Assessing the Effects of Residual Stresses on the Fatigue Strength of Spot Welds
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Assessing the Effects of Residual Stresses on the Fatigue Strength of Spot Welds Residual stress at the nugget's edge was taken into consideration in the evaluation of fatigue strength for spot welds of various shapes and dimensions BY D. H. BAE, I. S. SOHN, AND J. K. HONG ABSTRACT. This paper presents a welded structures, accurate stress analysis tion, he proposed material properties be method for fatigue strength assessment of and systematic fatigue strength assess- considered in modeling the weld nugget spot welds, which incorporates the effects ment are needed. Since it is very difficult and the methodology for modeling the of welding residual stress. Residual stress to determine directly the fatigue design electrode. Kim and Eager (Ref. 10) inves- analysis with a welding thermal history criteria for actual structures, it is a typical tigated transient temperature response was evaluated first, and then stress analy- practice to assess fatigue strengths using during spot welding using high-speed cin- sis for fatigue was performed. mock-up specimens with structural and ematography and infrared emission mon- First, the residual stresses of spot welds mechanical characteristics similar to the itoring. Huh and Kang (Ref. 6) developed were calculated using a nonlinear finite el- actual structures (Refs. 1, 2). a three-dimensional (3-D) FEA model for ement analysis (FEA). To validify the FEA Many investigators have numerically electro-thermal analysis of the resistance results, the calculated residual stresses and experimentally assessed fatigue spot welding process. were compared to those measured by X- strength and provided considerable data For fatigue analyses, the stress cate- ray diffraction. The residual stress distrib- on the fatigue strength of various spot- gories are generally evaluated by using a utions showed good agreement between welded joints (Refs. 2, 3). To apply the nominal stress, a structural hot spot stress, calculations and measurements. Then, to data into the fatigue design of actual spot- and a notch stress with consideration of evaluate the fatigue strength of spot welds, welded thin sheet structures, welding stress concentration effects. The choice of stress analyses were performed under ten- residual stress should be properly consid- stress category depends on the method sile loading on various dimensions and ered since it affects fatigue crack initiation used to express the fatigue strength data in shapes of spot welds. and propagation at the nugget edge of a the fatigue assessment (Ref. 11). Among Based on the results, the stress ampli- spot weld. Nevertheless, there are very few these categories, nominal stress and notch tude ((Ya-res)' which took into considera- fatigue strength assessments that consider stress can be considered as the mechanical tion welding residual stress at the nugget welding residual stress because welding parameters for fatigue strength assess- edge of a spot weld, was calculated using a residual stress analysis is quite compli- ment of a spot-welded joint. modified Goodman equation. Using the cated (Refs. 4-6). In this paper, the maximum stress stress amplitude (Ya-res at the nugget edge, Research on welding residual stress range at the edge of the spot weld nugget, the load range (AP)-fatigue life (Nf) rela- analysis is summarized as follows: Tsai et instead of nominal stress, was correlated tions from the fatigue tests can be re- al. (Ref. 7) proposed an FEA model for with the fatigue strength of a spot weld. A placed by the Ga.res -Nf relations. weld nugget generation in the resistance nonlinear FEA was conducted to simulate It was found the proposed stress am- spot welding process. Anastassiou et al. welding residual stress generated by ther- plitude (Ga_res) provided a systematic and (Ref. 8) performed an experimental inves- mal cycles during the spot welding process accurate evaluation of fatigue strength of tigation on welding residual stress and mi- and then the calculated residual stresses spot-welded joints with various dimen- crostructure distribution in spot-welded were compared with experimental data sions and shapes. steel sheets and showed residual stress dis- measured by the X-ray diffraction method tribution was high tensile stress at the cen- to validate the numerical results. Based on Introduction ter and compressive stress near the notch the results, the stress amplitude (O'a_res) root where a fatigue crack initiates. Nied with consideration to welding residual Fatigue strength of spot-welded joints (Ref. 9) investigated the FEA modeling of stress at the edge of the spot weld nugget affects the structural rigidity and durabil- the resistance spot welding process for nu- was calculated using a modified Goodman ity of spot-welded structures, and thus it is merical stress analysis. In this investiga- equation. Then the fatigue load range an important factor in determining safety (AP)-fatigue life (Nf) relations obtained and structural integrity. To determine de- from the fatigue test for spot welds of var- sign criteria for long fatigue life for spot- KEY WORDS ious dimensions and shapes were placed into the stress amplitude (cra.res)-fatigue life (Nf) relation. D. H. BAE ([email protected]) is Professor, Spot Welds School of Mechanical Engineering, Fatigue Strength SungKyunKwan University, Suwon-City. Kyunggi- Residual Stress Welding Residual Stress Do, South Korea. 1. S. SOHN Maximum Principal Stress Analysis (issohn~ mail.osan-c.ac.kr) is Professor, Dept. of Finite Element Method Machine Design Engineering, Osan College, X-Ray Diffraction FEA Model and Assumptions Osan-City, Kyunggi-Do, South Korea. J. K. HONG is Research Scientist, Battelle Memorial Spot welding is a material joining tech- Institute, Cohtmbus, Ohio. nology using electrical resistance heat of ml:l$.1 JANUARY 2003 A 250 I = 2 l Thermal Expansion Coemcient I --4 ~ 200 8o- ~150 1 ............ .........i ..............1 v~' o ~100 ==>- Young's Modulus p~ 50 :.,,..." >- J 0 0 200 400 600 800 1000 Temperature (°C) Fig. 1 -- Three-dimensional nonlinear FEA model used for welding resid- B ual stress analysis of a spot-welded joint. 1.6 0.055 1.4 nducti~ty 0.05 1.2 0.045 metallic material. During the resistance Fig. 2 (Ref. 12). The ~ 0.04 spot welding process, expansion and yield strength of shrinkage of the material occurs due to the SPCC rapidly 0.8 ..... )..........eat 0.035 o thermal cycle. Thus, welding residual decreased when "6"= 0.6 0.03 the temperature 3 stress and electrode indentation remain at o~ 0.4 0.025 3 the spot weld after the spot welding reached more than vo • process. 400°C and the spe- 0.2 0.02 In this paper, welding residual stresses cific heat increased 0 200 400 600 800 1000 were analyzed both numerically and with temperature Temperature ('(3) experimentally. The numerical analysis but rapidly changed procedure for determining welding near 800°C. The Fig. 2 -- Temperature-dependent material properties employed in nonlin- residual stress on spot welds consists of thermal expansion ear FEA. A -- Mechanical properties; B -- thermal properties. two parts: one is thermal analysis and the coefficient linearly other is mechanical analysis. Once the increased to 200°C temperature is calculated from thermal and thermal conduc- analysis, then the mechanical analysis tivity decreased at is conducted with the corresponding more than 200°C temperature histories. and slightly increased at more than 800°C. Results of Numerical Residual The 3-D FEA model employed in the Young's modulus decreases almost lin- Stress Analysis present work is shown in Fig. 1. Solid brick early with temperature. elements were used for upper and lower Generally, a welding nugget is gener- The temperature profiles taken at the plate modeling. The total number of ele- ated by thermal cycles of heating and cool- nugget center, the nugget edge, and loca- ments and nodes are 4300 and 6151, re- ing of the spot welding process. In this tions 5 mm away from the nugget center spectively. The weld nuggets were pre- analysis, however, nugget temperature of the inner and outer surfaces of the pared using multipoint constraints of the distributions were calculated using the plate are shown in Fig. 3. The peak tem- element nodes on the contact surface to heat block element within ABAQUS, perature was around 1300°C at the nugget prevent the incursion between the upper which can be referred to Huh and Kang center of the inner surface, 700°C at the and lower elements during the welding (Ref. 6) and RWMA (Ref. 13).ABAQUS, nugget center of the outer surface, and process. A sufficiently fine mesh at the a commercial FEA package, was em- 400°C at the nugget edge of both inner nugget was generated to obtain more ac- ployed for the transient temperature and and outer surfaces. curate results. Each plate was divided into subsequent residual stress analysis. The temperature distribution on the four layers to show the nugget generation process. The boundary conditions of the FEA model were the same as those of the Table 1 -- Chemical Composition of SPCC (wt-%) test condition. The material used for both FEA model C Si Mn P S Ni AI Fe and fatigue specimen is a cold-rolled sheet SPCC 0.12 0.01 0.127 0.015 0.007 0.025 0.045 Rem steel called SPCE, widely used for an au- tomobile body. The chemical composition and mechanical properties of the speci- men at room temperature are given in Table 2 -- Mechanical Properties of SPCC Table 3 -- Welding Condition of SPCC Tables 1 and 2, respectively. The spot welding condition was defined as the same Tensile Yield Elongation Electrode Welding Welding as in an actual automobile body assembly Strength Strength Force Current Time facility, which is described in Table 3. The (MPa) (MPa) (%) (N) (kA) (Cycles) temperature-dependent material proper- SPCC 307 168 47 Welding ties applied for the FEA are shown in condition 1962 8.3 15 WELDING JOURNAL Eiz~..'l 1400 ~'~-'" ~--@"-Q + Outer-center 1200 .:' • + Outer-nuggetedge O 1000 :' "o o Outer-5mm ::' ---o-- Inner.center :~ 800 ..: • ..-D-.