Week - 4 - Crash Box Simulation

AIM:

The objective is to perform crash test for the crash box and validate the results.

EXPLANATION:

The crash box is a component installed on the front side of the vehicle. The main purpose of the crash box is to observe the Impact/collision energy so that for front-side collisions, it absorbs the shock energy and reduces the damaging effect. The crash box is effective in low-velocity collisions.

PROCEDURE:

In this report, the virtual simulation of crash box model is performed in LS-DYNA, so first import the file in LS-PRE POST as shown in fig1. LS-DYNA by default do not follow any unit system and has to be defined by the user. So, in this model g-mm-ms unit system is followed.

FIG 1 - CRASH BOX MODEL

Next, create a rigid wall for the collision by MODEL AND PART > KEYWRD > RIGIDWALL_PLANAR as shown in fig2.

FIG 2 - RIGIDWALL KEYWORD

MATERIAL -

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

Material	Density	Youngs modulus	Poisson ratio	Yield stress	Ultimate strength	Tangent Modulus
Steel	7500 kg/m3	183GPa	0.27	110MPa	180 MPa	2000 Pa

TABLE 1 - MATERIAL PROPERTIES OF CRASH BOX

FIG 3 - MATERIAL CARD OF CRASH BOX

SECTION -

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

The fig 4 shows the section card of crash box. Since the crash box mesh is shell, shell section card is assigned to the crash box.

FIG 4 - SECTION CARD OF CRASH BOX

PART -

In part keyword assign the respective material and and section ID to the crash box as shown in fig 5.

FIG 5 - PART CARD OF CRASH BOX

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. Since, Rigid wall keyword is used to define the wall, there is no need to specify the contact type between crash box and wall. Single surface contact is used to define self contact of crash box as due to impact, crushing of box takes place and fig 6 shows the contact type for crash box.

FIG 6 - CONTACT CARD OF CRASH BOX

In order to plot the cross sectional forces in the middle of the box generated due to collision, create section plane in the middle as shown in fig 7.

FIG 7 - CROSS SECTIONAL PLANE

Next, in order to plot the kinematics of a particular node, go to MODEL AND PART > KEYWRD > DATA BASE > HISTORY NODE and in order to identify the node ID, go to ELEMENT TOOLS > IDENTIFY > NODE.

FIG 8 - HISTORY NODE KEYWORD

INITIAL CONDITIONS -

In this model, since the crash box impacts the wall, a velocity of 13.9 mm/ms is given in the normal direction as an initial condition as shown in fig 9. This was done by MODEL AND PART > ENTITY CREATION > INITIAL > VELOCITY.

FIG 9 - INITIAL VELOCITY

HOURGLASS ENERGY -

In order to plot hourglass energy, hourglass keyword is to be defined. This was done by MODEL AND PART > KEYWRD > CONTROL > ENERGY.

FIG 10 - CONTROL ENERGY CARD

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 1.5ms is given.

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.

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.

ANIMATION -

FIG 13 - CROSS-SECTION FORCES AT MIDDLE OF THE CRASH BOX

FIG 14 - ACCELERATION OF NODE

FIG 15 - STRESS ALONG THE DIRECTION OF IMPACT

FIG 16 - MEAN STRAIN ALONG THE DIRECTION OF IMPACT

FIG 17 - ENERGY V/S TIME PLOT

RESULTS FOR CRASH BOX OF 1.5 THICKNESS -

Fig-13 and Fig-18 shows the cross-sectional forces generated due to Impact. It is observed that since forces don't depend on the thickness, the plot is same for both the Crash box.

FIG 18 - CROSS-SECTION FORCES AT MIDDLE OF THE CRASH BOX OF 1.5 THICKNESS

Fig-14 and Fig-19 shows the acceleration plots for the middle nodes. The acceleration for both the plots are same as forces generated are same .

FIG 19 - ACCELERATION OF NODE OF THE CRASH BOX OF 1.5 THICKNESS

Fig-15 and Fig-20 shows the stress generated due to Impact. It is observed that since 1.5 thickness box has high stiffness, so less stress is generated for 1.5 thickness box compared to 1.2 thickness box.

FIG 20 - STRESS ALONG THE DIRECTION OF IMPACT OF 1.5 THICKNESS BOX

Fig-16 and Fig-21 shows the strain generated due to Impact. It is observed that since 1.5 thickness box has high stiffness, so less strain is generated for 1.5 thickness box compared to 1.2 thickness box.

FIG 21 - STRAIN ALONG THE DIRECTION OF IMPACT OF 1.5 THICKNESS BOX

FIG 22 - ENERGY V/S TIME PLOT OF 1.5 THICKNESS BOX

CONCLUSION :

The Crash box simulation was performed and the results are validated. By analysing the results, the design engineer may opt for design changes if the results are not satisfying the criteria and again simulation has to be performed with the new design. If the results satisfy the requirement for public usage then the design will be approved for Production.