Week - 4 - Crash Box Simulation

Crash Box Simulation

Aim: To Setup the Crash Box Simulation model for impact test using LS-Dyna

Objective: 

1. To rigidwall created using *RIGIDWALL_ keyword.

2. To create Material for the crashbox is steel. 

3. To define for Part such as material and section ID.

4. To create Boundary conditions such Initial velocity for the crashbox should be around 50 kmph.

5, To define interface for crushbox.

6. To define the Control keyword so that we will control simulation like time of simulation, printing results every defined time etc.

 

Introduction on Finite Element Analysis:

The finite element method is a numerical technique for obtaining approximate solutions to a wide variety of engineering problems. FEA (Finite Element Analysis) is done in three phases.

1. Pre-processing

2. Solution

3. post-Processing

Drop test: Crash box (vehicle collision), an energy absorbing device installed in order to reduce repair costs in low-speed vehicle collisions.

Crash-boxes  play an important role in automotive parts. It is not only to protect car occupants and external pedestrian safety, reduce the degree of damage effect, but also to reduce the damage to vehicles and other components. It can also reduce the repair cost of vehicle in the crash occurred.

Procedure:

1. Import file.k: In order to open a file which is a keyword file in .k file type by  file>open>LS-DYNA keyword file. In figure 1.1 and 1.2 shows the imported file.

                                                                                                                    Fig.1.1: Path 

                                                                                                                    Fig.1.2: Imported file

Also we can see that Keywords used in the imported file (ELEMENTS, KEYWORDS,NODE,PART,TITLE) Show in the figure 1.3.

                                                                                                                    Fig.1.3:Keywords 

Modeling in LS dyna

2. Creating Boundary : In order to run model we need boundary condition such as constraints and initial parameters such as velocity etc, so to create fixed plate where the phone is going to impact steps.

a. Initial velocity: Create velocity 50 kmph at which the cush box will move. and convert 50 kmph to mm/ms so it is equal to 13.888mm/ms 

b. for creating velocity we have to create node set ID  (Model and Part>Create(Cre)>Set> set node)

c. For velocity go to Initial> velocity. Select NSID and input velocity, here the crush box will move in -ve x direction.

3. Material Creation: For every part the material is necessary in order to run simulation. also the unit system with we have to input data.

a. Here the unit system is g-mm-ms, all the data which is input must be in same unit system.

b. Create material for the crush box 024_picewise_linear_plasticity)

4. Section Keyword:  In order to distinguish b/w the type of element we have to define the section keyword. for shell and solid

a. Section is created by (Model and Part>section(all)>shell). here we have to define thickness and type of element formulation.

b. We have to run simulation for diffrent thicknes. so created 2 cards for 1.2mm anf 1.5mm

5. Assigning Material and section: one done with Material and section we can assign those data to the parts by directly clicking on part in (Model and Part>part) so we can see part list and each part has the option to select for Section ID and MATID.

6. Initial velocity: In order to move the crush box twords rigid wall at velocity of 13.68mm/ms in negative direction. here select the node set and input velocity in which direction it should move.

7. Define Contact: In order to impact take place b/w crush and and rwall there should be contact definition or else crush box will fold and intersect itself in order to overcome AUTOMATIC_SINGLE_SURFACE IS USED. Only slave nodes are enough to define AUTOMATIC_SINGLE_SURFACE

8. Rigid wall:  Create Rigid wall from  Entity create> Rigid wall >cre> planar

9. Control Keyword: The keyword control cards are optional and can be used to change defaults activate solution options such as mass scaling adaptive remeshing and an implicit solution however it is advisable to define the CONTROL_TERMINATION card. This key word is used to control simulation by time, 2ms

Hourglass is the defined as deformation occurs with out strain.

10. Output Request: Using data base pannel we can request such as animation file energy data,contact forces etc.

BINARY3DPLOT - The complete state dump interval as given in *DATABASE_BINARY_D3PLOT serves as the time step to be used in the material model driver run. Plotting information is saved in core for the interactive plotting phase.

ASCII_option- This option is used to reade the data of following 

GLSTAT - Reade the energy like KE, PE, IE

NODEOUT- reade node data such as displacement, acceleration

MATSUM - Every material part energy due to deformation

RCFORCE - contact force b/w them

secforce -  section force

SLEOUT - sliding force due to contact 

SPCFORCE - single point contact force.

after setting tun simulation.

Results: Setup the Crash Box Simulation model for impact test using LS-Dyna and simulating the same is done.

1.1. for thickness 1.2mm From the image bolow we can see due to impact at the time of contact with plate the maximum Stress (4.514e+2 at 0.79ms)is produces. 

1.2. for thickness 1.5mm From the image bolow we can see due to impact at the time of contact with plate the maximum Stress (4.525e+2 at 0.79ms)is produces. 

Energy:

1. At initially before hitting RWALL, there is zoro RWALL FORCE, once the crush box hits the Rwall the Rwall force starts building and reaches maximum force and reach 0 velocity

3.  After that rigid wall forces are droping down beacuse velocity droping. Because of thickness changes the rigidwall forces will change this is due to mass change.

1. Initially the velocity is maximum so the Kinetic energy is maximum. when impact takes place the KE is absorbed by part, so that KE is decreasing and internal energy is increasing.

2. At the time of contact the suddent decreasing in KE and increasing in IE. Total Energy varies as per TE = IE-KE

3. Once deformation reaches max, due to PLAT wall stops velocity TO ZERO and rebound back with some velocity so same time the KE increasing,

In 1.5mm has more energy then the 1.2 because KE is mre for 1.5 and due to impact the energy required to stop the crush box is more so that more internal energy is created.

Cross-sectional force: By using cross sectional force we can find what is the force that travelling at particular place by creating section.

1. For 1.2 the maximum cross sectional force is 7.85e+3 N 

2. For 1.5 the maximum cross sectional force is 14.e+3 N 

In 1.5 the cross sectional forces are more because its more stiffer then 1.2mm for same velocity. 

Acceleration plot of a node: 

Acceleration remains constant before impact. Due to impact sudden drop in velocity and leads to noise in Acceleration

 Maximum directional stress and strain along the length of the crashbox:

effective stress and strain. the maximum value of stress under the von-mises criterion is 451 Mpa and effective strain nearly 0.0075 units.