Bus superstructure analysis according to the E.C.E regulation securing passenger safety

A bus is one of the most modern motor vehicles which is used for people's mass transportation ranging from either low to high ranges. In parallel to technological improvements the transportation vehicles are equipped with the active safety systems such as ABS, EBD, ESP, etc. and the passive safety systems such as airbags, embedded door reinforcements, foldable pedals, etc. According to the research made until today the highest casualties and injuries occur during a rollover accident. And it is seen that- in general - these casualties are not because of the severity of the accident but mostly because of lack of strength of the bus superstructure. Thus, to maintain enough body strength preventing casualties the E.C.E R66.00 regulation was put into force by U.N.E.C.E (United Nations Economic Commission for Europe) since the year 1997. The E.C.E R66.00 regulation is basically a tilting test where the bus is put on a 800 mm high platform and the platform is slowly rotated until the bus starts to free fall. Shortly after starting a free fall the bus structure (roof edge) hits the ground and the body starts to absorb the occurred kinetic energy during elastic-plastic deformation. While experiencing the deformation the vehicle also slides on the ground generating some friction energy. The most critical deformation zones are the junctions of sidewall pillars and the transverse profiles. These zones are called plastic hinges and they experience the highest level of plastic strain compared to other deforming parts. For this reason plastic hinges are the primary zones to focus on while trying to improve the design for a good rollover crash performance. The regulation applies to class M2 and M3 vehicles carrying at least 16 passengers. Thanks to this regulation, for the busses operating in European Union member countries, structural intrusion in the so-called survival space of the vehicle is prevented during a rollover accident and this precaution secures passenger and the crew safety. This regulation is in force also in Turkey and the produced bus vehicles are obliged to satisfy the requirements set forth by the regulation. There are two major methods applied by the bus manufacturers towards the approval of E.C.E R-66 regulation. In the first method a physical test set-up is created to rollover a full vehicle prototype from a prescribed 800 mm high platform. Upon the completion of the test the measurements are evaluated and the distance from the deforming body to the survival space at the worst instant is investigated. In the second method a simulation methodology (with finite elements method) is developed through a series of validation studies and by using relevant FEM software the bus rollover test is performed in computer environment. The validation studies include material coupon tests to obtain true stress-strain curves, material characterization with 3-point-bending tests applied on particular body structure components (i.e. breast knot or roof-edge knot) and test vs simulation correlation applied on a so-called body section of the bus superstructure. The FEM model parameters and the material model are fine-tuned until reaching a good level of approximation. As for validation, ground friction coefficient is also one of the most important parameters influencing the accuracy of the simulation results and therefore it is also investigated by performing particular test and simulation studies to obtain a reliable simulation methodology. Today, the vast majority of the bus manufactures follow the simulation method because the first method (Physical test on a full-scale vehicle) is both expensive and not reproducible. In other words it is difficult and so much time consuming to repeat the tests for obtaining an optimal design. And also it is rather expensive not only because of test set-up and measurement matters but also it is inevitable that each test results in getting a scrap vehicle eventually. However, the simulation method definitely enables the engineers or researchers to perform optimization studies once they provide a reliable simulation model. The virtual environment empowers innovation and gives the user a feasible opportunity to try out the ideas to attain lightweight yet high strength superstructure satisfying the rollover safety requirements. The required superstructure energy absorption percentage is increased by %90 (compared to R-66.00) with the E.C.E R-66.02 regulation which is released by U.N.E.C.E and which will come into force starting November 9th, 2017. This new version of the regulation orders to introduce half weight of the seated passengers and the survival space is redefined to occupy a space throughout the vehicle axis taking also the foremost and rearmost pillars into play. The countermeasures to be taken to satisfy the new regulation requirement might cause a body weight and cost increase. Besides, considering the maximum allowed axle weight, it might even result in reducing the maximum number of passengers or allowed luggage weight just to satisfy the new requirements coming with E.C.E R-66. In the context of this thesis the parts comprising the bus body are designed to have more strength yet reduced weight and high strength steel materials are used where necessary. The dynamical FEM model (Stiffness, mass and mass distribution is represented at a high level of accuracy) of the designed bus structure is prepared with the specialized preprocessing software Ansa and the rollover crash analyses are performed with the commercial code Ls-Dyna. Ls-Dyna is mainly an explicit-dynamic FEM code developed by Livermore Software Technology Corporation located in California, U.S.A. It is especially developed for low-to-hypervelocity impact events. It has an explicit time integration scheme based on central difference numerical method which is mathematically well suited for better capturing the consequences of highly transient physics. That is why it is globally widespread among the crashworthiness and safety departments of various industries. To establish a reliable FEM simulation, a simulation vs test correlation study is performed on a bus body section using the prescribed initial conditions. In addition to that, the mass center of gravity (CG) of a full-scale bus is measured with physical tests and the mass distribution of the dynamical simulation model is fine-tuned to accurately match the real CG. Upon obtaining a proven simulation methodology the computer simulation is performed and the deformation results are compared to those from the physical test. With all the effort given, a safe bus superstructure design is obtained which conforms the E.C.E R-66.02 regulation and does not require reducing the maximum allowed number of passengers or the maximum allowed luggage weight.