Week - 5 - Modelling Spotwelds

OBJECTIVE

To model spotwelds for the given assembly of parts and run a test to compare results between spotwelds modelled using beam and solid elements.

Conditions:

1. The spotwelds should be modelled using beam elements and solid elements separately.

2. The axial and shear force should be compared among beam and solid elements.

3. The same material card using *MAT_SPOTWELD should be used for both beam and solid elements.

4. The best simulation should have a failure at half of the termination time.

MODEL IMAGE

PROCEDURE

1. Firstly, on LS PrePost, the .k file of the model is imported via File > Import > LS-DYNA keyword file.

2. After importing, we can rename the parts of the model. This is done by accessing the keyword manager on the right toolbar and going to PART On selecting it, we are met with the PART keyword input form, where we can edit properties of the parts in the model. For now, we can assign the name and the PID (Part ID). We shall look into SECID (Section ID) and MID (Material ID) separately.

3. The part contains shell elements and we are to assign thickness to it. So, we can access the keyword manager again, select the 'all' option, scroll to SECTION and select SHELL under it. The SHELL section will be used to define the brackets. We can assign a section ID with ELFORM 16 and thickness 2.5mm.

4. Similarly, to create a material for the brackets, we can go to the keyword manager again, select 'all', and scroll down to the MAT card, under which we can select the 024-PIECEWISE_LINEAR_PLASTICITY. This card is used to define the brackets' material. We can assign the material ID (MID), density (RO), Young's Modulus (E), and Poisson's Ratio (PR) here (as of steel in this case). Once we are done, we can click 'accept'. (Here, I have also defined SIGY (Yield stress) and ETAN (Tangent Modulus))

While we are here, we can also create a material card for the spotwelds we will be creating. We can select the 100-SPOTWELD card and enter the following details:

There are multiple ways to define failure criteria for spotwelds. There's time-based failure (TFAIL), strain-based failure (RFAIL), shear stress-based failure (NRS), axial stress-based failure (NRT) and more. Here, we have taken time-based failure and inputted 5 ms, which would be exactly half of the simulation time.

Now, we can go back to the part card and assign the MID and SECID of the parts as discussed before.

5. Next, we can go ahead and create the spotwelds using the element generation tool in the mesh section of LS-Prepost. This tool lets us create different kinds of elements. Our focus here is to create beam and solid elements. There shall be two files saved and compiled as a result with this element type being the difference.

Mesh > EleGen > Beam > Two_Node_Sets. We can then select the two nodes that are to be connected using the beam spotweld.

A similar process is followed to create solid welds using the following option:

Mesh > EleEdit > Create > Hexa.

All the 8 nodes for the hexa element are selected.

We will then need to define section cards for each case. This is the section card for the beam elements using *SECTION_BEAM:

The following is one for the solid spotwelds with the help of *SECTION_SOLID:

5. Moving on, we can now define the boundary conditions. The aspects we need to define are the moving body and a rigid wall.

Firstly, we can assign a prescribed motion condition to one of the brackets. The idea is to slide one over the other, with the other constrained. To do so, we need to select the 'Create Entity' tool from the right toolbar as shown. In the entity creation window, select 'Boundary' and 'Prescribed motion' under it. Then, using the same node selection process, we shall select all the nodes of the bracket that is to be moved (or a set of them can be created and that assigned instead). Then we can assign a simple displacement curve (This needs to be done via the keyword manager through *DEFINE_CURVE as shown).

Since it will be moving in the x-direction, we need to select '1' for DOF and assign the previously created curve ID for LCID. We can then click 'Apply' and then 'Done'.

We can then constrain the end nodes on the other end of the non-moving bracket using the spc option under boundary in the same create entity menu. The nodes will be constrained in all DOFs.

6. Moving on, we need to define the self-contact for the assembly (excluding the welds). To do so, we can go back to the keyword manager, choose to show all keywords, go to the CONTACT keyword and select AUTOMATIC_SINGLE_SURFACE. Here, we just need to define the slave of this contact property using the part ID. Here, I have defined the card using a set - the set containing both brackets. So instead of assigning contact individually, the self contact is assigned to the set of two brackets. To enable this option, we need to select '2' for SSTYP (Since I defined as a part set). Once we do that, we can pick the slave, which would be the set of brackets. In addition to naming the contact and assigning a CID (contact ID), we needn't worry about anything else here. We can click 'accept' and then 'done'.

Next, we need to define a contact card that would define the interaction between the welds and the brackets. For beam elements, I created a set for the beam elements and assigned the *TIED_SHELL_EDGE_TO_SURFACE card. The slave set is the weld set and the master set would be the set of brackets defined earlier.

A similar process is followed to define the contact between the solid welds and brackets. But here, the *SPOTWELD card is used instead.

7. Next, we can create a CONTROL card to specify the end time of the simulation. This can be assigned via the TERMINATION card option under the CONTROL keyword in the complete list of keywords in the keyword manager. We can assign a value of 10ms to capture the entire simulation.

Finally, we can assign a couple of DATABASE keyword cards for the outputs - namely ASCII_option and BINARY_D3PLOT with a DT (Time interval between outputs) of 0.5ms. The GLSTAT, MATSUM, SLEOUT, NODOUT, RWFORC, SECFORC, SWFORC attributes are selected under ASCII_option.

The following screenshot shows the GLSTAT and MATSUM options selected. Energy values are written on a part-by-part basis in MATSUM and energy plots overall are evaluated using GLSTAT.

We are also required to query specific output requests. Firstly, we are to generate the strain outputs. To do that, we can create a keyword by going to Keyword manager and selecting DATABASE>EXTENT_BINARY. The DATABASE_EXTENT_BINARY card with STRFLG =1, would be used to compute the elastic strain in the model.

Then, selecting DATABASE>HISTORY_NODE lets us request outputs at specific nodes. Here, we are to compute the acceleration at node 502088 in the assembly.

Next, we can select DATABASE>CROSS_SECTION_PLANE to request output. This would be used to compute the cross-sectional forces in the model. With the help of the create entity tool, we can define the plane to create the card as shown below.

8. Once done, we can save it as a .k file and solve it using LS-RUN. The keyword file is inserted and we can click the play button to run the simulation. The number of cores to be utilised can be changed if needed.

Once it is finished without errors (aka Normal Termination), we can reopen LS-Prepost to view the results.

RESULTS

Normal Simulations

Beam Spotweld

Solid Spotweld

Von Mises Stress
We can input the binary d3 plot keyword file and plot the contour animation for Von Mises stress. To do that, we need to go to the post-processing deck and click on the fringe component tool. We can select the particular contour characteristics to plot from the given list here. The following are the contour animations of Von Mises stresses for each case:

Beam Spotweld:

Solid Spotweld:

Von Mises Strain

Just as we did for stress, we can go to the same fringe component tool to access strain rate options, where we can select the required Von Mises strain options to generate the contour animations.

Beam Spotweld

Solid Spotweld

ENERGY PLOTS

To generate plots, we need to access the ASCII tool in the post-processing deck. Here, we can select the type of plot data (glstat, matsum, etc) and pick the plots to be loaded accordingly. For energy plots, we shall be selecting glstat and the energies to plot would be internal, kinetic, total, hourglass, and sliding energies.

Beam Spotweld vs Solid Spotweld

CROSS-SECTIONAL FORCE GENERATED IN THE MIDDLE OF THE COMPONENT

For this plot, we need to select 'secforc' from the list of plot data in the same ASCII tool window (instead of glstat last time). When we do that, we have the option of selecting the particular section we created, of which we can select the output we are looking for. Here, I went for resultant force.

Beam Spotweld vs Solid Spotweld

ACCELERATION PLOT OF NODE 502088

As in the previous plot, we can select 'nodout' this time (instead of 'secforc'). Loading it lets us select node 502088 in the next section, in addition to selecting the required plots. Here, we are going for just resultant acceleration.

COMPARISON OF SHEAR FORCES GENERATED IN WELDS

For this output, we need to select 'swforc' from the same ASCII plot window and select all the spotwelds created. In the bottom selection menu, we can select shear force and plot the graph. A similar process is followed for Axial Forces, which is shown in the next section.

COMPARISON OF AXIAL FORCES GENERATED IN WELDS

OBSERVATIONS

There is an obvious difference in the simulation, in that the beam spotwelds affects the motion of the moving bracket, whereas the solid elements don't affect the movement.

Also, since time-based failure was used for the spotwelds' material card, the plots change results drastically at the 5ms mark, due to the welds failing. As expected, stress concentrations occur at locations of weld attachments.

The solid welds generate a maximum stress of 2.528 MPa, which is slightly higher than the beam spotweld assembly (2.387 MPa).

Looking at the energy plots, we can see that the solid spotwelds generate more internal energy than the beam spotwelds (almost three times). Additionally, this affects the total energy in the system, with the kinetic energy being almost similar in both cases.

Just as in the case of energies, more cross-sectional forces are generated in the assembly with solid spotwelds (Maximum around 12.2kN) as well as nodal acceleration for node 502088 in the same assembly.

The shear forces generated in the welds are higher as well in the case of solid spotwelds (hovering along 1.75kN) whereas the beam spotwelds produce higher axial forces (highest being very close to 800N).

CONCLUSION

The given assembly was simulated in two cases and the required outputs were generated. The welds were given a time based failure to fail at the halfway mark of the simulation. Using different weld element types strongly affected the results, with most of the energy and force plots being higher in the case of the assembly with solid spotwelds.