Week - 5 - Modelling Spotwelds

Spotwelds can be created between the two shell components by creating a beam element that is connected at both 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 spot weld.

MAT_SPOTWELD (100)

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 increased integration points and more accuracy.

SECTION_SHELL

In the above card, TFAIL of 2.5 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. *INITIAL_VELOCITY

initial velocity card to used to give some initial velocity to the model so that the model's kinetic energy  will be absorbed by impact with rigid wall

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.

a. 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.

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)

.CONTROL_TERMINATION

This keyword is used to specify the end time of the simulation which in this case is set to 10 ms

. RIGID WALL

The planar rigid wall is created as shown.

DATABASE

All output parameter requests are defined using this card.

a. 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 1.0, which represents that for each 1.0 ms the output data of the analysis is written onto the d3plot file.

b. 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 1.0 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

ELOUT- element_result

ASE-2: SOLID SPOTWELD

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

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.

Boundary Condition

Contact_automatic_single_surface:-

CAONTACT_TIED_SHELL EDGE_TO_ SURFACE

MAT_Elastic(100)

SECTION_BEAM

Result:-

Solid_Spotweld_Result:-

CONCLUSION:

• When a beam element is used for a spotweld, the axial load is higher than normal which may lead to earlier failure if force-based failure criteria are used.
• And, due to the high axial load in the beam element localized stress is also higher. so solid element is more suitable to model the spotweld it has 4 nodes in contact and has better axial and shear capabilities.
• the 3D solid element is more suited to represent spotweld better than the 1D beam element.