极端工况双矩形腔静压推力轴承动态特性

静压推力轴承动态特性受润滑油黏度、油膜厚度和油腔面积等因素影响, 极端工况运行过程中经常承受阶跃载荷或正弦载荷作用, 突加载荷将导致静压推力轴承动态特性改变, 表现为轴承的抗冲击能力和恢复平衡所需时间的变化. 为获得高速重载微间隙极端工况条件下双矩形腔静压推力轴承动态特性, 分别在不同油膜厚度、不同润滑油黏度以及不同油腔尺寸条件下对双矩形腔静压推力轴承的动态性能进行理论分析, 探讨了阶跃载荷作用下润滑油黏度、油膜厚度和油腔面积对轴承动态性能的影响, 揭示了油膜动态变化规律, 探究了正弦载荷作用下双矩形腔静压推力轴承的稳定性. 结果表明: 润滑油黏度、油膜厚度和油腔尺寸变化对其动态性能有很大的影响. 润滑油黏度越大、油膜厚度越小、油腔面积越大突加载荷作用下润滑油膜抵抗冲击的能力越强, 旋转工作台受到突加外力作用下恢复至平衡状态所用时间越短. 双矩形腔静压推力轴承油膜具有较大的阻尼系数, 轴承具有极强的抵抗正弦加载作用的能力      

混凝土重力坝爆炸荷载数值分析及抗爆性能研究

利用LS-DYNA非线性有限元程序，基于Eulerian和Lagrangian耦合的方法，研究了RHT本构模型模拟的混凝土板在爆炸荷载下的动力反应，并且将数值结果与现场实验结果进行比较，由此说明了RHT本构模型模拟爆炸荷载下混凝土动力反应的有效性。研究了2 t TNT炸药在距离坝体上游面10 m不同起爆深度的情况下，有泡沫混凝土保护层和无泡沫混凝土保护层的坝体动力响应及其损伤状况。计算结果表明，无泡沫混凝土保护层时，坝体上游面主要损伤区域位置总是随起爆深度的增加向坝体底部移动；当上游表面有泡沫混凝土保护层时，坝体上游表面的损伤明显变小，下游面的损伤较无保护层情况也明显减小。表明泡沫混凝土能够有效减小混凝土大坝在爆炸荷载下的损伤，在提高混凝土大坝的抗爆性能方面起到很好的保护作用。     

Dynamic and buckling analysis of polymer hybrid composite beam with variable thickness

This work deals with a study of the dynamic and buckling analysis of polymer hybrid composite(PHC) beam. The beam has variable thickness and is reinforced by carbon nanotubes(CNTs) and nanoclay(NC) simultaneously. The governing equations are derived based on the first shear deformation theory(FSDT). A three-phase HalpinTsai approach is used to predict the mechanical properties of the PHC. We focus our attention on the effect of the simultaneous addition of NC and CNT on the vibration and buckling analysis of the PHC beam with variable thickness. Also a comparison study is done on the sensation of three impressive parameters including CNT, NC weight fractions, and the shape factor of fillers on the mechanical properties of PHC beams,as well as fundamental frequencies of free vibrations and critical buckling load. The results show that the increase of shape factor value, NC, and CNT weight fractions leads to considerable reinforcement in mechanical properties as well as increase of the dimensionless fundamental frequency and buckling load. The variation of CNT weight fraction on elastic modulus is more sensitive rather than shear modulus but the effect of NC weight fraction on elastic and shear moduli is fairly the same. The shape factor values more than the medium level do not affect the mechanical properties.     

无轴承旋翼柔性梁载荷特性研究

应用Hamilton原理建立了双路传力的无轴承旋翼运动方程。采用均匀入流模型,基于直升机飞行平衡条件,建立了无轴承旋翼柔性梁载荷的计算模型,并通过算例验证了模型的精度。利用该模型,研究了全机重心位置、机身气动阻力以及平尾安装角对柔性梁载荷特性的影响,给出了各因素对柔性梁载荷的影响趋势,得出了降低柔性梁载荷的方法。数值结果表明:2cm左右重心位置的变化能够引起9%~11%的柔性梁载荷变化量,15%气动阻力的增加会导致约9%的柔性梁载荷的增大;2°平尾安装角的变化引起约10%柔性梁载荷的变化量,3°平尾安装角的变化引起约26%柔性梁载荷的变化。     

基于扩展多面体的离散单元法及其作用于圆桩的冰载荷计算

对于具有复杂几何形态的多面体单元,线性接触模型不能准确地计算不同接触模式下的作用力,且接触变形和作用力方向也不易判断.基于闵可夫斯基和(Minkowski sum)方法的扩展多面体单元能够准确描述非规则颗粒单元的几何形态,并可精确计算单元间的接触碰撞作用.该方法具有接触判断简单、计算效率高的特点.它将基本多面体和扩展球体相叠加以形成具有光滑棱边和角点的扩展多面体单元.考虑扩展多面体单元相互作用过程中角点、棱边和平面之间的不同接触模式,发展了相应的非线性黏弹性接触模型. 该接触模型将不同接触模型下的法向刚度统一表述为单元接触中接触点处等效曲率半径的函数;黏滞力和切向弹性力接触模型则借鉴球体单元非线性接触模型的处理方法. 为检验扩展多面体的可靠性,对碎冰区冰块对圆桩结构的冰载荷进行了离散元分析. 采用沃洛诺伊(Voronoi)切割算法获得了碎冰的初始随机分布状态,并考虑了海冰在运动过程中的海水浮力和拖曳力.计算表明该扩展多面体单元可描述海冰在海流拖曳下的运动过程以及圆桩结构的动冰力特性.在此基础上进一步分析了冰速和冰块尺寸对圆桩冰力的影响,并确定了冰力在圆桩上的分布规律. 最后,讨论了目前扩展多面体单元在计算冰载荷方面的局限性和改进方法.     

Deformation of thin straight pipes under concentrated forces or prescribed edge displacements

The purpose of this paper is to present an efficient analytic method for obtaining the deformation of thin straight pipes, subjected to prescribed edge displacements or concentrated loads.The approach uses the mixed formulation where unknown functions are combined with trigonometric terms. A variational procedure is used to obtain the system of ordinary differential equations. For the applied load a Fourier approach is used to represent the load as an analytical function. For the prescribed displacement, three solutions for the ovalization are evaluated and a method based on energy contribution of each term is used to obtain their superposition.In contrast to finite element method the proposed method is efficient and can be applied to other boundary condition problems leading to continuous displacement and stress fields with a low number of unknowns. Comparisons with experimental and finite element procedures show good agreement that enhances the merits of the analytical solutions proposed.The value of this method is based on solving the differential equations rather than using commercial codes. So far, the solution of prescribed edge displacements has been limited to one term. This paper discusses how to add further terms using the mixed formulation, thus, presenting a novel procedure.     

A new Monte Carlo model for predicting the mechanical properties of fiber yarns

Understanding the complicated failure mechanisms of hierarchical composites such as fiber yarns is essential for advanced materials design. In this study, we developed a new Monte Carlo model for predicting the mechanical properties of fiber yarns that includes statistical variation in fiber strength. Furthermore, a statistical shear load transfer law based on the shear lag analysis was derived and implemented to simulate the interactions between adjacent fibers and provide a more accurate tensile stress distribution along the overlap distance. Simulations on two types of yarns, made from different raw materials and based on distinct processing approaches, predict yarn strength values that compare favorably with experimental measurements. Furthermore, the model identified very distinct dominant failure mechanisms for the two materials, providing important insights into design features that can improve yarn strength.     

Response of moving load due to irregularity in slightly compressible,finitely deformed elastic media

The present paper has been framed to study the stresses produced on the rough surface of a slightly compressible, finitely deformed half space due to a normal moving load. The surface of the medium is irregular with parabolic type of irregularity. The perturbation method is applied to find the displacement field. The normal and shear stresses have been obtained in closed form and discussed numerically by means of figures. It has been observed that the shear stress developed at different depths below the surface depends on the irregularity depth, frictional coefficient and irregularity factor of the rough surface of the medium. Also, surface plots have been drawn to analyze the combined variation of non-dimensional stresses and irregularity factor against depth.     

Experimental investigation of hydrodynamic loads and pressure distribution during a pyramid water entry

In the maritime environment slamming is a phenomenon known as short duration impact of water on a floating or sailing structure. Slamming loads are local and could induce very high local stresses. This paper reports a series of impact test results and investigate the slamming loads and pressures acting on a square based pyramid. In this study the slamming tests have been conducted at constant velocity impact with a hydraulic high speed shock machine. This specific experimental equipment avoids the deceleration of the structure observed usually during water entry with drop tests. Three velocities of the rigid pyramid have been used (10, 13 and 15 m s⁻¹). Time-histories of local pressures, accelerations and slamming loads were successfully measured. The relationship between the pressure magnitude and the impact velocity is obtained and the spatial distribution of pressures on pyramid sides is characterized. The impact velocity was found to have a negligible influence in predicting the maximum pressure coefficient.     

RV减速器主支撑角接触球轴承混合润滑分析

针对RV减速器角接触球轴承承受预紧力、轴向力和径向力等联合外载荷作用的工况,分析得出了内、外圈滚道接触界面的接触区几何参数和接触载荷.在此基础上,综合考虑了角接触球轴承的接触区宏观几何、接触载荷、真实表面粗糙度、瞬态效应等因素,建立了角接触球轴承混合润滑数学模型,分析了在不同工况下角接触轴承的润滑状况及表面以下应力分布.结果表明:随着载荷的不断增加,钢球与内圈沟道之间的油膜厚度会不断减少,导致干接触面积迅速扩大,接触点表面以下最大应力增大;转速的增加会使油膜变厚,干接触面积缩小.该结果对角接触球轴承的实际工程应用具有重要借鉴意义.