Side crash analysis

Aim:

Car side crash analysis using Hypermesh and Hypercrash

Objective:

The objective for the front crash are as follows

1. Check the unit system used in the model and change to either (Mg mm s) or (Kg mm ms)
2. Create the Type 7 interface for the model with the friction value 0.2 and recommended parameters.
3. Check for the interaction and penetrations.
4. Correct rigid bodies if any issues.
5. Create a rigid wall with friction 0.1
6. Compare the model weight with the full-scale 300k nodes model and use added masses to reach the target weight of 700kg while getting CG about the required range.
7. Put the initial velocity of 35mph.
8. Use a model checker to ensure good quality.
9. Time-step :0.5 to 0.1 microseconds.
10. Give Run time for the simulation as 80 ms.

Methodology :

1. Open the file in Hypermesh and check the unit system used.
2. Create the interface for the crash.
3. Create a Rigid wall for the side crash.
4. Add the appropriate amount of mass to the model to obtain the COG at the required location.
5. Create the Initial velocity condition.
6. Create the section at the cross member.
7. Create output for intrusions at the three mentioned locations.
8. Peak velocity at the inner node of the door.
9. Check for the errors in the final model.
10. Giving the required time step value and the engine run value.

Introduction:

Some side impacts involve a vehicle traveling sideways into rigid roadside objects such as trees or poles. Often this is the result of a loss of control on the part of the driver, owing to speeding, misjudgment of a corner, or because of a skid in slippery conditions. Such accidents are severe and the frequency of death or serious injury is high.

In some tests, a car is propelled sideways at 32 km/h against a rigid, narrow pole. The car is placed at right angles to the direction of motion, at a small angle away from the perpendicular. Where a vehicle is equipped with a center airbag to protect against the front seat occupants hitting each other, two average male side impact dummies are placed in the front seats. Where there is no center airbag, a single dummy is placed on the driver’s seat.

This is a very severe test of a car’s ability to protect the driver’s head. As the loading on the car is so localized, deformation can be very high and the pole can penetrate deeply into the passenger compartment. Without effective protection, the pole would strike the head resulting in serious injuries. Head protection airbags – often curtain airbags mounted above the side windows but sometimes seat-mounted thorax/head airbags – have become a common solution but great care is needed to ensure effective performance of such devices.

Procedure :

Step 1: Opening the file and checking the Unit system.

The Starter file (.0000 rad ) is imported into the Hypermesh window. To import the file

File >Import>solver deck>import the 0000.rad file.

Now it’s important to ensure that the model follows a certain unit system. To check the unit system for the current model.

Model browser> cards >begin_card

The unit system used here is all good kg mm ms.

Step 2: Creating the interfaces

Type 7 is a general-purpose contact used for most of the contact conditions.

This works on the Penalty method which states that when the slave mode enters or touches the gap of the Master node, a spring element or resistive force is created a the point of contact.

And the reaction forces are then calculated by the solver when the contact develops based on the initial velocity and initial parameters. Also, the time step is decided based on the spring element.

The advantage of Type 7 is that stiffness is not constant and increases with the penetration preventing the node from going through the shell mid surface.

To create the Type 7 interface

Open the Groups in the model browser > right-click on it > Create > rename it to Type 7.

Now choose the Slave entities as the components and select the components. Then choose the Master entities as the components and select the components.

Some important criteria used under the type 7 interface.

Istf- 4 (interface stiffness calculated from both master and slave node characteristics )

Igap- 2 - variable gaps (how the gap size is calculated)

Idel- 2 (element deletion )

Gap_min - 0.5 (to decide the minimum gap to activate the interface condition )

Irem_gap- 2 (deactivation of slave node if element size is lesser than the gap values )

Inacti - 6 (action to take if initial penetration exist)

This Inacti is one of the crucial criteria that helps to decide the steps to take in case the penetration has occurred with the master node.

Now why this penetration has to take care of? The answer is to avoid the unstable run due to the time step.

The time step is calculated as

Delta T=0.5(g-p)/(dp/dt)

g- gap thickness

p- penetration

dp/dt= speed of sound through material.

Now if the g<p, the numerator value will be negative that in turn cause the time step value to be negative. This negative time step will cause an unstable simulation run.

So, the Inacti criteria push the node to the point using some form scale factor multiped with the gap thickness of the master node.

Step 3: Creating the Rigid wall

The Rigid wall as the name suggests that it is rigid and not deformation occurs. It is created in order to provide the wall for the slave node beyond which they are not allowed to go.

To create the Rigid wall open the Solver browser.

In the solver browser Right click>> Create >> Rwall >>CYL. A cylinder infinite wall is used to perform the side pole crash test.

A new rigid wall card is created, under which insert the required values.

Select the geometry Type as the Infinite plane

Click on the XM, YM, ZM(X, Y, Z coordinates as base node M)> Select the node at the plane rigid walls need to be created.

Select the normal at the axis and based on the direction of normal give a minus(‘’-”) sign in the Z direction (see the global coordinate )

Give the Slide value as 2

Friction value as 0.1

Dsearch distance as 1000.

Step 4: Reach the model weight to 700kg and Obtain the COG.

The COG of the model is calculated based on the total mass and its distribution over the model.

Here the overall weight of the model has to be matched around 700kg. In order to add the mass in the model.

In Solver Browser >>ADMAS>>Right click>> Create. A new add mass card is created and rename it to the appropriate name. Under the card Select the mass type as 0( mass is added to each node separately ).

Select the gnd_Id >> select the entities id>> nodes. Select the nodes and add the mass value to be added on the nodes selected.

Now check the Total mass and the COG of the model. To check the mass and cog go to Tools>> Mass details >>mass, cog, and inertia.

Maintain the Cog at the appropriate location and match the total mass to 700 kg

Plot the obtain Cog point in the model.

geometry>nodes > create

Step 5: Create the initial velocity.

Initial velocity has to be applied to the car model with which it will impact the rigid wall. To create the initial velocity.

Solver browser > right-click>>create >> Boundary conditions>> select the INVEL>> give the card a name.

Under the card,>create the gnd_id>> select the entities _id as nodes by displayed and proceed.

Enter the Velocity value in the direction to be applied. Here the velocity is apple at the Y

direction 15.64

Step 6: Creating the sections :

The section is created at a particular part in order to see forces and stresses developed in the particular part or section of the model in the crash process.

Now here the section has to be created at the cross member of the car model.

To create the section go to the Solver browser >>right-click>> create >> SECT>> In the entity Editor, give the N1, N2, N3 as the nodes >> a skew is created at this location.

After that, we need a local coordinate system for this section. For that, we create the Frame.

Create >>Frame >>Mov>> select the node in the same direction as selected for the section (origin,axis, plane)

Now go to the section card earlier created >>Frame_id >> select the frame system as created above.

Enter the Delta T value as 0.1 and alpha value as 1.67.

Under the Grshell_id >> right-click >> create >> select the entities _id as elements on the area where the section is created.

Iframe as 12 i.e On cog

Like these, the section is created at the other cross member.

Calling the section in the output :

The section created needs to be called at the output so that it can be analyzed.

To call out the Section. Model browser >> Output blocks >>right-click>> create >> rename it to the cross section.

Select the Entity_id as Cross-section and the cross-section is done.

Step 7: Creating the intrusions :

The intrusion is created to know the displacement or velocity of the particular part of the whole assembly before and after the crash

Here the intrusion is created for the B-pillar, Hinge and, file tank.

To create the intrusions at the specific node location, first, we need to have a Moving Skew.

Go to the Solver Browser >> right-click>> Create >> skew >> mov

Select the nodes as origin, axis, planes.

This intrusion is created in the output_blocks.

Model browser >> Output_blocks >> right -click>> create >> name it to intrusion Hinge

Like the rest of the parts, intrusions are created.

Step 8: Creating the peak velocity :

The peak velocity is crated at the inner door of the car.

To create the peak velocity we need the skew

Go to Solver browser >> Create >>skew>>mov >> select the nodes >> create

Now go to the MOdel browser>>output Block>> Right-click>>create >> rename to Peak velocity

Click on the Entity_id>>select the node on skew point >>click on the Iskew>> select the skew created

Step 9: Check for the penetrations :

The model has to be checked for the penetrations if there are two components elements are penetrating with each other.

To check for the penetrations, go to the Tools menu >> Penetrations check >> in the entity editor, click on the selection and click on the group>>Give the group. >> check

There is no penetration found in this model.

Step 10: Checking for the errors :

The model has to checked for errors if there are any errors and incompatible conditions present.

Menu bar >>Tools >> model checker >>radioss block

Step 11: Giving the time step and run time values :

After the Model check is done, apply the time-step value tfreq=0.5.

Goto > Tab bar > cards > ENG_ANIM_DT.

After editing the time step, apply the run-time value Tstop=80.

Goto > Tab bar > cards > ENG_RUN>>Tstop>>80

Errors encountered:

The errors faced here are only the Empty cards that are created will selecting the nodes for the add mass or maybe for skew. So these errors can be solved by deleting the cards in Hypercrash.

Other errors is the Unsupported cards that are formed while the file is exported from the Hypercrash to the Hypermesh. This file also needs to be deleted in the Hypermesh otherwise it shows as the error in the final run.

Basic steps for simulation :

• Import the starter file (rad_0000) into Radioss block
• Provide the proper material and failure to the component(Rupture plate). Since here it’s already provided with Law 2 as material and failure Johnson card.
• Then go to Analysis > Radioss> load the starter file > radioss. Simulate with no errors.

• Open the output file (rad_0001) to check for the energy error(less than 15%) and mass error(less than 1%).
• Now open the Hyperview and check the simulation using H3d file
• Open the Hypergraph 2D to observe the energy plots with the T01 file.

Results and observations :

Animation :

The animation shows the Rigid wall as a pole, the side crash test has a drastic impact on the car's other side. This means that the inside compartment passenger won't be safe in this condition if the material and structure of the car remain the same. Maybe this test result can be improved by using some more structural members or improving the material of the car BIW. Based on the value we see for E and rho the material is steel. The max. Von mises stress value is 0.44Gpa.

The plastic strain means that the element has deformed and absorbed the forces. So the animation shows that max plastic strain is formed in the region of the B-pillar and the Rocker attached to it which maybe means a good sign that this strain is not formed to the other side of the car.

Also in the animation, one of the strips attached to one of the cross-members gets detached from which may have helped to increase the load-bearing or force capacity of that cross member.

Graph plots :

Energy plots :

The energy plot for internal, kinetic, total along with the hourglass and contact energy is shown in the above graph.

The hourglass energy is very minimal in this case which is a good sign. This is due to the use of good shell formulation here QEPH shell which has a good amount of integration points that helps in reducing the hourglass effect.

The contact forces and hourglass energy has to be less than 15% of the total energy which the graph shows as a needed result. The contact force is minimal but it slightly increases after the 50ms of the crash. Though the overall contact force is less than the 3%

of the total energy which is kind of desirable and a good sign.

The kinetic and internal plots show the mirror image. That means the decrease in kinetic and an increase in internal energy at the same time.

This graph shows the total energy as an almost straight line because it is the summation of the kinetic and internal energy.

Section forces:

The section forces show that cross-section 1 has fewer fluctuations in the resultant force as compared to the second cross-section.

Also, cross-section 1 shows a very high value for the resultant force around 43 as compared to the other sections (4.3). This may be because the second section had one more strip attached to it which got detached later.

The second section plots more fluctuations in the forces even after hitting the peak value at around 4.3.

Intrusions Displacement plots :

Fuel tank

B-pillar

Hinge:

The intrusion plot for the fuel tank, B-pillar and, hinge shows the displacement value increases gradually with time.

The initial distance between the node and the skew measured was around to be 1300 approx. So as seen from the graph plots the maximum amount of intrusion was found to be in the Fuel tank followed by the B-pillar and the hinge has the minimum amount of intrusion. The door has a minimum due to the structural member attached to it which has deformed less and so the less displacement for the hinge. But for the Fuel tank, the intrusion was more because there was no cross member that can reduce the amount of displacement.

Measure to reduce the fuel tank intrusion :

One simple step that can reduce the intrusion is to change some design aspect for the fuel tank that means a cross member that to some stiffer or with proper material can help in reducing the intrusion for the fuel tank. Material and design has to proper because the intrusion in the B-pillar was quite significant though it had the Cross-member.

Peak velocity for inner Door:

The peak velocity for the inner side of the door hits its peak value of 16.8mm/ms at around 7 ms that is when the impact actually happens.

The velocity then decreases and fluctuates between 1-10mm/ms for the rest of the time in the crash.

Conclusion :

The side crash with the pole shows the devasting or kind of not desirable results in animation for the safety of the passenger. Though the plastic formation in the car shows a good result as it does form to the other end. All the energy and force plots