Week - 4 - Crash Box Simulation
Aim: To simulate a crash test for a crash box with varying thickness under initial velocity using LS-Dyna.
OBJECTIVE:
- To create the material card.
- To create a section card for thickness 1.2 mm.
- To assign material and section card to the part.
- To define a rigid wall.
- To assign initial velocity to the crash box.
- To create a cross section.
- To define a contact interface.
- To define control card.
- To define the output database as both binary D3 plot and ASCII.
- Model checkup and run the simulation.
- To create a section card for thickness 1.5 mm.
- To run the simulation.
PROCEDURE:
Open the LS-PrePost software, and then we have to import the required file i.e. new_crash_box, which is in .K format. So, in order to import the .K file format model in LS-PrePost, we have to follow:
File--->Import---->LS-DYNA Keyword File---->File: select file--->Import.
Once model is opened, it will look like as shown picture in below.
In order to check the keywords contained in the file we have to follow:
Model and Part---->Keyword Manager---->check: Model
From above we can notice, we have shell element present in the model. Also according to analysis point of view, we need only shell element. Also we have not find any material and section card defined in the model.
Before creating these cards, first we will check duplicates of element. If find, we will remove them first then proceeds further.
To check duplicate elements:
In order to check duplicate elements. First we have to check duplicated node by following:
Element Tools---->Duplicate Nodes---->Select node: by area---->Show Dup Nodes
If duplicate nodes are found then we have to merge these nodes because one, two or three elements belong to same node. So, 0 duplicate nodes are found as shown in above picture. Also, we can say model is cleaned.
- To create the material card.
Now we have to assign material card for crash box. In order to assign material we have to follow steps shown below
Model and Part---->Keyword Manager---->All---->MAT.
For crash box we have to assign MAT_ELASTIC. This is Material Type 1. This is an isotropic hypoelastic material and is available for beam, shell, and solid elements in LS-DYNA. In order to assign material we have to follow
Where,
MID: Material identification.
RO: Mass density
E: Young’s modulus
PR: Poisson’s ratio
- To create a section card for thickness 1.2 mm
In this section, the element formulation, integration rule, nodal thicknesses, and crosssectional properties are defined. All section identifiers (SECIDs) defined in this section must be unique, i.e., if a number is used as a section ID for a beam element then this number cannot be used again as a section ID for a solid element. The keyword cards in this section are defined in alphabetical order. Now, we are having solid elements, so we will use section solid.
In order to create section solid we have to follow steps as shown:
Model and Part---->Keyword Manager---->All---->SECTION.
First we will create section shell for crash box as shown in picture below with thickness 1.2 mm.
Where, SECID is Section ID.
ELFORM is Element formulation.
SHRF is Shear correction factor which scales the transverse shear stress.
NIP is Number of through thickness integration points.
- To assign material and section card to the part.
Once material and section card is created for new_crash_box. Now we have to assign both cards to their respective parts as shown in picture below.
- To define a rigid wall.
The RIGIDWALL provides a easy way to define a contact between a rigid surface and nodal points of a deformable body, called slave nodes. A rigid wall is defined by a surface type and a group of slave nides. Each wall can be fixed or moving. A rigid wall is a kinematic condition.
So a rigid wall is created ahead of crash box. So that crash box should crash into a rigid wall created. And rigid wall should create at some distance from crash box, because crash box have to move with some initial velocity. Also, we have to take care about rigid wall normal vector direction. It should be facing crash box. In order to create rigid wall we have to follow.
Model and Part---->Create Entity---->Rigidwall.
- To assign initial velocity to the crash box.
Define initial nodal point velocities using nodal set ID’s. This may also be used for sets in which some nodes have other velocities.
In order to assign initial velocity, first we have to create node set id for crash. In order to create node set id we have to follow:
Model and Part---->Create Entity---->Set Data---->Set Node.
Once node set ID is created for crash it will look like as shown in picture below.
Now we have create initial velocity to selected set nodes of crash box as shown in picture below.
Model and Part---->Keyword Manager---->INITIAL---->VELOCITY.
- To create a cross section
The purpose of creating cross section is to define a cross section for resultant forces written to ASCII file secforc. So we have to create cross section at middle of crash box in order to get values of forces along the length of the crash box. In order to create cross section we have to follow:
Model and Part---->Create Entity---->Database---->Cross Section.
- To define a contact interface.
The keyword *CONTACT provides a way of treating interaction between disjoint parts. Now we have to define contact interface. Before defining contact card, first we have to create set data segment, in which we will create segment for crash box (in which we will select those elements which are going to interact during simulation).
Define a set of segments with optional identical or unique attributes. For three-dimensional geometries, a segment can be triangular or quadrilateral. For two dimensional geometries, a segment is a line defined by two nodes and the GENERAL option does not apply. In order to create set of segments we have to follow:
Model and Part---->Create Entity---->Set Data---->SET_SEGM.
Once segments are created, it will look like as shown in picture below.
In order to create contact card, we have to select contact automatic single surface for crash box. In which slave is crash box.
This special feature was originally implemented to allow for the calculation of eigenvalues and eigenvectors on geometries that are connected by a contact interface using the automatic single surface options. It also can be used in a more general sense to tie a set of parts based only on their proximity to each other in the initial geometry.
In order to create contact card we have to follow:
Model and Part---->Keyword Manager---->CONTACT.
Once card is created and all parameters are assigned, contact card will look like as shown in picture below.
- To define control card.
We use the reduced integration point formulation method which introduce the hourglass energy in the simulation. To reduce hourglass energy, it is necessary to enable the hourglass card in the energy card from keyword card and enable hourglass card from control keyword.
8.1 The purpose of creating control energy card, provide controls for energy dissipation options. In order to assign control energy card we have to follow:
Model and Part---->Keyword Manager---->CONTROL---->ENERGY.
Once control energy card is set, card will look like as shown in picture below
Where,
HGEN Hourglass energy calculation option.
RWEN Rigidwall energy dissipation.
SLNTEN Sliding interface energy dissipation option
RYLEN Rayleigh energy dissipation option (damping energy dissipation):
IRGEN Initial reference geometry energy option
8.2 Now we have to create control hourglass card. In order to assign control hourglass card we have to follow:
Model and Part---->Keyword Manager---->CONTROL---->HOURGLASS.
Once control hourglass card is set, card will look like as shown in picture below
Where,
IHQ Default hourglass control type.
QH Default hourglass coefficient.
8.3 The main purpose of control termination card is to stop the job. In order to assign control termination card we have to follow:
Model and Part---->Keyword Manager---->CONTROL---->TERMINATION.
Once control termination card is set, model will look like as shown in picture below.
ENDTIM: Termination time. Mandatory.
ENDMAS: Percent change in the total mass for termination of calculation.
This option is relevant if and only if mass scaling is used to limit the minimum time step size.
- To define the output database as both binary D3 plot and ASCII.
The database definitions are optional, but they are necessary to obtain output files containing results information. The ordering of the database definition cards in the input file is completely arbitrary. The purpose of database binary is to request binary output file.
DATABASE_BINARY_D3PLOT ensures d3plot files in which it stores stress strain contours of the entire deformed model. In order to create DATABASE_BINARY_D3PLOT we have to follow:
Where, DT defines the time interval between output states.
ASCII can generate different outputs such as force generated in a contact like total energy, material energy etc. In which we will request GLSTAT, MATSUM are been requested as shown in picture below.
GLSTAT is global statistics and energies.
SLEOUT is sliding interface energies.
SECFORC is cross section force data.
RCFORC is resultant interface force.
- Model checkup and run the simulation.
Now model will be check for errors, if found it needs to correct. Warnings we can ignore. Once model check is run, it will look like as shown in picture below.
To run simulation:
Before running simulation we have to edit the K file and have to edit the KEYWORD ID, so that after simulation all files should be generated with proper format as shown.
Now we have to run simulation, in order to run simulation we have to use LS-Run. Once LS-Run is opened, then we have to assign all required parameters then simulation will process as shown in picture below.
During simulation run window will pop up as shown in picture below.
If normal termination occurs after running, it means the model is ready for post-processing.
In order to see simulation results, we have to open output file .d3plot file in LS-PrePost.
Contour plot for Von Misses stress with 1.2 mm thicknes
ENERGY PLOT for crash box with 1.2mm thickness:
- To create a section card for thickness 1.5 mm.
Now we will create section shell for crash box as shown in picture below with thickness 1.5 mm.
And steps to create section card is similar as explained in earlier step.
Where, SECID is Section ID.
ELFORM is Element formulation.
SHRF is Shear correction factor which scales the transverse shear stress.
NIP is Number of through thickness integration points.
To run simulation:
Before running simulation we have to edit the K file and have to edit the KEYWORD ID, so that after simulation all files should be generated with proper format as shown.
Now we have to run simulation, in order to run simulation we have to use LS-Run. Once LS-Run is opened, then we have to assign all required parameters then simulation will process as shown in picture below.
During simulation run window will pop up.
If normal termination occurs after running, it means the model is ready for post-processing.
In order to see simulation results, we have to open output file .d3plot file in LS-PrePost.
Contour plot for Von Misses stress with 1.5 mm thicknes
ENERGY PLOT for crash box with 1.5mm thickness:
CONCLUSIONS
- Crash box simulation has been done successfully with 1.2 mm and 1.5 mm thickness respectively.
- All the output requests have been calculated such as von misses stress, energy plots.
- After comparing results, we can analyses that simulation results varies as per thickness changes.