CAE-Companion-2018-2019

Modeling of Materials & Connections WISSEN CAE

Spot WeldModeling for Crash Simulation

Joints are often the weak points of a vehicle when overload occurs e.g. in crash situations. They join the single compo- nents to the load-bearing body in white. The crash simulation needs reliable and applicable tools for the prediction of the load bearing capacity and energy absorption of all kinds of joints to ensure the crash safety of vehicles. Joints are modeled with simplified models in crash simula- tions of whole cars due to efficiency. The simplified models should be able to reproduce the deformation and failure behavior as well as the energy absorption of the joints with less computational cost but with adequate accuracy. Simplified modeling techniques for point-shaped, line-shaped and plane joints are available in different crash codes and still newmodels are developed because of an increasing variety of new joining techniques. The procedure of determination of damage and fracture parameters of the models is a more or less standard procedure of inverse simulation. The procedure of calibration of model parameters of spot welds is shown in this article. Definition of spot weldmodel Here, as example a solid element is used for the geometric representation of a spot weld as one possibility for the simpli- fied modeling of spot welds. Figure 1 shows the dimensions and the position of a solid element representing one spot weld. The solid element is bound to the shell elements in the mid position of the sheet metal using tied contact definitions The weld nugget diameter d = 5.4 mm and the metal sheet thickness t 1 = t 2 = 1.5 mm give the height h = ( t 1 + t 2 ) /2 and the element edge length 4 / πd L 2 e  of the hexahedron.

The failure model is given according to

2

2

2

2

  

   

  

  

   

   

  

   

F f n n

F f s s

M m

M m

t t

b b

1

where f n , f s , m b and m t are the actual normal and shear force, bending and torsion moment calculated in the hexahedron, respectively. F n is the critical normal force, F s the critical shear force, M b the critical bending moment andM t the critical torsion moment at fracture. Because of minor importance the critical torsion moment is neglected. The exponents are all equal and set to 2 what results in a quadratic, equal distrib- uted superposition of normal, shear, bending and torsional loading in a mixed loading case. Procedure of failure parameter determination The three remaining failure parameters are determined by simulation of specimen tests of spot welded tension, lap- shear and peel specimens. The finite element models of the specimens are shown in Figure 2. The stepwise procedure of calibration of the failure parameters, the critical normal force F N , the critical shear force F s and the critical bending moment M b is described in Table 1. First the tension specimen test is simulated using the spot weld model. Under tension loading the easiest case occurs, because f s and m b are zero. If the global maximum force measured in the test is reached by the calculated global force, the local value of f n is evaluated and gives the value of the critical normal force F n . In the second step the peel specimen test is simulated. If the calculated global force reaches the measured value of maximum force the local values f n and m b of the hexahedron are evaluated, f s still remains zero. These values are put in the failure model using the already determined value of F N and the critical bending moment M B can be calculated by easy transforma- tion of the equation. In the third step the lap-shear specimen is simulated. The values of f n and f s are evaluated if the calculated global force reaches the value of the measured maximum force in the lap-shear test. F S can be calculated by putting these values for f n , f s and the already determined value for F N in the failure equation, because m b remains zero. With this procedure the triple of parameters of the failure model is determined which is specific for the tested material, spot weld diameter and loading velocity.

Figure 1: position and size of spot weld model hexahedron

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