Wave-in-deck experiments with focussed waves into a solid deck

This paper examines impact forces resulting from wave-in-deck processes from two separate series of experiments: one with a generic solid deck model, and the other with a combined jacket and deck model, both were conducted with and without an I-beam grillage in-place below the solid deck. A range of inundation levels from 2.1 to 7.1 cm at 1:80 scale is considered. The focus is on global impact forces, which are considered more relevant for integrity assessment of overall bottom-founded structures when survivability is in question and local slamming is not addressed. The objective is to characterise the resultant impact forces as well as to investigate whether there is any interaction between the flows through the jacket and hitting the deck. Focussed wave groups were generated to impinge on the models which were suspended from a carriage over a towing tank. The motion of the support carriage mimics uniform current in-line with the incident waves. Both undisturbed surface elevations as well as impact force time histories were measured. From the first series of tests, a large increase in peak forces as well as high frequency oscillations (force spikes) is observed with the grillage in-place. As soon as the jacket model is in-place for the second series of tests, albeit with a different mounting support arrangement, such a large difference vanishes, which could likely be due to the effect of frequency-dependent transfer functions. We provide experimental evidence of the presence of the jacket in modifying the wave impact on the deck through a significant reduction in the total horizontal impulse. The effects of current on the wave impact forces are also investigated. A simple analytical model based on a momentum argument is used to describe the scaling of horizontal peak force with currents and inundation levels. Finally, the importance of the short duration force spikes as well as vertical impact loads on a real structure at full-scale is discussed based on the same analytical model.     

Magnetically rotational reactor for absorbing benzene emissions by ionic liquids

A magnetically rotational reactor （MRR） has been developed and used in absorbing benzene emissions. The MRR has a permanent magnet core and uses magnetic ionic liquid [bmim]FeCl4 as absorbent. Benzene emissions were carried by N2 into the MRR and were absorbed by the magnetic ionic liquid. The rotation of the permanent magnet core provided impetus for the agitation of the magnetic ionic liquid, enhancing mass transfer and making benzene better dispersed in the absorbent. 0.68 g benzene emissions could be absorbed by a gram of [bmim]FeCl4, 0.27 and 0.40 g/g higher than that by [bmim]PF6 and [bmim]BF4, respectively. The absorption rate increased with increasing rotation rate of the permanent magnet.     

Computer simulation of nanotube contact

We develop procedures of numerical solution of nanostructure contact problems, which are based on the time discretization of nonlinear equations of molecular mechanics. The matrices and vectors of these equations are determined by using the Morse law of covalent atomic interaction, the fictitious rod elements to take account of angular variations between neighboring atomic covalent bonds, and noncovalent Van der Waals forces to take account of contact interactions between the graphene-like nanostructures. The procedures developed were included into the computational package PIONER, which was used to solve the problem of contact/self-contact of two nanotubes under conditions of dynamic equilibrium. We showed that the type of contact interaction significantly depends on the impact velocity of nanotubes. For a relatively small impact velocity, the nanotubes “adhere” to each other with a small deformation of their walls, due to the action of the Van der Waals attractive forces. As the impact velocity increases, the nanotubes fly apart because of the action of noncovalent repulsive forces. As the impact velocity continues to increase, there is a strong deformation of nanotubes with instantaneous “adhesion” of opposite ends and further separation of tubes. We show that taking account of the noncovalent forces of interaction between the opposite parts of the nanotube walls prevents their self-intersection; in this region of the nanotube contact, ovalization of their transverse cross-sections occurs.     

VIBRATING-UPLIFT ROCKING MOTION OF CAISSON BREAKWATERS UNDER VARIOUS BREAKING WAVE IMPACT FORCES

IntroductionCaissonbreakwatersarewidelyusedincoastalengineeringandhavetheadvantagesthatcanbeusedindeepwateranddevelopedtomulti_functiontypes.However,thewavesareeasilybrokenandmayproduceextremelyhighimpactforceonacaissonbreakwater.Inrecentdecades,therehavebeenmanyfailureexamplesofbreakwatersunderbreakingwaveimpactintheworld[1- 3].Thedurationofbreakingwaveimpactisveryshortandtheresponsesofcaissonbreakwatershaveobviouslydynamicbehaviorsunderbreakingwaveimpact,butthestaticmethodisstillusedinthedes…     

Interface Forces in Laterally Impacted Steel Tubes

In engineering mechanisms, solid bodies frequently come to a contact with a variety of geometric and kinematic situations. There has been a trend to express the interaction of the contacted bodies in the form of equivalent interface forces. The interface force represents the level of load transferred from one body to another. In a static or quasi-static contact problem, the interface forces could be sensibly evaluated by integrating the normal and the shear stresses over the common interfaces. In a dynamic contact, the interface stresses and subsequently the interface forces happen to be more complicated. They are, furthermore, affected by other parameters such as inertia forces, stress waves propagation, the material strain rate dependency and damping. This paper reports interface forces recorded in a series of experimental impact tests on axially pre-compressed steel tubes along with those from the numerical simulations of the tests. To monitor the so called impact loads, two small steel rings as load cells have been built in the striker. Based on the experimental and numerical results, the concept of equivalent interface forces in the impacted tubes has been verified. It has been highlighted that the interface forces (or the impact loads) may vary on the striker or the specimen themselves, depending on the measuring locations. Effects of axial compressions on the interface forces picked up by the striker load cells, impact loads imparted to the specimen supports and on the impact duration have been discussed. It has been reported that the initial compression applied to the tubes does not remain constant during and after the impact event. The amount of variations also depends on the initial level of the tube compression.     

含摩擦与碰撞平面多刚体系统动力学线性互补算法

基于接触力学理论和线性互补问题的算法, 给出了一种含接触、碰撞以及库伦干摩擦, 同时具有理想定常约束(铰链约束) 和非定常约束(驱动约束) 的平面多刚体系统动力学的建模与数值计算方法. 将系统中的每个物体视为刚体, 但考虑物体接触点的局部变形, 将物体间的法向接触力表示成嵌入量与嵌入速度的非线性函数,其切向摩擦力采用库伦干摩擦模型. 利用摩擦余量和接触点的切向加速度等概念, 给出了摩擦定律的互补关系式; 并利用事件驱动法, 将接触点的黏滞-滑移状态切换的判断及黏滞状态下摩擦力的计算问题转化成线性互补问题的求解. 利用第一类拉格朗日方程和鲍姆加藤约束稳定化方法建立了系统的动力学方程, 由此可降低约束的漂移, 并可求解该系统的运动、法向接触力和切向摩擦力, 还可以求解理想铰链约束力和驱动约束力. 最后以一个类似夯机的平面多刚体系统为例, 分析了其动力学特性, 并说明了相关算法的有效性.      

平面棱柱铰内考虑摩擦效应的接触分析

研究了多体系统中平面棱柱铰内的摩擦接触分析。在忽略铰内的碰撞效应的前提下,根据接触力系与约束反力的等效关系,以及各个角点接触力自身满足的互补关系,得到了接触力系关于摩擦力的线性互补方程。结合库仑摩擦定律,确定了铰内角点处的接触反力以及摩擦合力;另外,由接触力曲线可以得到铰内的接触形式,以及各个接触形式发生改变的时刻。通过ADAMS算例验证,这些接触形式转换的时刻就是铰内发生碰撞的时刻,为高效率地研究铰内碰撞提供了可能。     

The Newton drag law for bodies formed by intersecting surfaces

The Newtonian shock layer theory for smooth surfaces has been developed in [1–7]. However, if a body consists of two intersecting surfaces, on each of which there is a regular shock layer, these two layers will interact at the line of intersection. Considering that the shock layer is hypersonic, we can again apply the Newton scheme to the impact of the particles with the second surface. As a result, concentrated forces arise at the line of intersection. An indication of the possibility of the appearance of such forces is given in [2]. In the following we present a method of calculating the concentrated forces and indicate the real flow of which they are the analogy.     

不同宽厚比H形截面钢梁抗冲击性能

基于经实验校核的非线性有限元模型,对受横向冲击作用的H形钢梁进行了有限元分析。设计不同宽厚比组配的H形钢梁,分析H形钢梁跨中受横向冲击的动态响应和应力发展过程,并研究宽厚比对H形钢梁抗冲击性能的影响,重点讨论了腹板厚度、翼缘厚度对冲击力平台值和峰值以及耗能的影响。分析结果表明,两端铰接H形钢梁在跨中受冲击载荷作用下的变形模式主要为弯曲变形。相同冲击能量下,冲击力平台值主要受翼缘厚度的影响,冲击力峰值主要受腹板厚度的影响。翼缘厚度对钢梁抗冲击性能的影响要大于腹板厚度。本研究可为不同宽厚比H形钢梁的抗冲击设计提供依据和参考。     

压电智能结构冲击定位方法的实验研究

基于压电智能结构，构成冲击荷载监测系统，冲击样本的应力波信号经Hilbert变换变为连续的瞬态时序信号后，提取时间和峰值等特征信息训练BP神经网络，在线监测时由神经网络根据压电响应信号映射冲击荷载的位置。实验表明，该方法定位精度高，稳定性好，用于层合板结构低能量冲击的健康监控是可行的。     

Rail wheel removal and its implication on track life: A fracture mechanics approach

Wheel/rail forces generated by wheel defects is known to be one of the contributing factors to track failure. Current strategy of removing wheels from service is dependent on the magnitude of the impact forces generated by these defects. These impact forces are estimated with wheel impact monitor established along a section of the railway track. This is also known as wayside monitoring. The impact load levels recorded by these monitors are recorded and the wheels that generate impact loads above a stated level (e.g. 400 kN) will be removed from service for maintenance. The question one poses is “What if the impact force generated by a given wheel is just below this level and stays at this level for extended period of time?” Will this, e.g. 380 kN, impact force do as much damage to the track as a 400 kN impact force if it is allowed in service? What are the implications of allowing a wheel that generate a seemingly acceptable level of 250 kN impact load to remain in service for extended period of time? In an attempt to answer these questions, a series of investigations were carried out to investigate the effect of impact loads on the propagation of a Vertical Split Head (VSH) defect found in the head of a 50 kg/m rail.     

The role of external influences in the formation of waves on the surface of a flowing-down film of a viscous fluid

The example of two non-stationary forces is used to study the impact of external influences leading to the occurrence of additional ponderomotive forces on the wave regimes of the film freely flowing down a vertical surface. The first case describes a ferromagnetic fluid film affected by the magnetic field, and the second case touches upon a dielectric fluid film affected by the electric field. For the given forces, in the case of small flow rates, the problem is reduced to the solution of a model equation for the perturbation of the film thickness. The numerical solutions of the problem are obtained, and several characteristic scenarios of evolution of periodical perturbations are considered. It is shown that changes in the boundaries of the region of linear stability of the unperturbed flow with a flat free surface under the influence of ponderomotive forces have a great impact on the flow.     

On the Use of the Restitution Condition in Flexible Body Dynamics

The difference between the classical treatment offlexible body impact and the treatment of impact in flexiblemultibody dynamics is due to several fundamental reasons. Inthe classical impact theory, simple structures such as beamsand plates are used. Infinite dimensional models can bedeveloped for these simple structural elements to study theimpact dynamics and the wave propagation problem. Flexiblemultibody impact problems, on the other hand, involve bodieswith complex geometry that cannot be modeled using infinitenumber of degrees of freedom. Furthermore, the classicalimpact theory has been mainly concerned with the impactbetween a rigid mass that moves without constraints beforeit impacts a simple flexible structure. This is not amultibody simulation scenario in which the impact occursbetween kinematically constrained bodies that are subjectedto impulsive constraint forces in addition to the impactforces. These constraint forces can influence the motion ofthe two bodies immediately after impact, and as aconsequence, the simple classical theory scenario of impactdoes not apply. It is the objective of this paper to discussthe use of the restitution condition in flexible multibodyimpact problems and demonstrate that the use of thisapproach does not exclude the classical formulation.Nonetheless, the impulse momentum balance approach can serveas an effective and efficient procedure for solving theimpact problem in finite dimensional models that do not obeythe classical wave theory. Energy results of simplestructural elements are presented in order to demonstratethe consistency of using the impulse momentum balanceapproach in solving impact problems in finite dimensionalflexible body applications.     

A penny shaped crack in a transversely isotropic material under non-axisymmetric impact loads

A solution is given for problems involving non-axisymmetric dynamic impact loading of a penny shaped crack in a transversely isotropic medium. Laplace and Hankel transforms are used to reduce the equations of elasticity to integral equations, and solutions are obtained for the three modes of fracture. The stress intensity factors are determined for a penny shaped crack loaded by concentrated normal impact forces and concentrated radial shear impact forces. The integral equations are solved by numerical methods, and the results are plotted showing how the dynamic stress intensity factors are influenced by the asymmetric loading.     

Effect of hydrodynamic forces on capillary pressure and relative permeability

Because of the influence of hydrodynamic forces, the capillary pressure measured at static equilibrium may be different from that which pertains during flow. If such is the case, it may not be permissible to use steady-state relative permeabilities to predict unsteady-state flow. In this paper, the idea that the total flux of a given phase may be partitioned into several individual fluxes, together with a new pressure difference equation, is used to explore the possible impact that the hydrodynamic forces might have on capillary pressure and, as a consequence, relative permeability. This exploration reveals that, provided the pressure difference equation is implemented properly, capillarity has no impact on the relative permeability curves for the homogeneous, water-wet porous media considered. Moreover, it is demonstrated that, if the hydrodynamic effects are neglected, very little error is introduced into the analysis.     

作大范围回转运动柔性梁斜碰撞动力学研究

为正确估计由于碰撞引起的多柔体系统动力学特性的变化,针对作大范围回转运动的柔性梁与一固定斜面发生斜碰撞的情况,在考虑刚柔耦合效应的多柔体系统动力学建模理论的基础上,利用假设模态法建立起重力场作用下的柔性梁一致线性化动力法向碰撞过程中系统的动力行为。基于Ｈｅｒｔｚ接触理论和非线性阻尼项建立法向碰撞接触模型,基于线性切向接触刚度建立柔性梁切向碰撞接触模型,提出的数值算法保证了计算结果的合理性,给出的仿     

Numerical modeling of the impact pressure in a compressible liquid medium: application to the slap phase of the locomotion of a basilisk lizard

The forces acting on a solid body just at the time of impact on the surface of a medium with very low compressibility, such as water, can be quantified at acoustic time scales. This is necessary in wide range of applications varying from large-scale ship designs to the walking or running mechanisms of small creatures such as the basilisk lizard. In order to characterize such forces, a numerical model is developed in this study and is validated using analytical expressions of pressure as a function of the speed of sound-wave propagation in water. The computational results not only accurately match the analytical values but are also able to effectively capture the propagation of acoustic waves in water. The model is further applied to a case study wherein the impact impulse required by the basilisk lizard to assist in its walking on the water surface is evaluated. The numerical results are found to be in agreement with the closest available experimental data. The model and approach are thus proposed to evaluate impact forces for wide range of applications.     

Elastic impact on finite Timoshenko beam

In this paper the analytical solutions of the impact of a particle on Timoshenko beams with four kinds of different boundary conditions are obtained according to Navier's idea, which is further developed. The initial values of the impact forces are exactly determined by the momentum conservation law. The propagation of the longitudinal and transverse waves along the beam, especially, the effects of boundary conditions on the characteristics of the reflected waves, are investigated in detail. Some results are compared with those by MSC/NASTRAN.     

Dynamic Inter-Particle Force Inference in Granular Materials: Method and Application

Inter-particle force transmission in granular media plays an important role in the macroscopic static and dynamic behavior of these materials. This paper presents a method for inferring inter-particle forces in opaque granular materials during dynamic experiments. By linking experimental measurements of particle kinematics and volume-averaged strains to forces, the method provides a new tool for quantitatively studying force transmission and its relation to macroscopic behavior. We provide an experimental validation of the method, comparing inter-particle forces measured in a simple impact test on two-dimensional rubber grains to a finite-element simulation. We also provide an application of the method, using it to study inter-particle forces during impact of an intruder on a granular bed. We discuss the current challenges for applying the method to both model materials and real geologic materials.     

Continuous contact force models for impact analysis in multibody systems

One method for predicting the impact response of a multibody system is based on the assumption that the impacting bodies undergo local deformations and the contact forces are continuous. In a continuous analysis, the integration of the system equations of motion is carried out during the period of contact; therefore, a model for evaluating the contact forces is required. In this paper, two such contact force models are presented, both Hertzian in nature and based upon the direct-central impact of two solid particles.At low impact velocities, the energy dissipation during impact can be represented by material damping. A model is constructed based on the general trend of the Hertz contact law in conjuction with a hysteresis damping function. The unknown parameters are determined in terms of a given coefficient of restitution and the impact velocity. When local plasticity effects are the dominant factor accounting for the dissipation of energy at high impact velocities, a Hertzian contact force model with permanent indentation is constructed. Utilizing energy and momentum considerations, the unknown parameters in the model are again evaluated. The two particle models are generalized to an impact analysis between two bodies of a multibody system.