Week - 3 Drop test Challenge

Objective:

To perform a complete simulation for a drop test of a cellphone from the given meshed model of a simplified cellphone and a plate mimicking the ground.

Procedure:

• The given model is imported onto the workpage of the LS-Pre-post using File>Open>LS DYNA Keyword file.

• The keyword file consists of three parts which include the cellphone, floor made of shell element and a floor made of block element. Since only one type of floor is needed, one of the floor part is deleted from the part option under keyword manager. Also the parts were renamed accordingly.
• By analysing this model its convenient to use the unit system in g/mm/ms.

Assigning material and section to the parts.

• Materials can be classified as elastic materials, elastic plastic materials, hyper elastic materials, viscoelastic materials etc, in which metals and plastics can be modelled using elastic plastic material and is assigned using MAT_024 (PIECEWISE_LINEAR_PLASTICITY) card from the keyword manager. If failure mode is highly complex or the material is a polymer, composite, thermoplastic, then MAT_187 card can be used.
• Here the material for the cellphone is choosen as steel and the propeties such as mass density, youngs modulus, poissons ratio, tangent modulus etc were assigned to the material card. Also MAT_20 (Rigid) card were used to assign the material (Concrete) properties for the floor.
• While defining material for floor, the CMO (Center of mass constraint option) was set to 1 for constraining to global direction, also CON1 and CON2 were set to 7 to completely contrain the motion from translation and rotation.
• A section id with solid card is created for both the part with (ELFORM=1) using Keyword manager>All>Section>Solid.
• Now these created material id and section id are assigned to the phone and floor respectively using the part option under keyword manager.

Assigning velocity to the cellphone.

• Before assigning the velocity to the the phone a node set containing all the nodes of the phone has to be created. This can be done using create entity option or by set option under keyword manager.
• The distance between the cellphone and floor is around 20mm and with a initial velocity of -19.79mm/ms the impact should happen at t=0.8ms.
• The velocity is assigned from the INITIAL _VELOCITY card under keyword manager and a velocity of -19.79mm/ms is assigened in Z direction.

Giving contact to cell phone and floor.

• Contact in LS DYNA is defined using contact surfaces and occurs when one surface of the outer surface of a model penetrates another segment.
• Here an AUTOMATIC_SURFACE_TO_SURFACE card from keyword manager is used and the contact is given between the cell phone and floor by considering the cell phone as the slave part and floor as master part.

Assigning termination time.

• *CONTROL_TERMINATION_CARD from keyword manager is used to define the simulation time using ENDTIME otpion.
• Since the cellphone drops with an initial velocity of 19.79mm/ms and the impact happens at 0.8ms. therefore the ENDTIME is assigned as 2ms (Additional time after the impact is added).

Database:

• DATABASE_BINARY_D3PLOT under keyword manager ensures d3plot files are written in which it stores the stress strain contours of the deformed system.
• ASCII under keyword manager can generate different output such as force generated in a contact, total energy, material engery etc. Here output for GLSTAT, MATSUM, RCFORCE are been requested.
• DT (Time intervel between the output) is an essential parameter which basically denotes the time intervel after which the state of solution need to be written and the value is kept as 0.01 for this analysis.

Model check up and running the simulation.

• The model is checked for any errors and if any error occurs it should be corrected further.

• After performing the check up the keyword file for the model is saved and is open in LS DYNA manager for solving.
• If normal termination is achieved after running, it means the model is ready for post processing.

Results:

The required keyword file for the drop test was created from the scratch and run the sumulation sucessfully and found out the stresses induced in the model.

• The following figures below shows the von-mises stress induced during the drop test which shows that high stresses are induced at the bottom of the cellphone just after the impact and gradually propagates towards the upper region due the shock wave.

Conclusion:

A complete simulation for a drop test was done from the given meshed model of a cell phone and a floor. The objective of this assignment was not to get a correct simulation but to get a feel of creating an input deck from scratch.

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File: https://drive.google.com/drive/folders/1p-Dm579CiM9T35O5bJvqyjAurBgCQ5Do?usp=sharing