Beam vs Solid Spotwelds

MODELING SPOTWELDS

INTRODUCTION:

Spotweld is an electrical resistance welding technique used to weld, on most occasions, sheet metals together by adjoining the contact surfaces using the heat obtained from electric current in the electrode.

Spotwelds can be modeled in LS-DYNA using specific keywords in-built in LS-PrePost that can replicate the functions of an actual spotweld through user-defined parameters.

OBJECTIVE:

The objective of this project is to model 7 spot welds to join two sheet metal components. These spot welds are modeled using beam elements on the first case and solid elements on the second. After successfully running the simulation the output parameters of the spot welds like axial and shear force experienced by the spot welds in both the cases are compared.

SETUP KEYWORD:

BEAM SPOTWELD

1. SPOTWELD ELEMENT GENERATION:

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. This beam element then can be modeled using a spotweld material like MAT_100 and a beam section of type 9.

To create a beam element: Mesh -> EleGen -> Beam -> Two_Node_sets

This series of commands lets you select the required nodes in both the components that may act as the end nodes of the spotweld.

NOTE: Uncheck 'Create part property' in the EleGen window to restrict the pre-processor from creating default properties for the spot welds.

2. *PART - *SECTION & *MAT

Shell Components: The shell components are modeled using a *SECTION_SHELL keyword with an Element formulation, ELFORM = 16, which is a- 'Fully integrated shell element' and a shell thickness of 1.5 mm

The Elastic material is used to model the shell section with the below parameters,

Spotweld: Beam Spot welds can be modeled using a beam section using the *SECTION_BEAM keyword and a *MAT_100_SPOTWELD material.

NOTE: While the MAT_100 material is consistent for modeling any type of spot welds, it poses few constraints like - "The material model applies to beam element type 9 and to
solid element type 1".

Hence we can model the beam spot weld element using a *SECTION_BEAM of type 9 with a thickness/outer diameter of 3 mm and a 4x4 Gauss quadrature for increases integration points and more accuracy.

The MAT_100 card is specified as shown below,

In the above card, TFAIL of 3 ms is specified, which is used to specify the time of failure of the beam spotweld element.

Similarly, other factors like Plastic strain, Force, and moment that could act as a threshold for the failure of the spotweld are specified in the below row.

3. *BOUNDARY

This keyword is used to define the boundary conditions for the analysis.

3.1. PRESCRIBED_MOTION_SET

A prescribed motion keyword is specified to the set of nodes on the top shell component (red). This keyword is used to apply velocity along the -ve x-axis to the set with a time-dependent velocity rate which can be specified using a LoadCurveID [Define -> Curve].

3.2. SPC_SET

The other end of the bottom shell component is grouped onto a new SET_NODES where the specified nodes are constrained in all direction i.e DOFs=0.

4. *CONTACT

This keyword is used to specify the Contact interface either between shell component and spot weld or self-contact of the shell elements with itself.

4.1. AUTOMATIC_SINGLE_SURFACE

This card can be used to specify contacts that could occur between the two shell components or the elements within.

This can be achieved by creating a SET_PARTS which contains both the top and bottom shell components and assigning it to SSID in the card while the SSTYP is set to 2.

NOTE: The master components MSID need not be assigned since the contact occurs within the elements within itself.

4.2. TIED_SHELL_EDGE_TO_SURFACE

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 vary.

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 (with SSTYP - 4, MSTYP - 2)

5. CONTROL_TERMINATION

This keyword is used to specify the end time of the simulation which in this case is set to 5 ms

6. DATABASE

This keyword specifies the output parameters that need to be extracted during the simulation.

6.1. BINARY_D3PLOT

This is the most important keyword where the output of the animation file is written (.d3plot).

For our simulation, we specify a time interval of 0.5, which represents that for each 0.5 ms the output data of the analysis is written onto the d3plot file.

6.2. ASCII_Option

Here the required output parameters that need to be computed are specified and written in a separate file. In our case, the following files are written for every time interval of 0.5 ms,

GLSTAT - Global Statistics involving KE, IE, hourglass energy, etc.

NODOUT - Nodal properties like displacement, acceleration, etc.

SLEOUT - Sliding interface energies

SECFORCE - Cross-sectional forces

SWFORCE - Spotweld Nodal Constraint Reaction Forces

6.3. CROSS_SECTION_SET

Since we specified, the Cross-sectional energy like X, Y, Z forces that need to be computed. This keyword lets us specify the SET to which the required forces need to be computed.

For this keyword, we specify the nodes and beam elements onto a set and assign them in this card.

6.4. HISTORY_NODE_SET

Here, the *SET_NODE containing the required nodes for which the Nodal output parameters that need to be computed are defined.

SOLID SPOTWELD

The above parameters are applied the same to model a Solid spotweld with few minor changes in the ELEMENT and SECTION keywords.

1. SPOTWELD ELEMENT GENERATION:

To create a solid element: EleTol -> EleEdit -> Create -> Hexa

The above commands let us select eight nodes from both the shell components which can be used to define the eight nodes of the solid Hexa element.

2. *PART - *SECTION & *MAT

Same as for beam elements the solid spot welds can also be modeled using a MAT_100_SPOTWELD material.

But the solid welds are modeled using a SECTION_SOLID keyword with an ELFORM of 1 (constant stress solid element) which is the only element formulation suitable with a MAT_100 material.

3. *CONTACT_SPOTWELD

Unlike the previous case, in this type, we will use a CONTACT_SPOTWELD keyword to define the contact between the solid Hexa elements and the shell components.

Here, the slave elements are the eight nodes of each solid element and the master elements are the shell parts (with SSTYP - 4, MSTYP - 2)

POST-PROCESSING:

The above keyword file is submitted to the LS-DYNA solver and the following results have been obtained,

A comparison is made between the beam and solid elements as shown below,

Stress X (Beam vs Solid)

From the above, fig we can visualize the Stress in the longitudinal direction experienced by the components at a time of 0.4 ms. It is obvious that the elements which constitute the spot welds will experience the highest stress which is about 33.7 Pa in case of beam element and about 123 Pa in cases of solid elements.

This result is feasible since the Hexa solid elements will experience more stress being a solid element connected to 4 nodes of each of the shell components.

Von-Mises Stress (Beam vs Solid)

Similar to the X-Stress, the Von-mises stress which is measured within the elastic limit is shown above. It attains a maximum of 38.4 Pa in the first case and 109.8 Pa in the second.

Axial Force (Beam vs Solid)

The axial forces experienced by both the beam and solid spotweld elements are shown above. Due to the presence of seven 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 (Beam vs Solid)

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

We can notice that unlike the case of beam elements, the solid elements experience an increase in shear force till 2 ms after which there is no change as elements are not further displaced due to the action of loading.

Displacement (Beam vs Solid)

In both the above cases, there is a gradual decrease in X-displacement due to the loading action which acts along the -ve X-axis. Whereas the displacement in Y and Z directions has a small deflection since the spot welds are intact and fail only after 3 ms which as shown above causes a higher displacement in both the axis.

Acceleration (Beam vs Solid)

The acceleration experienced by a mid-node is shown above. The effect of acceleration due to change in velocity is experienced only after 3 ms which is because at 3 ms the welds fail causing a huge displacement of the shell components away from each other due to the effect of loading.

CONCLUSION:

Hence from the above-obtained results in both the cases, we can conclude that stress experienced in the case of solid welds is greater compared to the beam weld elements. Further, we can also conclude that solid spotweld elements offer greater resistance to the action of loading compared to the beam element as it gives more stiffness to the weld elements.