| The spot welding technology which is the fundamental method connects the thin steel plate component, is widely apply by its high-efficiency, simple operation, and easy to realize in the machinery automation. Vehicle body structures are formed by thousands of spot welds. If the weld spots are not well arranged as a reason of cursorily design or technics, the number of useless weld spots will increase, and every single increasement of a weld spot will lead to an increasement of the cost of the product line by 30,000 dollars. From the mechanical point of view, the weld spots will result in a more serious stress concentration, thus the initiation and growth of a fatigue crack may become easy. How to gain a reasonable balance between life and economic benefits,is an important issue in Welded Structure design. So it is of important to study how Spot pitch influence on the static strength and fatigue life of a weld structure.Body panels are usually connected by a number of spot welding. Welded components are diverse in cross-section shape, but the welding area are two overlaped plates. This paper investigates on three typical beam structure which are different in cross-section shape. These beam structures are widely applied in auto body. The weld spot is modeled by a rigid beam element in finite element analysis. In this paper, the diameter of s weld nugget is considered as 6mm, with a minimum distance to the plate edge s=12mm. The spot distance investigated in this paper is varying from 7mm to 120mm.The research aspects of this paper are:model the three typical structure with shell elements as the panels and cylinder bar elements as the weld spots. The number of weld spots on every beam structure varies from 7 to 100, and the weld spots are uniform distributed in welding area. Three different static strength analysis and a fatigue life calculation will be submitted for each model. Comparative the results, then decide a best spot pitch, which meets the longest service life and the greatest economic efficiency.This paper launched the following aspects :(1)Set up the three-dimensional geometric models of the three typical beam components in CATIA. Then set up the finite element analysis model in MSC.Patran. The panels are modeled by shell elements while weld spots are modeled by cylinder beam elements.(2)Each beam components are constraint at one end. Three different subcases are taken into consideration: two different lateral force and a torque is apply at the other end in each subcase. Then submit the three subcases to MSC.Nastran to finish the three static strength analysis. Record the max stress value of each analysis. Sort the max stress by spot pitch and group them by the cross-section of the typical beam components.(3)Plot the curves of max stress vs. spot pitch in matlab. It is easy to find out how the max stress varies as the spot pitch getting larger and larger from the curves. Considering all the three subcases for each typical beam component, determine a reasonable spot pitch range, whose value of the max stress in the three subcases are acceptable.(4)Process the fatigue analysis. Assume that the components are subjected to two loadings: bending and torsion. The two loading are fluctuating in a same frequency, but their phasic are different. As a result of that, the stress of the shell element comes into a complicated state: the two principal stresses will not change proportioned as the time varies, and their orientation will rotate at the same time. In this situation, the critical plane method will be applied to solve such problem. This theory assumes that the fatigue damage will accumulate only the critical plane. In this paper, a critical plane which perpendicular to the max principal stresses will be employed. The writer has programmed the procedure, and this step will be finished by the procedure. Plot the curves of service life vs. spot pitch in matlab, determine a reasonable spot pitch range, whose service life is acceptable.(5)Consider into both of the spot pitch ranges obtain from the static strength analysis and the fatigue life calculation, then take the intersection as the best spot pitch range. If there is no intersection between the two, a Compromise range will be employed.(6)Compare the spot pitch ranges between the three typical cross-section beams, check that whether they varies in the same way, and try to find out ageneral spot pitch range for the material 08Al.One of the innovations of this paper is taking the effect of spot pitch into account in the study of the welding strength. As a contrast, previous studies almost only involved the strength of weld-spots and the components. In production, weld-spots are arranged by experience, and the optimization of spot pitch was rarely mentioned.Another innovation of this paper is the adoption of critical plane method in calculating fatigue life of welding structure. In this paper, equivalent stress is used in calculation, so errors of fatigue life are inevitable. But it has an advantage over uniaxial fatigue.This article takes the finite element analysis as a key point, studies how the spot spacing influence component strength. The study shows that for 08Al, a reasonable spot pitch range is 30-35mm. the conclusion of this paper sets a good guide for weld spot arrangement.
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