Structural Crashworthiness of Railway Vehicles Manuel S. Pereira1
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Structural Crashworthiness of Railway Vehicles Manuel S. Pereira1 1IDMEC, Department of Mechanical Engineering, Instituto Superior Técnico, Technical University of Lisbon, Avenida Rovisco Pais 1049-001 Lisbon, PORTUGAL, [email protected] Abstract As a form of transport, rail has always maintained lower accident levels than its counterparts, the car and the airplane. The railway industry, however strives for still better safety measures to be implemented – a fact which intensifies with each accident that occurs. In the last years the continuous developments in technology, computer modeling capabilities and know-how have opened up various research activities in the passive safety area, also known as crashworthiness. Whereas the objective of active safety systems (such as signalling and automatic train protection systems) is to avoid accidents, passive safety only comes into effect in the event of collision and its objective is to significantly reduce the severity of accidents. Rail vehicles can be designed to behave in ways that minimize the injuries of passengers and crew during collisions. Crashworthy vehicles contain in-built design features, which are not relevant in normal train operations, but protect the train occupants should an accident occur. A methodology has been developed within the projects TRAINCOL, SAFETRAIN and SAFETRAM for improved passive safety in railway transportation. This methodology includes: i) A review of past accidents, identification of reference collision scenarios and evaluation of their consequences. Risk assessment for improved passive safety was also considered; ii) The establishment of a set of reference collision scenarios for main line train and city tram operations. Together with statistical and risk analysis, design feasibility studies played a major role in the definition of the reference scenarios; iii) The development of a general framework for structural crashworthiness design train and in city and periurban tram vehicles and iv) Guidelines for design validation procedures through modeling, component and real size dynamic testing. Improved passenger and crew safety issues have been systematically addressed. Passenger and crew accommodations on a number of railways within Europe have been reviewed from the standpoint of interior safety. Conclusions about typical acceleration pulses and appropriate values of injury criteria for the railway industry have also been established. One of the most important achievements of these projects was then to demonstrate the feasibility of optimized carbody structures to present an improved safety level to occupants, within acceptable cost and masses for the defined construction solutions. A European Standard is now being completed providing the framework for determining the crash conditions that railway vehicle bodies should be designed to sustain based on the most common accidents and associated risks. It also defines suitable passive safety features to meet the requirements. Introduction Occupant safety is dependent on the configuration and severity of the accident, as well as the degree of crashworthiness engineered in the overall vehicle design. Train vehicle occupant survivability in a given crash scenario is a function of the kinematic behavior of the entire train set, the integrity and collapse characteristics of the structure of each vehicle and the overall interior configuration of a compartment and occupant/surfaces contact characteristics. Train crash events can be basically depicted into two phases: In a first phase, normally referred to as primary collision, the initial kinetic energy is progressively dissipated by means of plastic structural deformation resulting from the crash generated impact loads. In this phase occupant compartment integrity and acceptable vehicle acceleration levels are the most important design requirements to be considered. In a second phase, normally referred to as secondary collision, the occupant will be subject to a great variety of potentially harmful occupant/interior or occupant/occupant contacts. Design requirements must involve the aspects of interior layouts, acceptable severity levels and biomechanical response to vehicle crash pulses. The friendliness of the compartment interior is obviously a major design issue. The energy generated in train collisions has to be dissipated by plastic crushing of designated structural arrangements developing forces, which in turn cause decelerations, which are directly responsible for the severity of secondary collisions. These designated structural arrangements are normally located at the extremities of the vehicles and are designed to absorb maximum possible energy and collapse in a controlled manner preserving the occupant compartment integrity. The crushable zones must be confined to reasonable lengths and the range of crush loads are to be consistent with passenger compartment buckling loads and static specified buffer loads [9]. Train collision are always assumed to occur under fully plastic impact conditions, which means that after collision both sets have the same velocity, which corresponds to a zero coefficient of restitution. For single vehicle collisions, such as the case of many tram configurations, the starting point for design requires the knowledge of the design collision scenario speed and the maximum allowed acceleration experienced by the occupant compartment. Closed form solutions are available for determining the required crushing length. Forces can subsequently be determined to specify the design requirements, in terms of a force-displacement curve for the extremities. For train collisions involving several vehicles the problem is considerable more complex. The energy distribution along train sets is strongly dependent on: • Train configuration, namely the number of vehicles • Mass distribution – masses of individual vehicles along the train set • Collision speed • Type of scenario. The characteristics of structural crashworthy vehicles are expressed in terms of force-displacement curves with different levels of plastic loads. Different force-displacement curves are used for inter-rake and inter-trailer zones and are referred to as EE and IE ends, respectively. SAFETRAIN project [2], [3] and [11] concentrated in the study of regional, intercity and high speed train sets configurations. SAFETRAM project [12] dealt with the specificities of city trams and periurban trams. Based on results of these projects, methodologies for improved passive safety, including the specification, design, testing and validation procedures for crashworthy rail vehicles, is outlined. Specific recommendations are proposed for passive safety development, minimisation of loss of survival space, train energy management issues, vehicle design, minimisation of severity of occupant injuries, methods of validation of crashworthy designs, static and dynamic testing and structural and passenger/crew numerical simulations. Review of accidents and choice of representative collision scenarios The principal aim of this task is to define the representative collision scenarios, their characteristics and parameters to be applied for modelling, design and test of crashworthy structures and other protection means (anti-overriding devices, obstacle deflectors, interior design). The main characteristics of collision accidents in Europe have been gathered into a representative database [8]. The quantity of data collected corresponds to approximately 60% of total European railway production (in terms of train kilometres, passengers transported, passenger kilometres and length of lines in service). The database forms a statistical population of collision accidents over the five years 1991 to 1995. The analysis showed that building practicable levels of energy absorption into passenger rolling stock would improve the passive safety of vehicles in head on collisions, rear on collisions, collisions with cars and lorries on level crossings and collisions with buffer stops. Risk analysis on the city tram accident data, collected through an inquiry was performed [5]. The purpose of the LRV statistics study was to identify relevant collision scenarios including an evaluation of their consequences, in terms of material damage and injuries and fatalities as applied to city tram operations. As there are few periurban trams in service and periurban traffic is relatively recent, all analysis refers to accident data involving only regional traffic. Risk and statistical analysis was conducted on the German and French accident data of ERRI B205 database, involving 248 and 329 accidents, respectively [10]. Based on the findings of the statistical analyses described herein the scenarios and train and tram configurations, presented in table1, have been selected. Rollling Collision scenarios Configurations stock Trains S1) Train vs. train train types were identified in single (multiple) units. S2) Train vs. buffer stops Type A – Main line train – mass 340 t – 8(16) vehicle S3) Train vs. deformable obstacle In level crossing train sets different statistical impact velocities were defined from Type B – Main line train – mass 412 t – 8(16) vehicle the B205 data base analysis. Each velocity covers, train sets respectively, 70, 80 and 90 % of accidents occurred. Type C – Regional train - mass 129 t – 3(6) vehicle train For scenario 1 : V70% = 30 km/h, V80% = 40 km/h, V90% = sets 55 km/h. Type D – Motor coach, single vehicle – mass 50 t 1(2) For scenario 3 : V70% = 80 km/h, V80% = 100 km/h, V90% vehicle train