Comparison of different Contact Interface Cards for Crash tube and the Behaviour of the Material

Objective :

To mesh the given bumper model using Hypermesh and simulate crash tube with different contact interface type using Radioss.

Explanation :

The given Bumper is meshed under given conditions.

BUMPER

Type 7: Node to Surface Contact

• A general-purpose interface and can simulate any types of impact between a set of nodes and a master surface.
• Slave nodes and master surfaces are defined.
• The slave nodes can belong to the master surface, hence the auto impact is possible.
• Node to the surface contact interface uses the Penalty method with variable stiffness.

Penalty Method:

• Whenever the node and surface are about to contact, the imaginary gap is created.
• This gap is named as gapmin. While the node and surface are so close to each other, a spring is created by the solver between node and surface.
• This spring has the stiffness, damping coefficient.
• Thus the behavior between slave node and master segments is treated as a spring that generates resistive force as a function of penetration.

NODE TO SURFACE CONTACT

• The slave is a node and the master is surface.
• There are some gap definitions, they are
• gm: master element gap

gm=t/2 , t is the thickness of the master element for shell element.

• gm=0 for the brick element.
• gs: slave node gap

gs=0, if the slave node is not connected to any element

gs=t/2, t is the largest thickness of the shell element connected to the slave node

• This method is the formulation of non linear stiffness, hence used more commonly in the industries.
• Since the method is non-linear, the time step is also less compared to the linear stiffness method.

The important interface parameters are

Igap      ----      Determines how the size of the gap is calculated

Gapmin  ----      Minimum gap for activation of the surface

Inacti     ----      Action to take if any initial penetration occurs

Istf         ----      Affects how the stiffness of the interface is calculated

Iform     ----      Friction Formulation

STmin    ----     Minimum stiffness to use in the interface

Idel       ----      What to do with the slave node and master segments if the elements get                              deleted

Igap : 0

• It is a constant gap method
• The gap is defined by Gapmin and is constant for all the contact. This is the default value.
• If the Gapmin value is not defined or not set, then Gapmin = min(tm,lmin/2)

Igap = 1

• It is a variable thickness method
• The gap varies according to the movement of the master surface and slave node
• the variable gap = max[(tm,(gs+gm)]
• This case will not give you an accurate result because the variable gap may not be correct all the time.

Igap = 2

• It is a variable thickness with scale factor

Igap = 3

• It is a variable thickness with a scale factor as well as the mesh size factor.
• The scale factor helps to avoid the resistive force in case if there is any kind of initial penetration.
• The mesh size factor helps to make mesh finer so that it creates the possibilities of all the node gap is detected or penetrated into the master surface gap
• Igap = 3 is preferred.

Inacti : Action to take if initial penetration exists

Inacti: 0

• No action takes place and running the simulation even though there is any initial penetrations.
• This is the best option if it is possible.

Inacti: 3

• Automatic removal of the initial penetrations
• It destroys the model geometry.

Inacti: 6

• The gap is reduced and scaled.
• When the scale factor is provided, the solver tries to reduce the gap that it won't affect the geometry and making it considered.
• Thus resulting in the best accuracy.

If there is a penetration, there is always the corresponding generation of resistive force.

The resistive force is non-linear with respect to the penetration of the slave node into the gap and is a function of value and time.

F = K*P+C(dp/dt)

Istf : Initial contact stiffness, K0

•  The stiffness of the node and master is selected depending upon the interface.

Hence

Iform : Sliding force computation

•  Iform = 1, viscous method
•  Sliding forces are computed using viscous parameters of the interface.
•  Iform = 2, Stiffness method
•  Sliding forces are computed using stiffness parameters of the interface, usually results in a bigger time step.
•

Idel :

• Idel =0, the master node and slave node are kept as such leading to instability of the simulation and the model
• Idel = 2, the master segment is removed from the contact when the link is deleted and the slaved node is deleted when it is free from the contact.

Comparison of Case1, Case 2 and Case 3:

CASE 1:

•  The simulation of the crash tube is simulated using Radioss.
•  The Contact interface with type 7 is ran as such and the following observation are made.
•  The following parameters are assigned default

CASE 2 :

• In this case, the Inacti is changed to 6
• Inact = 6 which meant,
• The gap is reduced and scaled.
• When the scale factor is provided, the solver tries to reduce the gap that it won't affect the geometry and making it considered.
• Thus resulting in the best accuracy.

CASE 3 :

• In this case, the recommended parameters are assigned as discussed earlier.
• Thus the following parameters are and the following changes are recorded

From the above simulation, we can able to see the changes that after assigning the values for Igap, the max stress values is reduced. This is due to less strain and deformation.

The stress is reduced because of the initial penetration is also taken into consideration. The Igap does the scaling accordingly and less resistive force is applied.

Rigid Wall Forces

The rigid wall forces are more for the TYPE2( recom values) compared to the other because of more reaction force has been offered by the rigid wall. This is due to the Sliding factor and also due to the setting of Igap=3. Thus creating more resistive force offered by the solver. The Type 2(recom value) is almost similar but at the time 0f 17milliseconds, The upper part of the crash tube is getting crashed and hence resulting in the more reaction force of the wall because there is no notch to observe energy. Wherever there is a notch, it produces less Rwall forces compared to the other parts of the component.

Contact Energy

The contact energy is due to the penetration of the elements. When the penetration is identified by the solver, it gives resistive force or energy resulting in the contact energy. The Type 7 (with recom parameters) is high compared to the normal cases because of the Igap=3 , whenever the penetration is there the solver tries to oppose using the resistive force which results in the increase in contact energy. The contact energy is increasing slowly from the time of 15 Milliseconds and drastically increases due to the Igap parameter.

The internal energy keeps on induced in the material. Whenever there is a hike in the graph it is due to the absorption of energy in the area where there is no notch. Thus notch helps to reduce the internal energy absorption. The type 2 (recom values) are high at the end due to the Igap = 3. The maximum internal energy is attained by the type 7(recom values).

CASE 4 :

Type 11: Contact interface

• It is the same as type 7 and still follows the penalty formulations.
• It is defined by gap definitions.
• It is defined by identifying the master and slave line(edges)
• The limitations of type 7 are overcome using type11 contact.
• Since it is a type 11 edge to edge contact, it creates more possibilities of detecting the intersection of slave and master.
• Most of the industries preferred with type 11.
• The crash tube is simulated using the type 11 with the recommended parameters and the following observations are made with the notch, without the notch, with a notch at the middle and notch at opposite face.
• The main drawback is due to an increase in computational time.

TYPE 11

The max stress-induced is 0.628 N/mm^2. Thus the stress is further reduced due to the parameters assigned which it just reacts whenever the edge of slave and master comes in contact, thus always gives the resistive force. and the time is reduced to 18 milliseconds. Thus the reaction is fast compared to the previous cases. This is because of the edge to edge contact and also increase in the stiffness of the material.

TYPE 11(recom values)

The kinetic energy is high at the start due to the motion of the elements and keeps reducing due to the impact of the tube through the wall. Thus at the time of 23milliseconds, the crash tube gets fully deformed and increase due to the reaction force offered by the wall.

The internal energy ascending till the time of 25 milliseconds and constant thus all there is no impact or no resultant force.

The contact energy keeps on increasing due to the penetration of elements and thus increase in the contact energy results in energy errors. This is because of the resistive force offered by the solver to remove the penetration.

Rwall Force(Type 11)

This is Rwall force shows hike is when the contact between the crash tube and the rigid wall is induced. This Rigid wall gives reaction force thus creating the spike. At the time of 20 milliseconds, the Rwall forces increase because the crash tube is completely deformed and more force is acting towards the Rigidwall thus putting the total weight into the Rigid wall.

Comparison between No Notch, Middle Notch, and Opp Face Notch :

• At first, the notch is removed using the align tool
• It is still in the same Type 11 contact interface.
• The simulation is executed and the following observations are made.
• Then next, the notch is created at the middle
• for the next simulation, the notch is created just opposite to each other face.
• The boundary condition card is deleted and is free to move in every direction.
• Thus the following observations are made.

No Notch                                                        Middle Notch                                                       opp notch

The simulation is executed and the following changes have been recorded.

With No Notch-- The maximum stress induces is 0.672 N/mm^2. This is because of no presence of a notch, the part gets bent and undergoes more stress compared to the other elements.

With Middle Notch-- Due to the presence of the notch the element undergoes deformation in the notch region and gradually transferring the stress to the other element. At the notch, the stress is concentrated. The max stress in the element is the 0.61N/mm^2

With End Notch -- The notch is present at the opposite faces when the element undergoes deformation, The elements in the notch deform first and transfer energy. Since the cross-section of the notch is less compared to the previous case, therefore little more stress is induced in the element. The max stress is in the element is 0.62 N/mm^2.

No Notch -- The internal energy is high at the start due to the absence of the notch and gradually increases throughout the impact.

Middle Notch-- The internal energy is less at start increasing and the hike is there at the time of 10 milliseconds. It is due to the presence of a notch.

End Notch-- The internal energy keeps on increasing till the time of 25 milliseconds and constant that is where the impact comes to and end remains constant throughout.

Without Notch --  Since there is no notch, the contact between the elements is high. The spike(8 milliseconds) in the graph is due to the folding of the element, there is more contact and thus resulting in more contact energy. The more the contact energy, the more the energy errors. The energy is dissipated from the system.

With Middle Notch-- The spike is due to the contact of elements in the folding region. and increases but less compared to the previous case.

With Opp Notch -- Since there is a tch at the opp face, the element gets contacted more at the start but at the end, only a few elements are getting contacted resulting in fewer energy errors. This also because the element is stiffer.

Rigid wall forces

Without Notch --- There is no notch present, so then there is no absorption of energy in the notch area thus transmitting the energy to the rigid wall which gives corresponding reaction force. It is clearly seen that the rigid wall forces are high at the start due to the reaction force.

With Middle Notch --- At the time of 9.6 milliseconds, there is a hike in the graph because it is due to the lower portion of the below notch is absorbed more energy and started deforming and thus applying more stress against the rigid wall.

With Opp Notch  ---   It is as same as the middle notch where the notch area is deformed earlier and opposing the more stress transferred to the rigid wall and almost tracing the curve of middle notch.

TH part for middle notch and No notch

The internal energy has been observed for the particular parts and the following observation has been made.

• There are four parts associated with the crash tube
• The pshell 1 and pshell 2 are made up of 3mm thickness
• pshell5 and pshell6 are made up of 2mm thickness.
• Thus the following observation of internal energy for each part had been made.

MIDDLE NOTCH

END NOTCH

The internal energy of each part has been observed, thus the pshell1 and pshell2 are the part at the top, whereas the pshell5 and pshell6 are the bottom-most parts. Now due to the load, the bottom-most part comes in contact with the rigid wall experiencing the reaction force thus the bottom-most part absorbs energy. The upper part remains constant because it keeps transferring the energy. And when the deformation completely took place till the notch, the upper part gets deformed which results in the increase of internal energy. At the time of 8 milliseconds, the upper part of the crash tube is absorbing energy.

Conclusion :

• Thus the following observation has been for different contact interface cards such as Type7 and Type 11.
• The changes that notch bring to the crush tube is significantly noted.
• The placement of the notch is also the ultimate factor.

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