LS-DYNA deck setup for cellphone drop test

Objective:

To set up a drop test simulation in Ls-Dyna.

Introduction:

LS-DYNA is an advanced general-purpose multiphysics simulation software package developed by the former Livermore Software Technology Corporation (LSTC), which was acquired by Ansys in 2019. The core-competency of LS-DYNA lies in highly nonlinear transient dynamic finite element analysis (FEA) using explicit time integration. LS-DYNA is used by the automobile, aerospace, construction and civil engineering, military, manufacturing, and bioengineering industries.

Many consumer products like cell phones, cameras, and other products are all subject to being dropped during their lifecycle. Impact analysis or drop testing is one of the most important stages of product design and development. Drop testing serves as an important indicator of quality and durability. Drop test simulations help an engineer understand the forces, stress, and displacements caused by dropping an object. Drop test simulation software helps manufacturers by speeding up the time to test a product, enabling higher levels of design quality and reducing the need for physical testing.

In this exercise, we will look at how to set up a basic non-linear dynamic simulation deck in LS-DYNA. For that, a simplified cellphone and a plate mimicking the ground model are used. We are not interested in how the phone travels from the hand to the plate, but only the period when the impact happens and a few milliseconds after that. Therefore, we will keep the two parts very close and provide an appropriate magnitude of velocity. The orientation of the cellphone is also important and there are specific guidelines on how to orient the products but for this assignment, we will ignore that.

Procedure:

The image below shows the simplified cellphone and ground model,

The cellphone is modeled using 3D hex elements and the ground is modeled using quad shell elements.

To prepare the model for simulation, we need to create some cards/Keywords to define and control the simulation. The keywords created are as follows:

*BOUNDARY_SPC_SET

This keyword is used to constraint the nodes. SPC stands for single-point constraint. For this keyword, first, a SET_NODE is created, which forms a set of all the nodes of the ground component by going to Entity Creation>Set Data>*SET_NODE.

Then, SPC_SET is created by going to Keyword Manager>All>BOUNDARY>SPC_SET. The ID of the nodal set created for ground nodes is entered in the NSID parameter. And, all the DOFs are constrained.

*INITIAL_VELOCITY

This keyword is used to assign some initial velocity to the selected nodes. For this keyword also, first, a SET_NODE is created for all the nodes of the cellphone model.

Then, INITIAL_VELOCITY is created by going to Keyword Manager>All>INITIAL>VELOCITY. The ID of the nodal set created for cellphone nodes is entered in the NSID parameter. An initial velocity of -5mm/ms is configured in the z-axis and is kept zero in all other directions.

Next, the *PART keyword is used to define the parts in the model. This keyword requires a material keyword to define the material properties of the part and a section keyword to define the element formulations for the part model.

*MAT

This keyword is used to define the material properties of the part. LS-DYNA provides multiple types of material laws. We are modelling the ground as rigid, therefore, the *MAT_RIGID (020) is used for the ground model. And, *MAT_PIECEWISE_LINEAR_PLASTICITY (024) is used for the cellphone model. The material parameters used are as shown in the images below,

*SECTION

This keyword is used to define the element formulation method used by LS-DYNA. The ground is modeled using the shell elements, therefore, the *SECTION_SHELL keyword is used. Similarly, for the cellphone model, the *SECTION_SOLID keyword is used.

*PART

This keyword is used to define the individual parts. There are two parts created, one for the ground and the other for the cellphone. For both parts, material ID and section ID is configured.

*CONTACT_AUTOMATIC_SURFACE_TO_SURFACE

This keyword is created to model the contact interface between the two parts, i.e., cellphone and the ground. For Slave ID (SSID) cellphone part ID is entered and for master ID (MSID) ground part ID is entered. SSTYP and MSTYP are configured to 3, so that LS-DYNA considers the part ID. All other parameters are kept as default.

*DATABASE

This keyword is used to configure the type of output that needs to be recorded from the simulations.

*DATABASE_GLSTAT is used to record the global history from the simulation.

*DATAASE_BINARY_D3PLOT is used to create the animation file of the simulation.

DT defines the time step to record the results.

Both keywords are configured as follows,

*CONTROL_TERMINATION

This keyword is used to control the simulation and define the simulation time. For our purpose, the simulation time (ENDTIM) is configured to 6ms. All other parameters are kept as default.

Results:

The stress contour in the cellphone is shown below,

Global Energies:

The results are of course not ideal, as the focus was not to model and simulate the real behaviour but to just learn how to prepare a simulation deck in LS-DYNA.

Conclusion:

The simulation deck for the drop test was successfully prepared and the simulation is run without any errors.