Week - 4 - Crash Box Simulation

AIM: To create the simulation of falling of mobile on the ground using proper input.

KEYWORDS: Contact energy, Hourglass, Contact, Boundary condition, Materials, Section, Initial Velocity, etc.,

SOFTWARE: LS-Dyna

OBJECTIVE:

• For the given meshed model check for the number of parts and rename accordingly.
• Creating suitable keywords like Material (Rigid & M24), Contact (Surface - Surface), Boundary condition (Rigid material), Section (Shell & Solid) & Control cards.
• Initial Velocity is given for the mobile part as 10 m/s.
• After creating the keywords, check for the model and save the keyword file.
• Run the keyword file in LS-run and simulate.
• In the simulation, get the contour plot of Von-Moses stress and Effective strain.
• Plot all the energies (Kinetic, Internal, Total, Hourglass & Sliding) vs. Time (s).
• The total termination time is given as 5ms.

INTRODUCTION:

Ansys LS-DYNA is the most commonly used explicit simulation program, capable of simulating the response of materials to short periods of severe loading. Its many elements, contact formulations, material models, and other controls can be used to simulate complex models with control over all the details of the problem. Ansys LS-DYNA has a vast array of capabilities to simulate extreme deformation problems using its explicit solver. Engineers can tackle simulations involving material failure and look at how the failure progresses through a part or through a system. Models with large amounts of parts or surfaces interacting with each other are also easily handled, and the interactions and load passing between complex behaviors are modeled accurately. Using computers with higher numbers of CPU cores can drastically reduce solution times. LS-DYNA consists of a single executable file and is entirely command-line driven. Therefore, all that is required to run LS-DYNA is a command shell, the executable, an input file, and enough free disk space to run the calculation. All input files are in a simple ASCII format and thus can be prepared using any text editor. Input files can also be prepared with the aid of a graphical preprocessor. There are many third-party software products available for preprocessing LS-DYNA input files.

Here, The given model has already meshed for shell components which as to be simulated by performing the Crash test of a Box to the rigid wall by giving an initial velocity of 13.33 m/s for Case I Elastic material and 50 m/s for Case II Plastic material. The Rigid wall has to be created and positioned accordingly. At the mid of the geometry section as created. In this simulation, we need to study the Energy balance, acceleration force, Section force, Max Stress & Strain developed along the direction. The actual geometry along with mesh which is shown below figure,

CASE I: Material behaves as Elastic

Figure 1: Meshed parts

Here, the crash box is constrained in all directions except in the X-direction which allows moving towards the rigid wall.

Rigid_Wall_Plane: Here, the rigid wall is created which is perpendicular to the box and based on X, Y & Z direction.

Cross-Section: Here, the cross-section is created at the mid of the geometry.

Boundary_condition: Here, the base part is constrained in all directions except in the X-direction.

Parts_definition: By assigning the section and material id for the respective shell part (Crash Box).

Section_definition for Shell: Here, Defining Element formulation (ELFORM-2), number of integration points, and thickness for the particular part.

ELFORM=2 (default element)-Belytschko-Lin-Tsay shell formulation

• Based on Reissner-Mindlin kinematic assumption (5 DOF in local coordinate system yield globally 6 DOF)
• Extremely effective: Formulated in velocity strains (rate of deformations) and Cauchy stresses.
• Bi-linear nodal interpolation
• One-point integration for efficiency reasons
• Updated, co-rotational Lagrangean formulation
• Hourglass control to counterbalance zero-energy modes
• warping stiffness (BWC flag on *CONTROL_SHELL)
• Warpage may be an issue; do not use in coarse models
• Nodal fiber vectors for improved warping stiffness
• Fastest kid in town but does not pass the patch test

Material_definition for Crash Box: Here, the material is preferred as Steel with Elastic Property and assigned all the parameters like density, young's modulus, & Poisson ratio.

Initial_Velocity: Here, the velocity is defined as 13.33m/s in X-direction for the Crash Box.

Contact definition: Here, The Crash Box is considered as either slave or master type depends on the impact on the rigid wall.

Control_energy definition: Here, Hourglass energy, stone wall energy, sliding energy, rayleigh energy, and initial reference energy are considered for energy balance.

Binary_D3Plot_Definition: Here, the time interval between the outputs is defined (DT) as 0.1.

ASCII_Option: Here, ELOUT, GLSTAT, NODOUT, MATSUM, RWOUT, SECOUTand SLEOUT are defined.

History_Node_Definition: Here, selecting one of the nodes from the created section part in the crash box to plot the acceleration and force acting in the X-direction.

Control_Termination: Here, the termination time is given as 10ms.

Keyword_Manager: Here, the required keywords are assigned and listed.

CASE II: Material behaves as Plasticity

Figure 2: Meshed parts with mass added

Here, the crash box is constrained in all directions except in the X-direction which allows moving towards the rigid wall along with the added mass.

Material_Definition for Crash Box: Here, the material is preferred as Aluminium with Plastic Property and assigned all the parameters like density, young's modulus, Poisson ratio, and Yield stress, etc.,

Constrained_Nodal_Rigid_Body: Here, all the end nodes in the crash box are defined by rigid in order to apply the mass.

Element_Mass: Here, Mass is added to the end of the box as 100 kg.

Initial_Velocity: Here, the velocity assigned for the crash box is 50 m/s.

Model_Check: After all the input parameters are given, checking for the model set-up.

LS-Run: Here, the file imported and run the simulation, after that plot the energy chart, Section force, Rigid wall force, effective stress, and strain.

RESULTS:

CASE I: Material behaves as Elastic with a thickness of 1.2mm

(I) PLOT KINETIC ENERGY VS. TIME:

T = 1.2 mm

In this simulation, the material assigned for the crash box is Steel which behaves as elastic with a thickness of 1.2mm. The box is moving at a velocity of 13.33 m/s and impacted to the rigid wall which rebounds completely without any crumbles due to elastic behavior. when the box hits the rigid wall at a time of 3.5 ms, the Kinetic energy falls suddenly from 0.1075 MJ to 0.106 MJ and fluctuates while it rebounds.

T = 1.5 mm

In this simulation, the material assigned for the crash box is Steel which behaves as elastic with a thickness of 1.5mm. The box is moving at a velocity of 13.33 m/s and impacted to the rigid wall which rebounds completely without any crumbles due to elastic behavior. when the box hits the rigid wall at a time of 3.5 ms, the Kinetic energy falls suddenly from 0.1345 MJ to 0.1325 MJ and fluctuates while it rebounds.

(II) PLOT INTERNAL ENERGY VS. TIME:

T = 1.2 mm

In this simulation, the material assigned for the crash box is Steel which behaves as elastic with a thickness of 1.2mm. The box is moving at a velocity of 13.33 m/s and impacted to the rigid wall which rebounds completely without any crumbles due to elastic behavior. when the box hits the rigid wall at a time of 3.5 ms, the Internal energy peaks suddenly from 0 J to 1.1kJ and fluctuates while it rebounds. The hourglass and sliding energy are eliminated because ELFORM2 is used which is fully integrated.

T = 1.5 mm

In this simulation, the material assigned for the crash box is Steel which behaves as elastic with a thickness of 1.5mm. The box is moving at a velocity of 13.33 m/s and impacted to the rigid wall which rebounds completely without any crumbles due to elastic behavior. when the box hits the rigid wall at a time of 3.5 ms, the Internal energy peaks suddenly from 0 J to 1.25kJ and fluctuates while it rebounds. The hourglass and sliding energy are eliminated because ELFORM2 is used which is fully integrated.

(III) PLOT ACCELERATION VS. TIME:

T = 1.2 mm

In this simulation, the acceleration varies from 3.5 ms which comes in contact with a rigid wall. The box is moving at a velocity of 13.33 m/s and impacted to the rigid wall which rebounds completely without any crumbles due to elastic behavior.

T = 1.5 mm

In this simulation, the acceleration varies from 3.5 ms which comes in contact with a rigid wall. The box is moving at a velocity of 13.33 m/s and impacted to the rigid wall which rebounds completely without any crumbles due to elastic behavior.

(IV) PLOT SECTION FORCE VS. TIME: In this simulation, the Section force fluctuates at the mid of the geometry (Section) which comes in contact with a rigid wall. The box is moving at a velocity of 13.33 m/s and impacted to the rigid wall which rebounds completely without any crumbles due to elastic behavior.

T = 1.2 mm

T = 1.5 mm

CASE II: Material behaves as Plasticity

(I) PLOT ENERGY VS. TIME:

T = 1.2 mm

In this simulation, the material assigned for the crash box is Steel which behaves as Plastic with a thickness of 1.2mm. The box is moving at a velocity of 50 m/s and mass added as 100 kg which is impacted to the rigid wall which crumbles due to plastic behavior moreover depends upon the velocity and mass added at the end of the tube. when the box hits the rigid wall at a time of 3.8 ms, the Kinetic energy falls suddenly from 9 MJ to 0 J and fluctuates and Internal energy increases with 0J to 9 MJ respectively.

T = 1.5 mm

In this simulation, the material assigned for the crash box is Steel which behaves as Plastic with a thickness of 1.5mm. The box is moving at a velocity of 50 m/s and mass added as 100 kg which is impacted to the rigid wall which crumbles due to plastic behavior moreover depends upon the velocity and mass added at the end of the tube. when the box hits the rigid wall at a time of 3.8 ms, the Kinetic energy falls suddenly from 9 MJ to 0 J and fluctuates and Internal energy increases with 0J to 9 MJ respectively. This simulation result is almost the same as the previous thickness (1.2 mm) but in the end, it not achieved as obtained due to mass added and it needs some extra energy to achieve that.

(II) PLOT RigidWall_Force VS. TIME:

T = 1.2 mm

T = 1.5 mm

(III) PLOT ACCELERATION VS. TIME:

T = 1.2 mm

T = 1.5 mm

(IV) PLOT SECTION FORCE VS. TIME:

T = 1.2 mm

T = 1.5 mm

(III) CONTOUR PLOT FOR ELASTIC CASE:

1. Effective Stress (T = 1.2 mm)

2. X-Strain (T = 1.2 mm)

3. X-Stress (T = 1.2 mm) :

4. Effective Stress (T = 1.5 mm)

5. X-Strain (T = 1.5 mm)

6. X-Stress (T = 1.5 mm)

(III) CONTOUR PLOT FOR PLASTIC CASE:

1. Effective Strain (T = 1.2 mm)

2. Effective Stress (T = 1.2 mm)

3. Effective Strain (T = 1.5 mm)

4. Effective Stress (T = 1.5 mm)

CONCLUSION:

The cellphone part is assigned as Aluminium (M-024) and the Base part is assigned as Steel (Rigid_M-020).

The initial velocity given for the Aluminium part is 10m/s.

When the cellphone part is falling freely at a velocity of 10 m/s in z-direction towards the ground.

The max von misses stress is developed at the tip is 0.123 N/m2 and max effective strain developed is 0.014 % deformed.