Thermo-Mechanical Coupled-Field Analysis of Car Front Bumper by Finite Element Approach

Transcription

1 Thermo-Mechanical Coupled-Field Analysis of Car Front Bumper by Finite Element Approach 1 Puttaswamiah S, 1 Assistant Professor, Department of Mechanical Engineering (Machine Design), EWIT, Bangalore, India Maruthi B H 2 Professor and HOD, Department of Mechanical Engineering (Machine Design), EWIT, Bangalore, India Dr. K Channakeshavulu 3 Principal, Department of Mechanical Engineering (Machine Design), EWIT, Bangalore, India Sharath S 4 PG Scholar, Department of Mechanical Engineering (Machine Design), EWIT, Bangalore, India Abstract: The automotive industry uses engineered polymers in a wide range of applications, as the second most common class of automotive materials after ferrous metals and alloys. A case study of the behaviour of car front bumper made of polymer material with respect to thermal and gravity loads were considered. The objective is to evaluate the mechanical behaviours such as displacements at the fit and clearance area due to thermal loading on the bumper and to determine the maximum Von-Misses stress for the applied load. For this purpose, the bumper system was categorized into five zones where deformation is critical. The analysis carried out was sequentially coupled thermo-mechanical analysis, elastic-plastic material model was considered. ANSA was used for preprocessing, ABAQUS/Standard FE solver best suited for nonlinear analysis was used for processing and METAPOST was used as the post-processor to view and analyze the data. Overall the results of the five critical zones were examined and the displacements found to be within the safe zone. The Von Mises stresses were also below the yield limit. Further gravity load was accounted to consider the mass of the bumper, which has a greater effect in Z deformation of the bumper and found displacements well within the safe zone. Keywords: Plastic, Automobile front bumper, Thermo-mechanical analysis, sequentially coupled analysis, deformation. I. INTRODUCTION Plastics have been instrumental in developing state-ofthe-art bumper designs and energy absorbing elements specialized in maximizing occupant and pedestrian safety. A plastic bumper usually weighs 50% less than one made of alternative materials, while absorbing four to five times more energy. While the disadvantage being the thermal expansion, heat causes things to expand. In the case of bumpers, what they are made of influences how much the bumper will grow when exposed to the sun. Thermal expansion coefficients are a direct measure of this expansion, and for aluminium, this number is low. Polymers tend to have coefficients which are many factors higher. This increased expansion causes more stresses on the protective structures of the automobile. The thermal expansion coefficients vary with different polymer materials, therefore this thesis intent to compare the thermal expansion coefficients and hence the stresses involved with each material and find out the optimum solution by using the finite element technique of analysis. Front and rear bumpers became standard equipment on all cars in What were then simple metal beams attached to the front and rear of a car have evolved into complex, engineered components that are integral to the protection of the vehicle in low-speed collisions. Today's plastic auto bumpers and fascia systems are aesthetically pleasing, while offering advantages to both designers and drivers. The majority of modern plastic car bumper system fascias are made of thermoplastic olefins (TPOs), polycarbonates, polyesters, polypropylene, polyurethanes, polyamides, or blends of these with, for instance, glass fibres, for strength and structural rigidity. The use of plastic in auto bumpers and fascias gives designers a tremendous amount of freedom when it comes to styling a prototype vehicle, or improving an existing model. Plastic can be styled for both aesthetic and functional reasons in many ways without greatly affecting the cost of production. Plastic bumpers contain reinforcements that allow them to be as impact-resistant as metals while being less expensive to replace than their metal equivalents. Plastic car bumpers generally expand at the same rate as metal bumpers under normal driving temperatures and do not usually require special fixtures to keep them in place. A Sequential coupled-field analysis This method involves two or more analyses, each belonging to a different field. You couple the two fields by applying results from one analysis as loads in another analysis. The type of analysis conducted for this project is the sequentially coupled-field analysis. The sequentially coupled approach is used when the stress and strain distribution in a model depends on the temperature field of the model whereas temperature 1195

2 distribution can be established without the availability of stress/mechanical strain distribution in the model. The thermal-mechanical problem is solved by first calculating the heat transfer analysis from which we get temperature distribution. For the stress analysis simulation, the nodal temperature data are read from the output file of heat transfer analysis simulation, and are applied on the model as a nodal temperature field in Abaqus/Standard by using predefined fields. The working steps for sequentially coupled approach are presented in below figure. A brief overview of the physical test conducted to account this structural deformation of the current geometry is done using a series of sodium lamps shunted in front and rear surface of the plastic bumper fascia with the auxiliary fittings. Heat from this source is compensated for different sources which increase the thermal potential of the structure. Figure 1: Working steps for sequentially coupled approach I. LITERATURE SURVEY Plastics play a major role in the automotive industry. As a result, the development of automotives without plastics is impossible. There are many advantages of using plastics for automobile such as: comfort, safety, low cost, weight reduction, corrosion and impact resistance, integration potential and design freedom. Because of these benefits, plastics have gained a permanent place for themselves in vehicle body design and will continue to play a prominent role in automotive applications in future. [1] Katarína SZETEIOVÁ explains in detail about the use of plastics in automotive market today and the average vehicle uses about 150 kg of plastics and plastic composites versus 1163 kg of iron and steel currently it is moving around % of total weight of the car. The automotive industry uses engineered polymer composites and plastics in a wide range of applications, as the second most common class of automotive materials after ferrous metals and alloys (cast iron, steel, nickel) which represent 68% by weight; other nonferrous metals used include copper, zinc, aluminium, magnesium, titanium and their alloys (Fig. 2). The plastics contents of commercial vehicles comprise about 50 % of all interior components, including safety subsystems, door and seat assemblies. [2] SAE paper presented by Yoshio Sugimoto of Kanto Auto Works Ltd shows the numerical solution of thermal displacement using ABAQUS. They have done experiments to prove that temperature significantly affects the material properties of plastic materials. [3] M. Avalle, M. Peroni and A. Scattina have investigated the influence of time and temperature on the behaviour of plastic material. Experimental tests were conducted to measure the strain rate at various temperatures, numerical analysis were also done to prove that mechanical behaviour of plastics is strongly influenced by temperature. [4] The type of analysis conducted for the current project is a nonlinear quasi-static analysis and we use implicit finite element method. F.J. Harewood and P.E. McHugh have compared the implicit and explicit finite element methods in depth and show that for problems with smaller nonlinearity and for simpler loading conditions implicit method is best suited. [5] II. PROBLEM DEFINITION This project investigates thermal loading effects on exterior plastic components of car. As a first approach to the problem it is decided to focus on the Front Bumper of a Car. The work in this project is performed to gain knowledge in FE-modelling of exterior plastic components of car. The project will cover solid mechanics in general but with a focus on: Thermal analysis, Element consideration, material testing, component testing, FE-guidelines for the bumper. III. METHODOLOGY In this project, computational FE method is used to determine the mechanical behaviour of the bumper system. Literature review on the bumper system was carried out to obtain the basic understandings of the project. Information like bumper material, heat source, and experiments conducted; and analysis methods were searched and reviewed. This project aims to develop a numerical method for thermal analysis of exterior plastic components of car. The study is intended to accurately model exterior plastic components and to contribute to a better knowledge of the behaviour of plastic parts under different loading conditions. All pre-processing analysis is performed using ANSA, finiteelement solver for Non-linear problems ABAQUS will be used to solve the analysis owing to the fact that the simulations carried out in this project can be categorized as 1196

3 nonlinear problems. Metapost will be used as the postprocessor to view and analyze the results. The mechanical properties of many plastics change with the change in temperature, E-Modulus being one of the important properties. Thus the temperature dependence of E- modulus of exterior plastics should be studied at the early stage. For the present analysis, Front Bumper of a car is selected due to its simple geometry and its suitable position which makes it easy to perform experiments. The shell model of the Front bumper with element size 5 mm is selected and the plastic clips are simulated with 1D elements including MPC Connectors owing to the fact that simulations are faster and easier with 1D element. The shell model of the Front bumper is attached to the fenders with various types of simulated 1D clips and the numerical analysis of the static and thermal deformation of the Front bumper is investigated. Finally using our knowledge of simulation of the plastic parts, the gap analysis of exterior plastic parts of car under different loading conditions is performed using numerical method. IV. FINITE ELEMENT METHOD FEA solution of engineering problems, such as finding deflections and stresses in a structure, requires three steps: 1. Pre-processing Table 1: Number of Shell Elements OVERALL NUMBERS OF SHELL ELEMENTs / PERCENTAGE (%) TYPE TRIA3 QUAD4 TOTAL NUMBER PERCENT (%) B Boundary Conditions The aim of this analysis is to calculate the displacements given by the thermal heating process, considering the temperature distribution calculated in the thermal simulation. The temperature distribution obtained in the first part of analysis is used for thermal loading. Ambient temperature is considered, 40 o C. Element type - Shell element : S (DOF- 6 Both Translational and Rotational) DOF: mounting points are constrained in all DOF. Design clearance is defined by using translator, radial and slide-plane connectors based on the regions. 2. Analysis 3. Post processing Pre-processing includes meshing and defining nodes, elements, material properties etc. For this analysis, preprocessing is carried out in ANSA. A Mesh model details The meshing of Bumper system is carried out in ANSA software. The finite element quality criteria considered in meshing is as provided in Table. Total number of elements and nodes used to create the FE model Bumper system is elements and nodes. Figure 3: Boundary conditions Connectors Translator connector works based on the local rectangular coordinates with a rigid motion in Y and Z & constrained motion of -1 to +1 in X. Radial thrust connector works based on local cylindrical plane with a radial clearance or constrained motion along radius of -1 to +1 & rigid motion along theta & Z directions Figure 2: Meshed model of car front bumper (front view) Figure 4: Boundary conditions Rigid elements The Hood, Fender and Bumper cross beams are considered as rigid (Element type: R3D) 1197

4 C Thickness details B Thermal Analysis Figure 5: Element thickness details V. RESULTS AND DISCUSSION A 1G Gravity Loading Figure 6: 1G Gravity Loading Total Deformation Plot The 1G gravity load is applied on all the elements in local global Z direction. This is done to ensure all the connections are proper and the huge relative movement if observed needs to be corrected between the parts. Density of all the materials plays a keen role in this first step of mechanical calculation. This deformed model is carried over to the next step to apply nodal thermal load. Figure 7: Thermal Analysis The first phase in the sequentially coupled thermal analysis is the Heat transfer analysis. On the onset of temperature using *Film card in Abaqus on the element sets, the nodal temperature distribution is requested from the output of the first step of this analysis. The overall nodal distribution of temperature is shown above. From the above contour the nodal distribution of bumper fascia is seen. Further the maximum temperature of 85 is observed on the top area of radiator grille, since the incidental solar radiance (thermal load) is more near to that zone. The temperature is conducted and convected to below area and there is a loss of heat which is clearly shown with respect to different zones. C Thermo-mechanical Analysis Table 2: Maximum Displacement at Fitting Areas due to Gravity load Fascia Grille Fender LED lamp Head lamp X Y Z Total From the table above, it is evident that the maximum displacements at all the fitting areas are below 0.5 mm as per the requirement. E ID E 1 E 2 E 3 E 4 E 5 E 5 LOCATION FRT BMP and FENDER HEAD LAMP and FRT BMP FRT BMP and WHEEL ARCH FRT BMP and MLDG-RAD GRILLE LED LAMP and FRT BMP (top) LED LAMP and FRT BMP (bottom) 1198

5 E 1 FRT BMP and FENDER Displacement plot in Y direction: Figure 8: E 1 FRT BMP and FENDER Max. Displacement is mm in X direction (Fitting direction) Max. Displacement is 0.19 mm in Y direction (Clearance direction) Max. Displacement is mm in Z direction (Clearance direction) The major directional displacements are within the nonvisible zone and are very low when compared to the overall surface area; also, it is within the clearance area and does not pose any problems. E 2 HEAD LAMP and FRT BMP Displacement plot in Y direction: Figure 9: E 2 HEAD LAMP and FRT BMP (Y direction) Displacements are measured at head lamp & fascia part in the mating regions. HEAD LAMP Max. Displacement is mm in Y direction Max. Displacement is mm in Y direction The Displacements observed in Y direction is more than the required displacement. This is not a problem for fitting or clearance but it will create more gap between the head lamp bracket and fascia and has an aesthetic concern. Adding clips at certain areas can prohibit this local movement. Further It needs to be checked with assembly. E 3 FRT BMP and WHEEL ARCH Displacement plot in Y direction: 1199

6 Displacement s are measured at Fascia & Grill in the mating regions. MLDG-RAD GRILLE Max. Displacement is mm in Y direction. FRT BMP Max. Displacement is mm in Y direction. The Displacements observed in X direction is more than the required displacement. This is not a problem for fitting or clearance but it will create more gap between the radiatior grille and fascia and has an aesthetic concern. Adding clips at certain areas can prohibit this local movement. Further It needs to be checked with assembly. Figure 10: E 3 FRT BMP and WHEEL ARCH The Max. Displacement is 0.96 mm in X direction (Fitting direction) The Max. Displacement is mm in Y direction (Clearance direction) The Max. Displacement is mm in Z direction (Clearance direction) E 5 LED LAMP and FRT BMP (top) o Displacement plot in Y direction: The major directional displacements are within the nonvisible zone and are very low when compared to the overall surface area; also, it is within the clearance area and does not pose any problems. E 4 FRT BMP and MLDG- RAD GRILLE Displacement plot in Y direction: Figure 12: E 5 LED LAMP and FRT BMP (top) (Y direction) Displacement s are measured at LED Lamp & Fascia in the mating regions. FRT BMP Max. Displacement is mm in Y direction. LED LAMP Max. Displacement is -2.60mm in Y direction. Figure 11: E 4 FRT BMP and MLDG-RAD GRILLE (Y direction) The Displacements observed in Y direction is more than the required displacement. This is not a problem for fitting or clearance but it will create more gap between the LED lamp bracket and fascia and has an aesthetic concern. Adding clips at certain areas can prohibit this local movement. Further It needs to be checked with assembly. 1200

7 E 5 LED LAMP and FRT BMP (bottom) o Displacement plot in Y direction: Figure 13: E 5 LED LAMP and FRT BMP (bottom) (Y direction) Displacement s are measured at LED Lamp & Fascia in the mating regions. FRT BMP Max. Displacement is mm in Y direction. LED LAMP Max. Displacement is mm in Y direction. E ID E 1 E 2 E 3 E 4 E 5 E 5 Table 3: Displacement data at the zones identified LOCATIO N FRT BMP and FENDER HEAD LAMP and FRT BMP FRT BMP and WHEEL ARCH FRT BMP and MLDG- RAD GRILLE LED LAMP and FRT BMP (top) LED LAMP and FRT BMP (bottom) PART FITTING DIRECTION DISPLACEME NT CLEARENCE DIRECTIONS DISPLACEMEN T X Y Z HEAD LAMP FASCI A GRILL FASCI A GRILL FASCI A LED LAMP FASCI A The Displacements observed in Y direction is more than the required displacement. This is not a problem for fitting or clearance but it will create more gap between the LED lamp bracket and fascia and has an aesthetic concern. Adding clips at certain areas can prohibit this local movement. Further it needs to be checked with assembly. D Von Mises Stress Figure 14: Von Mises stress 1201

8 The maximum average stress over the fascia is 15 MPa, which is lesser than the yield limit of 19 MPa. The stresses near the connections area and on the triangular elements are not considered. VIII CONCLUSION Sequentially coupled thermo-mechanical analysis was carried out on the front bumper system of the car which is made of plastics. Elastic-Plastic material model was considered. The mesh model for the analysis was created which satisfied all the quality criteria, hence the results are accurate. Loads and boundary conditions are accurately simulated to obtain the realistic loading conditions. The results depict the various displacements that have been due to the isothermal expansion of the plastic bumper. The deformation of the bumper is influenced by the geometry and the boundary conditions. The main area of interest for this thermal expansion is around the fit and clearance regions, as these regions have less initial gaps and higher deformation lead in overlapping. Gravity load was accounted to consider the mass of the bumper, which has a greater effect in Z deformation of the bumper. Overall the results of the five critical zones are examined and the displacements are slightly within the safe zone. The Von Mises stresses at critical regions were calculated and found to be well below the yield limit. VIII SCOPE FOR FUTURE WORK The procedure adopted to carry out this analysis paves the way to account thermal expansion of various industrial appliances used under thermal and electrical loads. All the plastic parts are of lower stiffness compared to metals, loads might be static or thermal. This analysis will help us to identify the stiffness for a thermal load. The results of this analysis can be taken as a reference to perform surface rigidity analysis of the bumper Further, slow speed frontal crash analysis can be done by considering the results of this analysis. REFERENCE 1. Katarína SZETEIOVÁ, Automotive materials Plastics in automotive markets today 2. FEM Analysis of Heat Deformation of Plastic Bumper, SAE Technical paper series Design and Analysis of an Automobile Bumper with the Capacity of Energy Release Using GMT Materials, World Academy of Science, Engg and Technology 52, Mechanical models of the behavior of plastic materials: influence of time and temperature, Latin American Journal of Solids and Structures 7(2010) Dr. Michael Fisher, Enhancing future automotive safety with plastics, Paper Number A continuum mechanics based four-node shell element for general nonlinear analysis, Eduardo N. Dvorkin and Klaus-Jürgen Bathe Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA 7. Introduction to Finite Elements of Engineering, Tirupathi R. Chandrupatla & Ashok D. Belegundu (PHI publications, 2011)