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Determination Of Load-Function And Formulation Of Separation Criteria For Spot-Weld Connections For Crash Simulations Disclosure Number: IPCOM000035590D
Publication Date: 2005-Jan-25
Document File: 3 page(s) / 104K

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By  Chelliah Madasamy, Tau Tyan, and Omar Faruque


Background: When a vehicle is subjected to a crash event, the spot weld connections in the body structure may undergo large deformation and plastification in critical areas. The degradation of these spot welds may sometimes control the characteristics of the crash performance of the vehicle. To design a vehicle for desired performances, the vehicle should be studied through CAE crash simulations.

Traditionally in CAE, the spot-weld connections are idealized as stiff springs that does not undergo deformation or separation during crash simulations. This type of CAE modeling depends on the crash testing, which often involves lot of time and money. Therefore, the improvement in the current CAE modeling process is required for future vehicle design.

Following are the difficulties associated with spot-weld modeling in the current method.

§         Difficulty in determining load function of the spot weld: The test data for spot-weld gives the information about the combined deformation of the component legs (Figure 1), flange and spot weld nugget. But, for finite element modeling, the deformation of spot-weld alone is required, which is however not accessible to measure in experiments. Therefore, a method to determine these characteristics of spot-weld nugget is required.

§         Inadequate spot weld separation criteria: In addition to determining the loading function of the spot weld, the separation criteria are also required to capture the real world characteristics. The available criteria does not account for speed (which is often necessary for crash simulation) and coupled loadings (combined tension and shear). There are no criteria available to represent the energy absorbed under the force-displacement curve.


Determination of spot weld load function

This section explains a simple and direct method to extract the experimentally non-accessible spot weld characteristics from the full component test data. Figure 1 shows the finite element model of spot welded component under pure tension. Figure 2 shows the load-displacement curve obtained from test data for pure tension. An initial run of the finite element model was conducted with relatively large spring stiffness and the corresponding force-displacement curve is shown in Figure 2. The displacement in the FEA model curve includes the displacement from axial elongation, bending of the flange, and spot weld nugget deformation. The net displacement from these two curves in Figure 2 is approximated as the displacement due to spot weld.  The force in the spot weld is the same as the force in the component from test data for any given displacement. The spot-weld force-displacement curve obtained for the spring element is shown in Figure 3. The portion of the force-displacement curve of the spot weld from th...