Hybrid-composite bus superstructure analysis according to the ECE R66-02 regulation

In recent years, increased sensitivity to vehicle safety has opened the way for the development of vehicles that are more durable and that keep passenger safety. Rulemakers have been trying to reduce the loss of life in rollover accidents, especially in the bus sector, by legal regulations, by increasing the pressure on vehicle manufacturers in recent years. For this reason, the regulation of ECE-R66 has been published under the United Nations roof and has become mandatory in many countries in order to test the structural integrity of large passenger vehicles in rollover accidents. On the other hand, an increase in vehicle safety leads to a certain weight increase in the vehicles, which in turn causes an environmental effect by increasing fuel consumption, as well as giving up the payload. The increase in vehicle weight pushes manufacturers to use lightweight and robust alternatives. Some of these are high-strength steels, magnesium alloys, and composite materials, which are the subject of this work. In this study, the design of the bus roll-bar, which will be made using the hybrid composite body profile, in accordance with ECE R66-02 regulation. T700 / 2510 carbon fiber / epoxy composite material produced by Toray Composite America (TCA) and tested by AGATE Program (Advanced General Aviation Transport Experiment) will be used as composite material. In the analysis, nonlinear explicit LS-DYNA finite element program is used. The reason for the use of T700 / 2510 material in the analyzes is that the material model has been verified by using this material in many academic workshops. In addition, MAT 54 material model of LS DYNA package program used in this study is able to catch damage modes of T700 / 2510 material well in many academic work. The aim of this study is to design the bus which carries the intercity passenger, using the composite materials besides the existing steel materials, designing according to the ECE bus rollover regulations by using the finite element method in computer environment. The ECE regulation, mandated by many countries around the world, is designed to provide passenger and driver safety in the passenger and driver area at the time of rollover accident by preventing the contact of the bus body and ceiling parts from interfering in this living space. In the first part of the work, crashworthiness and vehicle weight reduction issues are explained as well as general information about bus classes, accident summaries and composites. A lot of examples were given to the study of composite buses in the literature which is the inspiration source of this study, and the ground for this study was established. When the bus accidents in Europe and USA are examined, it is statistically revealed that even though the number of rollover accidents is small, it causes the greatest loss of lives. Such accidents usually occur on intercity roads and cause fatal consequences. When the collision resistances of the composite materials of this study are examined, it can be seen that they can absorb more energy than the conventional materials such as steel and aluminum. The widespread use of composite materials will provide both vehicle safety and lighter and environmentally friendly vehicles. In the second part, the regulation of ECE-R66 is explained in general terms and some test conditions such as the modeling of the survival space and the measurement of the center of gravity are given. The ECE-R66 regulation is a test designed to rollover an 800 mm high platform onto a concrete floor. The center of gravity of the overturned vehicle, inertia, mass must be identical to the actual production tool. Passenger weight should be fixed to each seat as 34kg. The initial speed during the test must be within the specified limits, the suspension system must be locked, and the doors and emergency exits must be in the closed position. The living area must be within the limits specified in the regulation. The manufacturer can model a larger living space if desired. In the third part of the work, some of the advantages at the collision strength of composite materials and given some information such as classes of composite materials, mechanics, types of damage, crash durability, energy absorption properties and production methods. In addition, a number of methods have been proposed for the production of the hybrid composite tilting bar, which is the subject of this study. In general, composite materials are materials that are formed by combining two or more materials, which are not sufficient for one purpose alone, in a macrostructure in certain conditions and proportions to provide desired properties. These materials that come together are insoluble in each other, even though they are homogeneous in the macro dimension, they are heterogeneous materials in micro dimension. Composite materials are used extensively in the automotive and aerospace industries due to their superior weight-to-strength ratio, corrosion resistance and vibration damping capabilities. In the fourth chapter, besides the finite element theory, information about linear and nonlinear analyses is given and general information about the software used in this study is explained. Linear analyzes are used for scenarios where deformations are at a minimum level. In this method, while the elastic modulus of the material is considered constant throughout the analysis, the stifness matrix is calculated at the beginning of the analysis and does not change. In non-linear analyzes, the stifness matrix is recalculated at each step to obtain more accurate results. The bus rollover analyzes are in the class of nonlinear dynamic analyzes and the real stress-strain curve of the material is used during the solution. Nonlinear dynamic problems are solved by explicit software. They are simpler solutions because there is no need stifness matrix in the solution. The main difference in explicit and implicit solutions are the time interval control. If the time interval is smaller than the critical time interval, the solution runs steadily. Basically, the wave must not pass more than one item at a time interval. The critical time interval depends on the speed of sound in the material and the size of the element used. The next section describes the verification and validation tests carried out in TEMSA for the steel structure. In the validation test, a specific bus section was rolled from a specific platform in accordance with the ECE-R66 regulation, and the results were validated by FEM analysis, while the 3-point bend test was performed on the specific pillar connection within the verification tests. In this study, physical tests can not be performed for the composite structure and the only analysis results are given. During the confirmation tests, the produced specific bus section was overturned from the 800mm platform to the concrete floor. The test results were compared with the results of the analysis using LS DYNA program in the computer environment and system approval was obtained. When the deformations that occurred after the test were examined, the results of analyzes were obtained in accordance with each other. Prior to the cross-section test covering the middle 2 glass compartment of the bus, 366 kg weight was added to make the deformations more visible, and tests and analyzes were done in this way. In the sixth part, numerical modeling of buses is explained in LS-DYNA as suitable for rollover analysis. In addition to the information about the material types, the survival space, the contact type and the energy values have been explained. While the deformed parts such as axles, engine, air conditioner are modeled with rigid materials, deformed steel structure is modeled with elasto-plastic material model. Composite materials were created with MAT54 Enhanced Composite Damage material model. Belytschko-Lin-Tsay with 4 nodes is used for shell elements as element formulation. In the analysis where real material curves are used, curves are generated by tensile tests. The passenger weights are rigidly connected to the seat connection areas, so that each seat weighs 34kg. In the last part, the structure of the bus composed from hybrid composite structure which has succeeded in the rollover analysis was examined and the differences between the energy equations for the bus made with steel material were examined. Hybrid composite rollover bars were designed on the front and rear of the vehicle and analyzes were completed in accordance with the ECE regulation. The bar on the front of the vehicle has 8 to 35 layers, while the rear roll bar has 5 layers. No contact surface between the composite structure and the steel profile is defined and the structure is considered integral. After many layers and analyzes, the design that succeeded the ECE directive became final. When the analysis results are examined, the energy charts and error rates are as expected, while all the side poles show adequate stiffness. The final version of the design is 100kg lighter than the entirely designed steel. As a result, in this study, a bus with a hybrid composite rolling bar was investigated to determine how light the bus made with the existing steel material could be made on the bus made with the selected T700 / 2510 carbon fiber / epoxy material by performing a tipping analysis according to ECE R66-02 regulation. In the analyzes, the composite materials were studied using the Chang-Chang damage criterion and a benchmark was made for future projects. Although it is not possible to attain the desired lightness, it is thought that the aim of appropriate geometry and weight can be attained by more rigid materials and design changes.