Quasi-static simulation approaches on rollover impact of a bus structure

Quasi-static simulation approaches on rollover impact of a bus structure DAN ALEXANDRU MICU, MIHAIL DANIEL IOZSA, CORNELIA STAN Automotive Engineering Department University POLITEHNICA of Bucharest 313 Splaiul Independentei st., 6th Sector, ROMANIA dan.micu@upb.ro; daniel_iozsa@yahoo.com; corraista@yahoo.com Abstract: The dynamic simulations of rollover behaviour of bus bodies require more computation time than the quasi-static simulations. This paper presents a study of several different virtual simulations of quasi-static loading test on body sections as an equivalent approval method of UNECE Regulation Number 66. The body section is analyzed by using the Transient Structural and Static Structural modules of Ansys. The scenarios analyze some new methods of deforming a structure by applying a force or a displacement to a plate which is in contact with a frame of the roof, or by applying different angle displacements directly to the frame. Resulting deformations are compared with a physical reference model, the absorbed energy is analyzed and a necessary mass of the structure is computed for each of the models. In some models the deformation is similar to the reference structure. Key-Words: quasi-static, rollover, bus, structure, Ansys, simulation, body section, residual space 1 Introduction Rollover strength has become an important issue for bus and coach manufacturers. Today the European regulation ECE-R66 is in force to prevent catastrophic rollover accidents [1]. Currently, the dynamic explicit integration is commonly used for impact and collision simulations. However, it may take tens of hours or even several days to carry out a single simulation for bus rollover, which greatly reduces the efficiency of product development [2, 3]. A preliminary procedure to determine the energy absorbing capability of a bus body section in rollover has been developed [4]. In rollovers, passengers can be ejected, partially ejected, or can become the victims of roof intrusion, all of which may be fatal. Passengers in bus or coach rollover accidents are farther from the center of rotation than passengers in car accidents [5]. According to the Regulation no. 66, the testing of body structures can be made by [6]: testing the entire bus; testing body sections; the quasi-static tests of body sections; the quasi-static tests on components. Long analysis times can be an impediment to achieving results and may limit the ability of the engineer to efficiently improve a model [7]. Paper [8] presents the results of two sets of simulations on a dynamic rollover test (ECE-R66) and on a quasi-static roof crush resistance test (according to FMVSS 220). Although both tests are used for the same purpose assessing the strength of the bus in the approval process their outcomes diverge. [8] Using an experimental test as a reference, the authors managed to analyze more scenarios and to establish the best Ansys simulation among them. In Section 2 two general parameters are determined: the height of the roof frame and the angle between the direction of the force and the longitudinal central vertical section of the structure. The development is presented in section 3 for five scenarios: in three of them the deformation force is applied to a plate which is in contact with a frame of the roof and in the other two the deformation force is applied directly to the frame of the roof at different angles. The structure of the bus section is analyzed through a quasi-static test using Transient Structural and Static Structural modules of Ansys. The models are compared with the reference model by three criteria. Finally, conclusions of the model that best complies with the criteria are presented in section 4. 2 General Parameters Testing a body section through quasi-static loading is specified as an alternative method for ISBN: 978-960-474-383-4 81

testing the rollover resistance of bus structures in Annex 7 of Regulation R66. By the quasi-static test specified in the Regulation an upward force is applied to a value at which the structure reaches the residual space. The force is equally distributed on the roof frame, whose direction is determined by the angle α made by the direction of the force and the longitudinal central vertical section of the structure, and calculated with the equation [76]: Considering the regulation specifications, according to which the load and deformation shall be measured to an accuracy of 1%, the number of steps for the calculation process is set to 100. After several simulations were performed in order to determine the force needed to deform the structure till it reaches the residual space, its value was amounted to 18000N. The structure deformation at the moment of contact with the residual space is shown in Figure 3., (1) where H c is the height of the roof frame of the structure, measured in mm from the horizontal support plane. Figure 1 shows a front view of the structure, above which the height was measured. In this case, Hc = 2667 mm. Fig.1. Measuring the height of the roof frame. The angle between the direction of the force and the central vertical longitudinal plane of the structure is α =72.5 0. 3 Models development Several attempts are presented. 3.1 Model E a force increasing to 18 kn is applied to the plate The force is applied to the surface of the block that comes into contact with the bus structure. The manner of applying the force is shown in Figure 2 (a front view and a side view). Fig.3. Structure deformation for the E model when coming in contact with the residual space. In order to determine the energy absorbed by the structure during its deformation, the scalar product of the vectors of force and displacement is computed. Figure 4 shows how energy varies with the deformation of the structure. Fig.4. Curve of energy - displacement obtained for model E. The regulation states that the amount of real energy absorbed by the body section (E BS ) must be bigger than the minimum energy (E min ) giving by the sum of energies absorbed by each frame [R66]. (2) (3) (4) a) Front view b) Side view Fig.2. Application of the force used for quasi-static testing. (5) where: E BS actual energy absorbed by the body section, the analytically determined value which corresponds to the time the structure comes into contact with the residual space; ISBN: 978-960-474-383-4 82

E min minimum energy required to be absorbed by the body section; E i the absorbed energy by the "i th " bay; E T total energy to be absorbed by the superstructure; m i mass of the "i th " bay; M mass of the vehicle; g = gravitational constant; Δh = the vertical movement (in metres) of the vehicle centre of gravity during a rollover. By replacing with relations 3-5, relationship 2 becomes: (6) The real structure mass was 2830 kg. At this weight the structure enters the residual space. The total energy required to be absorbed and equivalent with the experimentally determined impact energy, according to relation (5), is 11606.7 J. However, by using equation (6) and considering the sum of the bays masses as being equivalent with the mass of the structure section, the maximum mass can be determined: (7) It follows that the mass of the structure section (M S ) is: Fig.5. Comparison between the model's deformation and deformation by experimental testing. The deformations of both models are similar. 3.2 Model F force up to 120 kn applied to the plate in the case of a structure made of linear material The same conditions as in previous model were used, but the material has been turned into a linear one. After several attempts, the force required to deform the structure till the survival space was determined at 97.5 kn. In order to check the shape of the deformation in comparison with the one of the experimental test, the force F applied to the model was increased to 120kN. Figure 6 shows the obtained deformation for the F model when the structure comes into contact with the residual space. (8) The resulting mass is used as a criterion for the evaluation of the simulation and it is compared with the experimentally determined structure s mass (2830 kg) and with the deformation of the model. In this case, the maximum energy absorbed, shown in Figure 4, is 3587 J. Therefore, the total mass of the bays and the maximum mass of the structure as assumed in (8) should follow the relationship: Fig.6. The deformation of model F when the structure comes into contact with the residual space Figure 7 illustrates the variation of energy with the deformation of the structure. ; (9) The structure mass obtained by using this method is excessively small. In the experimental test, the structure of 2830 kg invaded the survival space only during a few moments of impact, and finally it was at a considerable distance outside the survival space. By increasing the pressing force of the plate to 19 000 N, the penetration into the residual space has been obtained. A comparison with the experimental deformation is performed in Figure 5. Fig.7. Curve of energy - deformation obtained from the use of force applied to the plate in the case of linear material. In this case, the maximum absorbed energy is 17565.6 J. This energy corresponds to a ISBN: 978-960-474-383-4 83

deformation similar to the experimental one; but to determine the mass according to relation (8), the energy at the moment of contact with the residual space. This, as shown in Figure 7, corresponds to step 80, whose energy is 12198.6 J. Therefore, the bays summed mass and the maximum mass of the structure as assumed in (8) should follow the relationship: (10) Relation (10) and the energy value, resulted when the structure came in contact with the residual space, assigned a truth-value to relation (2); thus model F would meet the requirements of the Regulation. However, the structure s mass in the experimental test is smaller and the highest deformation of model F corresponds to an energy of 17565.6 J, as shown in Figure 8. Fig.8. Deformation of model F and deformation of the structure in the experimental test. It can be seen from Figure 8 that the deformation appearance of model F is similar to the deformation in experimental tests. 3.3 Model G deformation obtained by imposed move of the plate The behavior of the structure was studied by moving with different inclinations a plate located on the roof frame. In the first stage only appearance of deformation was studied and the case was further studied in more details after its deformation matched the experimental test. To determine the energy absorbed by the structure, the program will return the values of the reaction forces on the three components of the global Cartesian system. The movement could be achieved by creating a local Cartesian system on one of the surfaces of the plate and by constrain (block) movements in the desired directions. Figure 9 shows the Cartesian system defined on the plate surface and the required direction of movement. Fig.9. The deformation method used for model G. The first test has been made by moving the plate with an angle of about 20 degrees between the direction of travel and the horizontal plane, and the resulting deformation is different from the experimental one. Another attempt has been made by orienting the plate to an angle of 17.55 0, as determined by Regulation. But the structure does not comply with the deformation resulted from experimental testing in this case, either. A third test was performed with the displacement angle of 13 degrees to the horizontal plane. Figure 10 compares the aspects of deformation when moving in a direction oriented at 13 degrees to the horizontal, by two different width dimensions of the plate. It must be noted that the aspect of the deformation is influenced by the width of the plate. a) b) Fig.10. The deformation of the structure for a displacement of the plate with an angle of 13 0. The resulting reaction force of the model G is almost three times greater than the force of model F. Energy - displacement curve for model G is presented in Figure 11. Fig.11. Energy - displacement curve for model G. The maximum amount of energy absorbed by the specimen E is 72371 J. The energy when the ISBN: 978-960-474-383-4 84

structure gets into contact with the residual space corresponds to step 80, and its value is 55737 J, as shown in Figure 11. Thus, the maximum mass of the structure should be: (11) The energy and the maximum weight determined for model G are extremely high; therefore, this option cannot be regarded as applicable for rollover tests. For additional verification of the model G, angle of 4 0 was also considered. Results were not good in this situation either. It must be noted that the deformation of the structure is different from the experimental one in all the cases where a plate was used. 3.4 Model H deformation of the structure obtained by imposing a direct displacement to the roof frame at an angle corresponding to quasi-static test In model H, a displacement of the roof frame was predefined (fig. 12); its vector direction corresponds to the angle determined by R66 regulation specifications on the quasi-static test and its values correspond to the sidewall penetration of survival space. (12) In order to check the movement of the side wall with respect to the residual space, the corresponding displacement components have been sequentially increased up to their limit, until the horizontal and the vertical movement have been set at the values: și (13) The deformation of the structure for model H from the application of the displacement vector to the roof frame is shown in Figure 13. Fig.13. The resulting deformation of model H. Energy - displacement curve of model H is shown in Figure 14. Fig.12. Displacement vector applied to the model H. Moving the entire longitudinal roof frame was imposed by applying a vector to the outside top edge. The integration time step needed for calculation process was set at 1x10-2 s for a total computation time of 1s. Thus an accuracy of 1% is obtained, as specified in the Regulation requirements, for both the imposed displacement and the resulting force. The movement control was achieved by using the Cartesian method, which define the values of the vector components in the three directions. Knowing the application angle of the vector, a relationship of dependence between the vertical (dz) and the horizontal (dy) component is determined, and the following relationship results: Fig.14. Energy - displacement curve of model H. The maximum energy absorbed by model H corresponds to the energy at the moment of contact with the residual space and has the value of 7030.5 J. Therefore, the maximum mass of the structure in (7) should respect the relationship: (14) The displacement components were increased to 450 mm for Y axis and 144 mm for Z axis, maintaining the same value for the angle of the displacement vector. The maximum energy resulted was 8335J. This value is smaller than the impact energy which was experimentally determined (11607J). The deformation resulted from these displacements is compared with the experimental model in Figure 15. The deformation appearances are different. ISBN: 978-960-474-383-4 85

Deformations have the same appearance that the absorption energy of the stricter is bigger than the impact energy. Maximum mass of the structure of in (7) should follow the relation: (15) Therefore, the model I cannot be used as a valid model of experimental test. Fig.15. Deformation of model H and deformation of the experimental model. 3.5 Model I experimental deformation of the structure obtained by imposing a displacement to the roof frame at an angle corresponding to the final deformation experimentally obtained The structure deformation has been analyzed for the case when the roof frame is established to have the same displacement as the final deformation of the experimental model: 353 mm horizontally and 25 mm vertically. In Figure 16 it must be noted that the deformation is similar to the one produced during the experimental test at the time preceding the coming into contact with the residual space. At this stage, the resulting energy has been studied too. Fig.16. Deformation of model I and deformation of the experimental model. Energy - displacement curve of model I is shown in Figure 17. E min=0,75mgδh Fig.17. Energy - displacement curve of model I. The absorbed energy for the deformation in model I corresponding to the time of getting into contact with the residual space is 14696.3 J. Therefore, according to relation (2), 14696.3 J 11606.68 J. Using this relationship it must be noted 4 Conclusion Five different Ansys models has been quasistatically analyzed and compared with a reference model: three of them were deformed by a plate and the others two by a force applied directly to a frame of the roof. Their deformation appearances, the absorbed energies and computed masses were the criteria of comparison with the experimental model. The model that best complies with the criteria of analysis is model H, whose deformation was obtained by imposing a direct displacement to the roof frame at an angle determined by R66 regulation specifications on quasi-static testing. References: [1] Pankaj, D., Rollover and roof crush analysis of low-floor mass transit bus, Master of Science Thesis, Department of Mechanical Engineering, Wichita State University, 2006. [2] Chen, X.D., Yang, J., Zhao, X.D. and Fan, X.Q., Development situation and application of finite element method, Manuf. Inf. Eng. China, Vol. 39, No.11, 2010, pp. 6 8. [3] Song, J.H., Zhang, G.R., Liu, J.H., Han, B., Li, G.F., Research on current situation and development tendency for the simulation of automobile collision, Shandong Transp. Technol., Vol. 4, 2003, pp. 60 62. [4] Kyaw, S.O., Indera, N., Rais, Z., Sandro, M., Computer Modeling of Energy Absorbing Capability of Bus Superstructure for Rollover Safety, Journal of KONES Powertrain and Transport, Vol. 18, No.2, 2011, pp. 331-338. [5] Kyoung-Tak, K., et.al., Design of a composite roll bar for the improvement of bus rollover crashworthiness, Elsevier, Composites: Part B, Vol. 43, 2012, pp. 1705-1713; [6] Regulation 66-UNECE. [7] Micu, D.A., Straface, D., Farkas, L., et.al., Integration of a Concept Beam in a Complex Structure, INCER Proceedings, Second Edition, Bucharest, Romania, June, 2013. [8] Bojanowski, C., Gepner, B., et.al., Roof Crush Resistance and Rollover Strength of a Paratransit Bus, 8 th European LS-DYNA Users Conference, Strasbourg, May 2011. ISBN: 978-960-474-383-4 86