Crash tube Analysis in Radioss using Hypermesh

Crash tube Analysis in Radioss using Hypermesh

Objective:

• Simulate crash tube with different notch replacements and with type 11 contact.
• Analyze the simulations by creating TH/part for components and plot various energies
• Mesh bumper assembly with no topological error

Description:

• Radioss consists of various contact types properties, in this particular scenario we are interested in
  • TYPE7,variable stiffness,node to surface contact
    • It is a general-purpose interface and can simulate all types of impact between a set of nodes and a master surface. The main advantage is that the stiffness is not constant and increases with the penetration preventing the node from going through the shell mid-surface. The gap used is on both sides of the shell mid-surface, and a cylindrical gap is added around the edges.

• • TYPE11,variable stiffness, edge to edge contact

    • There are situations where the Type 7 (node to surface) interface will not “see” the contact due to edges crossing each other at angles. In that case of edges crossing, Type 11 interface can be added to capture the physics correctly

      • Type 11 is defined by identifying master and slave lines (edges).
      • Type 11 works similar to Type 7 in terms of Penalty formulation, Gap definition, Search method.
  • TYPE2, Tied contact
    • A tied contact is defined by a group of slave nodes and a master surface. The slave nodes are kinematically tied to the master surface within a defined search tolerance, dsearch

• Both contacts work on the penalty method in which the behavior between slave nodes and master segments as springs that generate resistive forces as a function of penetration.
• The spring stiffness is automatically computed according to the material and property of both the master segment and the slave node.

Bumper Assembly Mesh:

Correcting topological errors:

The model consists of free surfaces and free lines(red lines) and no contact between components:

• Free surfaces can be joined together by toggling the surfaces of crash box component
• Inside bumper, a free surface in the middle is connected by trim with surf command and toggle
• contact between components is achieved by spot weld after meshing the whole component

Mesh of Bumper assembly:

• Assembly is meshed with an element size of 6mm using 2D auto mesh.
• The contact between components is made by using spot weld from 1D as a connector, which can also act as type2 contact.
• The mesh is as follows:

Crash Box analysis:

Crash box components are initially set to following material, contact, Boundary conditions:

There are a lot of important parameters to observe but as now we are interested in contact interfaces since our analysis is on the between contact and notch placements.

Important parameters to observe Vs Recommended properties for safe use:

From the parameters we can observe that some are set to default which may results in some errors, to avoid those recommended properties are used as follows:

• In addition to this Type 11 contact has been created to look after edge crossing elements.

Simulations:

• To analyze what is happening when there is a change in parameters and change in notch position we are going to different simulations as follows:

• For Igap=3 ,Fscale_gap=0.8 and prMeshSize =0.4(default) has been adopted.

Results and Observations:

Rigid wall forces:

The forces which have been transferred from the crash tube on to rigid wall are Rigidwall forces.

Sim1,2,3:

• Rigid wall force plot, displacement and velocity plots for master node plots have been plotted.
• Rigid wall forces are increased when there is a transfer of force from the crush tube and decreases when there is no transfer of force(i.e, force is utilized within the crash tube).
• Here we can observe that Rigid wall forces are increased in sim-2,3 compared to sim-1 which can be explained by master node displacement graph.
  • In sim-2,3 the master node displacement is less which indicates, compression is more in sim-1 which implies force is utilized in compression, whereas in sim-2,3 compression is less which implies the transfer of forces has happened.
• There is also a forward shift of graph in sim-2,3 which can be explained by the master node velocity graph.
  • In sim-2,3 the velocity decreased fastly due to that there is a forward shift.

Sim4,5,6:

• Here it is a different case the peaks of rigid wall forces are varying with the position of notch rest all parameters are the same for each case.
• This is an interesting thing to observe, depending on the geometry of the crash tube there are variations of forces transferring to the rigid wall.
• Model with notch at opp.sides exhibits better transfer of forces.

Contact Energy:

• Contact energy is added to the system depending upon the type of contact, number of elements involving in the contact, geometrical changes and dynamic situations happening while simulation.
• From the above contact energy graphs, variation in the graphs is explained by the factors mentioned above. Since there are changes in the position of notches variation has been observed in sim-4,5,6. whereas in sim-1,2,3; there is a huge variation between sim-1 and sim-2,3 due to change in Igap.
• Since it is an extra added energy to withhold the contact there should be an energy error in the system, for example: in the above plot, the energy error for simulation one, it has been observed that energy error is varying inversely with respect to contact energy.

Internal Energy:

• Internal energy is the amount of energy stored within the elements during simulation.
• In sim-1 internal energy stored is more compared to the other two due to the change in Igap. Earlier it was Igap=0, now it is changed to Igap=3.
• Due to the change of gap, stiffness has been varied between master and slave nodes at the time of self impact between elements. According to that variation, the transfer of energy from master to slave via contact has been varied.

• In sim-4,5,6 all the parameters are the same except notch positions, from the above plot it can be observed that notch at opposite faces has the maximum internal energy compared to the other two.
• Hence the change in notch placements has a greater influence on internal energy.

PART ENERGY:

• The internal energy of each part for all simulations has been plotted above.
• When an element under loading, it tries to stores energy within itself(i.e, internal energy) after maximum storage the surplus will result in the deformation of the element.
• This behavior of whole parts has been plotted above, whenever there is a rise in the graph it implies storing of energy and the constant line represents deformation either compression/ expansion.
• All the above graphs exhibit almost the same kind of behavior except small variations in sim-5 which are resulted due to Notch in the middle section.

Conclusion:

• Change in interface parameters and notch positions has a greater influence on the rigid wall forces which implies the transfer of loads to rigid walls. From the above graphs, it is inferred that simulation 5(notch at the middle section) exhibited better transfer of forces.
• Change in contact energy was observed due to the change in Igap parameter and notch positions due to this energies energy error has been observed which is inversely varying with respect to contact energy.
• From the internal energy plots, it is clearly observed that notch at opposite faces shows optimal results having maximum internal energy.

Drive link:-https://drive.google.com/open?id=1nbTfd_NCb83e9KKCQfh7jEBpzs4WQbMp