Week - 3 Drop test Challenge

OBJECTIVE

The objective of this assignment is not to get a correct simulation but to get a feel of creating an input deck from scratch. I have been provided with a mesh file and a complete simulation of the drop test is made and analysed below.

PROCEDURE:

Generally, Drop test method is performed on the components to check the structural integrity of a product. By performing these tests, the weaker areas of a product can be visualized and the design engineer has a scope to redesign the product. LS-DYNA by default do not follow any unit system and has to be defined by the user. So, in this model kg-mm-s unit system is follwed. 

In this model, drop test of a cell phone has to be performed by inputting required parameters. First import the file in the   LS-PREPOST tool as shown in the fig 1.

FIG 1- CELL PHONE MODEL

The model consists of two parts as shown in the fig1, the green part represents the cell phone case and red part indicates the floor or base on which cell phone is dropped. The material properties has to be assigned to these parts by MODEL AND PART > KEYWRD > MAT.

MATERIAL - 

• MAT_24_PIECE WISE LINEAR PLASTICITY keyword is assigned to phone as per the material properties as shown in Table1.

TABLE 1 - MATERIAL PROPERTIES OF PHONE CASE

FIG 2 - MATERIAL CARD OF CELL PHONE CASE

• MAT_20_RIGID  keyword is assigned to the base and it is constrained in all directions.

FIG 3 - MATERIAL CARD OF FLOOR

SECTION - 

The Section keyword is assigned to these parts by MODEL AND PART > KEYWRD > SECTION.

The fig 4 shows the section card of phone case. Since the phone mesh is solid, solid section card is assigned to the phone case.  

FIG 4 - SECTION CARD OF PHONE CASE

The fig 5 shows the section card of floor. Since the floor is meshed by shell elements, Shell section card is assigned to the floor with shell thickness of 10mm.  

FIG 5 - SECTION CARD OF FLOOR

PART -

In part keyword assign the respective material and and section ID to the respective parts.

FIG 6 - PART CARD OF FLOOR

FIG 7 - PART CARD OF PHONE CASE

CONTACT -

The purpose of Contact keyword is to make sure that the two parts do not pass through each other. Based on the type of interaction different contact keywords are defined. The fig 8 shows the contact type in this model.

FIG 8 - CONTACT KEY WORD CARD

INITIAL CONDITIONS -

In this model, since the phone is dropped from a certain height, a certain velocity of 60 mm/s is given in the negative z-direction as an initial condition. This was done by MODEL AND PART > ENTITY CREATION > INTIAL > VELOCITY.

FIG 9 - INITIAL CONDITION

TERMINATION -

In control keyword, global settings for the entire analysis can be done. Control_Termination keyword is used to assign the time for the simulation. In this model end time of 0.5sec is given.

Control- CONTROL_ENERGY, CONTROL_HOURGLASS and CONTROL_TERMINATION

The CONTROL_ENERGY is used to make sure that the hourglass energy and sliding energies are calculated for the energy balance check.

The CONTROL_HOURGLASS card is used to make sure that the hourglass effect is minimal or nonexistent.

The CONTROL_TERMINATION card is used to input the simulation run time as 10ms and minimum time step as 0.01ms.

DATA BASE -

This keyword allots different types of output files which is used for post processing purpose. In this model 2 types of keywords are defined- 1)Binary_D3 plot  2)ASCII_OPTION. The Binary_D3 plot visualize the data only in the form of plots, where as in ASCII_OPTION, the ouput data can be visualized in plots and text files.

FIG 10 - Binary_D3 plot KEYWORD

MODEL CHECK -

After providing all the necessary input conditions, the final step of pre-processing unit is to check for errors and this was done by MODEL CHECK > KEYWORD CHECK. In the fig 11 as there are no errors in the model, next step is to do solving. 

FIG 11 - MODEL CHECK

Next, run the file in LS-DYNA - LAUNCH MANAGER, fig 12 shows the computational time, number of CPU required in order to run the model and NORMAL TERMINATION indicates the simulation was done without any errors.

FIG 12 - LAUNCH MANAGER

RESULTS :

Next, the results can be validated by importing the d3plot file in the LS-PREPOST.

Fig 11 and fig 12 shows the von misses stress developed in the parts due to impact. Since, the base is considered as RIGID body, no stress is developed in the base plate.

FIG 13- BEFORE IMPACT

FIG 12 - AFTER IMPACT

Fig 13 shows the stress developed in z-direction due to impact. In the drop test, since the phone case is dropped in the z-direction, max stress is developed at the impact region indicated in RED colour.

FIG 13- STRESS DEVELPOED ALONG Z  -DIRECTION AFTER IMPACT

Internal Energy:

1. Since the phone is dropped from a hight, initially, the internal energy is zero.

2. After the phone hits the ground, the reaction force imparts energy to the phone and thus internal energy increases to achieve the maximum value.

3. After the phone bounces off the ground, the fluctuations in internal energy can be noted due to the impact loading.

Kinetic Energy:

1. Since an initial velocity is given to the phone, the kinetic energy is maximum at the begining.

2. Kinetic energy becomes zero the moment the phone touches the ground and then increases from zero as the phone bounces off the floor

Hourglass Energy:

1. Due to the nature of selectively reduced shell and solid elements, hourglass effect is not produced.

Total Energy:

1. Total energy is the sum of all the energies like Internal energy, hourglass energy, kinetic energy, external work etc.

Result:

The drop testing of a phone was done in LS-Dyna and the contours are successfully plotted.

 Conservation of energy, principle of physics according to which the energy of interacting bodies or particles in a closed system remains constant. The first kind of energy to be recognized was kinetic energy, or energy of motion. In certain particle collisions, called elastic, the sum of the kinetic energy of the particles before collision is equal to the sum of the kinetic energy of the particles after collision.

The same theory applies here. When the phone is in free fall and about to hit the wall, the is kinetic energy which is maximum until 3ms when collision happens. The energy upon hitting the wall is transferred to internal energy was minimum until 3ms and then rises as the body losses its kinetic energy. The energy upon crush causes the phone to rebound because of the elastic nature of the crush

 The total energy of the body remained the same which validates the theory of conservation of energy. Also, no hourglass controls were needed as there is not much deformation in this experiment. The hourglass energy is practically nonexistent as no occurrence of hourglass is seen.

CONCLUSION

This simulation helped demonstrate the creation of a typical LS-Dyna solver deck input file. The simulation was successful, and the conservation of energy was demonstrated during the drop test.