Assignment 5-RADIOSS Interfaces & Study of Effect of Notches Challenge

OBJECTIVE:

1. Create the mesh for bumper assembly, mesh size should be 6mm. 
2. Run the Crash tube model with 6 different cases as given below and compare.

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

Interfaces (or) contact modeling:

The interfaces are defined as a model how the parts interact, when they come into contact with each other. While discussing the interfaces, there

are four types of interfaces widely used. They are:

1. Interface penalty method
2. Type 7, Variable stiffness, Node to surface contact
3. Type 11, Variable stiffness, edge to edge contact 
4. Type 24, Linear stiffness, node to surface & surface to surface contact

Other type is Type 19, which is the combination of both type 7 & type 11.

1. MESH THE GIVEN BUMPER MODEL

Procedure:

Model 1: 

Meshing the given model(Bumper):-

• select the user profile

Import the hm model of bumper system

•  The next step is to view the component by a topo. in order to check for geometrical errors. After switching it to the topo, the geometry in the graphical display 

 

Geometry clean up:

As we can see in the figure below there are surfaces detaches from the crash box

 

we can fix this geometrical errors using tools in quickedit panel , Geometry -> Quick edit 

go to project points and project points from the separated surface to the free edge of main surface

and use replace point option to make connection between surfaces,

and bumper beam mid surface is not intersected to the upper and lower surfaces as shown in figure below

we can fix this using Geometry -> surface edit -> trim with surfs/planes ->  selfintersecting surfaces ,select both surfaces and trim

After trim, as we can see the image below T-junction formed (yellow colour edge )

Meshing:-

• Before meshing, the parameters are element size is 6mm in the auto mesh panel should be selected. And then by selecting the surfaces by clicking on mesh, the mesh will be generated. go to 2D -> AutoMesh

• After meshing the entire component, it appears as 

CHECK FOR FREE EDGES:

• we have to check for free edges (red lines) in the component.
• Edges Tool will help us find this error:
• Tool >> Edges [Shift + F3] >> Elem's >> Select by box selection >> Find edges
• If free edges are present, connectivity won't be established between the surfaces.
• As a result, when force is applied on any surface it won't be distributed to the neighboring surface due to a connectivity issue.

we can create 1d connections between crashbox and bumber system, go to 1D -> connectors -> spot

select the nodes we wanted to create connections between and select the 2 cpmponents

After creating spot weld connections between crashbox and bumber system

Now we can reflect the half part to get the complete bumber system model, goto Tools -> Reflect , 

Complete Meshed bumper system

Running Simulations on the crush tube

There are 6 cases to be carried out and all of them have mostly common procedures, except the interface/model changes, which are outlined as follows:

1. The crush tube model will be imported into Hyperworks via the 'import solver deck' option in the upper toolbar, or through File > Import > Solver Deck.

2. As per the given requirements, the changes shall be made in this step, either through the model browser in Hyperworks, or by switching to Hypercrash via Applications > Hypercrash in the upper toolbar. Specific instructions will be explained within each case.

Accessing Hypercrash:

3. After the changes are made, we can move to the analysis section in Hyperworks and select 'Radioss' (if Hypercrash was used to make changes, the model needs to be exported first through File > Export in Hypercrash. Then it needs to be imported again in Hyperworks, after which the radioss tool can be accessed).

4. Before carrying out radioss analysis, we can go ahead and create the TH/PART. This will only need to be done once, for the 1st case, since this will be reflected on the remaining cases since each case will be making use of the starter file from the previous case.

Firstly, we can move to the solver tab. This can be enabled through View > Browsers > Hypermesh > Solver. Then, we simply need to right-click in the solver interface and go to create > TH > PART to create TH/PART.

While editing the TH/PART attributes, we need to select all the components as shown.

5. In the radioss section, we can click 'save as' to save the file if it hasn't been saved yet. Care must be taken to include '_0000.rad' in the file name since it's the starter file. After that, we can check the connectors option and input '-nt 4' in the options bar before clicking 'Radioss'. This starts the Radioss simulation, which will run for a few minutes.

Radioss solver running:

6. After the simulation is complete, we can switch to Hyperview through the client selector.

In Hyperview, we will need to import the h3d variant of the file. After importing, we can then select the 'contour' tool to switch to the Von Mises contour so we can analyze the stresses that form within the crush tube in the simulation.

7. Next step is to carry out energy error and mass error checks and this is done by analyzing the RADIOSS engine output file. This can be accessed from the same directory as the starter and engine files and is denoted by the '.out' extension. We need to check the file that contains '_0001.out' by using any text editor.

8. We can then go ahead and generate the plots, using the same client selector, we can switch to Hypergraph. In the Hypergraph client, we are asked to import the required file, which is the T01 variant of the file.

On importing, we can build the plots using the variables for the plots (Graph 1: Global Variables - Internal energy, Kinetic energy, Hourglass energy, Contact energy, Total energy AND Graph 2: Rigidwallforce - total resultant force)

CASE 1: Run the crash tube model as it is

For the first case, we are required to run the simulation on the crush tube as is. The instructions outlined previously are followed. In this case, we will be discussing the Type 7 interface when inacti is set to 0 (Default). In Hyperview, the following simulation can be seen:

From the animation we can clearly see that the deformation taking place at the middle notch first due to applied velocity and also as shown in the below figure

• Once mid notch deforms the next deformation occurs in the second notch i.e Near to rigid wall this is because the thickness is less at bottom 
• When the notch elements are undergoing deformation 
• Here notch elements deforming inside and the rest flat elements are coming outside. This region does not require more force to deform the elements

From the above animation, it is evident that the presence of the notches results in stress concentrations in those regions and as a result, buckling occurs there first. This will happen in all the following cases involving notches. The region below the highest notch is completely deformed first. After the deformation of the lower part, buckling moves onto the upper part, where buckling takes part next until the crush tube is completely sandwiched between the rigid wall and the source of the force application.

Moving on, we can take a look at the output files to discern possible errors:

As we can see, the energy error is -3.8%. It is acceptable since it is below -15%. In addition to that, there is no mass error (0%)

Taking a look at the energy and rigid wall plots:

1. Form the above graph we can see that when the tube is in its initial position the velocity is maximum due to this kinetic energy is maximum

2. But when the tube is hitting the rigid body the kinetic energy is absorbing by the elements so that the kinetic energy goes on decreasing 

3. However the kinetic energy decreasing internal energy increasing

4. So, once the deformation reaches maximum due to rigid wall   

5. Form the graph rigid wall forces are decreasing once it reaches to peak because velocity drops this is due to rigid wall forces velocity decreases so, kinectic energy become zero and internal energy become max 

6. Initially the contact energy is at zero when the velocity applied to the tube starts deforming due to this deformation the penetration taking places 

7. To avoid those penetrations the forces are building as per the given GAPmin due to these forces we can see this contact energy

8. The total energy remains constant (no loss or generation of energy), kinetic energy decreases while internal energy increases. Hourglass energy and Contact energy are negligible small

The kinetic energy goes on decreasing as per the buckling endured by the crush tube. It falls to 0 at the 27ms mark, meaning there is no deflection in the model at all at that point. There is a slight increase afterward due to a slight rebound (reaction force) of the tube.

Internal energy keeps increasing over the course of the simulation as expected, with the tube continuously absorbing energy, which in turn results in deformations. Once the deformation was complete, it plateaued around the 27ms mark.

Contact energy is a type of energy that is formed when one element comes in contact with the other or neighboring elements. The opposing force created by this element forms the contact energy. In this case, it increases after 20 ms, at this point the full model is buckled in such a way that penetration of nodes occurs. With the entire tube pressing on itself, buckled elements are forced into contact. It slightly decreases and stabilizes after the 27ms mark, probably due to the rebound discussed earlier.

The increase in contact energy affects the overall energy of this system, which is why there is a decrease in the total energy at around the same time.

Looking at the RWall forces plot below, it is evident that the rigid wall immediately experiences some of the force. It slightly increases until the 20ms mark where there is a steep increase in the force due to the crush tube having mostly deformed. It was not able to absorb any more energy and this was being transferred to the rigid wall. After the 27ms mark, the tub was deformed completely and that explains the steep decrease in RWall forces.

• Before velocity acting on the tube, there are zero rigid wall forces
• Once the velocity acting on the tube connects with a rigid connection then the forces start applying on the body then the elements were deforming
• Initial stage rigid wall forces start building opposite to the velocity 
• From the graph we can see that till 20-millisec elements deforming easily because of notches this will require less forces, But after 20 millisec the graph is suddenly increasing because its required more forces to deform, So the rigid wall reaction forces increasing till the final displacement 

SELF CONTACT FORCE

The self contact force from the TH inter card helps identify the forces due to the Type 7 contact formulation.

The max self contact force is 1068.08 N, which occurs at the end of the simulation, when the crush tube is completely deformed and maximum number of master and slave segments are in contact with each other.

CASE 2: Change the Inacti=6 and run

In this case, we shall be using Hypercrash as well. So, continuing past the second step outlined in the procedure, we can import the given crush tube model using File > Import.

After importing, we can go ahead and switch to the model browser tab. After that, we can move on to contact interface > self contact. We can then right-click 'self contact' and select the option 'see in panel'.

Doing so lets us edit its attributes. We can go ahead and change the inacti value to 6, as per the requirement.

After that, we can export the model with an appropriate file name and continue the steps outlined (from step 3), after this saved model is imported again in Hyperworks.

After analysis, switching to Hyperview, the animation is as follows:

There isn't any discernible difference between case 1's and case 2's animations, they're both very similar. Even though the Inacti parameter was changed, it didn’t have any effect on the simulation as there was no initial penetration to manage in the first place.

Next, we can take a look at the output file:

As we can see, the values are the same as in case 1. The energy error is the same -3.8% (also the same mass error of 0%). Moving on to the plots:

There isn't much to be said as the graphs are exact copies as that of case 1. The only thing we have learned from the energy plot is that there is no initial penetration since there is no difference between case 1 and case 2 energy plots (since the simulation was tweaked to detect such penetrations through the inacti function). If any initial penetrations existed, internal energy would start from a non-zero value.

RWall forces also exhibit the same characteristics as of case 1:

The forces being plotted in the Rigid Wall Forces plot are the Usual Forces applied to the crash tube by the Rigid Wall. So as the contact energy increases, the Rigid Wall forces will also increase, and we can see that the Rigid Wall Forces also decrease at the point where the Contact energy flat and finally when the specimen loses contact with the Rigid Wall, the Rigid Wall Forces become Zero.

SELF CONTACT FORCE

The self contact force from the TH inter card helps identify the forces due to the Type 7 contact formulation.

The max self contact force is 1068.08 N, which occurs at the end of the simulation, when the crush tube is completely deformed and maximum number of master and slave segments are in contact with each other.

CASE 3: Create the type 11 contact and run

In this case, we are to introduce the type 11 interface to the case 2 solver deck file. After importing it in Hyperworks, following step 2 of the general procedure, we are to make changes in the model browser tab.

Right-clicking the section, we can select Create > Contact. After that, we can define this newly created contact and assign it the following values:

For both Line_id (S & M) attributes, all the components of the model will be selected. The other values are recommended settings for Type 11.

Then, as usual, the file can be processed using the radioss solver. The remaining steps are followed as given.

Here is the simulation of case 3:

And here are the output file values:

Energy error is still within the -15% limit at -4%. Mass error is non-existent. Moving on to the plots:

The internal energy, which is representative of the energy absorbed, increases with time, as the tube makes contact with the rigid wall. We can observe a small peak and a drop off at around 27 seconds, which is the time when the tube is completely crushed and no more energy can be absorbed by the system.

The hourglass energy is zero, since the QEPH 24 element formulation has integrated hourglass control.

Here, the deformation characteristics are similar to what was observed in previous cases. There is no discernible difference between the energy and rigid wall plots of this case and previous cases. Therefore, we can conclude that there was no edge-to-edge penetration in this model.

SELF CONTACT FORCE

• The next graph obtained is, contact forces with respect to type 7 contact. The obtained contact forces graph for type 7 contact. The contact forces will be constant and reach the peak value when all of a sudden the nodes come in contact with the master elements within the tube model. And the graph falls back when the model attains reaction against the rigid wall.

• Then a graph is obtained for the type 11 contact. The obtained graph for type 11 contact. This graph depicts the edge-to-edge contact. There will be fluctuations, as the edges of the elements come into contact gradually. ANd reaches a peak value, after the entire collision. And decreases thereby due to the little expansion of the model after hitting the rigid wall.   

CASE 4: Remove both notches and remove boundary condition on rigid body node then run

For this case, we are required to remove the notches and boundary conditions. To do that, after importing the solver deck file (starter file for case 3), we can make use of one of the mesh edit functions available on Hyperworks - the align node tool (F7).

With this tool, we need to select the '1st end' and '2nd end' - which are basically reference points for node alignment. The algorithm picks the singular straight line running through these nodes as reference and aligns the nodes that require alignment, which in this case will be notch removal. After that, we can save the file as 'case4_0000.rad'.

Without notches:

We can now switch to Hypercrash and import 'case4_0000.rad'. Moving on to the model browser tab, right-click 'boundary conditions' and select 'delete'. Doing so removes the boundary condition as per the requirement for this case.

Now we can export this file in the same name and continue to the radioss simulation procedure on Hyperworks. The remaining steps are the same continuing from step 3 in the main procedure as outlined before.

Taking a look at the simulation:

Since there are no notches present, the buckling started at the contact area of the rigid wall and the tube. Whereas, in previous cases, the initial buckling or deformation would happen at the notches.

Here are the output values, which are acceptable numbers:

Taking a look at the energy plot:

The kinetic energy decreases from maximum with an increase in internal energy. The characteristics are similar as in previous cases, with the internal energy's slope increasing after the 20ms mark, probably due to most of the elements coming in contact with other elements due to the buckling, and due to the fact that the crush tube is almost completely crushed. Although, compared to previous cases, the contact energy is slightly lesser. This is probably due to the lack of notches in the model.

Looking at the rigid wall plot:

The total resultant force peaks at almost 1200 kN. Just as in the case with internal energy in the previous plot, there is a steep increase at 20ms mark here as well. At this point, a large amount of force is being transferred to the rigid wall considering the crush tube is almost completely deformed to absorb any more energy.

SELF CONTACT FORCE

• Then the obtained contact force graph. There is a reduction in the contact forces. Because the notches & boundary conditions are removed. So, the energy will be observed by the elements will be faster when compared to the previous cases with the notches.      

since the removal of notches results in a phased deformation of the crush tube from the lower to the upper point, where none of the component edges come in contact with each other.

CASE 5: Create a new notch in the middle, select the whole section and run

For this case, we shall be using the starter file of case 4. So there are no notches or boundary conditions. But the case requires us to create a notch through the middle section of the crush tube. For this, we shall be making use of the translate tool in Hyperworks. After importing the starter file in Hyperworks, we can go to tools > translate.

We can either select elements or nodes to translate. After assigning the direction of translation (using an axis or a vector), we then need to assign a magnitude. Depending on the global axis, we then have to either click 'translate+' or 'translate-'. This creates the depression.

After creating Middle notch

Now, we need to run the analysis through the radioss engine and generate the simulation. This is what it looks like:

Just as in the first 3 cases, the notch is the region where the tube undergoes deformation first. Then the region closer to the rigid wall is deformed next, which would be the lower half of the tube. Around the 10-12ms mark, when the lower half is mostly deformed, the upper half undergoes proper, visible deformation very rapidly until the entire tube is crushed completely. In addition to that, based on the nature of the notch, there may be anomalies. Since it was created manually.

Taking a look at the readings in the output file, we can see that the values are acceptable:

Moving on to the energy and rigid wall plots:

Contact energy: 

• Contact Energy is the energy acting opposite to the system to prevent the penetration acting on the system. When the force is applied to the body, the crush tube starts to deform but the penetration happening is low in the initial stage. The contact energy will below.
• Once the deformation is maximum, the elements will be forced to penetrate but the opposite force which is contact energy will also increase which helps in the prevention of penetration. Once the deformation ends, the contact energy will become zero and will stay constant thereafter.
• In this case, the contact energy incurred is more when compared to case 1 and case 2. This is due to only one notch at the center which increases the rate of deformation, which intern increases the rate of penetration.

• The obtained graph for internal energy. There will be a change in the internal energy. Because, As there is a notch only at the middle. The elements will be impacted without any deviation, as of in the previous cases in which the notch is present at the beginning. There will be a drop in the internal energy at the middle as shown in fig. Due to the model moves back after hitting the rigid wall. And there will be a rise again. Because the model again hits with the elements after notch. 

CASE 6: Create a new notch with nodes only from opposing 2 faces and run

• In this case, the new notch is created in the middle again. But only at the two opposite faces.

 

 We can then carry out the analysis using radioss as outlined in the main procedure. The simulation is as follows:

• Then the obtained 0001.out file is opened. To view the obtained results in the form of values. The obtained results for the last cycle

They have acceptable energy and mass error values.

• Then the simulation is run. After running the simulation, the obtained h3d file is imported into hyperview to view the simulation. The obtained simulation appears as shown in the below animation.  

 

• Deformation takes at the middle notch of the model due to applied velocity due to stress concentration near the notch
• After complete deformation of the notch. The next deformation occurs at the bottom which is near to the Rigid wall is because the thickness is less at the bottom which is 2.1mm 

• Rigid wall force

The max force is 1361.06 N, which coincides with the point at which the the internal energy peak is achieved. This represents the point in the simulation, at which the crush tube is completely deformed, and the rigid wall has to react the forces due to the impact motion of the rigid body.

The value of the force drops rapidly thereafter, since the entire crush tube bounces away after the impact and loses contact with the rigid wall.

•  contact force

 

It can be observed, that the self contact energy has a peak at around 20 seconds, when most of the tube is completely deformed. We can see a secondary peak at around 24 seconds, which is due to the tube being fully compressed at this point, which leads to maximum contact of between adjacent elements.

• Energy plot

The internal energy, which is representative of the energy absorbed, increases with time, as the tube makes contact with the rigid wall. We can observe a small peak and a drop off at around 27 milli seconds, which is the time when the tube is completely crushed and no more energy can be absorbed by the system.

The hourglass energy is zero, since the QEPH 24 element formulation has integrated hourglass control.

OBSERVATIONS AND RESULT

Notches are the regions where deformations happen first , due to them being regions of stress concentration. But it does help to control and predict the location of deformations. This in turn plays a role in how the absorbed energy spreads throughout the tube. A notched tube results in higher contact forces but a smoother transmission of load.

In this assignment, different types of contacts were used to analyze the simulations of the crash tube. Under a constant load, the behaviour was observed by varying the geometry through the removal and creation of notches.