Crash simulation of Spotweld modeled assembly using LS-DYNA

Objective:

Spot welding is the most common technique for connecting metal parts of an automotive body.

A few thousands of these spot welds are applied to a car body-in-white structure during an automated assembly process. Therefore it’s obvious that the dynamic behaviour of an automotive body is highly affected by these connections.

The main aim of this challenge is to study how a spot weld behaves during crash simulation of a sheet metal assembly using LS-DYNA.

Also, to compare and validate the results for spot welds made up of beam element and solid element.

The unit system followed in this challenge is gm-mm-ms.

Modelling spotweld:

Crashworthiness is described as the ability of a vehicular structure to absorb kinetic energy and minimize injury to passengers during collison,and is considered among the important criteria in determining vehicle design with regard to passenger safety.

The FE simulations help engineers how components interact in an assembly and how the failure mode and mechanism is developed during impact loading.

Among many different possibilities to model spot welds in crashworthiness applications, three modelling techniques have been investigated: A single beam element, four hexahedron element and a single hexahedron elements (solid element).

Obviously, using a beam element is the easiest discretization effort to model spot welds with finite elements. Here, we found that the overall stiffness depends strongly on the position where the beam nodes are connected to the shell surfaces. Secondly, the torsion stiffness of the beam cannot be activated, since the corresponding degree of freedom is not available in the attached shell formulation.

Considering the spot weld discretization with four hexahedron elements, the small element size would decrease the time step too much and the additional mass to counterbalance this is not acceptable.

The third modelling technique consists of one single hexahedron. Here, the available options to model failure were found not accurate enough in most cases.

In this challenge, spot welds made up of beam element and a single solid element is used for the analyses.

There are totally seven spotwelds defined in the assembly in a symmetric manner.

The beam and solid spotweld elements are created from the Element generation tab and are modelled in their respective same locations for each cases.

Modelling rigidwall:

Rigidwalls are analytical representation of surfaces or volumes,frequently used to represent stiff structures in FE modelling, that are either stationary or in motion.

A stationary planar rigidwall is created ahead of the frontal portion of the crashing model at an approximate distance using Create entity>Rigidwall option and choosing an appropriate node and plane to define the normal direction of the rigidwall.

The created rigidwall is translated to some distance in the direction of impact velocity of crashing model.

The crashing model is made to hit the stiffened rigidwall at an impact velocity of 13.88 mm/ms.

Materials setup:

Although many material models exist for metal, one of the most robust models is *MAT_024 or *MAT_PIECEWISE_LINEAR_PLASTICITY.

This material model is the standard workhouse and is the recommended starting point for elastic-plastic deformation of metals and general plastics since it can also handle viscoelastic behaviour.

The spot welds with beam and solid elements are modelled with *MAT_100 or *MAT_SPOTWELD material card.

Here, the spotweld failure timing is defined so that we are purposefully making the spotweld to fail at half of the termination time in order to analyse the behaviour of spotweld during failure.

The crashing sheet metal model and the spot welds are defined with steel material properties.

In our model, instead of defining an arbitrary stress versus strain load curve ,we define Tangent modulus for elastic-plastic behaviour of steel material.

The tangent modulus is useful in describing the behaviour of materials stressed beyond the elastic region.

The *MAT_024 and *MAT_100 materials are referenced to the respective components of the assembly using Part option.

The properties of steel in the following figures is represented in gm-mm-ms unit system.

Sections setup:

In LS-DYNA, the FE model of crashing sheet metals are modelled with shell sections with thickness 1.2mm thickness.

The spotwelds made up of beam elements are modelled with beam sections with appropriate cross section CST= 1 and using an element formulation ELFORM_9 (especially for spotweld beam elements).

The spotwelds made up of solid elements are modelled with solid sections with an element formulation ELFORM_1.

The shell section and the spot weld sections are referenced to the respective components of the assembly using Part option.

Initialization setup:

In our model, the crashing asssembly is made to hit the rigidwall at an impacting velocity of 13.88 mm/ms.

For this. *INITIAL_VELOCITY card is used to apply velocity for the crashing assembly in the direction of impact.

A node set ID is created for the crashing assembly by selecting its nodes using part selection option.

The input velocity condition is applied to the created node set for crashing assembly.

Contact setup:

Contact provides a way of treating interaction between different components or disjoint parts.

Contact in LS-DYNA is defined using contact surfaces.It is useful for impacting one part over the other.

Most impact and crash simulations require self contact to be defined.

A self contact is established in our model for both the top and bottom shells using *AUTOMATIC_SINGLE_SURFACE contact card.

Single surface contact is the most general type of contact used in LS-DYNA program that automatically searches all the external surfaces within a model to determine any penetration has occured.

Also, *CONTACT_SPOTWELD card is used to define contact interface between spotwelds that are created in specified location and the shell parts(top and bottom).

A new part set IDs are created for the shell parts and the spot welds.

While defining the spotweld contact, the spotwelds are treated as slave and the shell parts are treated as master.

This is common for both types of spot welds (solid and beam).

Control setup:

In LS-DYNA, *CONTROL_TERMINATION card is used to define the simulation time using ENDTIM option.

In this challenge, the simulation is set to run for 20 ms.

Thus, the spotweld failure timing is defined as 10 ms. This means that the spotweld is made to fail at time 10 ms during the simulation in order to analyse its behavior.

Model checking and solving the setup:

One of the most important pre-processing tasks is to ensure the validity of a CAE model before submitting it to the solver for analysis.

A good and rich suite of checks reduces engineering errors, increases confidence in the simulation results, and minimizes the duration of simulation cycle by eliminating multiple back and forths from the solver to the pre-processor.

After getting cleared up with the errors if any, the model is saved.

The saved keyword file is set to run in LS-DYNA manager.

A normal termination of the setup is achieved and the results are analysed for post-processing.

Post processing results:

The requested outputs for d3plot,history and ASCII options are now ready to view in LS Prepost for post processing.

1.Animation of the spotweld model

The above animation evidently depicts that the crashing assembly model hits the rigidwall and the spotweld elements are failing at some point of time.

2.Energy plots:

1. Energy plots for Spotweld beam element modelled assembly:

2. Energy plots for Spotweld solid element modelled assembly:

From the above plots of graph,it is evident that the model crashes into to the rigidwall at time 7ms.Therefore the kinetic energy of the system drops deeply which is transformed into the internal energy gained in the assembly model.

Also, we come to know that the total energy of the model decreases significantly at time 10 ms.This means that the spotweld with beam and solid elements fails at time 10ms.

There is only a minor difference between the above two plots which basically occurs due to the independent properties of spotwelds with beam and solid elements.

3.Axial and shear forces plots:

The following graphs are plotted for middle-positioned spotweld element.This is done because results from any one of the corresponding spotweld element is sufficient for analysis.

1. Axial and shear forces plots for middle - positioned Spotweld beam element:

2. Axial and shear forces plots for middle - positioned Spotweld solid element:

From the comparison of above two graphs, it can be seen that the axial force generated in the beam element of the mid-positioned spot weld is higher than that of the solid element and the shear force generated is quite higher in the case of solid element spotweld than that of the beam element.

This is because of solid spotweld element having a single element size of one of the shell elements which basically has a higher cross sectional area value than that of the beam element used in our model.

4.Von-Mises stress contours:

The fringe component of the Von-Mises stress is changed from dynamic to static to visualize the variation of stress in the entire model during the simulation.

1. Von-Mises stress contour of Spotweld beam element model assembly:

2. Von-Mises stress contour of Spotweld solid element model assembly:

From the above two figures, it can be seen that the Von-mises stress in the Spotweld with solid element model assembly is higher than that of the Spotweld with beam element model assembly. This is mainly due to the stress developed in the spotwelds during the crash simulation in both the cases.

Conclusion:

Spotweld modelling techniques with beam and solid elements are used in the model assembly to perform crash simulation in LS-DYNA.

The investigations on contact formulations are studied that are necessary for the correct evaluation of spotweld forces and stresses.

From the results, it can be concluded that the assembly with Spotweld solid element has the higher effective stress level upto the failure.

Thus, Spotweld single solid element has better stiffness than that of beam element when considering the effective stress levels.