Week - 5 - Modelling Spotwelds

Objective: To Model a Spot weld with Beam and Solid Element in Sheet Metal and compare the results with different parameters.

Model: The upper and lower metal sheets have meshed with shell elements. These two Sheet metals have to be connected with spot welds using beam and solid elements separately.

Basically the spot weld can be modelled by the following ways,

1. Using beam element and tied contact
2. Using solid element and spotweld contact.
3. Using constraint spotweld
4. Using constrained generalized

In This Simulation, We discuss Spotweld Using beam element and tied contact and Solid element and Spotweld contact.

Spotweld Using beam element and tied contact:

Procedure:

Beam Element Creation:

Spotwelds can be created between the two shell components by creating a beam element which is connected at both the ends to the nodes of both the shell elements

Select Element Tool>Element Editing>Create>Element Type-Beam >Select node1,2> Accept >Done. Element ID and Part ID is defined

Material Model

We are supposed to Create Two material cards one for The sheet metal and other for spot weld

• We are using MAT_ELASTIC card for Sheet metal using Steel Properties

1. MID Material identification. A unique number or label not exceeding 8 Characters must be specified
2. RO Mass density.
3. E Young’s modulus.
4. PR Poisson’s ratio.

• For Spotweld We are using MAT_SPOTWELD card and using Steel material properties failure criteria such as effective plastic strain, resultant stress, axial stress are assigned after few calibrations so that the spot weld fails at half of the termination time.

1. MID-Material identification. A unique number or label not exceeding 8 characters must be specified.
2. RO-Mass density
3. E -Young’s modulus.
4. .PR-Poisson’s ratio
5. SIGY- Initial yield stress
6. TFAIL Failure time if nonzero. If zero this option is ignored.
7. EFAIL-Effective plastic strain in weld material at failure
8. NRR-Axial force resultant 𝑁𝑟𝑟 at failure.
9. NRS -Shear Force resultant 𝑁𝑟𝑠𝐹 at failure
10. NRT -Force resultant 𝑁𝑟𝑡𝐹 at failure
11. MRR -Torsional moment resultant 𝑀𝑟𝑟𝐹 at failure.
12. MSS -Moment resultant 𝑀𝑠𝑠𝐹 at failure
13. MTT -Moment resultant 𝑀𝑡𝑡𝐹 at failure.

Section Creation :

Section Creation Define Section Properties for different Element

• Since the Sheet Metals are meshed using 2d element we will define them with SECTION_SHELL card

1. SECID Section ID. SECID is referenced on the *PART card. A unique Number or label must be specified.
2. ELFORM Element formulation options.= 16 Fully integrated shell element (very fast),
3. SHRF Shear correction factor which scales the transverse shear stress.= 1
4. NIP Number of through-thickness integration points = 2
5. T1 Shell thickness = 1.5mm

• Since Spotweld are made of beam element we will use SECTION_BEAM card

1. SECID Section ID. SECID is referenced on the *PART card. A unique Number or label must be specified.
2. ELFORM Element formulation options.= 9 spotweld beam
3. SHRF Shear correction factor which scales the transverse shear stress.= 1
4. QR/IRID Quadrature rule or rule number for user defined rule for integrated beams. = 4 (4x4 Gauss quadrature)
5. CST Cross section type = 1 Tubular
6. TSI Beam Thickness = 2.0mm

Assign the material and section to the shell and beam elements respectively

Boundary Creation:

Two Boundary Condition is defined For this model

1. Boundary SPC
2. Boundary Prescribed motion

• Boundary SPC (Single point Constrained )

A Set of nodes in one side of the sheet metal is selected and there all degree of freedom is Constrained (fixed).

• Boundary Prescribed motion

A set of node on the other side of the sheet metal is selected and Displacement along x axis is defined for that node in form of Load curve.

Load curve for the displacement

Contact Creation:

• For Sheet Metal

Once the spotweld failed in the sheet metal there is change the two sheet metal get penetrate itself or penetrate each other so it recommend to use AUTOMATIC_SINGLE_SURFACE Contact . The only Slave can be defined in this type of contact

• For Spotweld

While modeling spot welds with MAT_100, the following contacts could be used,

• *CONTACT_TIED_SHELL_EDGE_TO_SURFACE
• *CONTACT_SPOTWELD
• *CONTACT_SPOTWELD_WITH_TORSION

The difference between them arises when factors like the type of spot weld element and forces involved varying. In the case of a beam element connected to the nodes of the shell surfaces, we will use the *CONTACT_TIED_SHELL_EDGE_TO_SURFACE card. Here, the slave elements are the nodes of the beam element and the master elements are the shell parts

Control Cards

• Control­_Termination is defined at what time the simulation ends or the end time of the simulation. It is set to 10 ms

Control_Energy It Provides controls for energy dissipation options.

• HGEN- Hourglass energy calculation option. Is set to 2 In Which hourglass energy is computed and included in the energy balance.
• RWEN- Rigid wall energy dissipation option: Is set to 2 in Which energy dissipation is computed and included in the energy balance
• SLNTEN- Sliding interface energy dissipation option Since the Contact is Defined it is set to 2 So That energy dissipation is computed and included in the energy balance

Database Creation

• BINARY_D3PLOT - It defines the frequency at which the animation file is to be created and is set to 1ms
• Cross-section Creation: One of the required outputs is Cross-sectional force at the Spotweld on the sheet metal for this a DATABASE_CROSS_SECTION_PLANE is created

• DATABASE_HISTORY_NODE Set of nodes of Spotweld were defined in the middle of the crash box to measure the acceleration

• ASCII_option keyword

The output request in ASCII format, The following keyword are Activated

1. ELOUT- Element Data
2. GLSTAT- Global Data
3. MATSUM- Material Energies and
4. SLEOUT– Sliding Interface Energies.
5. NODOUT- Nodal point data
6. SECFORC- Cross-section forces
7. SWFORC – Spot weld Force

Model Check

Spotweld Using Solid element and Spot Weld contact:

Everything is the same as Modelling the Beam Elements but, Type Element, Section Properties and contact is different in the type of spot weld

Solid Element Creation

Select Element Tool>Element Editing>Create>Element Type-HEX >Select node 8> Accept >Done. Element ID and Part ID defined

Section Creation

Since the Spotweld are made up of Solid element we will use SECTION_SOILD card

1. SECID Section ID. SECID is referenced on the *PART card. A unique Number or label must be specified.
2. ELFORM Element formulation options.= 1 Constant stress solid element

Contact Creation:

For this type, we will use a CONTACT_SPOTWELD keyword to define the contact between the solid Hexa elements and the Sheet metal.

Here, the slave elements are the eight nodes of each solid element and the master elements are the shell parts

Model Check

Results for Spot Weld Using beam element and tied contact :

The beam as failed at 5ms which is half the termination time

Von Mises Stress

Max Stress: 1302MPa

Effective Strain :

Max Effective Strain : 5.644e-5

Energy Plot:

Max Internal Energy: 180 J

Max Kinetic Energy: 5 J

• The Kinetic energy at 10ms is 5e+3 N-mm or 5 N-m(J), All of the kinetic energy was being applied to tear (axial force) the beam elements and thereby the beam elements absorbing more amount of energy.
• At the point of Failure, the Total energy recorded was 175 J and it reached a maximum of 225 J
• The internal energy was 180 J at the end of the Simulation.

Sliding & Hourglass Energy :

Sliding & hourglass energy are very small Compare to Internal energy since Full integration formulation is used they were no hourglass effect in the model thus result in very less hourglass energy

Cross-Section Force:

Peak Force : 2.5e+4 N

Acceleration at Midnode:

Peak Acceleration : 800mm/ms^2

The effect of acceleration due to change in velocity is experienced only after 5 ms which is because at 5ms the spot welds fail to cause a huge displacement of the sheet metal away from each other due to the effect of loading.

Axial Force for Spotweld:

Peak Axial Force: 30e+2 N

The axial forces experienced by the Beam spotweld elements are shown above. Due to the presence of eight-spot welds present, few elements will experience a negative axial force because the force is not evenly distributed among each spot welds. Due to the position of spotweld and orientation of force acting upon it, few spot welds will experience a higher axial force compared to the rest.

At 5ms the Axial Force of all the spotweld Drop till it reached zero, This shows all the spotweld is failed at 5ms, because it reaches maximum axial stress

Shear Force for Spotweld:

Peak Shear Force: 20e+2

The Shear stress experienced in each spotweld is shown above. Since shear force depends on the force along with the opposite directions, every weld element will experience shear stress.

Once the Spotweld reaches maximum shear stress values the Spotweld fails and shear force drops to zero, As you can after 3ms it slowly drops and reaches zero at 6ms

Results for Spot Weld Using Solid element and Spot weld contact :

The failure of the solid element spotweld is exactly at the half the termination time - 5ms

Von Mises Stress

Max Stress: 2339 MPa

Effective Strain

Max Effective Strain: 1.014e-04

Energy Plot:

Max Internal Energy: 659 J

Max Kinetic Energy: 5.3 J

• The Kinetic energy at 10ms is 5.3 J , All of the kinetic energy was being applied to tear (axial force) the beam elements and thereby the beam elements absorbing more amount of energy.
• At the point of Failure, the Total energy recorded was 670 J and it reached a maximum of 712 J
• The internal energy was 659 J at the end of the Simulation.

Sliding & Hourglass Energy :

Sliding & hourglass energy are very small Compare to Internal energy since Full integration formulation is used they were no hourglass effect in the model thus result in very less hourglass energy

Cross Section Force:

Peak Force: 5.8e+4 N

Acceleration of MidNode

Peak Acceleration : 2000mm/ms^2

The effect of acceleration due to change in velocity is experienced only after 5 ms which is because at 5ms the spot welds fail to cause a huge displacement of the sheet metal away from each other due to the effect of loading.

Axial Force for Spotweld

Peak Axial Force: 13.7e+2 N

The axial forces experienced by the Solid spotweld elements are shown above. Due to the presence of eight-spot welds present, few elements will experience a negative axial force because the force is not evenly distributed among each spot welds. Due to the position of spotweld and orientation of force acting upon it, few spot welds will experience a higher axial force compared to the rest.

Shear Force for Spot Weld:

Peak Shear Force: 70e+2 N

The Shear stress experienced in each spotweld is shown above. Since shear force depends on the force along with the opposite directions, every weld element will experience shear stress.

Comparison :

• Stress & Strain

The Solid element spot weld has higher stress and strain than beam element spotweld we know the Hexa solid elements connects to 4 nodes of each of the shell component which is why it experiences more stress being a solid element

It clearly shows the solid element spotweld have higher internal energy compare to the beam element spotweld which implies the solid element spotweld absorbs more energy than beam element

• Cross Section Force and Acceleration

The internal energy of the solid element is more so the solid element absorbs more energy than beam element but as the spotweld failure there is will huge displacement in sheet metal which as solid element spotweld this huge displacement results in more acceleration at the mid node during spotweld failure

As we can see The Cross-section force for a Solid element is more. This is because the Hexa solid elements connect to 4 nodes of each of the shell component. Which makes them stiffer.

• Axial & Shear Force
  
  

Here beam element is 1d element with some approx. thickness which is less than the solid element the tensile resistance is maximum in beam element compared to the solid element that’s why the axial force is more in beam element spot-weld

Since Shear Stress Depends on the thickness of the element and we know the solid element is around 3.5mm thickness and beam element is lesser so the solid element will have more shear strength and the beam element

Conclusion :

• We have Model Two types of Spotweld in the sheet metal separately and compared the results
• The Spotweld failure criteria values were calibrated according to fail at half of the termination time.
• As we saw The Solid element spotweld experience more stress than beam Element and The Internal energy of the Solid element is very higher than the beam element.
• And we can also conclude from the result that the solid element is stiffer than the beam element and offer great resistive force against the losing
• So the Solid element is preferable for a realistic condition