Week - 5 - Modelling Spotwelds

AIM

To Run a crash box simulation using beam spotweld and solid hex spotweld and compare the results (axial and shear forces)

OBJECTIVE:

The objective of this project is to model spot welds to join two sheet metal components using beam elements on the first case and solid elements on the second and compare the axial and shear force experienced by the spot welds in both cases.

LS-DYNA

SETUP:

CASE-1: BEAM SPOTWELD

SPOTWELD ELEMENT GENERATION:

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.

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 increased integration points and more accuracy.

The MAT_100 card is specified as shown below,

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.

INITIAL VELOCITY

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

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.

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 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 (with SSTYP - 4, MSTYP - 2)

All contacts are frictionless since we are only concerned about the axial and shear forces in the spot welds.

CONTROL_TERMINATION

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

RIGIDWALL

the planar rigid wall is created as shown.

DATABASE

All output parameter requests are defined using this card.

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.

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

ELOUT- element results

CROSS_SECTION_SET

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

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

HISTORY_NODE_SET

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

node-set is created with all spotweld nodes.

CASE-2: SOLID SPOTWELD

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

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.

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 compatible with a MAT_100 material.

CONTACT_SPOTWELD

Unlike the previous case, in this type, we will use a CONTACT_SPOTWELD keyword to define the contact between the solid hex 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)

RESULTS:

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

Energy plots:

It can be seen from the internal and kinetic energy that most of the energy is absorbed in beam spotweld due to internal motion between the two c-section beams.

SECTION FORCE in C-section beams

For beam spotweld: Most of the impact is taken by the front part while the back part experiences only a small amount of the impact force, which in reality won't happen.

For solid spotweld: It can be seen that both the front part and the back part experiences impact force

Von-Mises Stress (Beam vs Solid)

Similarly, the Von-mises stress which is measured within the elastic limit is shown above. It attains a maximum of 4.6E3 Pa in the beam case and 2.6E3 Pa in the solid element.

Axial Force (Beam vs Solid)

FLANGE AREA spotweld:

WEB AREA spotweld:

The axial forces experienced by both the beam and solid spotweld elements are shown above.

• beam experiences more axial load compared to the solid element. since it is connected through one node only the weld is not stable.

Shear Force (Beam vs Solid)

FLANGE AREA spotweld:

FLANGE AREA spotweld:

The Shear stress experienced on each spotweld in both the cases is shown above. solid element experiences more shear load compared to beam element because the solid element has 4 nodes in contact while beam has only one.

so compared to the beam, the solid element is more stable and is more suitable for modeling spotweld.

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.