CRASH BOX SIMULATION USING LS-DYNA

Objective:

A crash box is a highly energy absorbing structure that crashes on application of loads and reduces impact on other components nearby.

The main aim of this challenge is to simulate how a crash box behaves during frontal impact into a rigidwall using LS-DYNA.

Also, to compare and validate the results for crash box with thickness values 1.2mm and 1.5mm.

The unit system followed in this challenge is gm-mm-ms.

Crash box:

A crash box is an important component in determining crashworthiness , and is placed between the bumper and side rails of an automotive vehicle.

Crashworthiness is described as the ability of the vehicular structure to absorb dynamic energy and minimize injury to passengers during a collision, and is considered among the important criteria in determining vehicle design with regard to passenger safety.

Traditionally, aluminum and steel have been used for making crash boxes in cars .

However, insistence on lightweighting combined with high stiffness and strength has led to composites gaining preference for use in cars, as all these properties are crucial in achieving better crashworthiness.

The FE simulation helps engineers how components interact in an assembly and how the failure mode and mechanism is developed during impact loading.

The crash box used here is made up of steel material with initially elastic property and prolonged to have plastic non-linear effect.

In this challenge, the crash box is set to collide with a rigidwall with an initial velocity of 50 kmph (i.e. 13.88 mm/ms).

The respective keyword file containing the model setup is imported into LS Prepost to start with the challenge.

Modelling rigidwall:

Rigidwalls are analytical representations of surfaces or volumes that are frequently used to represent stiff structures that are either stationary or in motion.

A stationary planar rigidwall is created ahead of the frontal portion of the crash box at an approximate distance using the option Create entity>Rigidwall and choosing an appropriate node and plane.

The created rigidwall is translated to some distance in the direction of impact velocity of crash box.

The crash box is made to hit the stiffened rigidwall at an impact velocity of 13.88 mm/ms.

Material setup:

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.

The crash box is modelled with steel material.

In our model, instead of defining an arbitrary stress versus strain load curve, we define the Tangent modulus for elastic-plastic behaviour of steel material.

The tangent modulus is useful in describing the behavior of materials that have been stressed beyond the elastic region.

When a material is plastically deformed, there is no longer a linear relationship between stress and strain. Thus, quantifying the "softening" or "hardening" of material that generally occurs when it begins to yield.

The steel material is referenced to the crash box using the part option.

The properties of steel in the following figure are represented in gm-mm-ms unit system.

Section setup:

The FE model of crash box is modelled with shell elements.

In LS-DYNA using the Keyword>Section tab, a new shell section is created with thickness as 1.2mm for the crash box.

For the second case, the simulation is set to run with shell thickness value of 1.5mm for the crash box.

The shell section is referenced to the crash box using the part option.

Initialization setup:

In our model, the crash box is made to hit the rigid wall at an impact velocity of 13.88 mm/ms.

For this, *INITIAL_VELOCITY card is used to apply velocity for the crash box in the direction of impact.

A node set ID is created for the crash box by selecting its nodes using part selection option.

The input velocity condition is applied to the created node set for crash box.

Contact setup:

Contact provides a way of treating interaction between different parts or disjoints parts.

Contact in LS-DYNA is defined using contact surfaces. It is useful for impacting one part over other part.

Most impact and crash applications require single surface contact to be defined.

A self contact is established in our model as there is only one surface.It is done by modelling *AUTOMATIC_SINGLE_SURFACE contact card.

Single surface contact is the most general type of contact used in LS-DYNA program that automatically searches all of the external surfaces within a model to determine if penetration has occurred.

Also, to record contact interface forces that occur in our model, *CONTACT_FORCE_TRANSDUCER card is established.

Control setup:

In LS-DYNA, *CONTROL_TERMINATION card is used to define the simulation time using ENDTIM option.

In our project, the model setup is set to run for 25 ms.

Model checking and solving the setup:

One of the most important pre-processing tasks is to ensure the validity of a CAE model before submitting it to the solver for analysis.

A good and rich suite of checks reduces engineering errors, increases the confidence in the simulation results, and minimizes the duration of a simulation cycle by eliminating multiple back and forths from the solver to the pre-processor.

After getting cleared up with the errors if any, the crash box simulation setup is saved.

The saved keyword file is set to run in LS-DYNA manager.

A normal termination of the setup is achieved and the results are analyzed for post-processing.

Post processing results:

The requested outputs for d3plot, history and ASCII options are now ready to view in LS Prepost for post processing.

1.Animation of crash box simulation

The above animation evidently depicts that the crash box hits the rigidwall and rebounces backwards in the same direction.

While rebouncing back, the crash box stores some amount of internal energy ,which is more or less equal to the impacting energy.

2.Energy plots for crash box of shell thickness 1.2mm:

The above plots of graph gives us the information about the energy distribution of crash box impacting a rigid wall.

It can be seen that at time 5ms, the crash box hits the rigidwall, from where its internal energy tends to increase.

The maximum internal energy is found to be 3.62 E+07 N-mm at time 6 ms.

Correspondingly, the kinetic energy of the system tends to behave in an opposite manner to the internal energy.Thus, decreasing with respect to time.

Since the crash box model produces significantly small amount of strain, the hourglass energy remains zero.

Since there is no chance for sliding type of contact behaviour in the model, the sliding energy also remains zero.

Total energy is the sum of all other energies. Hence, the total energy of the system decreases relatively with respect to time.

3.Cross sectional force generated at middle of the crash box:

The above plot gives us the information about the cross sectional force generated at the middle of the crash box.

Initially before solving, a planar cross section is created at the middle portion of the crash box using the *DATABASE_CROSS_SECTION_PLANE card.

The output request for cross sectional force is created from 'secforc' ASCII option.

It can be seen that the maximum cross sectional force at middle of crash box is found to be 9.52 E+04 N at time 6ms.

4. Comparison of acceleration plots between crash box at mid-node of shell thickness 1.2mm and 1.5mm

(a)For the case of 1.2 mm thickness,

(b)For the case of 1.5 mm thickness,

The above two plots depict the resultant acceleration vs time variation for crash box at mid-node of shell thickness 1.2mm and 1.5mm.

For the case of 1.2mm thickness, the maximum resultant acceleration is found to be 0.688`(mm)/(ms^(2)` at time 7ms.

For the case of 1.5mm thickness, the maximum resultant acceleration is found to be 0.576`(mm)/(ms^(2)` at time 22ms.

5. Comparison of directional stress between crash box of shell thickness 1.2mm and 1.5mm

The legend in the fringe animation is changed to static from dynamic to visualize the variation of stress in entire model during the simulation.

(a)For the case of 1.2 mm thickness,

1. X- stress

2. Y- stress

(b)For the case of 1.5 mm thickness,

1. X- stress

2. Y- stress

The following table summarizes the maximum directional stress of crash box simulation.

From the above table, it is evident that the maximum X- directional stress is more for crash box of shell thickness 1.5 mm than that of 1.2mm.

Also, the maximum Y- directional stress is more for crash box of shell thickness 1.2mm than that of 1.5mm.

6. Comparison of directional strain between crash box of shell thickness 1.2mm and 1.5mm

The legend in the fringe animation is changed to static from dynamic to visualize the variation of strain in entire model during the simulation.

(a)For the case of 1.2 mm thickness,

1. X- strain

2. Y- strain

(b)For the case of 1.5 mm thickness,

1. X- strain

2. Y- strain

The following table summarizes the maximum directional strain of crash box simulation.

From the above table, it is evident that the maximum X- directional strain is more for crash box of shell thickness 1.2 mm than that of 1.5mm.

Also, the maximum Y- directional strain is more for crash box of shell thickness 1.2mm than that of 1.5mm.

Conclusion:

Crash energy absorbers are important structural elements which plays an important role during accidents of automotive vehicles.

A simulation on crash box impacting a rigidwall with 13.88 mm/ms (i.e 50 kmph) velocity is performed using LS- DYNA.

It can be seen from the simulation that the crash box bounces backwards after hitting the rigidwall.

The simulation is carried out for crash box shell thickness values of 1.2mm and 1.5mm, and the post processing results are analyzed.

From the obtained results of stress levels for both the cases, it is found that the crash box with shell thickness 1.5mm has better stiffness and energy absorption rate as stress increases during impact than that of 1.2mm thickness.