Antonio De Iorio

Transverse strength of railway tracks: part 2. Test system for ballast resistance in line measurement

© Gruppo Italiano Frattura 2014. This is an Open Access article distributed in accordance with the terms of the Creative Commons Attribution (CC BY 4.0) license, which permits others to distribute, remix, adapt and build upon this work, for commercial use, provided the original work is properly cited. See: https://creativecommons.org/licenses/by/4.0/

Transverse strength of railway tracks: part 1. Planning and experimental setup

Several studies have been carried out until now by various Research Agencies and Railway Administrations to quantify the effects of the track-bed geometrical characteristics on the transverse strength of the track. Unfortunately, not all the possible scenarios in terms of track components, track-bed cross profile, operating conditions etc. have been investigated and not all the relevant variables have been directly measured. Therefore data available from the literature have different degrees of reliability. With the aim of enlarging the knowledge on the track stability and covering much of the possible relevant scenarios, an experimental research program has been developed in the framework of a cooperation between RFI, Italcertifer and DII. In order to perform the investigation under quite general conditions and to reduce the experimentation costs, n. 28 significant scenarios have been identified and reproduced on as many independent track segments. By applying on each track segment a transversal load, the strength of the ballast-sleeper interface has been determined. The results relative to the first four scenarios are presented in terms of applied load vs. lateral track displacement diagrams and in more synthetic numerical tables.

On the ballast–sleeper interaction in the longitudinal and lateral directions:

In service, railway tracks must withstand the transverse and longitudinal forces that are caused by running vehicles and thermal loads. The mechanical design that adopts any of the track models available in the technical literature requires that the strength of the track is fully characterised. In this paper, the results of an experimental research activity on the sleeper–ballast resistance along the lateral and the longitudinal directions are reported and discussed. In particular, the work is aimed at identifying the strength contributions offered by the base, the ballast between the sleepers, and the ballast shoulder to the global resistance of the track in the horizontal plane. These quantities were experimentally determined by means of an ad hoc system designed by the authors. Field tests were carried out on a series of track sections that were built to simulate scenarios in which the ballast was removed from the crib and/or the shoulder. The results of this study indicate that the strength percent contributions from the crib, the sleeper base, and the shoulder are, respectively, equal to about 50%, 25%, and 25% in the lateral direction, and 60%, 30%, and 10% in the longitudinal direction. Moreover, the comparison of the acquired data with literature results reveals that a detailed knowledge about the testing conditions and the activated ballast failure mechanisms is needed in order to correctly use the test data for the design purpose.

An Analytical Model for the Identification of the Threshold of Stress Intensity Factor Range for Crack Growth

The value of the stress intensity factor (SIF) range threshold for fatigue crack growth (FCG) depends highly on its experimental identification. The identification and application of are not well established as its determination depends on various factors including experimental, numerical, or analytical techniques used. A new analytical model which can fit the raw FCG experimental data is proposed. The analytical model proposed is suitable to fit with high accuracy the experimental data and is capable of estimating the threshold SIF range. The comparison between the threshold SIF range identified with the model proposed and those found in the literature is also discussed. identified is found to be quite accurate and consistent when compared to the literature with a maximum deviation of 5.61%. The accuracy with which the analytical model is able to fit the raw data is also briefly discussed.

Transverse strength of railway tracks: part 3. Multiple scenarios test field

In the present paper the design and construction choices of a test field for the ballast lateral resistance measurement, in order to produce data useful for the development of a numerical model able to simulate the service critical conditions of a continuous welded rail track, are described. Some construction details described herein allow to better understand the methodological approach followed in the design of experiments, the tests management philosophy as well as of the accuracy achieved in their implementation.

Development of Predictive Models for Fatigue Crack Growth in Rails

Fatigue failures of rails often occur at the rail foot, since the geometry of this zone gives rise to stress concentrations under service loads or defects during rail manufacture and installation. In this paper, the fatigue behavior of cracks at the web/foot region of a rail is analyzed numerically. Analytical models in the literature for a semi-elliptical surface crack in a finite plate assume that the geometry of the front remains semi-elliptical during the whole propagation phase and the ellipse axes do not undergo translations or rotations. Fatigue tests show that this is not the case for such cracks in rails. A predictive model for crack growth has been developed by assuming an initial small crack at one probable initiation point between the web and foot of the rail in reference to a service condition loading. SIF values have been estimated by means of the finite element method and the plastic radius correction. The results attained were compared with crack growth experimental data.

Selection and ranking of the main beam geometry of a freight wagon for lightweighting

The traditional freight wagons employ I-beam sections as the main load-bearing structures. The primary loads they carry are vertical (from loading units) and axial (from train traction and buffers). Ease of manufacturing has played an important role in the selection of the I-beam for this role. However, with lightweighting increasingly becoming an important design objective, an evaluation needs to be done to assess if there are other existing or new section profiles (geometry) that would carry the same operational loads but are lighter. This paper presents an evaluation of 24 section profiles for their ability to take the operational loads of freight wagons. The profiles are divided into two categories, namely ‘conventional – made by wagon manufacturers (including the I-beam)’ and ‘pre-fabricated’ sections. For ranking purposes, the primary design objectives or key performance indicators were bending stress, associated deflection and buckling load. Subsequently, this was treated as a multi-criteria decision-making process. The loading conditions were applied as prescribed by the EU standard EN 12663-2. To carry out structural analysis, finite element analysis was implemented using ANSYS software. To determine the validity of the finite element analysis results, correlation analysis was done with respect to beam theory. Parameters considered were: maximum stress, deflection, second moment of area, thickness, bending stiffness and flexural rigidity. The paper discusses the impreciseness related to the use of beam theory since the local stiffness of the beam is neglected leading to an inaccurate estimation of the buckling load and the vertical displacement. Even more complicated can be the estimation of the maximum stress to be used for comparison when features such as spot welds are present. The nominal stress values computed by means of Navier equation lead to an inaccurate value of the stress since it neglects the variations in the local stiffness, which can lead to an increase in the bending stress values. The main objective of the paper is the applicability of particular section profiles to the railway field with the aim of lightweighting the main structure. Sections commonly adopted in civil applications have also been investigated to understand the stiffness and strength under railway service loads. The common approach reported in literature so far makes use either of the beam theory or topological finite element approach to determine the optimised shape under the action of the simplified loading conditions. Although the previous approaches seem to be more general, the assumptions made affect the optimisation process since the stress state differs from that attained under the actual service load in the real structure. In this paper, the use of complex shape cross sections and detailed finite element models allows to take account of the real behaviour in terms of stiffness distribution and local stress effects due to manufacturing features like welds. The structural assessment carried out with the detailed models also allows for the proper comparison among the considered sections. Analysis of the results showed that three out of the 24 section profiles have the highest potential to be fitted as the main load-bearing beams for freight wagons, with the pre-fabricated Z-section being the optimum of the three.

A decomposable freight wagon frame suitable for bimodal transport systems that uses a fast locking mechanism

This paper describes a novel connection system which allows fast, safe and reliable coupling and uncoupling of two sections of a freight wagon body as part of a bimodal transport system. The connection system allows the time for loading and unloading a truck trailer to be reduced to almost one-half of the current duration. The connection system is composed of a cylindrical clamping pin with three conical shoulders at one end and an axial slide. A crank mechanism in a cylindrical guide is used to lock the clamp. The crank is connected to the pin and symmetric piston rods are connected to the moving clamp, other auxiliary members constrain the clamp.

Fatigue Crack Growth Simulation in Railway Axles

In order to carry out the railway axle design according to the “Damage Tolerance” philosophy, reliable crack-growth models for these kind of components are necessary. Indeed, damage tolerance principles have received more and more attention from railway technical community, thanks to its ambitious task concerning the inspection intervals prevision of railway components subjected to non-stationary loading conditions. In this paper, a simple routine is exposed that is able to calculate the characteristic parameters of the Linear Elastic Fracture Mechanics (LEFM) for a generic cracked mechanical component. Such parameters are then used in a two parameters propagation law to estimate the necessary time for a crack to become critical.