Analysis on the automotive Crash box using contact interface Type-7 simulation Type-11 along with Type-7 using Hypercrash and Radioss

OBJECTIVE

In this report, we mesh a Bumper assembly.
Perform simulation on Crash tube and compare the Type-7 simulation results, Type-11 along with Type-7 and different kinematic condition simulation results.

Create six different cases 

  1. Run the crash tube model as it is, which is given.
  2. Change the Inacti=6 and run.
  3. Create the type 11 contact and run.
  4. Remove both notches and remove boundary conditions on a rigid body node then run.
  5. Create a new notch in the middle and run.
  6. Create a new notch with nodes only on opposite 2 faces and run.

Plot Rigid wall forces, contact forces, internal energy and compare results for all cases.

Procedure/Explanation

1. Bumper system meshing

The bumper is meshed with an average mesh size of 6 mm and did spot welding to join two parts of the bumper.

The mesh flow and connectivity of mesh should be good for the proper transfer of energy from one part to the other part.

2. Function of crash tube

  • It is located between the bumper and chassis. It helps to reduce impact energy during a collision.
  • During a crash, shock energy converted into strain energy and internal energy by the crumpling of the crash tube. Instead of bending it should be crumple and absorb a large amount of energy.

3. Simulation of crash tube

We have a crash tube on which we perform crash analysis comparing the Type-7 simulation results, Type-11 along with Type-7.

The total length of crash tube is 304.50 mm.
Initial velocity is 13.3 mm/ms at the rigid body.

4. Interface/Contact modeling means how parts interact when they come into contact with each other.

Type-7 deal with modeling contact between a master surface and a group of slave nodes with the non-linear stiffness.

The main advantage of interface type 7 is that the stiffness is not consistent and increases when the node trying to penetrate and resist from going through shell mid surface.

Case 1

We have a crash tube has given, which contains two notches near to the rigid wall and having Type-7 self contact.

  • Igap=0: It takes a constant gap which is equal to Gapmin.
  • Gapmin=1 mm, This is user-defined or if it is not defined or equal to zero then
    Gapmin= min(tm,Lmin/2)
    tm
    is the average thickness of the master shell element
    Lmin is the smallest side length of all master segment
  • Inacti = 0: automatically set to 1000 (default) No action is taken on initial penetration and just run the simulation
  • Istf= 4 By this we calculate Initial contact stiffness Ko= min. (Km, Ks)
    km= master segment stiffness
    and Ks= equivalent nodal stiffness
  • Iform= 2, Friction based on stiffness formulation: sliding forces are computed using stiffness parameters of the interface
  • Stmin=1 kN/mm, Minimum stiffness to avoid too soft contact
  • Idel=0 i.e. set to default value 1000, No deletion of an element while failure

Case 2

Model is the same as case 1, here we change to recommend parameters for contact Interface. 

  • Igap=3: To avoid initial penetrations, the solver considers the variable gap with the gap scale correction factor and also the size of the mesh is taken into account.
  • Fscalegap = 0.8. Gap scale correction factor. The gap nodes and segments keep on changing because of constant vibration generate while calculating the force.
  • Gapmin=0.5
  • Idel= 2 i.e. when one element is deleted, the master segment is removed from the contact and slave nodes became free are also removed.
  • Inacti=6,  It adjusts the master segment to remove the initial penetration and it increases stability and gives space to nodes for vibration.
  • Istf= 4

Case 3

In this case, we create a new Contact interface Type 11.

Type-11 deals with the edge to edge contact with the non-linear stiffness. Here we define all elements are a slave and all are master as well.

  • Igap=1000; This is default value so it takes a constant gap which is equal to Gapmin.
  • Gapmin=0.45
  • All other interface parameters are the same as the recommended ones which we use in case 2 for type 7.

Comparison of case 1,2 & 3

The total energy should remain constant but it is decreasing in all cases.
We can see that it decreasing less in case 1 as compared to case 2 and 3 because we can see contact energy increases less in case 1. Contact energy uses system energy to play its role.

Igap value changes to 0 to 3, when it is zero the gap slave and master remains constant throughout the simulation that gap is equal to gapmin. But when we take Igap is 3 the solver considers the variable gap.
The variable gap varies according to this formula:

Variable gap= max.{Gapmin, min[Fscale*(gs+gm) , %mesh size *(gs_l + gm_l) , Gapmax]}

gm = master element gap, gs = slave node gap
gm_l= 
Length of the smallest edge of the master element
gs_l= Length of the smallest edge of element connected to the slave node
Gapmin is given by the user and the size of the mesh is also taken into consideration to avoid initial penetration, the default value is 0.4 of percent of the mesh size.

 

After crumpling the penetrations of the element occurs and but in case 1 the gap doesn't change so there is less contact energy.
Contact energy follows the same behavior in all cases. As in case 1, it is less than case 2 & 3. Due to Igap= 3, more elements come under the minimum gap for impact activation. So as crash tube crumble surfaces are come in contact due to which nodes trying to penetrate into master segment then stiffness increases the oppose to penetration and at that moment the energy generates that energy is called the contact energy.

In case 1 we give inacti = 0 which means that there is no action taken for the initial penetration and in case 2&3 we give Inacti= 6 in which master segment sift by 5% of (gap - initial penetration). There is no initial penetration. So there no major change in cases 2 & 3 and it follows the almost same curve.

As the crash tube impacts the ground the internal energy rises. We observe in a simulation that in case 2 & 3 it rebound early from case 1 that why in case 1, it stores more energy w.r.t time.
case 2 & 3 release its internal energy while rebound.

Rigid wall forces are the forces which act on the rigid wall while the crash tube is crumple.
As the crash tube crumple and make a curve in the surface at that moment there is change (increases & decreases) observed in rigid wall forces.
We observe from all three simulations that in case 2 and 3, crash tube rebound one frame earlier from case 1 from this we can say that the rigid wall force is more in case 2 & 3. we also see in the graph.

Case 4

Delete the boundary condition (which arrests translation motion of X & Y direction and Rotational motion of X, Y, Z direction) of master node of a rigid body and remove the notches & make a plane surface. All the interface parameters are the same as case 3.

Case 5

In this case, boundary conditions are not there and we create a new notch in the middle & keep all other parameters the same and run the simulation.

Case 6

Here, boundary conditions are not there and we delete previous case notch and create a new notch with nodes the only on opposite 2 faces and Keep all other properties the same as the previous case and run the simulation.

                         CASE 4 without notch                                   CASE 5 all side notch in middle                               CASE 6 opposite side notch

The Clear difference in Stress value we can see. It went decreases from case 4 to case 6. Here without Notch case 4 has higher stress value.

As all crash tubes are made up of the same material, so crash tube without notches is better because more energy it absorbs in itself less it transfer further.

We can observe that in case 6 (Opposite side notches) has higher contact energy and case 4 (crash tube without any notches) has less than case 5 & 6 both.
This is because we can say that in case 4, lesser elements come under the minimum gap compare to cases 5 & 6. So less energy is required to oppose penetration by increasing stiffness.

 

As motion starts the internal energy is more w.r.t. time in case 4 (without notches) as compare to cases 5 & 6. That means a crash tube without notches absorb energy at a faster rate.
As the deformation takes place the internal energy increases. We can say that, at the end when the whole crash tube crumpled, the internal energy gets constant.

By this, we can conclude that a crash tube without notches is better than the other two cases 5 & 6.

 

For all three cases, Rigid wall forces fluctuate in the beginning, as the crash tube crumples, as the curve formed on the surface of the crash tube, rigid wall force increases.
But when the tube crumpled fully after that it keeps on compressed then more the surface collide with the wall and we get a hike in rigid wall force increase drastically.

Due to the opposite side notches are present in case 6, crash tube deforms more and more the elements are colliding to the rigid wall.
Higher the rigid wall force we get, the lesser energy absorbed by the crash tube. Hence we prefer the crash tube without notches (case 4) because it absorbs more energy.

Rigid wall forces again decrease because of the crash tube rebounds.

Internal energy graph for all four parts of the crash tube for case 4.

TH/part gives the time history of particularly every four parts. we can plot graphs for any part of the component and get to know the details on that particular part.

  • The bottom part of the crash tube undergoes the deformation first when the load is applied.
  • so according to that 5 pshell_2mm & 6 pshell_2mm denotes in graph, we observe an increase in internal energy w.r.t. time. It getting constant in between because at that time the upper part of the crash tube starts deforming.
  • Around 20th ms, the upper part also crumples completely then the whole crash tube keeps on crashing.
  • For the upper part after 8-9th ms, we can observe that internal energy keeps on increases upto 26th-27th ms.
  • The graph becomes constant again, crash tube stores as much as energy in the form of internal energy.

 

CONCLUSION

  • Performed meshing on the Bumper assembly.
  • Performed crash analysis on the crash tube and compared the simulation results of Type-7 contact interface with & without recommended properties and Type-11 along with Type-7.
  • Compared simulation without a notch, all side notches, and opposite side notch in the last three cases. From that, we find a crash tube without notches is better than others because it absorbs more amount of energy and transfers less force to the rigid wall.
  • we get time history for all four-part individually and analyze the internal energy graph w.r.t. time for that. As tube getting crushed according to that internal energy of the part getting increases.

Analysis on the automotive Crash box using contact interface Type-7 simulation, Type-11 along with Type-7 using Hypercrash and Radioss

 

google drive link:

https://drive.google.com/open?id=1JlsVhlzVPfAUZjCGlZG-DQmHQHh6zZ3Y

 

 


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