To Perform Crash Simulation and Study the Performance of Crash box with varying thickness Using LS-DYNA

Objective: To Perform a Crash Simulation on a Crash Box with Different Thickness and Derivate and compare the following output.

• Cross-sectional forces at the midplane.
• Acceleration of a node at the midplane.
• Maximum directional stress and strain along the longitudinal axis.
• Energy plots
• Comparing acceleration and stress/strain plots for the Different shell thickness.

Model:

The given crash box is a simple rectangular channel-shaped component, which is been meshed already.

Material Model:

We will use Steel Material Properties For the Crash Box

Although many material models exist for metals, one of the most robust models is *MAT_024 or *MAT_PIECEWISE_LINEAR_PLASTICITY. This material model is the standard workhorse and is the recommended starting point for elastic-plastic simulation of metals and general plastics since it can also handle viscoelastic behavior (i.e., strain-rate dependency).

And for defining this material we are using MAT_024 (MAT_PIECEWISE_LINEAR_PLASTICITY), This is Material Type 24, which is an elastoplastic material with arbitrary stress

1. MID Material identification. A unique number or label not exceeding 8 Characters must be specified
2. RO Mass density.
3. E Young’s modulus.
4. PR Poisson’s ratio.
5. SIGY Yield stress.
6. ETAN Tangent modulus, ignored if (LCSS.GT.0) is defined.

Section Creation:

Section Creation Define the section Properties for different Element

Since there is only one component is this model which is a shell element. For Shell Element we use SECTION_SHELL

1. SECID Section ID. SECID is referenced on the *PART card. A unique Number or label must be specified.
2. ELFORM Element formulation options.= 2 Belytschko-Tsay
3. SHRF Shear correction factor which scales the transverse shear stress.= 1
4. NIP Number of through-thickness integration points = 5
5. T1 Shell thickness = 1.2mm

Contact Creation

In case of crash scenarios where the Crash box crashes into the rigid wall there is change crash box buckles and penetrate itself. So It is recommended that AUTOMATIC_SINGLE_SURFACE has to be considered. The only Slave can be defined in this type of contact.

Rigid Wall Creation:

A *RIGIDWALL_PLANAR keyword has been created on where the box impacts. The Rigid wall was positioned 265mm away from the crash box. The direction of the normal of the generated rigid wall should be in the opposite direction of the velocity.

Boundary Condition:

The Initial Velocity is defined to the Crash Box

We assume in this case the initial velocity is 50kmph i.e 13.85 mm/ms

Control Cards

• Control­_Termination is defined at what time the simulation ends or the end time of the simulation. It is set to 5 ms

• Control_Energy It Provides controls for energy dissipation options.

  HGEN- Hourglass energy calculation option. Is set to 2 In Which hourglass energy is computed and included in the energy balance.

  RWEN - Rigid wall energy dissipation option: Is set to 2 in Which energy dissipation is computed and included in the energy balance

  SLNTEN - Sliding interface energy dissipation option Since the Contact is Defined it is set to 2 So That energy dissipation is computed and included in the energy balance

Database Creation

• BINARY_D3PLOT - It defines the frequency at which the animation file is to be created and is set to 0.1ms

• Cross-section Creation: One of the required outputs is Cross-sectional force at the middle of the crash box for this a DATABASE_CROSS_SECTION_PLANE

• DATABASE_HISTORY_NODE Three nodes were defined in the middle of the crash box to measure the acceleration

• ASCII_option keyword

The output request in ASCII format, The following keyword are Activated

1. ELOUT- Element Data
2. GLSTAT- Global Data
3. MATSUM- Material Energies and
4. SLEOUT– Sliding Interface Energies.
5. NODOUT - Nodal point data
6. SECFORC - Cross-section forces

Keywords

Model Checker :

Result - Crashbox with 1.2mm Thickness

Directional Stress - X-axis:

Max Stress = 302.35MPa

Directional Stress - Y-axis:

Max Stress = 480.9MPa

Strain Rate in X-axis:

Max Strain Rate = 1.268

Strain Rate in Y-axis:

Max Strain Rate = 0.1536

Energy Plot :

Max Internal Energy: 9000J

Max Kinetic Energy: 11000J

Hourglass & Sliding Energy :

Cross-Section Force :

Peak Force During Impact: 12 KN

Acceleration of Node at Midplane of the crash box :

Peak acceleration during impact: 5 mm/ms^2

Result - Crashbox with 1.5mm Thickness

Directional Stress - X-axis:

Max Stress = 300.01MPa

Directional Stress - Y-axis:

Max Stress = 426.6MPa

Strain Rate in X-axis:

Max Strain Rate = 1.337

Strain Rate in Y-axis:

Max Strain Rate = 0.1304

Energy Plot :

Max Internal Energy: 11000J

Max Kinetic Energy: 14000J

Hourglass & Sliding Energy :

Cross-Section Force :

Peak Force During Impact: 14 KN

Acceleration of Node at Midplane of the crash box :

Peak acceleration during impact: 4 mm/ms^2

Comparison:

• Energy Graph:

If we notice the internal energy of the crash box with 1.5mm Thickness which has more internal energy than a crash box with 1.2mm thickness which means the crash box with 1.5mm will absorb more energy than the crash box with 1.2mm.

The hourglass energy in both cases is more than 15 % so there are two way to sort it out first to use fully integrated formulation in shell properties and other is to use hourglass control method

• Directional Stress & Strain Rate:

As we See the Stress acting on the Crashbox with 1.5mm Thickness is less because the thickness is more and it makes the crash box more stiffener As the stiffness of the crash box increases due to the increases in thickness so eventually the deformation is deceased.

• Cross-Section Force and Acceleration at Midplane:

As we can see The Cross-section force for a Crash box with 1.5 mm thickness is more. This is because the Crash box with 1.5 mm thickness has extra mass compared to 1.2mm shell thickness with the same velocity values in both the cases. So the extra mass is responsible for producing extra force values at the same velocity.

It Shows the peak acceleration is more in the crash box with 1.2 thickness and We can see the acceleration values for X direction are negative. This is because the crash box is stopped by a rigid wall and thus deaccelerates. Whereas the resultant acceleration is the magnitude hence positive and almost equal to X-direction acceleration value because Y and Z direction accelerations being very small in comparison.

Conclusion :

The crash Box with different thickness is been simulated and the results were dedicated and we can come to the conclusion that Crashbox with 1.5mm thickness is better than 1.2 mm thickness because the crash box with 1.5mm has more internal energy which absorbs more energy and stress is less and strain rate is more which mean more deformation and more energy is absorbed.