To Study the Effect of Grooves in the Crash box Performance - Using Hypermesh Radioss

Objective: To Simulate the Crashbox with Two Contact Interface and To Study the Effect of Grooves in the Crash box Performance

Case 1: With Type 7 Contact Interface and Inacti = 1000 NO action

Case 2: Type 7 and Inacti=6.

Case 3: Type 7 and Type 11 contact interface.

Case 4: CrashBox Without any Grooves.

Case 5: Create a new Groove in the middle of the Crash box

Case 6: Create a new Groove with on opposing 2 faces alone

The Crash box is a device mounted between the front bumper and mainframe of the car in order to absorb energy during
a collision. It buckles when the axial compressive force exceeds the limit. Energy is absorbed during buckling and damage
to the mainframe is avoided.

Axial Load: When the initial peak force is the force that is required for the structure to start its deformation the subsequent peak forces are the result of the following deformation when the folding pattern occurs. When the folding mechanism has started the bending stiffness is reduced and that reduction can lead to bending failure. The maximum load resistance of the column during folding is dependent on the corner crush strength. To keep a stable collapse the global buckling stress must be greater than the maximum load resistance of the corner.

Model :

Properties :

• The upper column (1 PSHELL1, 3 PSHELL 2 ) of the Crashbox has 3mm Thickness

• The Lower Column(5pshell_2mm, 6 pshell_2mm) has a thickness of 2mm.

Material:

MAT 2 PLAS-JOHNSON CARD is used with the Material Properties of Steel.

Boundary Condition :

Initial Velocity: 46.8Km/hrs = 13mm/ms

Contact :

Seam weld (Type 2 Contact Interface)

Case 1: With Type 7 Contact Interface and Inacti = 1000 NO action

Interface type 7 is a general-purpose interface and can simulate all types of impact between a set of nodes and a master surface.

The Penalty Method Used In Interfaces 7

The penalty method treats the behavior between slave nodes and master segments as springs that generate resistive forces as a function of penetration. Spring elements are created between the slave node and its projection. The spring stiffness is automatically computed according to the material and property of both master segment and slave node

Interface Type 7: Nodes to Surface Contact

• Automatic node to surface contact interface using the penalty method with variable stiffness
• A gap is defined around the master segment and the slave nodes
• The contact gap can be constant or variable
• Resistive forces are applied to keep slave nodes out of the contact gap
• The interface time step is computed to ensure stability

Type 7 Properties :

Igap(Determines how the size of the gap is calculated): 1000

Gapmin(Minimum gap for activation of the interface ): 1

Inacti(Action to take if initial penetrations exist): 1000(No action)

Istf(Affects how the stiffness of the interface is calculated): 4 Kmin=(Km, Ks)

Iform(Friction formulation): 2 Stiffness

Stmin(Minimum stiffness to use in the interface): 1

Idel(What to do with slave nodes and master segments if an element fails (deleted) that they are attached to): 1000(no deletion )

Output files:

Number of Cycle	ELAPSED TIME	Energy Error	Mass Error
83691	225.91 s	-3.8%	0

The Energy error is -3.8% which is less than 15% so, the Result is Accepted.

Results :

Von Mises Stress:

Max Von Mises Stress = 6.964e-01

Graph :

• Energy Graph :

• Part Internal Energy:

1 PSHELL1, 3 PSHELL 2 - Upper part of the Crash box 5pshell_2mm, 6 pshell_2mm - The lower part of the crash box

• 1 PSHELL 1 IS UPPER PART OF TE CRASH BOX WITH THICKNESS 3MM the internal energy is zero till 10ms is because the buckling or deformation takes place at the bottom part of the crash box only after the 10ms the buckling starts at the upper part of the component
• As we Know the 1PSHELL 1 AS 3mm thickness and pshell_2mm as 2mm thickness when the thickness of the crash box is more they will have more energy absorption capacity than the less thickness part that's why the internal energy for the 1PSHELL 1 has max internal energy compare to the pshell_2mm

• Contact Energy:

Contact energy forces basically the energy required or work done to remove the penetration of the nodes on to the master surface. We can see the peak in the plot at a time after 25 the ms. After the 25th ms the full model is fully deformed in such a way that some layers of elemental try to overlay or penetrate other elements. in this incase the nodes penetrate to the surface of the master the penalty stiffness will create a non-linear stiffness and thereby pulls out the nodes to the surface of the element Thus requires a large amount of energy to stop penetration at 25 ms.

• Rigid wall Force:

The initial Peak Force to Start the buckling is 85J

Rigid wall force is the reaction force of the rigid wall when the Crashbox is Collide with wall Which means when the force is applied on the crash tube, some amount of energy is taken by the crash box material. So the force which is applied at the top of crash tube basically by a rigid body is not transferred entirely to the wall or it is not completely applied to the wall instead some amount of the force is used in deformation of the Crash tube, but after 20ms the rigid wall force increases and reach the peak at 26ms, this because after 20ms the force applied on the crash box, the crash box is not able to absorb energy since it has reached the maximum deformation most the force is transfer to the rigid wall and after 26ms the velocity becomes zero, We can see a slight rebound of the deformed crash tube.

Case 2: Type 7 and Inacti=6.

In this case, we will use the same type 7 contact Interface but the Inacti (Action to take if initial penetrations exist) is set as 6 (The gap reduced and Scaled)

If any initial penetrations exist in the model.in such cases the solver will try to reduce the gap with scale. So it allows little penetration but when the penetration reaches the scaled value the contact gets activated and it pushes out the nodes to the surface of the element.

Output Files:

Number of Cycle	ELAPSED Time	Energy Error	Mass Error
83691	260.77s	-3.8%	0

The Energy error is -3.8% which is less than 15% so, the Result is Accepted.

Results

Von Mises Stress

Max Von Mises Stress:6.964e-01

Graphs:

Energy Graphs:

Contact Energy

The graph looks same as the case 1 contact energy, this interprets that there is no initial penetration of node in this model if there were any initial penetration they would have been initial energy or work done to push those nodes away from the master but the initial contact energy starts from zero which mean there is no initial penetration. Max Contact Energy: 2010 J

Part Internal Energy:

Rigid wall Force :

Case 3: Type 7 and Type 11 contact interface.

Since we know the type 7 doesn't solve edge to edge interface we are going to Add-Type 11 Edge to Edge interface, Just like Type 7 we need to define a master and salve as the Edge, just as type 7 interface The Penalty Method Used In Interfaces 11 but we need to give the Gapmin value a little less than what we provide in TYPE 7. Provide Igap as 0.8 remaining settings as the type 7

Output File:

Number of Cycle	ELAPSED Time	Energy Error	Mass Error
81906	274.37 s	-4.1%	0

Since additional Contact is added the Number of Cycle and Elapsed Time is increased than previous case and Energy error is also increased but they didn't exceed more than 15%, so the results are accepted

Results:

Von Mises Stress:

Max Von Mises Stress: 6.735e-01

Graphs :

Energy Graphs :

Energy curves don't show any difference with the above cases. Here also the energies are following the same path as the above cases. The energy conservation is there. The new energy is added to Stop Edge to Edge penetration there is a slight change in total energy.

Contact Energy :

Since the Edge to Edge penetration is very less for this model there is no huge change in contact energy but there is a slight change in the max value during the 25ms the max value as increased slightly from the case2 because of the type 11 interfaces. Max Contact Energy: 2125 J.

Part Internal Energy :

Rigid Wall Force:

Contact Interface doesn't affect the reaction force of the rigid wall So the Graph is the same as the case2.

Case 4: CrashBox Without any Grooves.

All the Grooves have Removed All properties and Boundary Condition Remaining the same as Case 3.

Output File:

Number of Cycle	ELAPSED Time	Energy Error	Mass Error
78220	275.74 s	-2.6%	0

The Energy error is -2.6% which is less than 15% so, the Result is Accepted.

Results:

Von Mises Stress:

Max Von Mises Stress 6.728e-01

Graph:

Energy Graph:

Contact Energy:

The contact Energy is lesser than the case 3 (Crashbox with Grooves ) this because the node to surface or edge to edge penetrations is very less than the case 3, Since the less penetration the work is done will be less compared to the case 3 which results in less contact Energy

Part Internal Energy:

If we Notice the upper Column has no change in Internal Energy But The Lower Column, the internal energy has increases compare to case 3 because In case 3 the column lower of the crash box had Grooves in them but in this case, Grooves are removed.

Max Internal Energy for Lower Column: 9500J

Rigid Wall Force:

If you notice the initial peak force to start the buckling is around 200J but in the case3 it around 90J its because When the load reaches a MAX point, buckling first begins to occur at the weakest plate of the column. since there are no Grooves in the crash box the buckling starts at the bottom of the crash box so, most of the force is transmitted to the rigid wall at the initial time.

Case 5: Create a new Groove in the middle of the Crash box

Triggers are geometrical differences in the structure that are designed to help the structure to collapse in the desired way. They are also known as crush initiators or beads or Grooves. If a member has triggers placed in the right places it can help the member to collapse in an axial folding mode

In this case, Grooves are placed in the middle of the cashbox

When the load reaches a certain point, buckling first begins to occur at the weakest plate of the column. This is called local buckling. Under the influences of this local buckling, the other plates buckle one after another, giving rise to a buckling wave. Since the Grooves is placed at the middle of the crash box that makes the weak point in the crash box and buckling first starts at the Grooves region.

Output Files:

Number of Cycle	ELAPSED Time	Energy Error	Mass Error
81760	271.33 s	-3%	0

The Energy error is -3% which is less than 15% so, the Result is Accepted.

Results:

Von Mises Stress:

Max Von Mises Stress: 6.314e-01

Graph:

Energy Graph:

Contact Energy :

Contact Energy is more for this case because of Grooves. During the folding, there is more nodes to surface penetration that required more energy to push them away. Max Contact Energy: 1750J

Part Internal Energy:

The Lower Column, the internal energy has Decreases compare to case 4 because In case 4 the column lower of the crash box had no Grooves in them but in this case, Grooves are Presents.

Max Internal Energy for Lower Column: 8000J

Rigid Wall Force :

If you notice the initial peak force to start the buckling is around 90J

The initial peak energy lower than the case 4 Since the Grooves is placed at the middle of the crash box that makes the weak point in the crash box and buckling first starts at the Grooves region and buckling starts at the middle of the crash box.

Case 6: Create a new Grooves with on opposing 2 faces alone

One more Groove is created on the opposite face.

Output Files:

Number of Cycle	ELAPSED Time	Energy Error	Mass Error
75319	244.41 s	-3.2%	0

The Energy error is -3.2% which is less than 15% so, the Result is Accepted.

Results:

Von Mises Stress:

Max Von Mises Stress = 6.657e-01

Graphs:

Energy Graphs:

Energy graph almost looks the same but there is a slight change in the internal energy, the internal energy seems a bit lower for this case compare to case5, which means the energy abortion capacity is lesser than the case5

Part Internal Energy:

The Lower Column, the internal energy has increased compared to case 5 because In case 5 the column lower of the crash box had full Grooves on them but in this case, Grooves are Present only in the opposite face so this Model more energy absorption capacity than Case 5.

Max Internal Energy for Lower Column: 8250J

Contact Energy:

The folding pattern is different in the region of Grooves since the Grooves are only in the opposite side of the crash box during buckling the side face folds so much the contact the lower part of the crash box this might increases the contact energy little bit compared to the case 5

Left side Case 6 and Right Side Case 5

Rigid Wall Force:

If you notice the initial peak force to start the buckling is around 160J

The initial peak force is a bit higher for this case compares to the case5 because the Grooves are only in the opposite face crash box that doesn't make the region weaker as case 5 so the initial force required for buckling to start is a bit more than case 5.

Conclusion :

The presence of Grooves increases the rigid wall force. An early absorption of energy is also possible with Grooves. This is why the Grooves are used in a crash tube so as to increase the crashworthiness parameters like crushing force efficiency and specific energy absorption. Interface forces also act until the release of the internal energy happens. We usually call these Grooves as triggers and these triggers. Grooves can decrease the peak crushing force thereby it increases the crushing force efficiency.

These initiators play a key role in controlling the buckling modes(that progressive folding we have seen) and smooth energy transfer during axial crash analysis. Triggers, which are expected to lower the peak crushing load and to actuate progressive folding and also increases crushing efficiency. The triggers are the prime initiators for the first elastic buckling mode of deformation

As we Notice in The Energy Graph The crash Box with Grooves in the middle has higher Internal Energy Compared to The other two Case We proves the Case5 Grooves is Optimize the Crash Box Performance

Model File:

https://drive.google.com/open?id=1mTHZegoirRFrC3VZPnx3X7y_z69mU4IL