International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:14 No:06 76

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1 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:14 No:6 76 Finite Element Analysis of Fatigue Life of Spot Welded Joint and the Influence of Sheet Thickness and Spot Diameter M.Venkatasudhahar a, *, N.Dilipraja a, P.Mathiyalagan b, S.Venkata Subba Reddy a, P.Sathyaseelan a, K. Logesh a a Assistant Professor, Dept. of Mechanical Engineering,Veltech Dr.RR& Dr.SR Technical University, Chennai, Tamilnadu, India. b Professor, School of Mechanical Engineering, Veltech Dr.RR& Dr.SR Technical University, Chennai, Tamilnadu, India a kmrflowers@gmail.com, a dilip_raja1@yahoo.co.in, b mathis95197@yahoo.co.in, a sallasubbu345@gmail.com, a sathyaseelan156@gmail.com, a klogesh7@gmail.com Abstract Spot welding of sheet metal is essential in various manufacturing sectors such as automobile, ship building, aerospace, construction, refrigeration and air-condition. In this paper the impact of sheet metal thickness and spot weld diameter on the fatigue life of sheet metals is discussed. Three different sheet thicknesses (1.5mm, 2 mm and 3 mm) and two different spot diameters (2.5 mm and 5 mm) were selected for the study. The loads given during fatigue analysis were 35,, 5, 7, N respectively. Pro-E was used for designing the specimen and Ansys was used for analysis of fatigue life. Finite Element Analysis was used for the study to determine the relationship between fatigue life on spot weld diameter and thickness of the sheet metal is discussed. It was found that for higher number of sheet thickness and spot diameters respectively and respectively lower load, the number of cycles taken for the specimen to rupture was greater. In this case were 9.E+6. Index Term Spot welding, fatigue strength, sheet thickness, spot weld diameter. 1. INTRODUCTION Sheet metals are widely and extensively used in almost every sector of manufacturing, production and construction. Few of these include automobile, ship building, aircraft manufacturing and commercial purposes such as construction, refrigeration and air-conditioning. In every of these manufacturing sectors the sheet metals are subjected to spot welding for creating of joints. The parts fabricated by these sectors are subjected to fatigue load. Mechanical of structures and components is a serious concern in all types of industries. Robert Stone [1] estimated that between 5 to 9 % of these s are due to fatigue. According to him fatigue is defined as The process of progressive localized permanent structural change occurring in a material subjected to conditions that produce fluctuating es and strains at some point or points and that may culminate in cracks or complete fracture after a sufficient number of fluctuations. Fatigue life is one of the most important properties when designing such components. Fatigue damage has long been an important aspect of designing automotive and aeronautical components, and it has been extensively studied. The expected life of a product can be determined in the design stage itself by considering various factors such as static and dynamic load, nature of working environment, nature of weld joint, etc. A typical car or truck may have more than spot welds [2]. The majority of structural components under actual working conditions, in the customer s environment, are subjected to random amplitude service loading, during its operating life. According to Single there are three major fatigue life methods used in design and analysis. These three methods are -life method, the strain-life method, and the linearelastic fracture mechanics (LEFM) method. The numerical techniques based on the linear-elastic fracture mechanics with input data from laboratory tests is often used to establish fatigue criteria. The Strain Life approach is widely used at present. In this method strain can be directly measured and has been shown to be an excellent quantity for characterizing low-cycle fatigue. Strain Life is typically concerned with crack initiation, whereas Stress Life is concerned with total life and does not distinguish between initiation and propagation. Fracture Mechanics starts with an assumed flaw of known size and determines the crack s growth as is therefore sometimes referred to as Crack Life. Fracture Mechanics is widely used to determine inspection intervals. In general, the fatigue process is characterized by three distinct regions [3]. Region I is associated with the growth of cracks at low intensity factor ranges and is commonly believed to account for a significant proportion of the fatigue life of a component. Region II is the stable crack growth region and has been extensively studied for its technological importance [4-17]. Rapid crack growth occurs in region III and this region is typically thought to account for a small fraction of the total life. In this paper the relationship between sheet metal thickness and spot weld diameter on the fatigue life of a welded sheet metal joint has been studied. Much research work on welded sheet metal had been carried out by researchers. The results based on mechanical properties, defects in joints, had been published in their papers. FEA on welded joints and relationship with fatigues had also been analyzed and published. A few of those papers and their contents are shown below Alenius [18] have investigated Spot weldability of dissimilar metal joints between stainless steels and non stainless steels The aim was to determine the spot welding parameters for the dissimilar metal joints and to characterize the mechanical properties of the joints. Bin Zhou [19] has presented a methodology for determining the cohesive facture parameters associated with pull-out of spot welds. Since of a spot weld by pull-out occurs by mixedmode fracture of the base metal, the cohesive parameters for

2 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:14 No:6 77 ductile fracture of an aluminum alloy were determined and then used to predict the of two very different spotwelded geometries. Helmut Dannbauer[] modeled has given an overview of some common methods and standards for the assessment of welding seams and spot joints including spot welds and self piercing rivets. It has been shown, how the influence of the mesh quality and the element size can be minimized. For spot welds force based concepts and based concepts have been presented. It has been shown, that based concepts usually deliver better results and can be applied for self piercing rivets too, but the effort for the local mesh refinement is very high and error prone. Jeremy L. Lucas [21] have designed tests with the intention of addressing the highway sign problem and determining a solution as quickly as possible with the limited amount of material available for testing. It must be noted that these results are based on testing sample connections from one new sign panel. There are many variables (including weld quality and pre-torque) involved in manufacturing and erection that add to uncertainties in the quality of the signs in service and the test sign. Unfortunately, the testing procedures described did not adequately model the loading conditions found in the field because the load ranges that found infinite fatigue life cycles are still higher than the fatigue loads due to wind gusts in the field. Radakovic D.J.[23] have performed an analysis which includes a combination of finite element modeling and fracture mechanics calculations was used to predict the weld modes in the shear-tension tests of resistance spot welds in AHSS grades. In the finite element model, the base material and heat affected zone and weld properties were assumed to be homogeneous. The homogeneous model predictions agreed well with the test data for cases where pullout s occurred. This is because the pullout s most often initiated in the base metal outside of the notch at the perimeter of the weld. However, when comparing the homogeneous model results to actual test data, the predicted loads for the interfacial fractures were not consistent. Palma E.S. [22] presented a method for design durability qualification of a vehicle body shell. Field test data were used to produce an accelerated durability test that retains the entire damaging real time load histories present in the original test cycle. Fatigue analysis methods are used to access and compare the fatigue damage imposed during durability test and laboratory (torsion) experiments. The literature review carried out on various areas of the project in fatigue analysis and spot welded joints suggests that various tests have been carried out for lap-shear, inplane rotation; coach-peel, normal separation, and in-plane shear tests. But very limited performance data on fatigue life of spot welded joints have been reported in the open literature. 2. DESIGN OF THE PART The first part of the study was to design the part. Galvanized Iron (STEEL 43) sheet was selected as the work piece material for the study. The mechanical features of the selected materials are important aspects of resistance spot welding process since they have great influences on the properties of the welded joint and the quality of the welded structure such as the strength, fatigue life and so on. The arrangement of spot welded structure under investigation is shown in the Fig.1. The parts were designed using Pro-E software and had the following dimensions. Length: 154 mm Width: 5 mm Thickness: 3mm, 2mm, 1.5 mm Fig. 1. Plate Specification The mechanical properties of the selected material is shown in the below table Property Young s modulus (E) TABLE I Material Properties Value 2 x 1 11 Pa Density ( ) 785 kg/m 3 Poisson s Ratio.3 Tensile strength Yield strength x 1 6 Pa x 1 6 Pa 3. ANALYSIS The objective of the study is two folds. First is to predict the fatigue life of spot-weld joint under imposed loading conditions. Second is to examine the influence of spot welded diameter and sheet thickness on fatigue life of the spot welded joint. Detailed finite element modeling of spot welds is not feasible for 3-5 spot welds in a typical automotive body structure. Instead of the detailed modeling of the spot welds, a simple spot welded specimen is chosen for the fatigue analysis. The analysis is carried out by repeatedly loading i.e., fatigue loading the geometry to predefined time steps. The finite element analysis was conducted to simulate the mechanical behavior of the spot weld process. A detailed finite element model of a spot welded joint is required to calculate the states near the joint. Simple information of the study is shown below. Number of spot welds = 1no Spot diameter variables = 2 no s Thickness variables = 3 no s Number of loading = 5 no s Three parameters were selected as variables i.e., spot weld diameter, thickness of the plates and load values. Characterizing the capability of a material to survive the

3 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:14 No:6 78 many cycles a component may experience during its lifetime is the aim of fatigue analysis. Stress life is based on empirical S-N curves and then modified by a variety of factors. However strain life is based upon the equation relating the strain life. The parameters of the Strain Life are values for a particular material that best fit the equation to measured results. The strain life relation requires a total of 6 parameters to define the strain-life material properties; four strain life parameter properties and the two cyclic -strain parameters. Thus by simultaneously solving Neuber s equation along with cyclic strain equation, we can thus calculate the local /strains including plastic response given only elastic input Common Decisions to Both Types of Fatigue Analysis Once the decision on which type of fatigue analysis to perform, Stress Life or Strain Life, there are 4 other topics upon which your fatigue results are dependent upon. Input decisions that are common to both types of fatigue analyses are shown in Fig.2. Fatigue Analysis Type/ life ing Type Constant amplitude, proportional loading Constant amplitude, non proportional loading Non Constant amplitude, proportional loading o Bin size Non Constant amplitude, non Mean Stress Effects Goodman Soderberg Gerber Mean curves Mean dependent Multiple r-ratio curves Fatigue Modifications Value of infinite life Fatigue strength factor scale factor Interpolation type o Log on o Semi log o linear Multi Axial Stress Correction Component X Component Y Component z Component XY Component YZ Von mises Signed von mises shear principal Abs Fig. 2. Fatigue Analysis Chart 3.2. Types of cyclic loading Unlike static, which is analyzed with calculations for a single state, fatigue damage occurs when at a point changes over time. In the above descriptions, the amplitude identifier is readily understood. The second identifier, proportionality, describes whether the changing load causes the principal axes to change. If the principal axes do not change, then it is proportional loading. If the principal axes do change, then the cycles cannot be counted simply and it is non-proportional loading Spot welding process Spot welding is one of the primary methods to join sheet metals for automotive components. The resistance spot welding (RSW) is the most important joining method for joining sheets of metal. According to studies, about 9 % of the welds used in an automotive body assembly are RSWs. A typical vehicle contains more than 3 spot welds. This number may reach 5 and sometimes in couch and bus bodies. The advantages of using spot welding are that it is a quicker joining technique, no filter material is required, and that the low heat input implies less risk for altered dimensions during welding. Table II shows the relation with number of cycles and the induced respectively. TABLE III Stress Vs No. Of Cycles Cycles Alternating Stress (Pa) 5.1 x x x x x x x x x x Steps in Analysis Procedure The following are the important steps involved in problem solving, which are as follows: Step 1: Building the model Pro-E wildfire platform is utilized to build the model of plate with the following dimensions. Length of the plate = 154 mm Width of the plate = 5 mm Lapped length = mm Step 2: Assembly The next step is the assembly of the plates in the assembly module of Pro-E wildfire. The bottom plate is imported in to the assembly window and kept as default reference. Then the top plate is imported at the same compass location and assembled. Step 3: Creation of spot weld 1. Enter Assembly mode and retrieve the assembly. 2. Click Applications > Welding. The WELDING menu appears. 3. Set up the welding environment. 4. Choose Spot from the WELD ROUTE menu. 5. A dialog box appears, listing elements of the weld feature that need to be defined. Spot Refs - Specify geometric references for the weld.

4 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:14 No:6 79 Penetration - Specify the penetration depth. Measurements - Create measurements used to control welding parameters 6. Locate the weld by referencing datum points. Create or select datum points for locating the weld using options in the SELECT POINT menu. Choose Create and create datum points, or Select and pick existing datum points. 7. When finished specifying reference points, choose done from the FEATURE REFS menu. 8. Enter the penetration distance. 9. To conclude feature creation, choose OK from the dialog box. Step 4: Meshing the model In this section ways to divide the model assembly in ansys is discussed, such that it has enough nodes, to make an accurate enough analysis. Before detailed FEA, optimization with element size ranging from.1 to.5mm concluded that an element size equal to.5mm yields optimum results. The conclusion was in agreement with the earlier studies which concluded that an element size in the neighborhood of.5 mm yielded satisfactory stable crack growth predictions under constant amplitude loading. Number of elements = 335 Number of nodes = 1711 Element type = Solid Element Midsized Nodes Step 5: Performing Fatigue analysis The fatigue analysis procedure is based on life method. Stress Life is concerned with total life and does not distinguish between initiation and propagation. Using the boundary condition the plate was modeled and defined to analyze the plate assembly, we need to apply the appropriate boundary conditions. The model is considered to be one single beam of cantilever type. I.e. At one end all the displacement degrees of freedom are arrested and load is applied at the opposite end in vertical direction. Two load cases are applied to the free end of the specimen: +ve N at each corner. The time at the end of the load step is 1 seconds. ve N at each corner. The time at the end of the load step is seconds. ing type is of constant amplitude. 4. RESULT AND DISCUSSIONS The finite element analysis is conducted to simulate the life of spot welded joints when subjected to fatigue loading. A finite element model is generated using the commercial software. The distributions in the weldment and their changes during the loading condition are determined. The fatigue analysis is performed using Ansys workbench to determine and strain results of the finite element model. The results of the maximum principal es are used for subsequent fatigue life analysis. The distribution of the spot welded specimen is presented in Fig.3. Fig. 3. Von-Mises Stress value In Fig.4, the maximum around the nugget is MPa, which is well below the tensile strength of the material. Fig. 4. Stress value around the nugget 4.1. For spot diameter of 2.5 mm with varying sheet thickness Analysis is carried out on the specimen for a spot diameter of 2.5 mm with varying sheet thickness. and fatigue life values for a spot diameter of 2.5 mm with a sheet thickness of 3 mm are shown in Table III. TABLE IIIII Case 1 (sheet thickness of 3 mm) E E E E E+4

5 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:14 No:6 TABLE V Case 1 (sheet thickness of 1.5 mm) 1.E+4 8.1E+4 2.E+5 8.E+5 1.E+6 Ts = 3 mm E E E E E+3 Fig. 5. Case 1 (Sheet thickness of 3 mm) A and fatigue life values for a spot diameter of 2.5mm with a sheet thickness of 2mm is shown in Table IV. TABLE IVV Case 1 (sheet thickness of 2 mm) Ts = 1.5 mm 1.E+3 7.5E+3 2.E+4 8.5E+4 1.E E E E E E+3 Fig. 7. Case 3 (Sheet thickness of 1.5 mm) 4.2. For spot diameter of 5 mm with varying sheet thickness Analysis is carried out on the specimen for a spot diameter of 5 mm with varying sheet thickness. and fatigue life values for a spot diameter of 5 mm with a sheet thickness of 3 mm are shown in Table VI. TABLE VI Case 1 (sheet thickness of 3 mm) E+6 Ts = 2 mm E E E+5 2.E+3 1.5E+4 8.E+4 2.E+5 5.E E+4 Fig. 6. Case 2 (Sheet thickness of 2 mm) A maximum and fatigue life value for a spot diameter of 2.5 mm with a sheet thickness of 1.5 mm is shown in Table V.

6 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:14 No:6 81 TABLE VII Case 2 (sheet thickness of 1.5 mm) Ts = 3 mm E+5 7.E+4 5.E+5 1.5E+6 5.E+6 9.E E E E+4 Fig. 8. Case 1 (Sheet thickness of 3 mm) E+3 A maximum and fatigue life value for a spot diameter of 5mm with a sheet thickness of 2mm is shown in Table VII. TABLE VII Case 1 (sheet thickness of 2 mm) Ts = 2 mm E E E+5 1.5E+4 8.E+4 3.E+5 9.E+5 2.E+6 Fig. 1. Case 3 (Sheet thickness of 1.5 mm) E E Effect of sheet thickness The number of cycles to is plotted against sheet thickness for spot diameter values of 2.5 mm and 5 mm. 1.E+7 1.5E+4 8.E+4 3.E+5 9.E+5 2.E+6 Fig. 9. Case 2 (Sheet thickness of 2 mm) Ts = 2 mm A maximum and fatigue life value for a spot diameter of 5mm with a sheet thickness of 1.5mm is shown in Table VIII. 1.E+6 1.E+5 Ts = 3 mm Ts = 2 m 1.E+4 Ts = 1.5 mm 1.E+3 1.E Fig. 11. Effect of sheet thickness for spot diameter of 2.5 mm

7 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:14 No: E+7 1.E+6 1.E+5 1.E+4 1.E Ts =3 mm Ts =2 mm Ts =1.5 mm Fig. 12. Effect of sheet thickness for spot diameter of 5 mm 5. CONCLUSION A spot welded joint on a Galvanized Iron (STEEL 43) sheet metal was analyzed using Finite Element Analysis. The variables selected for the study were sheet metal thickness (1.5 mm, 2 mm and 3 mm), diameter of the spot weld (2 mm and 3 mm) and fatigue load. It was found that lower load i.e., 35 N induced lower to the joint and thereby increased the fatigue life of the joint. The respective fatigue life for the specimen with varying thickness revealed that for lower load and higher sheet thickness the life expectancy was higher. Notably 9.E+6 for 3 mm sheet, 2.E+6 for 2 mm sheet and 7.5E+5. This reveals that the due to fatigue was lower for thicker sheet metal which in this case was 3 mm. Noting the relationship between sheet metal thickness and weld spot diameter, the following information was gathered. Within a same spot diameter, i.e., 2.5 mm. The fatigue life was higher for a 3 mm joint. However comparing the result between the two different spot diameters, it was found that the 5 mm spot diameter with 3 mm sheet metal thickness had still higher fatigue life, i.e., 9.E+6. It can be concluded that for a joint to have higher fatigue life, the criteria required is sheet metal thickness should be high, weld spot diameter should be high and applied load i.e., fatigue load should be low. In this study the values for the respective parameter were 3 mm sheet metal thickness, 5 mm spot diameter and 35 N loads. The fatigue life for this case was 9.E+6. [9] Wu F, Ni C, 6 Eng. Fracture Mech.DOI: /j.engfracmech [1] Meshii T and Watanabe K 3 Nuclear Eng. Design [11] Borrego L P, Ferreira J M, Pinho da Cruz J M and Costa JM 3 Eng. Fracture Mech [12] Kim J and Shim D Int. J. Fatigue [13] Shin C S and Cai C Q 7 Int. J. Fatigue [14] Amrouche, Mesmacque G, Garcia S and Talha A 3 Int. J. Fatigue [15] Jiang Y and Feng M 4 In: Fracture Methodologies and Manufacturing Process, uly 25 29, 4, San Diego, California, ASME PVP-Vol. 474, PVP4-2297, [16] Ding F, Feng M and Jiang Y 7 Int. J. Plasticity [17] Alam M S and Wahab M A 5 Int. J. Pressure Vessels Piping [18] Alenius M., pohjanne P., Somervuori M., and Hanninen H. (6), Exploring the Mechanical Properties of Spot Welded Dissimilar Joints for Stainless and Galvanized Steels, International of Welding, pp [19] Bin Zhou, Thouless, M. D. and Ward S. M. (6), [] Helmut Dannbauer, Christian Gaier, Klaus Hofwimmer (5), Fatigue Analysis of Welding Seams and Spot Joints in Automotive Structures, International of Fatigue, pp Jeremy L. Lucas and Thomas E. Cousins (5), Determination of Remaining Fatigue Life of Welded Stud Details on Overhead Aluminum Sign Panels in Virginia, Department of Civil and Environmental Engineering, Virginia Polytechnic Institute and State University. [21] Palma E.S. and Vidal F.A.C. (2), Fatigue Damage Analysis on Body Shell of a Passenger Vehicle Journal of Materials Engineering and Performance, Volume 11(4), pp [22] Radakovic D.J. and Tumuluru M., (8), Predicting Resistance Spot Weld Failure Modes in Shear Tension Tests of Advanced High-Strength Automotive Steels, Journal of Welding Reasearch, vol 87, pp Predicting the Failure of Ultrasonic Spot Welds by Pull-out from Sheet Metal, Department of Material Science and Engineering, University of Michigan. REFERENCES [1] Robert Stone, Fatigue Life Estimates Using Goodman Diagrams [2] M. M. Rahman; Rosli A. B., M. M. Noor, M. S. M. Sani, M. R. M. Rejab, (7) fatigue analysis of spot-welded joints using finite element analysis approach, Regional Conference on Engineering Mathematics, Mechanics, Manufacturing & Architecture (EM3ARC). [3] Shigley J E, Mischke C R and Budynas R G 4 Mechanical Engineering Design 7th ed. McGraw-Hill Companies, Inc., New York, NY. [4] Molent L, Jones R, Barter S and Pitt S 6 Int. J. Fatigue [5] Huang X and Moan T 7 Int. J. Fatigue [6] Sansoz F and Ghonem H 3 Mater. Sci. Eng. A [7] Korda A, Miyashita Y, Mutoh Y and Sadasue T 7 Int. J. Fatigue [8] Ding J, Hall R F, Byrne J and Tong J 7 Int. J. Fatigue