Week - 4 - Crash Box Simulation

LS DYNA – Crush Simulation

By Enos Leslie

Mechanical Engineer

18th  August 2024

OBJECTIVE

The project aims to understand the crashworthiness of the crash box design and evaluate the effect of thickness on its energy absorption and structural integrity during an impact.

PROCEDURE

The crush box is made up of a shell rectangular box of quad elements. I order to perform a crush, there is a need for a solid wall. Due to this , a rigid wall is created using create entity RIGIDWALL. Planar rigidwall is created by selection 1n + NL. The normal of the rigid wall is place opposite to the direction of impact. The image of the setup is seen below : 

MATERIALS

• 024 - PIECEWISE_LINEAR_PLASTICITY (Steel_Non_Linear_Material)

Density = 0.0078500 g/mm3 ||  Young Modules = 2.000e+05 Mpa  ||   Poisson ratio = 0.28 || SIGY = 180 ||

SECTION 

Two sections would be created for optimising the thickness for post analysis

BOX_SECTION_1.2 || ELFORM =6 ||  T = 1.2mm || 

BOX_SECTION_1.5 || ELFORM =6 ||  T = 1.5mm || 

BOUNDARY CONDITIONS

• INITIAL_VELOCITY_GENERATION

Nodes Set= box_velocity_nodes  ||  VX = -16 mm/ms  

CONTACT

For the self contact of the box after impact:

Automatic_single_surface || Self_Contact_Box

CONTROL

Energy : HGEN = 2 || RWEN = 2  ||  SLNTEN = 2 || RYLEN = 2 ||

CONTROL_TERMINATION : ENDTIME = 1.5 ms ||

TIMESTEP :  TSSFAC = 0.9 || 

DATABASE

ASCII_option : ELOUT (dt=0.1 , binary = 1), GLSAT(dt=0.1 , binary = 1), NODEOUT(dt=0.1 , binary = 1), RCFORCE(dt=0.1 , binary = 1), SECFORCE(dt=0.1 , binary = 1), SLEOUT(dt=0.1 , binary = 1)

Binary_D3plot : DT = 0.1

Cross_Section_Plane : The cross_section plane is created at the midsection of the box. This is relevant for measuring the forces generated in the middle of the box.

RESULTS

From the animation above, it can be seen the box hits the wall and bounces off. The deformation looks good and energy and forces would be analysed below. Analysis is going to be made by varying the thickness of the box between 1.2 and 1.5.

|| 1.2 thickness Assesment ||

 #Cross-sectional force generated in the middle of the crashbox (1.2 mm):

Force generated around the middle section is 55900 N

#Acceleration plot of a node  in the middle of the crashbox (1.2 mm)

Node 9392 was selected and the acceleration plot produced max acceleration of 1390mm/ms2 at 0.7ms which is the box time box makes contact with the wall.

 #Maximum directional stress and strain along the length of the crashbox(1.2mm):

Effective Stress reached a maximum of 100E-03 ,this value the maximum pressure induced in the crush box and this occurred at 0.8ms which was the last point till the box began to bounce off. The plastic strain began to increase linearly at 0.6ms which was the time of impact and reached a max value of 0.286E-03. It can be seen that after the crush bounces off the strain remains constant which shows plastic deformation.

#Plot of all energies (1.2mm)

At 0 ms, the whole of the total energy is Kinetic energy. But as the simulation progresses and the impact between the Crash box and the Rigidwall takes place, at approx 0.6 ms, the Kinetic energy starts getting converted into Internal energy. The Internal energy reaches its peak and the kinetic energy reaches its lowest value when the crash box is on the verge of getting rebounded from the rigid wall. The crash box then moves with a constant rebound velocity which is reflected in the energy plot as internal energy gets converted to kinetic energy and remains constant through the rest of the simulation. Hourglass energy is zero which is good for accurate results.

|| 1.5 thickness Assesment ||

 #Cross-sectional force generated in the middle of the crashbox (1.5 mm):

Force generated around the middle section is 64800 N

#Acceleration plot of a node  in the middle of the crashbox (1.5 mm)

Node 9392 was selected and the acceleration plot produced max acceleration of 1250mm/ms2 at 0.7ms which is the box time box makes contact with the wall.

#Maximum directional stress and strain along the length of the crashbox(1.5mm):

Effective Stress reached a maximum of 85E-03 ,this value the maximum pressure induced in the crush box and this occurred at 0.8ms which was the last point till the box began to bounce off. The plastic strain began to increase linearly at 0.6ms which was the time of impact and reached a max value of 0.286E-03. It can be seen that after the crush bounces off the strain remains constant which shows plastic deformation.

#Plot of all energies (1.5mm)

At 0 ms, the whole of the total energy is Kinetic energy. But as the simulation progresses and the impact between the Crash box and the Rigidwall takes place, at approx 0.6 ms, the Kinetic energy starts getting converted into Internal energy. The Internal energy reaches its peak and the kinetic energy reaches its lowest value when the crash box is on the verge of getting rebounded from the rigid wall. The crash box then moves with a constant rebound velocity which is reflected in the energy plot as internal energy gets converted to kinetic energy and remains constant through the rest of the simulation. Hourglass energy is zero which is good for accurate results.

Results Discussion on the Effect of Thickness on Crash Box Performance

The crash box simulations for the two different thicknesses—1.2 mm and 1.5 mm—provide valuable insights into how material thickness influences structural behaviour, energy absorption, and overall performance during an impact.

1. Cross-Sectional Force

1.2 mm Thickness: The force generated in the middle section of the crash box was approximately 55,900 N.

1.5 mm Thickness: The force increased to around 64,800 N.

Analysis: The thicker crash box (1.5 mm) generated a higher force during the impact, indicating greater resistance to deformation. This increase in force is expected, as a thicker structure can better withstand the applied loads, requiring more energy to deform the crash box.

2. Acceleration

1.2 mm Thickness: The maximum acceleration recorded was 1,390 mm/ms² at 0.7 ms, coinciding with the time of impact.

1.5 mm Thickness: The maximum acceleration decreased slightly to 1,250 mm/ms² at 0.7 ms.

Analysis: The slightly lower acceleration in the thicker crash box suggests that the additional material helps dissipate the impact energy more effectively. The thicker structure likely absorbs the impact more efficiently, reducing the peak acceleration experienced by the structure.

3. Maximum Directional Stress and Strain

1.2 mm Thickness: The effective stress reached a maximum of 100E-03, while the plastic strain reached a maximum of 0.286E-03.

1.5 mm Thickness: The effective stress was slightly lower, reaching 85E-03, with the same maximum plastic strain of 0.286E-03.

Analysis: The reduction in maximum stress in the 1.5 mm crash box indicates that the thicker material experiences less stress under the same impact conditions. This suggests that the thicker structure distributes the impact load more effectively. However, since the plastic strain remains the same, it implies that both thicknesses undergo similar levels of permanent deformation, with the 1.5 mm box managing the stress more efficiently before reaching that deformation.

4. Energy Absorption

1.2 mm Thickness: The energy plot shows a complete transfer of kinetic energy into internal energy at around 0.6 ms, with internal energy peaking as the box rebounds.

1.5 mm Thickness: The energy plot for the 1.5 mm box follows a similar trend, but the maximum kinetic energy observed is higher compared to the 1.2 mm box.

Analysis: While both thicknesses demonstrate efficient energy absorption, the higher maximum kinetic energy in the 1.5 mm crash box indicates that it starts with more energy to absorb. This suggests that the thicker box is more robust, allowing it to handle and absorb greater impact energy before rebounding. The ability of the 1.5 mm box to maintain higher kinetic energy also implies that it has a larger capacity to convert this energy into internal energy, enhancing its overall effectiveness in impact scenarios.

Conclusion

Increasing the thickness from 1.2 mm to 1.5 mm significantly impacts the crash box's performance:

Higher Resistance to Force: The 1.5 mm crash box generates more force, indicating enhanced structural integrity under impact.

Reduced Peak Acceleration: The thicker box experiences lower peak accelerations, which suggests better energy dissipation and reduced impact severity on adjacent components.

Lower Stress Levels: The thicker crash box experiences lower maximum stress, decreasing the likelihood of material failure.

Improved Energy Absorption with Higher Kinetic Energy: The 1.5 mm crash box starts with higher kinetic energy, which it effectively converts into internal energy, demonstrating superior energy absorption and impact resistance.

Overall, the thicker crash box design offers improved energy absorption, better stress management, and enhanced overall crashworthiness, making it more effective in protecting critical components during impact events.