저자명 송정한 
년도 2008 
The electric resistance spot welding process is an indispensable assembling process of steel auto-panels in the automobile industries since its introduction in 1950’s. As a modern auto-body contains several thousands of spot welds, the strength of spot welds under dynamic loading conditions is extremely important in the evaluation of the integrity and crashworthiness of auto-body members. Due to the geometric complexity and material non-homogeneity factors involved in spot welds, it is impractical in auto-body crash analyses to model every spot weld accurately using detailed solid elements. Currently, in auto-body crash analyses, a common method of modeling a spot weld is to use a rigid link or a beam element. The onset of failure of a spot weld is determined using a failure model which takes into account the forces acting on the spot weld. However, the failure of a spot weld is assumed to be independent of the strain rate in the vehicle crash simulation due to the lark of both an effective testing methodology and available data to model the failure behavior of spot welds accurately. Under a rapid collapse in a crash situation, the failure behavior of a spot weld can differ substantially from a statically loaded case. The immediate implications of the quasi-static behavior of spot welds in crash analyses, thus, lead to inaccurate numerical results of the load and energy absorption of an auto-body.
This paper deals with dynamic failure of spot welds under combined axial and shear loading conditions. Quasi-static and dynamic failure tests of spot welds in three different steels of mild steel, high strength steel and advanced high strength steel (AHSS) were conducted at seven different combined loading conditions. A universal testing machine of INSTRON 4206 was used in the quasi-static failure test and a high speed material testing machine was utilized in the dynamic failure tests at the intermediate strain rate of 1/sec, 10/sec and 100/sec, as the strain rate most relevant to an automotive crash event lies in the intermediate strain rate regime. In order to obtain the failure load with the proportional loading path in terms of the axial load and the shear load, test fixtures and a specimen were newly designed and fabricated with the information from finite element analyses. The proposed apparatus guarantees a greatly improved failure mechanism from two aspects: the first involves the improved constraint condition using a pin-joint between the pull bar and the fixture; and the second involves the prevention of unfavorable rotation by the nugget using the guide plates. The proposed test fixtures and specimens make it reliable to calculate the axial and the shear failure load using simple trigonometric functions of the applied failure load and the initial loading angles.
Failure loads and failure behavior of spot welds were evaluated using the experimental data. The experimental results indicate that failure loads do not show a monotonic increase or decrease with increasing loading angles, as reported in earlier studies in the literature. The failure load decreases as the loading angle increases when the loading angle is less than 30°, whereas the failure load increases as the loading angle increases at the interval from 45° to 90°. The experiment also revealed that a spot weld fails with different failure modes as the loading angle changes. When the pure axial load acts on the spot weld at the loading angle of 0°, shear failure mode was observed around the circumferential boundary of the nugget. For combined axial and shear loading conditions, failure is initiated with the localized necking in the interface between the HAZ and the base metal. From the failure loads measured in the experiment, the failure contour of spot welds at different strain rates were constructed by decomposing the failure loads into the two components along the axial and shear directions and decomposed ones were plotted in the force domain. The failure contour of the spot weld expands with increasing strain rates.
In order to describe the failure behavior for the finite element analysis of auto-body components, a failure model has to be proposed by interpolating failure contours obtained from failure tests accurately. Although some failure models of a spot weld have been proposed in quasi-static case, relatively few studies reported regarding the failure model of a spot weld under impact loading conditions. In this paper, a dynamic failure model was newly proposed from the assumed stress field around the spot weld which satisfies the force and moment equilibrium conditions. The proposed failure criterion takes the shape of a β-norm failure contour and the strain-rate sensitivity values of the axial and shear failure load are assigned as an exponential function of the logarithm of the strain-rate. The proposed failure model provides a fairly accurate interpolation of the failure contours of spot welds obtained from the experiment at intermediate strain rates, as the axial and shear failure load are coupled using the shape parameter of β. In general case of FE analyses for welded steel structures, it is suggested that the β value of 1.45 with the axial failure load and the shear failure load obtained could be used for a failure criterion without losing the accuracy of the numerical calculation.
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