DROP TEST OF A CELLPHONE USING LS-DYNA

Objective:

Drop test simulation plays an important role in investigating impact behaviors and identifying weak points of a cellular phone at design stage since actual testing is expensive and time consuming.

The main aim of this challenge is to simulate how a cellular phone, placed just beneath a plate mimicking ground, behaves during impact and a few milliseconds after that using LS-DYNA.

The input such as velocity of cellphone is provided using *INITIAL_VELOCITY card with appropriate magnitude, .

The unit system followed in this challenge is kg-m-s.

Drop test:

The most common failure of cellular phones is probably the impact due to accidental drop, so the manufacturers aim to develop products that are impact resistant and can survive the drop test from specific height.

Both physical test and finite element (FE) simulation are employed in the industry at different stages. At the design stage, FE method plays a more important role since actual drop test can only be conducted at the end of design cycle and provides little feedback to improve the design.

The FE simulation will help designers understand how components interact in the assembly and how the failure mode and mechanism is developed under the impact loading.

Physical test should be conducted on prototypes since field tests are still the most reliable source of information of the assembly during drop test and the only method of final validation of the FE analysis.

The respective LS DYNA keyword file is imported into LS Prepost to start with the challenge.

The shell part of the plate is deleted so that the problem is left with only solid elements.

The cellphone is oriented in such a way that its breadth edge is perpendicular to ground.

Materials setup:

• Modelling Rigid bodies (*MAT_RIGID)

This is one of the most powerful modeling techniques within LS-DYNA. By setting bodies (i.e., parts) to use *MAT_RIGID, where deformation and stresses are not of interest, significant CPU savings can be realized.

In the background, LS-DYNA retains the surface mesh of the part for contact behavior and calculates an inertia matrix to simulate the dynamic behavior of the body.

What is useful with this approach is that the body still retains its inertia and physical characteristics as it interacts with other bodies within the simulation but at a fraction of the numerical cost of dragging around a fully deformable body.

Rigid bodies cannot have constraints applied to them. To constrain a rigid body, the CMO flag is set within the *MAT_RIGID card.

The plate is modelled with rigid material with standard concrete as the material characteristics.

The rigid material is referenced to the plate using the part option.

The properties of standard concrete material are given below.

• Isotropic elastic failure material

This is Material Type 13. This is a non-iterative plasticity with simple plastic strain failure model.

This is a very low cost isotropic plasticity model for three-dimensional solids.

The cell phone is modelled with Steel material.

The steel material is referenced to the cell phone using the part option.

The properties of Steel are as follows

Contact setup (AUTOMATIC_SURFACE_TO_SURFACE):

Accurate modeling of contact interfaces between bodies is crucial for the prediction capability of the finite element simulations.

Contact in LS-DYNA is defined using contact surfaces. Contact occurs when one segment of a model's outer surface penetrates another segment.

A contact is setup between ground and cellphone with slave part as cellphone and master part as plate.

With AUTOMATIC_SURFACE_TO_SURFACE contact, interpenetration is enforced and the slave surface nodes are immediately moved to accommodate the profile of the master surface.

Initialization setup (*INITIAL_VELOCITY):

The most common initialization load is *INITIAL_VELOCITY card option. For example, for any type of drop test, the structure is given a uniform initial velocity and then allowed to instantaneously hit the target.

In this task, the cellphone is given an appropriate initial velocity of 25 m/s in falling downwards direction (negative Z direction in our model).

A node set ID is created for the cellphone by selecting all the nodes of cellphone using part selection.

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

Section setup:

The FE model of cell phone and plate are modelled with hexahedral elements.

Both the cellphone and plate are assigned with section card SOLID with element formulation as constant stress solid element(ELFORM = 1).

In LS-DYNA, solid element stresses are reported at the centre of the element.

The solid sections of cell phone and plate are referenced to the parts of the setup respectively.

Control setup:

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

In the test, the simulation is set to run for 1 second, typically a drop test timing of a phone has.

Solving the setup:

The model is checked for any errors before solving.

After getting cleared up with errors, the drop test setup is saved and is ready to run in LS DYNA manager for solving the scenario.

The saved keyword file is selected as the input to run.

A normal termination of the run is achieved and is ready to analyse for post processing.

Post-processing results:

The drop test scenario is visualized by an animation accessed through the binary plot of the solved setup.

The stress and strain developed in the model can be visualized.

Conclusion:

In this project, the drop test of a cellphone on the ground is conducted by a simulation in LS DYNA.

The provided FE model of the cellphone drop test setup is loaded with appropriate boundary conditions and materials assignment.

However, the cellphone FE model does not contain various components like coverglass,LED display and back housing rather modelled with simple solid hex elements.

The case setup is set to run and the results are analyzed successfully.