Propagation of perturbances generated in classic track, and track with Y-type sleepers

Railway track with classic and Y-shaped sleepers or slab track is composed of two rails that are assumed to be infinitely long and joined with sleepers by viscoelastic pads. Numerous assumptions are used in railway-track modelling, leading to different simplifications. The two-dimensional periodic model of track consists of two parallel infinite Timoshenko beams (rails) coupled with equally spaced sleepers on a viscoelastic foundation. Nowadays the interest of engineers is focused on slab track and track with Y-shaped sleepers. The fundamental qualitative difference between the track with classic and Y-shaped sleepers is related to local longitudinal symmetric or antisymmetric features of the railway track. The sleeper spacing influences the periodicity of the foundation elasticity coefficient, mass density (rotational inertia) and the effective shear rigidity. Track with classic concrete sleepers is affected much more by rotational inertia and shear deflections than track with Y-shaped sleepers. The increase of the elastic-wave velocity in track with Y-shaped sleepers and more uniform load distribution will be proved by analysis and simulation. The analytical and numerical analyses allows us to evaluate the track properties in a range of moderate and high train speeds. However, the correct approach is not simple, since the structure of the track interacts with the wheels, wheelsets and vehicles, which constitute complex inertial loads. We note that the growth of amplitude in selected velocity ranges depends strongly on the track type.     

Strength-optimal running surface contour for track links

The various stresses resulting from weight, driving and tracking forces are transmitted over the contact point between track roller and track link running surface which is, compared with the dimensions of a track-type undercarriage, very small. The design of this contact point and hence the running surface profile of a track link, therefore, is particularly important. References were analyzed with the aim of discussing theoretical diversions on contact problems between plain and curved bodies to get the basis for an optimized design of the running surface profile of track-type undercarriages. Therefore stresses at point and line contact, i.e. for curved and plane running surfaces, are compared by Hertz' theory. In the case of line contact, the maximum Hertzian stress and thus also the maximum effective stresses strongly depend on the actual tilted position of the track roller to the track link running surface. The lowest stress is applied in the case of ideal line contact which is only theoretically interesting. Already at small angles of misalignment, the stresses for line contact are higher than for point contact. In field operation, at constantly changing angles of misalignment, the edge stresses cause the formation of curved running surfaces due to plastic deformation. If track links are already designed with such a running surface contour, the result will be constant stresses over the whole service life which are largely independent of the angle of misalignment avoiding excessive stresses caused by roller misalignments.     

An improved analysis for thermal track buckling

In a previous analysis of track buckling by the author, a simplifying assumption was made that the axial resistance which the ballast exerts on the rail-tie structure is constant. In the present paper this assumption is dropped, by presenting the axial resistance as a bi-linear response, which agrees better with the corresponding test data over the entire range of axial displacements. The obtained analytical results were evaluated numerically for the track parameters currently in use and the results are compared with the results of previous analyses, in order to establish the effect of the introduced analytical improvement.     

Receptance behaviour of railway track and subgrade

Summary Vertical dynamic behaviour of a railway track on an elastic halfspace or on a layered halfspace is investigated by a frequency domain analysis. The results are compared with those for a simpler model, where ballast and subgrade are considered as a viscoelastic foundation. In the low- and medium-frequency range up to 250 Hz, great differences are observed between the results of the halfspace model and the results of the viscoelastic foundation model. This is because the damping due to wave propagation and coupling between sleepers cannot be modelled correctly by a viscoelastic foundation. Contradictions observed in the past between measured and calculated results can be explained with the new halfspace model. For frequencies higher than 250 Hz, the influence of the subgrade is negligible, so that here the simpler viscoelastic foundation model can be used. Received 5 January 1998; accepted for publication 7 April 1998     

Fast dynamic modeling for off-road track vehicles

The paper presents a simple, fast, and reliable dynamic model for an off-road track vehicle operating on terrain with obstacles. The method has been proven previously for wheeled-vehicle formulation. The model is based on a discrete body dynamics (DBD) method, which leads to simplistic linear decoupled motion equations. In this method, joints and bodies with relatively small mass are replaced with stiff springs and dampers, eliminating the system’s constraints and reducing the number of system bodies; this is important for accelerating the simulation runtime of the track vehicle model. The track in this approach is based on modeling each link as a point-mass. Two consecutive links are connected by stiff springs and dampers. This approach reduces the calculation time and increases system stability. The track–soil interaction was modeled using Bekker’s and Janosi’s formulation (Bekker, 1956; Hanamoto and Janosi, 1961). Specific soil properties were obtained for each link–soil interaction from soil classification and GIS. The link–ground contact was determined from topographic surface and adjustment of the force and direction acting on the track. The results of the simulation using the DBD method were compared with Siemens' VL commercial multibody dynamics program and with experiments reported in the literature. Results using the proposed method were found to be similar to the commercial program based on published experiments. The solution runtimes obtained for unpaved soil were two orders faster with the DBD method compared with the Siemens' VL program. The model was written as an independent software infrastructure, enabling easy integration with any other software component, such as a control system. The algorithm is in a suitable form for parallel processing calculation to speed up the runtime simulation close to real-time.     

长型浮置板轨道隔振系统理论分析(I)——弥散曲线及临界速度

浮置板轨道结构是近年来减振降噪效果最好的轨道结构形式.将整个轨道结构看作一个隔振黑匣子,钢轨与浮置板分别用欧拉梁来模拟,推导动力方程,用傅氏变换将其转换到频率-波数域中,求得系统的弥散方程,并给出轨道结构临界速度计算方法.定义了反映浮置板轨道结构参数的质量比例ξ_(m)、刚度比例ξ_(h)及阻尼比例ξ_(c)等三个系数.在给定实际中板上结构物理参数的条件下,深入讨论了三个比例系数对于系统弥散曲线以及临界速度的影响,进而为浮置板轨道共振频率及临界速度的初步设计提供建议.     

Comments on recent developments of asymptotics

Summary Technical aspects of asymptotic procedures are correlated with recent literature on asymptotics.

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Sommario Aspetti tecnici dei procedimenti asintotici sono posti in relazione con recenti pubblicazioni di asintotica.

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Paper presented at the 8th National Congress of AIMETA Turin, September 29–October 3, 1986 (invited lecture).     

Recent developments in enhanced heat transfer

Enhanced heat transfer has evolved into an important component of heat transfer experimentation and theory. The accumulated literature includes thousands of references. To give an overview of the current state of this important technology—for the past ten years, representative developments in each category of enhancement techniques are cited and commented on. The discussion is divided into the literature, passive enhancement techniques, active enhancement techniques, and compound enhancement techniques.     

On the developments of computational mechanics

Meccanica - The impact of computational methods in structural mechanics has been enormously increasing in the last two decades. A new discipline, now labelled as Computational Mechanics, has...     

Recent developments in multibody system dynamics

For the dynamical analysis of discrete mechanical systems the method of multibody systems is well qualified. The resulting equations of motion are solved by simulation codes and displayed by computer animation. The paper presents an approach for the efficient computation of motions and reactions.General Lecture presented at the 10th Italian National Congress of Theoretical and Applied Mechanics; AIMETA, Pisa, October 1990.     

Soil response to track and wheel tractor traffic

Experiments were conducted on a Eudora silt loam to determine the effect of tracked and wheeled tractor traffic on cone penetration resistance and soil bulk density at three different soil-water content levels. Treatment plots were ripped to a depth of 0.45 m and irrigated 5 days prior to the experiment. Significant differences in penetration resistance and bulk density were observed between the treatments within the plowing depth (0.30 m). After the tractor passes, the average penetration resistance recorded was about 7.5% higher and the soil bulk density was about 3% higher in the wheel treatment plots. However, at the soil-water content level close to Proctor optimum (15% w/w), no significant difference was observed in the average penetration resistance of the two treatments.     

Interaction of railway vehicles with track in cross-winds

This paper presents a framework for simulating railway vehicle and track interaction in cross-wind. Each 4-axle vehicle in a train is modeled by a 27-degree-of-freedom dynamic system. Two parallel rails of a track are modeled as two continuous beams supported by a discrete-elastic foundation of three layers with sleepers and ballasts included. The vehicle subsystem and the track subsystem are coupled through contacts between wheels and rails based on contact theory. Vertical and lateral rail irregularities simulated using an inverse Fourier transform are also taken into consideration. The simulation of steady and unsteady aerodynamic forces on a moving railway vehicle in cross-wind is then discussed in the time domain. The Hilber–Hughes–Taylor α-method is employed to solve the nonlinear equations of motion of coupled vehicle and track systems in cross-wind. The proposed framework is finally applied to a railway vehicle running on a straight track substructure in cross-wind. The safety and comfort performance of the moving vehicle in cross-wind are discussed. The results demonstrate that the proposed framework and the associated computer program can be used to investigate interaction problems of railway vehicles with track in cross-wind.