DURABILITY ANALYSIS OF HCV CHASSIS USING FPM APPROACH

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1 DURABILITY ANALYSIS OF HCV CHASSIS USING FPM APPROACH Shailesh Kadre Principal CAE Analyst Mahindra Engineering Services #128/A, Sanghavi Compound, Chinchwad Pune, Shreyas Shingavi CAE-Analyst Mahindra Engineering Services #128/A, Sanghavi Compound, Chinchwad Pune, Swapnil Patil CAE-Analyst Mahindra Engineering Services #128/A, Sanghavi Compound, Chinchwad Pune, Manoj Purohit Senior CAE-Analyst Mahindra Engineering Services #128/A, Sanghavi Compound, Chinchwad Pune, Abbreviations: FEA: Finite Element Analysis, FPM: Fatigue Process Manager, RLDA: Road Load Data Acquisition Keywords: Cab mount, FPM, Conventional load cases, RLDA, Inertial Loading, Optimization Abstract In conventional FE analysis of chassis, full frame model is used. Durability analysis is performed for historical loads such as inertial, racking and twists loads. Also, for performing component level analysis, full frame model is used to retain accuracy related to boundary conditions. In the proposed simplified approach the forces obtained through road tests were effectively used to come out with durability load cases for the component level analysis. Stress analysis was performed followed by detailed strain based fatigue life calculations with the help of HyperWorks Fatigue Process Manager (FPM), which uses strain - life method. It was observed that this approach produces realistic results with considerable cost savings on pre and post processing efforts as well as through reduced solution time. The simplified approach was tried on cab mounting system of a truck assembly. All the CAE results were validated against the test data. Good test correlation was observed between test and FEA results. The results obtained in this methodology were also compared with the other commercially available FE solvers. Introduction Growing competition in automotive market makes it more and more necessary to reduce the development time and cost of the product development process. One of the most costly phases in the vehicle development process is the field durability test and high expenses for this phase can be attributed to the number of prototypes used and time/efforts needed for its execution. Also, multiple iterations during designing, building and prototype testing are no longer affordable against the time and cost constraints for developing a competitive product. Today, analytical tools in the form of computer simulation [1] [2] have been developed to such a level that they reliably predict performance. Hardware prototypes cannot be made in early design phase, however, today with the use of CAE tools virtual models can be created to accurately represent physical models and to take right decisions at the right time. In Heavy Commercial Vehicle (HCV) cab mounting system is utilized to isolate driver from road generated vibrations. The vehicle cab is typically mounted on the chassis with the help of four supports. At the rear end it is mounted on Cab crossmember center channel. Isolators are used to mount the cab on this center channel. The center channel is attached to the frame through cross member end bracket. The front of the cab is mounted on frame using an assembly of two brackets namely cab side mounting bracket and frame side cab mount. The cab side mounting bracket is connected to frame side cab mount through a bushing to provide vibration isolation. The frame side cab mount is bolted to the frame rail. The front cab mount also provides facility to tilt the cab for inspection/maintenance of the under-cab systems. The cab mounting system with its main components is shown in Figure 1. Simulation Driven Innovation 1

2 FIGURE 1: SCHEMATIC DIAGRAM OF CAB MOUNTING SYSTEM [NOTE: ALL THE IMAGES SHOWN ARE PROPRIETARY AND SHOULD NOT BE USED, FOR ANY PURPOSE, WITHOUT PRIOR PERMISSION OF NAVISTAR INC.] Objective and Methodology Sub-systems like cab mounting, fuel tank mounting, battery box mounting are prone to failures because of heavy dynamic loads during vehicle operation. To evaluate the structural performance of these sub-systems of vehicle it is necessary to determine g levels of acceleration at these systems. Also, once the test data is available, it can be used for correlating it with the FEA results and then load history can be used for reliable analysis to develop the product. The methodology helps in moving towards virtual testing and to avoid/reduce physical testing of the sub-systems to adhere to the constraints on time and cost. Objectives of the simulation are, 1. To carryout strength analysis of the cab mounting system. 2. To correlate the test results with FEA results using road load data. 3. To determine fatigue life of the components of the cab mounting system. 4. To optimize the cab mounting design. Methodology: Following steps are were followed : 1. Peak inertial event analysis using OptiStruct [3] to find out the stresses. 2. Fatigue life calculation based on RLDA using FPM. 3. FEA and test result correlation. 4. Topology optimization using OptiStruct for mass reduction. 5. Comparison of fatigue life estimates for various design iterations. Simulation Driven Innovation 2

3 Analysis of Cab mount System Method 1: Stress Analysis using conventional peak inertial loading The baseline design of cab mount is shown in Figure 2. FIGURE 2: BASELINE DESIGN Results for Baseline Design As stated in objective, design must satisfy inertial loading acceptance criteria, so linear static analysis is performed. Both the ends of truncated frame are constrained in all directions. Acceleration is measured during testing on the cab mount in all the directions. Based on the magnitude of measured acceleration in each direction, inertial loads are scaled and results are re-calculated. These stresses are then compared against acceptance limit and conclusion is drawn. The von Mises stress result summary for peak inertia loading is given in table II. TABLE II.: STRESS RESULT SUMMARY Component Maximum von Mises stress (MPa) lateral longitudinal vertical Frame side Cab mount 2 *YS 0.7 * YS 1.3 * YS Cab side mounting bracket 0.9 * YS 0.3 * YS 0.2 * YS Note : YS = Yield Strength of the material From the above table it is observed that stresses for Frame side cab mount are higher than the Yield stress limit. Hence, current design is not satisfactory as stresses are much above the target acceptance criteria. So, design changes are necessary to increase the structural strength. Stress contour plots for failure events are shown in Figure 3 and 4. Simulation Driven Innovation 3

4 FIGURE 3: STRESS CONTOUR OF FRAME SIDE CAB MOUNT FOR LATERAL INERTIAL LOADING FIGURE 4: STRESS CONTOUR OF FRAME SIDE CAB MOUNT FOR VERTICAL INERTIAL LOADING Simulation Driven Innovation 4

5 Method 2: A) Stress Analysis using "Time Slicing" Methodology for Inertial loading This method is useful when the load-time history data is available.the peak load magnitudes for the frame side cab front mount were taken from the load time history. The load steps are developed by taking the peak loads in respective directions by "Time Slicing" the load data. The details of the load magnitudes in lateral, longitudinal and vertical directions are given in Table III. Figure 5 shows the load-time history measured with the help of load cell for a particular event. FIGURE 5: LOAD TIME HISTORY DATA MEASURED BY LOADCESS IN VERTICAL DIRECTION Note : LT is the magnitude of the Load at time T TABLE III:. DETAIL OF THE LOAD MAGNITUDE Load Magnitude/ Direction Force in (N) Loadcase1 Loadcase 2 Loadcase 3 Loadcase 4 Loadcase 5 Longitudinal LT1 LT2 LT3 LT4 Vertical LT5 LT6 LT7 LT8 Lateral LT9 LT10 LT11 LT12 LT13 The result summary of the peak load analysis for the frame side cab front mount is given in Table IV. From the analysis it is observed that maximum von Mises stresses on the frame side front cab mount and the frame rail are below the yield strength of the respective materials. Therefore, it can be concluded that these components are adequate for peak loading. Figure 6 shows the von Mises stress plots for the frame side front cab mount. TABLE IV:. STRESS RESULT SUMMARY Cab front mount components Maximum von Mises Stress (MPa) Loadcase1 Loadcase 2 Loadcase 3 Loadcase 4 Loadcase 5 Simulation Driven Innovation 5

6 Frame Side Front Cab Mount 0.2 * YS 0.3 * YS 0.3 * YS 0.2 * YS 0.1 * YS Frame rail 0.1 * YS 0.2 * YS 0.2 * YS 0.1 * YS 0.1 * YS FIGURE 6: STRESS CONTOUR OF FRAME SIDE CAB MOUNT FOR LOADCASE-1 B) Fatigue analysis (Conventional Approach) : The result summary of the fatigue analysis for the frame side front cab mount for Loadcase-1 is given in the Table 5. From this table, it is observed that number of failure blocks for these components are above 3. Therefore, it can be concluded that these components are adequate for the fatigue loadcase analysis. TABLE V. FATIGUE ANALYSIS RESULT SUMMARY Component Name Frame Side Front Cab Mount Maximum von Mises Stress (MPa) High Stress Location E (GPa) Loadcase-1 Cycles Calculated with commercially available Fatigue Solver Life in Blocks Remarks 0.2 * YS Kt= Infinite Infinite >> 3 Method 3 1. Life prediction using FPM (Fatigue process Manager) approach: In order to calculate fatigue life of the cab mounting system it is necessary to take into account all the relevant parameters like material behavior, load time history data collected during the RLDA, notch factors, cycles to failure, etc. FPM was found to be quite suitable to quickly setup the fatigue analysis task. As the name indicates fatigue manager itself manages all the complex material and load data required for the life prediction. The step by step guidance provided in FPM helps in quickly adopting all the necessary input parameters. Simulation Driven Innovation 6

7 Based on the loading history, FPM predicted life in no. of cycles. Predicted life is compared with target life and conclusion is drawn: TABLE VI. FATIGUE LIFE SUMMARY Component Name E Life in Cycles by FPM Life in Blocks by FPM Remarks Frame side Cab mount E >>3 From the above table it is observed that life of Frame side cab mount is higher than the target blocks of life (3 blocks). Hence, current design is satisfying acceptance criteria. The life estimated using FPM and life observed during testing are comparable with each other. Stress Contours for frame side cab mount is shown in Figure 5. FIGURE 5: STRESS PLOT OF FRAME SIDE CAB MOUNT FOR FATIGUE LOADING 2. Optimization of Frame side cab mount : Based on the analysis results for peak inertia loadcase and fatigue analysis results, it is observed that the location of maximum stress is same for all the cases, whereas other regions of the bracket show stresses much lower than the target stress limit. This indicates that there is a scope to optimize the frame side cab mounting bracket. Hence, topology optimization is carried out using "OptiStruct" for worst load. With Mass reduction as an objective and maximum stress as a constraint, optimization is performed. Figure 6 shows the strain energy plot for topology Optimization. Simulation Driven Innovation 7

8 FIGURE 6: STRAIN ENERGY PLOT FOR TOPOLOGY OPTIMIZATION Based on the strain energy results it is decided to put additional material near small rib as this was the common region which shows high stresses for all the load cases. Transparent area indicates the potential region for material reduction. Hence, based on this plot wall thickness of bracket is reduced by 15% and along with reduction in thickness for larger rib. For new design approximately 12% mass reduction was achieved. The above mentioned structural analysis was repeated for ensuring the structural adequacy of the system. Based on the results of new design, it is observed that the light weight bracket is not only structurally adequate under peak inertia loading but also satisfies fatigue failure criteria. Benefit Summary 1. It is very easy to graduate to FPM applications for a HyperMesh user. 2. If the tokens available are adequate, it is possible to use FPM modules without any additional investment. 3. All complicated parameters required for life prediction can be easily combined together to get accurate results with the help of Fatigue Process Manager (FPM). The step by step approach of including input data is user friendly and helps in getting accurate results. 4. OptiStruct is very good tool for optimization. Conclusions It can be said that FPM is very useful tool to calculate life of component using available load history. OptiStruct yields very good solution which assures adequate strength of the component along with minimum cost. This methodology of product development saves time and cost needed for quick and reliable component design. Simulation Driven Innovation 8

9 ACKNOWLEDGEMENTS The author would like to thank the following for their valuable technical inputs: 1. Ajit Jha, Strength and Durability Analysis Manager, Navistar Inc., USA. 2. Deepak Nidgalkar, Head- CAE, Mahindra Engineering Service Ltd. REFERENCES [1] Gillespie, T. D., Fundamentals of Vehicle Dynamics, Society of Automotive Engineers Inc., USA, 1992 [2] Michael Blundell and Damian Harty, Multibody Systems Approach to Vehicle Dynamics, Elsevier Butterworth-Heinemann, 2004 [3] Reference guide of OptiStruct [4] Reference guide of MotionView 9.0 of Altair HyperWorks [5] Reference guide of RADIOSS, MotionSolve, and OptiStruct ABOUT THE COMPANY Mahindra Engineering Services Ltd., a Mahindra Group company, provides design analysis testing solutions to number of clients which includes Navistar Inc.' and Mahindra Navistar and Automotive Limited (a Joint Venture between Mahindra & Mahindra and Navistar Inc. to develop and market commercial vehicle product line). Simulation Driven Innovation 9