Strength Analysis and Simulation of Multiple Spot-Welded Joints

Transcription

1 Proceedings of the SEM Annual Conference June 1-4, 2009 Albuquerque New Mexico USA 2009 Society for Experimental Mechanics Inc. Strength Analysis and Simulation of Multiple Spot-Welded Joints Xin Zhang, Bing Liu F.tech R&D North America Inc, 1191 Horizon W Court, Troy, OH xzhang@ftech-rd.com ABSTRACT Resistance spot welding (RSW) is used widely to cut weight and lower cost in automotive industry. It is important to improve ultimate strength of spot-welded joints for automotive safety. The finite element models (FEM) for multiple spot-welded joints under tensile-shear load are investigated based on experimental results. The effect factors of multiple spot-welded joints strength are analyzed including spacing between two welds, edge distance, weld size and thickness based on finite element analysis (FEA). The analysis shows that weld size and thickness are primary factors affecting the strength of the joints for a given material. The optimized parameters are also discussed to improve structure strength based on effect factor analysis. 1. Introduction Resistance spot welding is used widely to connect the sheet metal for automotive body, even aircraft. With the higher requirement of automotive fuel economy, it is also used to connect automotive subframe in order to reduce the weight and lower the cost. For automotive body, there are about spot welds, and for a typical front subframe, there are more than 200 spot welds, it is very important to improve spot welded joints strength so that the subframe can endure specific loads during the automotive service life. A spot-welded joint can be considered to consist of three zones (Figure 1), the fusion zone (FZ), the heat affected zone (HAZ), and the approaching base metal zone (BM). Besides the welding metallurgy effect on the strength of spot-welded joint, the strength of spot-weld joint is also influenced by weld size, sheet thickness, edge distance, and spacing between the spot welds [1]. Due to the spot welding processing, the HAZ near FZ has the highest hardness, which means the area has the higher yielding and ultimate strength [2, 3]. According to spot welding failure location, there are interfacial nugget failure and pullout nugget failure. Pullout nugget failure is dominant for big weld size, which appears in HAZ and nugget remains intact. For tensile-shear load, the nugget rotation is the main cause of the pullout failure for spot-welded joint, the rotation of nugget makes high stress around nugget in load direction. Because of the complexity of spot-welded joint analysis, the finite element method has been extensively used by researchers in this field. Pan and Sheppard [4] applied a three-dimensional finite element analysis to determine the critical local stress intensity factor solutions for cracks with an elliptical shape emanating from

2 the main notch around the nugget in lap-shear specimens. Deng [5] developed elastic and elastic-plastic three-dimensional finite element model to analyze stress fields around the nuggets in tensile-shear and symmetrical coach peel specimens to understand the effects of the nugget size and the thickness on the interface and nugget pullout failure modes. The model is helpful to understand the spot weld, but the HAZ effect on the strength was not discussed in the models. BM BM HAZ FZ HAZ Figure1. Spot weld cross section Because failure load is directly related to ultimate strength and easy to obtain in practical test, the failure load is used to evaluate the structure strength in the paper. Three finite element models of spot-welded joint are investigated to compare their performance. For solid model and umbrella model, the HAZ influence is considered. The results are correlated to test results to find a better model for investigating effect factors of spot-welded joint failure load. The optimal weld size, spacing and edge distance are also discussed. 2. Spot weld FEM models The metal sheets are connected by spot-welded joint, all of forces and moments are transmitted through the spot-welded joint, the FEM model should represent these load transition. There are a variety of finite element models of spot-welded joint for different objectives. For strength and stiffness, the rigid umbrella bar, multiple rigid cylinder bars, and solid nugget model are applied [6, 7]. For NVH analysis, the rigid bar and hexa spot weld model are used. For crash analysis, the spring and rigid spot weld model are applied [8]. For solid model, it can represent spot weld real structure, give detailed stress field distribution including nugget, if nugget is not interested, then nugget is modeled as rigid to improve simulation efficiency. For umbrella model and solid model, HAZ can be considered into the model, which can evaluate the HAZ effect on spot-welded joint strength. Beam model is simpler, it is convenient to apply hundreds or thousands of spot welds in practical application. (a) (b) (c) Figure2 Spot weld model (a) single beam model, (b) umbrella spoke model, (c) solid model In the paper, the two sheets are modeled by shell element, though shell element got the underestimate results compared to solid element [8], shell element is widely used in industry application due to computing efficiency. The beam model, umbrella model and solid detail model are discussed for strength evaluation, as shown in Figure 2. Timoshenko beam (B31) allows for transverse shear deformation, it can be defined by the properties

3 of rigid, elastic material or elastic-plastic material. The disadvantage of rigid property is the weld size effect can not be considered. Due to the welding process, the mechanical properties at the FZ and HAZ close to FZ have been changed and are different from base material, even hardness is about 1.5 times that of base material, [9] the failure of spot weld is mainly from nugget rotation under tensile-shear load, which produces higher stress around nugget. As mentioned above, the material properties in nugget and HAZ have been changed due to metallurgy and hardening. It is difficult to get FZ and HAZ mechanical properties, but hardness is more convenient to obtain. Based on the hardness measurement, the HAZ mechanical properties are decided by a scaling factor of 1.5 of yield curve for the tested material. FZ material is defined by elastic properties. 3. Result Comparison between FEM and test Tested specimen was made from two U-shaped channels and joined by two spot welds. The base dimensions and layout of the tensile-shear specimen and the cross-tension specimen are shown in Figure 3(a) and Figure 3(b). The length of each U-channel is 150 mm, the width (flange to flange) is 100 mm, and the height is 35 mm. S represents the distance between the two welds, E represents the edge distance from the edge of the spot weld to the end of the U-channel for the tensile-shear specimen shown in Figure 3(a), D represents the weld size, T represents the sheet thickness, as shown in Table 1. Figure 3(b) shows a tensile-shear specimen. The material is DP590 which is extensively used in automotive. Table 1. Test specimen parameters S E D T Dimension, mm (a) (b) Figure 3. Test specimen layout (a) tensile-shear specimen, (b) Cross-tension specimen The FEM setup is shown in Figure 4. The solver is ABAQUS/explicit, general contact is considered. Figure 5 is the Von Mises stress field distribution of three FEM models. The highest stress is found at location 1 due to nugget rotation in Figure 5(b) and (c), where crack initially start, as shown in Figure 7. Compared to test result in Table 2 and Figure 6, both solid model and spoke model show good correlation, but spoke model gives a higher results, beam model gives a result which is lower than tested result. Though model a curve fitting is not good, the result error is 9%, which is acceptable in practical application. The results show the detail model can disclose the failure mechanism of the spot-welded joint and investigate effect factors.

4 Table 2. Test result and FEM calculations Test Model a Model b Model c Failure load, kn Constraint DOF 1-6 Constraint DOF 2-6 Figure 4. Tensile-shear FEM model set up 1 1 (a) (b) (c) Figure 5. FEM model stress distribution under tensile-shear load (a) beam model, (b) umbrella model, (c) solid model Test FEA-Solid FEA-Beam FEA-Spider 40 Load, kn Displacement, mm Figure 6. Failure load comparison between FEM calculation and test result

5 4. Analysis of factors affecting failure load of spot welded joints It was noted that spot welding parameters affect spot weld performance [1]. Based on the above analysis, the solid model is used to investigate weld size, thickness, edge distance and spacing under the tensile-shear load. The spacing between two nuggets is set to 10 mm, 20 mm and 60 mm separately; the edge distance is 5 mm and 21 mm compared to base model, spacing is 44 mm and edge distance is 11 mm. The results are shown in Table 3. Figure7. Model c failure mode Table 3. Spacing effect on failure load under tensile-shear load Spacing, mm Failure load, kn Based on the results, failure load is affected by the spacing between two welds, when the spacing becomes smaller, it shows lower failure load, which is because there is stress superposition between the two welds, the nearer the two welds, the higher the stress around the two welds. From Table 3, when spacing changes from 10 mm to 20 mm, the failure load increases 13.5%, and from 20 mm to 60 mm, the failure load just decreases 0.7%. Therefore, it is important to optimize spot weld layout to achieve the balance between cost and performance. For edge distance influence, small edge distance lowers the failure load, which also affects the failure mode, it may tear the edge base material out, see Table4. In table 5 and Table 6, it is apparent that the thickness and weld size have primary influence on the failure load, the failure load increases 66% when thickness changes from 1.0 mm to 1.6 mm; and there is average 15% increase for every 1.0 mm weld size change. It is effective method to using bigger weld size to meet failure load requirement and less weight. Table 4. Edge distance effect under tensile-shear load Edge distance, mm Failure load, kn Table 5. Thickness effect under tensile-shear load Thickness, mm Failure load, kn

6 Table 6. Weld size effect under tensile-shear load Weld size, mm Failure load, kn Application The basic spot weld finite element models have been discussed, there is an example of front subframe which includes 210 spot welds, considering efficiency of modeling and computing, the beam model is used to simulate the spot-welded joints. In order to test side members strength, a push force in the X-direction was applied until the side members buckled, the appropriate spot weld joints along the side members are very important, finite element model is set up for the simulation shown in Figure 8(a), side member material is JAC440, compared to test deformation mode (Figure 8(b)), there is a good correlation for failure modes, the buckling locations are exactly same, The error of loads between calculation and test is -5%. From above discussion, beam model does not have good fitting curve with test curve, but if spot-welded joints are not mainly objective to research, then they can be simulated by beam model, which will not affect strength performance. In practical application, it is not realistic to set up detail model, the beam model is usually used to model spot-weld joint. (a) (b) Figure 8. Front sub-frame side member strength simulation (a) and test (b) 6. Conclusions Three types of FEM have been investigated under tensile-shear load, HAZ mechanical properties should be considered in the FEM, which can improve FEM accuracy. Detail solid model is better way to investigate spot weld failure mechanism. Although beam model displacement-load curve does not have good correlation to test curve, the failure load error is acceptable, moreover, it is convenient to apply where a number of spot welds required. Based on the detail solid model, weld size, thickness, edge distance and spacing are also analyzed. It is found that the sheet thickness and weld size have primary effect on the strength, small spacing and edge distance should be avoided, and 45 mm spacing is recommended.

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