STRENGTH ANALYSIS OF FRAME STRUCTURES OF MEDIUM MONOCOQUE BUS USING FINITE ELEMENT METHOD AND TEST RESULTS *Khairul Jauhari a,b *, Mahfudz Alhuda b * amaster of Mechanical Engineering, Faculty of Engineering, University of Diponegoro Jl. Prof. Sudarto, SH Tembalang, Semarang, Indonesia 50275 bagency for the Assessment and Aplication of Technology Center for Machine Tools, Production & Automation. Balai MEPPO Gedung Teknologi II No. 251 Kawasan Puspiptek, Serpong, Tangerang Selatan, Banten, Indonesia 15314 *E-mail: khairul.jauhari@bppt.go.id, huda13@yahoo.co.jp ABSTRACT Monocoque body structure is widely used as the vehicle frame structure of a large passenger car such as city bus. This structure distribute the load through overall of body structure of the vehicle, so that thickness of the material can be reduced, consequently the total weight of the vehicle is reduced and it will save fuel consumption. In addition, since the impact forces in the collusion are also distribute through overall of body frame, this structure is also more safety for the passenger. In this paper, the strength of the monocoque body structure in medium size bus is analyzed due to various loading conditions. The finite element method and experiment result approach are used to calculate the stress strain through the structure. As a result, the combination of both techniques is proposed to the maximum stress and deflection that occurs on the monocoque frame due to symmetrical and asymmetrical static load below allowable values. Keywords: Bus, Experiment result, Finite element modeling, Monocoque, Static load 1. INTRODUCTION Mass transit system is a form of transportation that are prevalent in the area major cities. One kind of mass transportation systems that are used in urban areas is a bus rapid transport (BRT). Characteristic shape of the mode of transportation is the bus type has several sizes according to the length dimension ranging from the size of a medium has a length of 6 meters to 8 meters, while for the large size of its length ranging from 9 meters to 12 meters. From the observation in several major cities in Indonesia, found that in most of the region using a medium size buses as a form of mass transportation systems. It is based on the condition of the road is relatively very crowded and congested. In addition to observations about the size dimensions of observation also made in the form of a frame structure, frame structure generally still use the chassis and body structure are integrated. However, the trend of technology for the structural frame of the vehicle when a passenger bus has started many uses monocoque body structure. The advantages of the monocoque body structure as a whole is able to spread the influence of the load on the body structure, the total weight of the vehicle to be lighter so they can save fuel, but it is very possible to make a lower floor, so expect very friendly towards passengers. The purpose of this paper is to analyze about the level of the power bus monocoque body structure due to the influence of the medium on the size of the loading condition that can be known reliability. Studies have been conducted to determine the optimal shape of the skeletal structure models of passenger cars a lot [1]. Model parameters are determined by the needs of the design specifications, the model is analyzed using the finite element method and the data is based on test results [2-5]. Simulation analysis is shown in the form of von misses stress distribution and deformation that are validated by the test result data. 2. METHODOLOGY Modeling method is used to analyze the strength of the monocoque frame structure bus started with data on a monocoque structure, medium bus characteristics, determining the location of the position of the center of gravity or center of gravity (COG), and the load experienced by the bus body structure both position and magnitude were subsequently analyzed using CAE software (Pro Engineer Wildfire 4) and validated with data obtained from actual test results. 2.1 Monocoque frames In this paper, the shape of the monocoque body structure is divided into three kinds, and for the first modeled as an arrangement of several stems that form a closed loop (closed ring) which is the main structure of the monocoque body. Second, the secondary structure is composed of several rod that serves as a binder 1889
between the main structure with one another. Third, the nature of the tertiary structure as reinforcement secondary structure. Monocoque body structure which is used as a model for this analysis has a total weight (GVW) 6,153 tons as can be seen in Figure 1. The frame structure consists of several types of hollow steel rod which measures 100 mm x 50 mm and 50 mm x 50 mm which is located on the main structure (primary), secondary and tertiary. Material model used in the form of structural steel (SS400) with properties of properties which can be seen in Table 1 as follows: Table 1. Property of SS400 material. No. Material properties SI unit 1. The density 7,850 kg/m 3 2. Modulus of elasticity 200 Gpa 3. Poisson ratio 0.3 4. Shear modulus 0.7 Gpa 5. 6. Yeild strength Ultimate strength 250 Mpa 460 Mpa Figure 1. Model of monocoque frame. 2.2 3D Finite element model The monocoque frame is modeled as a 10-noded tetrahedral element (Tet-10) and brick-noded. Study experimentally and numerically in a straight type rectangular hollow rods were able to demonstrate that simple, more accurate meshing geometry quality and the number of elements produced less and less. Shape analysis carried out in the form of a linear static stress analysis to find the effect of stress distribution and shape deformation patterns due to the influence of static load. In this paper the finite element model is obtained to form a monocoque frame meshing with 76,134 and 217,424 nodal elements that can be seen in Figure 2. The boundary conditions are used as a form of representation that represents the real condition of the monocoque frame models. The boundary conditions represent pin-mounting bracket on the frame monocoque suspension, where the location of the axis is not allowed on the translation axis direction (axis) but only allowed the occurrence of any rotation on the axis. Loading is done on several conditions, namely when the bus is at a stationary state in which the load is derived from components and passenger weight itself (due to heavy load component), when the bus has a condition which occurs when braking (deceleration due to loading) and when the bus drove on the conditions of play (in torsional loading). Based on the results of finite element modeling, the determination of boundary conditions and the determination of some loading conditions, static analysis model of the monocoque frame is simulated with the help of CAE software (Pro Engineer Wildfire 4). 1890
Figure 2. Model meshing of monocoque frame. 2.3 Model testing Tests conducted included in the category of experimental testing, which in a way is stress-strain test that occurs in frame structures can be directly known in real accordance with the work load. The sensor used is a strain gauge, where the sensor is able to measure changes in strain (deformation) in the structure up to the order of microns. In general, due to the load, then the structure will undergo deformation / alteration. Deformation is so small in accordance with the planned security numbers by the manufacturer. Changes that occur as a function of the next load is detected by the sensor (strain gauge). The strain sensor will measure the deformation that occurred and the information will be detected as a change in resistance value changes. Changing the value of resistance is also so small that the Wheatstone bridge method is used to measure the changes that occur. All the changes that followed were recorded by using a dynamic strain meter equipped with a filter to ensure that measurable change is purely due to the mechanical changes during operation. The entire measurement data stored in the hard disk (notebook), so the tests performed can be illustrated as in Figure 3 and 4 as follows: Wheatstone bridge Figure 3. Schematic of experimental measurements. 1891
Figure 4. Strain gauge mounting location. 3. RESULTS AND DISCUSSION 3.1 Due to the weight of components Simulations based on the assumption that the bus was in a stationary condition. Monocoque framework is modeled as a simple beam modeling is weighed down under the influence of gravity experienced by the beam component. Reaction force on the axle is determined by the sum of the forces and moments due to the weight of components and component position. To facilitate the calculation done that, the expenses incurred in order to be regarded as including the severity of the load acting on a point. The point load is statically equivalent to the actual distribution of the load experienced by the vehicle. Weight components that occur considered as point loads acting on the frame. Passenger load of 2,240 kg divided into three parts load forces acting on the position where the integrated deck. Table 2 shows the mass and the weight of the component and its position along the frame. Axle reaction force occurs vertically at the center position of the suspension-mounting bracket. The calculations show that the reaction force on the front axle by 901 kg (8,841 N) and the rear axle of 3,267 kg (32,048 N). Large reaction force is still below the maximum capacity of the second axle where the front axle (3,000 kg) and rear axle (8,000 kg) compared with the technical specifications axle. Used as, the basis for the calculation of reaction force axle derived from the calculation only result of the weight of the components and load passengers when the bus in a stationary condition. In actual conditions, when the bus drove on a flat road, the reaction force is greater than the quiescent state when it is caused by the unevenness of the road surface in the form of steps or holes. Table 2. Mass and position of the load on the chassis monocoque. No. Componen Mass (kg) Load Position of ROH (mm) t (N) 1. Engine 350 3,433-1,400 2. Gear box 150 1,471-1,070 3. Pay load 840 8,240-500 4. Fuel tank 80 785 875 5. Pay load 1,400 13,734 2,587 6. Cab 421 4,136 5,450 1892
7. Cab 551 5,405 775 Figure 5 below shows the location of the position of the strain gauge simulation results for stress distribution and shape of the monocoque frame deformation patterns due to the influence of a heavy load vertical component. The greatest stress distribution simulation results is subject to strain gauge position no. 8 of 19.697 MPa, while for the same position actual stress that occurs at 19.236 MPa in the area to the left of the rear underframe. Stress characteristic pattern shape between finite element modeling simulation results with the test results have almost the same shape graph, it is shown in Figure 6 below. Figure 5. Contours Stress and deformation patterns due to the imposition of weight. Experimental Simulation Strain gauge position Figure 6. Graph of actual vs. stress characteristics simulation on vertical load conditions. 3.2 Imposition due to braking (deceleration) Simulations based on the assumption that the bus arrives at the braking. Reaction force on the axle is determined by the load is transferred from the front axle to the rear axle in accordance with the proportion that experienced a slowdown in the amount of 5.48 m/s 2 [6]. The calculations show that the reaction force on the front axle of 1,340 kg (13,141 N) and the rear axle of 2,829 kg (27,750 N). Figure 7. Below shows the location of the position of the strain gauge simulation results for stress distribution and shape of the monocoque frame deformation patterns due to the effect of the vertical load on the braking condition. The greatest stress distribution simulation results lies in the region to position strain gauge no. 7 of 24.626 MPa, while for the same position actual stress that occurs at 17.136 MPa in the area to the left of the rear underframe. Stress characteristic pattern shape between finite element modeling simulation results with the test results have almost the same shape graph, it is shown in Figure 8 below. 1893
Figure 7. Contours stress and deformation patterns due to the imposition of braking. Experimental Simulation Strain gauge position Figure 8. Graph of actual vs. stress characteristics simulation on the condition of the braking load. 3.3 Imposition of asymmetric (torsional) Simulations based on the assumption that the bus was driving conditions while turning. Reaction force on the axle is determined by the load transfer between the axle of the front axle to the rear axle and an axle between the members in proportion to the speed of the wheel bolsters experienced in the amount of 34.06 km/h [8]. The calculations show that the reaction force on the front axle left and right in a row by 32 kg (315 N) and 869 kg (8,526 N). So is the reaction force on the left and right axle rear of -62 kg (-607 N) and 3,329 kg (32,655 N). Negative reaction force on the rear axle to the left is what causes the asymmetric load. Figure 9. Below shows the position of the strain gauge layout simulation results for stress distribution and shape of the monocoque frame deformation patterns due to the influence of asymmetric load at the time of driving conditions while turning. Greatest stress distribution simulation results lies in the area of the strain gauge position no. 1 of 22.057 MPa, whereas for the same position actual stress that occurs at 25.074 MPa at underframe area near the back right side suspension bracket. Stress characteristic pattern shape between finite element modeling results with the test results have almost the same graphic form on underframe area, it is shown in Figure 10 below. 1894
Figure 9. Contours stress and deformation pattern for asymmetry loading. Experimental Simulation Strain gauge position Figure 10. Graph of actual vs. stress characteristics of the simulated asymmetry load conditions. For all three models loading conditions have been described previously, indicated that the monocoque frame experiencing the greatest stress by FEA simulation and experimental results of actual testing when loading due to the weight of (19.697 MPa at St. 8) and (28.266 MPa at St. 12), the condition of braking of (24.626 MPa at St. 7) and (26.796 MPa at St 1) and asymmetric loading condition for (22.057 MPa and 25.074 MPa at St 1). 4. CONCLUSION In this paper has been carried out starting from modeling to analysis of both static modeling and finite element simulation of the test results, where the monocoque frame undergone three static loading conditions, namely the loading due to the weight of components (vertical load), the current state and the imposition of asymmetric braking when driving due to conditions while turning. The maximum of actual stress occurs at the area near the rear underframe suspension bracket of 26.796 MPa. This value is acceptable because it is still far below the allowable stress (183.75 MPa) of the material SS400. 5. REFERENCES [1] Reimpell, J., Stoll, H., W. Betzler, J., 2001, The Automotive Chassis, Second Edition, Elsevier Science. [2] Gursel, K., Turgut, G.S., 2010, Analysis Of The Superstructure Of A Design Bus In Accordance With Regulation ECE R66, Journal of Science, GUJS : 71-79. 1895
[3] Teo Han Fui, Roslan Abd. Rahman, 2007, Static And Dynamic Sructural Analysis Of 4.5 Ton Truck Chassis, Jurnal Mekanikal, No 24: 56-57. [4] MAN Design Ltd, 2004, Design Of Commercial Vehicle Chassis And Body Structure. [5] Capoco Design Ltd, 2001, Side Impact Analysis Of A Passenger Bus Using Ansys/LS-Dyna, Vol.72 : 143-167. [6] Asmara Brata, K., 2004, Perancangan Dan Pengembangan Rangka Bus Pada Chassis Central Truss Frame Dengan Analisa Beban Puntir Statis Pada Kondisi Jalan Datar, Tesis Teknik Mesin, UI, Jakarta. [7] D. Thomas, Gillespie, 1996, Fundamental Of Vehicle Dynamic, First Edition, Society Of Automotive Engineers, Inc., Werrendale PA. [8] Sutantra, I. Nyoman, Sampurno, Bambang, 2010, Teknologi Otomotif, Edisi Kedua, Guna Widya, Surabaya. 1896