Week - 3 Drop test Challenge

OBJECTIVE

To simulate a mobile drop test event on LS-Dyna using the given .k file - which contains basic representations of a mobile and floor containing nodes and elements.

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. This model contains a shell part AND a solid part representing a floor. I went ahead and deleted the shell to simplify 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. Since we don't have any 2D elements in the model (we deleted the floor shell elements), we can reaccess the keyword manager, select the 'all' option as shown, scroll to SECTION and select SOLID under it. The SOLID section will be used to define the mobile and floor parts since they contain solid elements.

We can name our SOLID section and assign a SECID - used to identify this property when assigning them in the PART keyword form. In addition to that, we can assign the element formulation (ELFORM), which we shall be leaving at 1 for the moment [constant stress solid element (default)]. Once we have finished creating/editing, we need to click the 'accept' button on the top.

4. Similarly, to create a material, we can go to the keyword manager again, select 'all', and scroll down to the MAT card, under which we can select the 020 RIGID card. Doing so lets us create a material with rigid properties. The rigid card is used to define the floor's material since we aren't concerned about the results on it. We can define the material ID (MID), density (RO), Young's Modulus (E), and Poisson's Ratio (PR) here. These are the most important ones to define. Once we are done, we can click 'accept'.

Now we need to define a material for the mobile phone. Since this needn't be a completely realistic simulation (the focus is to simply create a fully functioning simulation), I went ahead and selected the 024-PIECEWISE_LINEAR_PLASTICITY card. We can then follow the same process as in the rigid card to define the properties and click 'accept'. In addition to the terms we came across previously, here we can go ahead and assign values for yield stress (SIGY) and tangent modulus (ETAN). Care must be taken to ensure the units are consistent.

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

5. Moving on, we can now define the boundary conditions. Since this is a drop test, the things we need to define are the fixed and moving bodies. So the focus would be on constraints and defining the velocity of the dropped object.

We can select the 'Create Entity' tool from the right toolbar as shown. In the entity creation window, we can go to boundary > Spc (Single Point Constraint). Here, selecting the 'cre' (create) option at the top, we can assign constraints for selected entities. With the help of the 'Sel. Nodes' window, we can pick an entire part during selection. We shall do that here when selecting and assigning constraints for each of the two parts involved.

For the floor, we shall be constraining all six degrees of freedom as shown. Then 'apply' is selected. (The steps to be followed have been numbered):

A similar process is followed for the mobile but in this case, we won't be constraining translational motion along the Z direction, due to obvious reasons. That means the 'Z' checkbox would be unchecked.

Also, when selecting the nodes by part, it creates a 'set' of nodes.

Now, we can assign a velocity condition to the mobile phone part. To do so, in the same entity creation window, select 'Initial' and 'Velocity' under it. Then, using the same node selection process, we shall select the mobile phone's nodes. Then we can assign a value for Vz. Since it will be moving in the negative z-direction, we can give it a value of '-10' denoting 10 mm/ms. We can then click 'Apply' and then 'Done'.

6. Now, we can define the contact between the two surfaces. To do so, we can go back to the keyword manager, choose to show all keywords, go to the CONTACT keyword and select either AUTOMATIC_NODES_TO_SURFACE or AUTOMATIC_SURFACE_TO_SURFACE.

Here, we intend on selecting the master and slave of this contact property using the part ID. To enable this option, we need to select '3' for SSTYP (Slave segment set or node set type) and MSTYP (Master segment type). Once we do that, we can pick the slave (mobile) and master (floor) by entering the respective PIDs (or) by selecting them from a PID list (by clicking the box with a period beside SSID and MSID). In addition to naming the contact and assigning a CID (contact ID), we needn't worry about anything else here. We can clcik 'accept' and then 'done'.

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 3ms to capture the entire drop test.

8. 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 0.1ms. The GLSTAT and MATSUM attributes are selected under ASCII_option. Energy values are written on a part-by-part basis in MATSUM and energy plots overall are evaluated using GLSTAT.

The following shows the properties assigned for the BINARY_D3PLOT keyword. Only the DT value needs to be assigned.

9. Now, we can save the keyword 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, we can reopen LS-Prepost to view the results. The D3plot output file that was generated is opened in LS-PrePost using File > Open > LS-Dyna binary plot. 

We can then select the 'fringe component' option in the toolbar to select result we'd like to simulate. In the following screenshot, I have selected Von Mises stress:

The play button in the animate window is used to play the simulation.

Also, the 'History' tool on the right toolbar can be used to generate plots of various time history components generated as part of the simulation. The following screenshot is of an energy-time history plot.

SIMULATION

ENERGY PLOT

OBSERVATIONS

The simulation shows the stresses generated during and after the mobile phone makes contact with the floor.

The plot shows the energy levels during the simulation. The large kinks in the plots are obviously formed when the phone makes contact with the floor. The kinetic energy suffers a massive dip due to a drop in velocity during the event.

CONCLUSION

The given .k file was solved after the necessary keywords were created and configured. The simulation was also generated and a plot was compiled as well. Through this challenge, the process of creating an input deck from scratch was also learned.