Study the effect of notches on crash tube model by simulating six different test cases using Hypercrash and Radioss

OBJECTIVE

• To simulate six different test cases of a crash tube experiment using Hypercrash and the solver Radioss.
• Analyse and compare all six outputs to study effects of contact interfaces

Test case setups

1. Run the crash tube model with default setup.
2. Change the Inacti=6 and run.
3. Create the type 11 contact and run.
4. Remove both notches and remove boundary condition on rigid body node then run.
5. Create a new notch in the middle ,select the whole section and run.
6. Create a new notch with nodes only from opposing 2 faces and run.

Model Setup

The model is imported in hypermesh and viewed.

Hypercrash is a dedicated preprocessor used for crash applications, hence the crash tube imported to hypercrash.

Initial velocity - 13.3 m/s in negative z-axis direction. The boundary conditions for the rigid body are defined and only z-axis translation is permitted all other motion are restricted.

Interface - TYPE 7 contact interface is defined as self contact.

Inacti value is set as zero

The material is defined as Steel for the tube and a rectangular rigid wall is defined at the bottom of the tube.

Case 1 - Run the crash tube model with default setup.

The model is run with default setup using Radioss solver. The output graph and animation are seen using hypergraph and hyperview correspondingly.

From the animation it is seen that the notch causes stress concentration and buckling of tube starts from notch area. Once the lower part is deformed then only area above the notch starts to deform

The rwall forces peak after 25 ms becuase at that time the tube has been completely crushed and further forces are transmitted to the rigid wall.

Energy curves

Internal energy is indication of energy absorbed by the tube as internal strain energy.

As the crushing begins the internal energy increases till 26.8 ms, at this point the tube has absorbed maximum energy and further energy are transmitted.

Thus the energy applied to the tube is not completely transmitted to the rigid wall and major part is converted into internal strain energy.

The Kinetic energy decreases as expected and at the end slight increase is seen due to rebound of the tube due to normal reaction forces. Total energy is equal to sum of kinetic and internal energy.

Contact energy due to the type 7 contact peaks at a time after 25 ms, at this point the the full model is buckled in such a way that penetration of nodes occur and the solver tries to remove these maximum penetrations by creating a spring with nonlinear stiffness between master and slave nodes.

Case 2 - Change the Inacti=6 and run.

In TYPE7 contact the value of Inacti is set to 6.

Inacti = 6 is recommended,in order to avoid high frequency effects into the interface.

Initial penetrations are calculted and gap is reduced to accomodate vibrations which inturn stablizies the timestep.

The rwall forces are not affected by the Inacti value change. The energy curves of case (2) are same as case(1). This indicates that there are no initial penetrations in the model. The Internal energy starts from zero, if any initial penetrations were present initial energy would start from a non-zero value (energy required to push the intail penetrations out)

Case 3 - Create the type 11 contact and run

In this case EDGE to EDGE contact is defined using TYPE 11 contact.

The interface properties are:

• Impacts occur between a master and a slave line.
• A slave line can impact on one or more master lines.
• A line can belong to the master and the slave side. This allows self-impact.
• This interface can be used in addition to /INTER/TYPE7 to solve the edge to edge limitation of interface TYPE7.

TYPE 11 contact parameters.

Igap = 1000

Inacti =6

siffness definition = 4

St min = 1 KN/mm^2

Idel = 2

Gap (Min gap) = 0.45 mm

Coulomb friction = 0.2

After 25 ms the rebound of the tube occurs due to the normal reaction force from the rigid wall. This causes nodal self contact thus TYPE7 contact energy rises to maximum and falls to a constant value at the end of simulation. In TYPE11 contact the contact energy is tiny on comparing with TYPE7 contact and falls to zero at 28 ms, thus it can be stated that there is very less self edge contact involved. This causes almost no change in rwall forces.

The energy curves are similar to case (1) & case (2). There is a slight decrease in total energy as small energy amount is lost in TYPE11 contact energy.

Case 4 - Remove both notches and remove boundary condition on rigid body node then run

In case (4) the notches are removed using aligh node option in hypermesh and the model is imported in hypercrash and the boundary conditions are removed.

Due to the removal of boundary conditions the rigid body can has all 6 degree of freedom can translate along all three perpendicular axis.

The output animation from .h3d file clearly shows that the rigid body has deviated from the straing line (z-axis). This causes one corner of the bottom part to slightly lift up and thus reducing the rwall forces.

The removal of notch causes the deformation to start from the bottom of the tube. In previous three cases the deformation started from notch area due to high stress concentration factor.

The rwall force plot shows that in case(4) the rwall forces are very low compared to previous 3 cases. This is caused because the boundary conditions on the rigid bodies are removed and it makes a bottom corner of the tube to lift and reducing force transmitted to the rigid wall

blue line - case(3) rwall forces

red line - case(4) rwall forces

As the kinetic energy decreases, the internal energy increases. The total energy decreases in the end as some amount of energy is lost as contact energy. Contact energy at the end occurs becaues the solver tries to remove self penetrations. As the top part of the tube deviates in the xy plane it is pressed against the tube part lying below. Thus there is a situation at the end of the simulation where a part of tube is firmly pressed against another part of tube, this increases the contact forces in the end.

Case 5 - Create a new notch in the middle ,select the whole section and run

In case 5 a notch is created at the middle of the tube.

Similar to first three cases with notch, in case 5 also the deformation starts from the notch area. Once the part below the notch is deformed then only top part of tube is deformed.

The energy plot reveals that energy is completely conserved. The kinetic energy is transformed into internal energy. The energy plot is similar to case 3.

The contact forces and rwall forces are as shown below

Case 6 - Create a new notch with nodes only from opposing 2 faces and run

In case 6 only two opposite faces have notch in the middle of the tube.

Due to the reduction of notch area the buckling effect induced by notch in case-6 is lesser than other cases such as case-5 where all four sides had notches.

This is explains the decrease of rigid wall forces in case-6 force plot compared to case-5 rigifd wall force plot

blue line - case 5

red line - case 6

Further, in case-6 only the sides with notches buckle and buldge outwards but in case-5 all 4 sides deform outwards.

The energy plots are similar to case-5 but there is a slight change in the rate of change of kinetic energy. Compared to case-5 the kinetic energy in case-6 decreases slowly, this is due to presence of notch only in 2 sides leading to decrease in buckling effect caused by the notch.

CONCLUSION

The contour plot of stress and strain are presented below for all 6 cases.

Von-Mises plot

Plastic strain plot

In cases where both TYPE 7 & 11 contacts are present the contact energy is higher than cases where only TYPE 7 contact alone exists.

Effect of notches

From the results it is seen that if notches present they tend to increase the buckling effect and thus causing the tube to deform with less amount of force.

On other hand notches also help in determining the deformation process. Thus notches have their adavntages as well as disadvantages.

Advantages of notches - They help in predicting the nature of deformation of a component.

Disadvantages of notches - They make the component fail much sooner due the stress concentration effect.

If notches are not present then it is not possible to determine the nature of strain formation as seen in case(4)

In automotive crash applications it is very important to know the nature of components to ensure safety of the passenger, thus notches must be placed such that it helps in determining the nature of deformation but doesnot make the component fail under low forces during impact.

Link for the files used - https://drive.google.com/file/d/1UZp8-4kTDV0l8gn79C59uEiqgqKPQ5YW/view?usp=sharing

Bumper model meshing

The bumper model is imported in hypermesh and meshed using automesh option under 2D meshing tab. Mesh size is defined as 6mm.

Since only one half of the model is given, the meshed model is mirrored to get the full model.

The crash tube is attached to the bumper using spot welds and seam welds.

Now this model can be imported in hypercrash to define parameters for a crash test.