Week - 4 - Crash Box Simulation

CrashBox Simulation

OBJECTIVE:-

To create a complete simulation file for crash analysis of crash box having two different thickness of 1.2 mm and 1.5 mm from the given FE model of crashbox and produce the following deliverables.

• Input. k file and output files (D3PLOT, GLSTAT, SLEOUT, RCFORC)
• Animation of the final simulation
• Cross-sectional force generated in the middle of the crash box (along its length)
• Acceleration plot of a node in the middle of the crash box (along its length)
• Maximum directional stress and strain along the length of the crash box (X strain, Y strain etc.)
• Plot of all energies (total, internal, kinetic, hourglass, sliding)
• Compare the accelerations and stress/strain plots of 1.2/1.5mm crash boxes

INTRODUCTION:-

A crash box is a highly energy absorbing structure that crashes on application of loads and reduces impact on other components nearby. A full fledged crash box is a highly sophisticated design but, in this case, a rectangular channel structure is considered the most simple crash box.

->Following are the different assumptions and information required:

1. The crash box should crash into a rigid wall created using *RIGIDWALL_ keyword.
2. Material for the crash box is Elastic which is steel in this case.
3. The unit’s system used is gm-mm-ms.
4. Thickness of the crash box is 1.2 mm.
5. Initial velocity for the crash box is around 50 kmph which is to be converted into gm-mm-ms.
6. The simulation should satisfy the condition that It crashes on the rigid wall and rebounces completely.

Once the simulation is completed, results are collected in the form of graphical data and another simulation is executed by increasing the thickness of the crash box from 1.2 mm to 1.5 m.

PROCEDURE:-

The section describes different stages that were performed to set up the simulation file using LS-DYNA. These stages can be broadly categorized into creating an input deck containing different cards such as contact, control, database, element, hourglass, initial_velocity, keyword, MAT, node, part, section, and title. Since LS-DYNA does not have a default system of units, therefore, the user needs to input material properties and physical dimensions according to a consistent system of units as per their choice and in this case, it is gm-mm-ms.

Import the .k file:-

Import the .k file on the physical screen of the software using File > Import > LS-DYNA Keyword File.

Generally we have some of the keywords already present in a basic .k file containing any model and these are PART, NODE, ELEMENT etc.

PART:-

First of all we have to check and identify the parts that we are provided with. Here we are provided with only one part with the name auto created.

Now we have to define the section and material for this part using SECTION and MAT keywords.

SECTION:-

We have defined SHELL section under SECTION keyword for the part provided with 1.2 mm thickness.

MAT:-

MAT keyword is used to define the material for any part and here we have defined ELASTIC material under MAT keyword.

Numerical values of material properties of steel are used here as input for material like density, young’s modulus and poisson's ratio.

Assigning section and material to the part:-

After creating section and material for the part, now we have to assign the both to the part in order the part to exhibit them. We go to PART and click on the radio button provided for both SECID and MID, then we assign section and property to their respective IDs.

Creating entities:-

Here we have to create one node set for crashbox and one rigidwall also one crosssection at the middle of the crashbox. All these 3 entities can be created in create entity tab.

Node Set-

Vector:-

A vector is created to define the normal direction of the rigidwall. This is important because the vector normal of rigidwall should be facing the coming crashbox otherwise the rigidwall would not detect the crashbox nodes, hitting the rigidwall and also this is the reason that we do not need to define a seperate contact for crashbox and rigidwall because defining vector direction of rigidwall normal is enough in this case for rigidwall to detect the crashbox.

Rigidwall:-

Cross Section Plane:-

Defining boundary conditions:-

After assigning the section and material to the part, now we have to define the boundary conditions for the simulation. According to the simulation, boundary conditions may vary but here, we have 2 boundary conditions. 1) Initial velocity of the crashbox, 2) Contact between crashbox and rigidwall but since rigidwall is not any physical entity here which means not a FEA entity and we have created it only for the purpose to get fulfilled so we do not need to define contact here.

INITIAL:-

Initial keyword is used to give initial velocity to the crashbox so that it move with this initial velocity and hit the rigidwall. we have used 50kmph or 13.89mm/ms as initial velocity in positive x direction.

CONTROL:-

This keyword is used to control the simulation outputs and simulation itself.

TERMINATION:-

The control termination function is enabled to specify the end time of simulation. The termination time is set for 10ms to capture the effect of crash box striking the rigid wall.

HOURGLASS:-

Hourglass (HG) modes are nonphysical, zero-energy modes of deformation that produce zero strain and no stress. Hourglass modes occur only in under-integrated (single integration point) solid, shell, and thick shell elements.

ENERGY:-

The control energy function is enabled for computing the hourglass energy, stonewall energy and sliding energy.

Defining required outputs:-

DATABASE:-

Simulation is performed to get required outputs and these outputs can be requested from the software to provide. There are some keywords that need to get defined under DATABASE KEYWORD in order to get outputs in the form of graphical data.

ASCII:-

The time step value of 0.08 ms is given to DATABASE_ASCII option for GLSAT, MATSUM, ELOUT, NODOUT, RCFORC, RWFORC, SECFORC and SLEOUT.

CROSS_SECTION_PLANE:-

DATABASE_CROSS_SECTION_PLANE card is used to compute the sectional force of the crash box.

BINARY_D3PLOT:-

Defining this card is must because this when this card is defined, it generates d3plot file as output after running the simulation and this d3plot file is the main simulation file which shows the simulation animation when played in LS-PrePost.

The time step value of 0.08 ms is given here.

EXTENT_BINARY:-

DATABASE_EXTENT_BINARY card with STRFLG =1, is used to compute the elastic strain in the model.

HISTORY_NODE:-

Here by defining this card, we are requesting nodal outputs like acceleration, stress, strain etc. for node no. 5130

HISTORY_SHELL:-

Here by defining this card, we are requesting element outputs like acceleration, stress, strain etc. for element no. 6640

The keyword file created is checked for errors using the option keyword manager > model check. The keyword file is saved using ‘.k’ extension and is made to run in the LS-Run solver.

It should show Normal Termination message after completion of run as shown in below image otherwise if it shows Error message then that should be encountered by the engineer.

Now the solver deck setup is complete and ready to run the Analysis. We need to run the analysis for 2 thicknesses, one 1.2mm and the other, 1.5mm. The Solver deck that we have created here is for a 1.2 mm thick model. Thus, we need to create another solver deck for the 1.5 mm thick model. This can be done very easily by simply changing the thickness value under the section definition in the Keyword file by opening it in Notepad. Then save it as a new file.

RESULTS:-

The D3plot output File is opened in LS-PrePost using option File > open > LS-Dyna binary plot.

The animation of Von-Mises stress for 1.2 mm thick crash box is as shown below.

Below images shows the comparision on the basis of various energies, stress, strain, acceleration etc. between crashbox having 1.2mm and 1.5mm thickness.

Cross Sectional Force Comparision:-

Max. Cross Sectional Force along the length of the crashbox in the X-dir is found to be 1.44e+5 (gm mm/ms^2) at 4.32ms while Min. Cross Sectional Force along the length of the crashbox in the X-dir is found to be -2.06e+5 (gm mm/ms^2) at 3.76ms.

Max. Cross Sectional Force along the length of the crashbox in the X-dir is found to be 1.73e+5 (gm mm/ms^2) at 3.84ms while Min. Cross Sectional Force along the length of the crashbox in the X-dir is found to be -2.65e+5 (gm mm/ms^2) at 3.76ms.

Acceleration Comparision:-

Above image displays the variation in the acceleration of a central node over the entire period of simulation. Max. Nodal Acceleration along the length of the crashbox in the X-dir is found to be 4.67e+3 (mm/ms^2) at 3.68ms while Min. Nodal Acceleration along the length of the crashbox in the X-dir is found to be -4.63e+3 (mm/ms^2) at 4.4ms.

Above image displays the variation in the acceleration of a central node over the entire period of simulation. Max. Nodal Acceleration along the length of the crashbox in the X-dir is found to be 4.59e+3 (mm/ms^2) at 3.68ms while Min. Nodal Acceleration along the length of the crashbox in the X-dir is found to be -4.73e+3 (mm/ms^2) at 4.4ms.

Energies Comparision:-

it can be seen that when the crash box hits the rigid wall, there is a reduction in the kinetic energy of the box while there is an increment of the internal energy of the wall and after that both energies become constant. This validates the principle of conservation of energy which states that energy can neither be created nor destroyed but can be converted from one form of energy to another. Moreover, from the plot it can also be seen that the hourglass energy remains very low throughout, indicating a good selection of both the hourglass and element formulation for the simulation setup. Basically Hourglass Energy upto 10% of the Internal Energy can be accepted which is achieved here.

X-Stress Comparision:-

X-Strain Comparision:-

The same simulation was run for Imposed velocity with Elastic material and Imposed velocity with Plastic material using some additional keywords defined like CONTACT_SINGLE_SURFACE and BOUNDARY_PRESCRIBED_MOTION_SET.

Imposed velocity with Elastic material -

Imposed velocity with Plastic material -

For 1.2mm Thickness

For 1.5mm Thickness

RESULT:-

The main aim of the Week 4 challenge was to create an input deck or setup file to perform the crash analysis of crash boxes with different thicknesses using LS-DYNA software and analyze the behavior of stress-strain and other results while and after the crash boxes hit the rigid wall. The crash simulations of the boxes were performed for scenario where the crash boxes hit rigid wall and rebounds back completely. From the GLSTAT plots, it was found that the simulation setup follows and validates the principle of conservation of energy expressing loss of kinetic energy of crash boxes conversely leads to the gain of internal energy of the rigid wall.