Optimization the node concept of the base structure –side wall, which was desinged according to ECE-R/66-02 regulaton on coaches

Buses are high in demand among public transportation vehicles segment due to being affordable and easily available. They are considered to be more critical and important in terms of security compared to other passenger vehicles because they can transport a large number of people at a time. There are various types of accidents on the highways and naturally, experts mostly focus on crash accidents. Such accidents occur almost every day. Many security systems have been developed about these accidents and most of them have succeeded. Studies show that the most destructive accidents with the highest death toll for buses are rollover accidents. This type of accidents are not common, but they are 5-6 times more destructive for passengers compared to other accident types. In this regard, U.N.E.C.E (United Nations Economic Commission for Europe) decided to establish standards for rollovers and regulation ECE R 66.00 was released in 1997. This regulation identifies a living space inside the vehicle's body frame in the event of a rollover and anticipates to save the passengers' lives within that space. Regulation ECE R66 is overall the first regulation that aims to protect passengers' lives in the event of a rollover. According to the regulation; a body frame with the specified components is released sideways from a platform which is 800 mm above the ground to roll over. After the rollover, it must be observed that the specified living space is not intruded and the body frame is not broken. In order to keep up with the advancing and changing conditions, the commission made the conditions stricter by releasing version ECE R66.01 and then ECE R66.02. Between Version 0 and 1, addition of passenger weights and 90% more energy absorption for the structure is requested. In Version 2, optional compliance for these conditions is requested for double decker buses. After these changes, the necessity to seriously enhance the vehicle body frames has emerged. Regulation controls these conditions with two different methods. In the first method; a physically complete vehicle is rolled over with the specified conditions identified in the regulation and the living space is checked. In the second method; an analysis software, whose reliability is confirmed with tests, is used to virtually roll the vehicle over and check if the conditions are met. Predictably, the first method is very costly and hard to implement. If the first try is not successful, making enhancements and repeating the test will be inefficient in terms of time and budget. In this respect, almost all companies use a computer software called LS-Dyna to simulate the conditions. Making adjustments and implementations on the software is quicker and more cost effective. A similar method was used in this thesis so instead of a physical vehicle rollover, the effect of the optimization to the whole vehicle is observed via LS-Dyna software. A vehicle family that is already compliant with the ECE R66.02 was used for the tests and optimization of the existing reinforcements were planned. It's observed that high strength and molded parts in the base structure-side wall joints of the vehicles were deemed as disadvantageous in terms of cost. Replacing these parts and finding lower cost structures as strong as these parts was planned. TRIZ method was used as a problem solving method to perform zonal examination and optimization of the structure. This method provides the user with previously implemented methods which were used in different areas, instead of creating a new solution, and directs suggestions. Among 13 different methods in TRIZ, 39x39 contradiction matrix method was used and the method suggested both existing solution (breaking down and rounding) and the solution used in the study (composite structure support). Thus, instead of adding existing high strength and molded parts to the body frame, supporting the composite structure solution was chosen. Tension-compression, 3-point bending and drop-tower tests were applied in ITU (İstanbul Technical University) Faculty of Mechanical Engineering Labs, to monitor the behavior of different types of composite structures. Initially, carbon fiber plates were suggested to be used as support in the base structure-side wall joint area, but the observations showed that this structure would not be sufficient. After that, 6 different solutions were tried. Eventually, five 0/90 layered glass fibers on the 3 surfaces that cover the structure and two 0/90 layered glass fibers on the bottom surface was decided as the optimal solution. The tests were confirmed by undergraduate ITU students in ANSYS program and information about the materials were gathered. These information were then used as entry data for the optimal material choice in the composite material catalog of LS-Dyna software. Existing structure and composite-supported structure were run in LS-Dyna software and the results were evaluated. Examination of the results showed that making some additions is needed and this study will be indicative for future studies. When the solution was examined in terms of costs, target savings compared to the existing structure was planned as 25% and weight savings were planned as 4%. 23% cost savings that was achieved with this study is close to the desired values and weight savings is also similar to the target value.