,研究了单元尺寸、界面强度等参数对模拟结果的影响机理。结果表明:非线性弹簧界面单元能够准确地模拟分层损伤的起始和扩展过程,使用非线性弹簧模拟界面可以减小计算模型规模,与内聚力单元相比计算效率提高约10倍;裂纹每扩展一个单元长度所需要的能量与单元尺寸成正比,单元尺寸越大,裂纹扩展所需要的能量越大;界面强度越低,初始裂纹尖端张开过程越平缓,模型的收敛性越好,可以通过降低弹簧单元界面强度来减小模型计算规模。     

线弹性广义界面层模型的研究

为了用界面层方法分析一般结构中的裂纹扩展问题,提出了广义界面层的概念.基于能量释放率G相等的原则,对界面层的本构关系进行了线弹性等效.在此基础上,重点推导了含Ⅰ型裂纹双悬梁(DCB)的载荷-位移函数,并借助Abaqus软件对双悬梁的分层问题进行数值模拟.理论推导和数值模拟的结果证明了广义界面层的线弹性等效是可行的.     

含界面脱粘压电复合材料层合板的非线性动力稳定性分析

基于Reddy提出的板高阶剪切变形简化理论,研究了含界面脱粘损伤压电复合材料层合板非线性动力稳定性问题.首先,建立了分层模型,推导了考虑几何非线性、阻尼效应、纵向惯性力和力-电耦合效应的Mathieu方程,并且给出了该方程解的解析表达式.其次,通过典型算例讨论了界面脱粘损伤以及反馈控制力对该层合板动力不稳定区域、纵向、横向共振频率和最大"牵引"深度的影响.由典型算例讨论可知:随着层合板界面脱粘损伤的扩大,其动力稳定性能逐渐减弱,其中在损伤较小时,反馈控制力对智能结构几乎没有影响;而在损伤比较大的情况下,反馈控制力将能有效地减少动力不稳定区域重合面积.     

考虑界面损伤层合梁板的层间应力分析

基于精确应力分析的广义六自由度板理论，应用变分原理和损伤力学中 的应变等效原理，考虑复合材料铺设层内和层间界面处的损伤效应，建立了具两种损伤模式 的复合材料层合板的三维非线性平衡微分方程，且运用有限差分法对考虑损伤简支层合梁板 的层间应力进行了求解.     

复合材料加筋板铺层优化设计的等效弯曲刚度法

借助于等效弯曲刚度法和遗传算法,建立了一种复合材料加筋板铺层顺序优化设计算法.等效弯曲刚度法基于层合板的弯曲刚度与其失稳载荷一一对应的关系,将任意铺层顺序的层合板等效成一个只有8层的对称铺层的辅助层合板,通过优化辅助层合板,得到层合板的最优弯曲刚度参数,最后以获得的最优弯曲刚度参数为约束应用遗传算法进行铺层顺序优化,获...     

板材冲压成形有限元数值模拟界面摩擦约束处理方法

基于弹塑性大变形板材冲压成形增量有限元法,采用线性弹簧单元提出一种空间三维板材冲压成形等效界面摩擦力的处理方法.这种方法的优点是可以反映界面摩擦力的同步、被动产生效果,同时它还可以保证约束后刚度矩阵的对称性.将提出的摩擦约束处理算法引入自主开发的板材成形模拟软件FASTAMP,计算了汽车油底壳的成形过程,计算结果的厚度分布与实际冲压件吻合比较好,验证了等效界面摩擦力约束处理方法的有效性.     

三角形截面梁柱构件的三维等效弹簧计算模型

传统有限元在分析梁柱构件时一般采用常应变单元和双线性单元,但此类单元在梁柱构件受弯分析中计算精度不是很高.本文根据梁柱构件的力学性能,在三维连续介质体受到轴向变形和弯曲变形的状态下,利用其轴向变形和弯曲挠度相同,得到具有相同宽度和高度且刚度等效的超静定桁架力学模型.然后,通过桁架杆的截面参数求得弹簧的刚度系数,从而得等效弹簧元模型.本文提出的等效弹簧模型计算方法简单,便于扩展到更为复杂的构件分析中.     

纤维增强混凝土材料的界面剪应力分布研究

针对纤维与混凝土界面的破坏过程,提出了几种简化的粘结-滑移本构模型,以双线性局部粘结-滑移本构模型为基础,在受力平衡和变形协调的基本原理基础上,推导了纤维脱粘过程中界面剪应力的解析解.采用弹簧粘结单元,通过数值方法模拟了纤维与混凝土之间的粘结-滑移过程,给出了纤维与混凝土界面脱粘过程中界面剪应力的分布、变化情况.对解析解、有限元计算结果和试验结果之间的差异进行了对比分析,验证了简化模型的合理性和有效性.     

Classical limits for the coefficient of variation for the normal distribution

The exact classical limits for the coefficient of variation c for the normal distribution are derived. The hand-calculating approximated classical limits for c having high accuracy are given to meet practical engineering needs. Using Odeh and Owen's computational method and Brent's algorithm, the tables for the r-upper exact classical limits of coefficient of variation for normal distribution are calculated for the different confidence coefficient , the sample size n=1 (1)30,40,60,120, the sample coefficient of variation =0.01(0.01)0.20. It is shown that if n8, 0.20, then the -upper exact classical limits c_u for c are slightly higher than the exact fiducial limits c_u,F for c. if n>8, 0.20, then c_u–c_u,F<5×10^–6.     

Examination of the computational model for the layer-removal method for residual-stress measurement

An analytical solution is presented to examine the accuracy of the ‘layer-removal’ method for measuring localized residual stresses. In this approach, strips, which may have been cut from a pipe or a plate, have strain-gage rosettes placed on one face and layers removed from the other face. The strain measurements are used to deduce the residual stress in the layers removed. The stress measured is that along the axis of the strip. It may vary rapidly with axial distance, as for example when the strip is taken from a welded part. The present analysis shows that the actual stress distribution may be quite different from that predicted by the computational model normally used in the layer-removal method. It shows that the difference increases as the ratio of the heighth of the strip from which a layer is removed to the half dimensiona of the localized residual-stress zone increases. It is recommended that the layer-removal method can be used for measuring residual stresses for cases in which the ratio ofh/a is less than or equal to unity.     

The difference methods for the solution of singular-perturbation for the elliptic-parabolic differential equation

In this paper it is discussed the difference method for the solution of singular perturbation problems for the elliptic equations, involving small parameter in the higher derivatives. As ε= 0 the original equations are degenerated into the parabolic equations.Authors constructed special difference schemes by means of the boundary layer properties of the solutions of these problems as well as investigated the convergence of this scheme and asymptotic behaviour of the solutions. Finally, a numerical example is given.     

A procedure for the design of nozzles used for the production of turbulent liquid jets

Where turbulent liquid jets are used for cutting and mining purposes the pressure generated by impact must be maximized. Initial jet behaviour has an important influence on subsequent jet impact pressures at medium range. Nozzle wall boundary layer history has a strong influence on the initial jet, and certain boundary layer features can be linked to poor jet performance. The procedure outlined in this paper was developed to eliminate new nozzle designs or changes in operating conditions on the grounds of badly behaved nozzle boundary flow. The design procedure consists of a potential flow analysis and a boundary layer analysis coupled to empirical correlations for boundary layers in accelerated flows. The procedure is exemplified by application to the design of a nozzle to be used for the specific purpose of mining china clay.     

On the conditions for the existence of a plane of symmetry for anisotropic elastic material

We consider the problem of determining necessary and sufficient conditions for the existence of symmetry planes of an anisotropic elastic material. These conditions are given in several equivalent forms, and are used to determine special coordinate systems where the number of non-zero components in the elasticity tensor is minimized. By the method presented here it is also shown that an elastic solid has at least six coordinate systems with respect to which there are only 18 non-zero elastic constants and cannot possess more then ten traditional and distinct symmetrics by planes of symmetry.     

Morawetz's method for the decay of the solution of the exterior initial-boundary value problem for the linearized equation of dynamic elasticity

In this paper, the behavior of the solution of the time-dependent linearized equation of dynamic elasticity is examined.For the homogeneous problem, it is proved that in the exterior of a star-shaped body on the surface of which the displacement field is zero, the solution decays at the rate t ^-1 as the time t tends to infinity.For the non-homogeneous problem with a harmonic forcing term, it is proved that for large times, the elastic material in the exterior of the body, tends to a harmonic motion, with the period of the external force.The convergence to the steady harmonic state solution is at the rate t ^-1/2 as t tends to infinity, and is uniform on bounded sets.     

Allowance for the unsteadiness of the flow in determining the critical cavitation number for marine propellers

A method of estimating the critical cavitation number for marine propeller blades is proposed. This method is based on the reduction of the three-dimensional unsteady problem to the three-dimensional steady problem and a series of two-dimensional unsteady problems.Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No.1, pp. 78–85, January–February, 1993.The authors are grateful to S. V. Kaprantsev for assisting with the experiments.     

A 3D DEM-LBM approach for the assessment of the quick condition for sands

We present a 3D numerical model to assess the quick condition (the onset of the boiling phenomenon) in a saturated polydisperse granular material. We use the Discrete Element Method (DEM) to study the evolution of the vertical intergranular stress in a granular sample subjected to an increasing hydraulic gradient. The hydrodynamic forces on the grains of the sample are computed using the Lattice Boltzmann Method (LBM). The principal assumption used is that grains remain at rest until the boiling onset. We show that the obtained critical hydraulic gradient is close to that defined in classical soil mechanics. To cite this article: M. Mansouri et al., C. R. Mecanique 337 (2009).