Week - 5 - Modelling Spotwelds

OBJECTIVE

To model spotwelds for given assembly using beam and solid elements and simulate it and compare the results. Identify the advantages of each method

PROCEDURE

The spotwelds can be created using 2 different methods i.e. using beam element and using solid element. The beam element has less complexity and can fetch only axial data whereas solid elements can give both axial as well as shear forces.

The study focuses on creating the spotwelds using both type of elements to assemble 2 plates. For better understanding of the results 5 spotwelds were created. The procedure is as follows.

Beam Elements

• Import the keyword file to LS-Dyna
• Now create beam elements using element generation -> beam -> two nodes. Repeat this operation and create 5 beam elements between two plates.
• Define section card for both plates and beam elements. Shell section for Plates and beam section for beam elements. The important thing is to keep the ELFORM to 9 and CST to tubular for beam elements.
• For material elastic steel is chosen for the plates. For spotwelds, dedicated MAT100 material card is used for the study. This card gives the flexibility of choosing the time based failure or load based failure. TFAIL =5
• Rename the parts and assign respective section and material to it.

• Now create Node sets for better definition of beam nodes. Nodes of beam elements, Nodes for assigning the boundary condition are the node sets to be created.
• Define a displacement curve as a source of prescribed motion. Define a boundary card using prescribed motion set to define this.

• Create two sets of contact. One for the beam elements and plates i.e. tied edge shell to surfaces and other for the self – contact of the plates i.e. automatic single surface contact.

• For termination, 10 ms is defined using Control termination card.
• Database card is used to define the different output cards like d3plot, glstat, matsum, nodout and swforc. This cards are defined with dt = 0.25

Solid Elements

• Import the keyword file to LS-Dyna
• Now create beam elements using element editing -> Hexa -> select 8 nodes. Repeat this operation and create 5 solid hexa elements between two plates.
• Define section card for both plates and Solid elements. Shell section for Plates and beam section for beam elements. The important thing is to keep the ELFORM to 1
• For material elastic steel is chosen for the plates. For spotwelds, dedicated MAT100 material card is used for the study. This card gives the flexibility of choosing the time based failure or load based failure. TFAIL = 5
• Rename the parts and assign respective section and material to it.

• Now create Node sets for better definition of beam nodes. Nodes of beam elements, Nodes for assigning the boundary condition are the node sets to be created.
• Define a displacement curve as a source of prescribed motion. Define a boundary card using prescribed motion set to define this.

• Create two sets of contact. One for the solid elements and plates i.e. Contact spotweld and other for the self – contact of the plates i.e. automatic single surface contact.

• For termination, 10 ms is defined using Control termination card.
• Database card is used to define the different output cards like d3plot, glstat, matsum, nodout and swforc. This cards are defined with dt = 0.25

RESULTS

From the subsequent results we will be able to conclude that simulation ends at time = 5 and we can see a substantial change in the flow at that point. This shows that simulation ran as per our expectation and spotwelds failed at the TFAIL given above.

Following aspects are compared

Energy

Based on the energy details in both the elements, we can see that energy is conserved and thus we can conclude that our simulation is correct.

Energy in the solid elements spotweld is more than that of beam element. This is due to additional details which solid elements bring in comparison to the beam elements and thus it can capture energy in one additional energy.

Displacement

Acceleration

Axial force

Shear force

Resultant force

Seeing the forces in both the cases, we can clearly see the detailed information of shear forces developed in welds in case of Solid elements while in beam elements, the shear forces are very low or not dominant. Also We know that shear forces are the dominant factors in failure of spotwelds and thus can conclude that Solid elements are better for modelling than beam elements. This is due to the specific characteristics of each elements and Beam elements are designed especially for Axial loading whereas Solid elements can carry loading in more direction

CONCLUSION

This activity helped us understand different methods to model the spotwelds in LS-Dyna. Also based on the results, it shows the aspects of both beam and solid elements. Beam elements are useful when only axial force is the concern and computation power available is limited whereas Solid elements provide detailed information but require more computational power and time to process

LINK

https://drive.google.com/drive/folders/1AJsFgxvIz2ue1qv17ji1KnFClqPnGCpR?usp=sharing