Week - 5 - Modelling Spotwelds

           Modelling Spotwelds with Beam and Solid Element using LS DYNA and LS PrePost

AIM: To model the spot-weld for the given assembly parts using beam and solid elements seperately. Also, perform the crash analysis for the same and compare the output result.

Procedure:

1.open the spot weld-assignment.k file in ls-pre-post.

In the opened file, the bottom and top sheet metal have meshed with shell elements. The bottom and to sheet metal have meshed with 10011 element/shell and 10296 nodes. The unit system followed for the unit consistency is gm-mm-ms.

2.Create a shell section card for the bottom and top sheet metal using the metal and part>keyword manager>SECTION>select the shell optin with element formulation=16 and thickness=1.2mm.

define the material for the bottom and top sheet metal select the 001=ELASTIC material card for the bottom and top sheet metal

Model and part>keyword manager>MAT>select the material card

3.Next define the material, properties like mass density, youngs modulus, poisson ratio, stress value for the material cards.

4.specifying the respective material and section ID to the bottom and top sheet metal part IDS.

5.Next step is to create a rigid wall in front of the top sheet metal part 15mm away by using the RIGID WALL-PLANAR card. this is done by selecting entity creation>Rigid wall>create The rigid wall type is selected as a planar and defined vector with one point and a normal i.e 1n+NL here selected the one node, and the normal is created as a rigid wall. This rigid wall is translated/offset by 15mm from crash box component 

the blue outer line shows the extract location of the rigid wall from  the crash box component.

6.Next step is to define a velacity of 50kmph(13.8889mm/ms) and assigned to each node of the selected bottom and top sheet metals, under Entity creation>initial>velacity>insert the option with respect to the part as shown below.

7.The spot-weld can be modeled as beam and solid elements by using the following techniques.

• To represent as a beam element, we use CONTACT-TIED-SHELL-EDGE-TO-SURFACE CARD
• To represent as a solid element, we use CONTACT-SPOT WELD CARD 

spot weld modeled using beam element:

The spot weld modeled using beam element is created by selecting the Mesh>element generation>Beam>Give part ID>Select Beam by TWO_NODE_Sets>Pick Node set1 and 2> Create>Accept>Done

Now the beam element act as a spot weld.

Defining the material card for beam element:

For the beam element, the material card is assigned as the MAT-100 that is especially for the spot weld material. The values are assigned and have we are taking a time-dependent failure concept rather than the force-dependent failure concept like effective plastic strain, maximum axial stress, maximum shear stress, force resultant, tortional moment resultant, moment resultant. so to keep this problem as time-dependent failure we given other failure term as a higher value.

Defining the section card for beam element:

for the beam element, the section property is assigned by using the SECTION_BEAM card. it is important to assign element formulation (ELFORM) as 9 because the MAT_100 spot weld material model only applies for ELFORM 9 formulation in the case of the beam element the cross-section type is selected as tubular since the default cross-section (rectangular) give zero cross-section area during the simulation and the beam thickness at both ends is given as 3mm.

Defining the contact card for beam element :

Here for the beam element, the contact type to be*CONTACT_TIED_SHELL_EDGE_TO_SURFACE card. Here the master IDwill be partID and the slave set ID will be nodes of beam weld element for defining the slave set select beam nodes so as to create the set and the steps is mentioned below.

Entity create>set data>*SET NODE>pick the nodes>Apply

for defining the Master set select both the metal sheet part and the steps is mentioned below.

Entity create>set Data>*SET_PART>PICK the part>Apply

so assign both the slave-set and the master-set as created by the above technique to the contact card *CONTACT-SPOTWELD card

slave-type-Node set ID

Master-type-part set ID

Defining the material card for solid element: 

For the solid element, the material card is assigned as the *MAT_100 that is especially for the spot weld material. The values are assigned and here we are taking a time-dependent failure concept rather than the force-dependent failure concepts like effective plastic strain, maximum axial stress, maximum shear stress, force resultant, torsional moment resultant, Moment resultant. so to keep this problem as time dependent failure we given other failure term as a higher value.

8.Next the termination time  is to be assigned by using *CONTROL_TERMINATION card. Here the time to end the simulation can be calculated analytically using the distance between the rigit wall and component i.e, 15mm and the velacity of the crash box i.e, 13,8889mm/ms

End time,t=(15mm)(13.8889mm/ms)=1.0799ms

The calculated time is 1.4399 ms but extra time is considered after the impact in simulation i.e, the crashing and the bouncing back of the crash box. so, the total time of 5ms is assigned as end time in the CONTROL _TERMINATION card.

9.Now the following output data is generated by using the DATA BASE card with a time step of 0.5ms are assigned for generating output data.

The selection of nodes and element/shell list is done by picking each node and element though creating entity>set data>set node for node set and creating entity>set data>*SET_SHELL for element/shell set.

10.Von Mises stress contour plot after the spot weld fails:

the Von Mises Yield criterion states that if the Von Mises stress of a material under load is equal or greater than the yield limit of the same material will yield. In both the simulation the component hits the rigid wall at 1.0799ms of the simulation, the spot weld fails, exactly half the termination time i.e, 2.5ms. there is no abnormal behaviour of element during the simulation since the element's materials are defined using the elastic property. After the spot weld fails we can see that the top bottom parts come into contact and causing the stress at the contact area and which is shown in above contour as a red-colored region.

From the lots above i.e, the study of the beam and solid weld here intially the sheet metal parts had a velacity of 13.8889mm/ms and as they impacted the rigid wallwe can able to see that the K.E of the component is been decreased and between in terms I.E of component is been increased higher and the hourglass energy is fall to zero as we controlled it in both the case study.

Here the nodes are selected from top and the bottom part of the sheet metal A-node represents the top node and B node represents the bottom part node. it can be seen that after the impact the acceleration value decreases due to a drop in velacity if the component i.e acceleration (-ve velacity). and then goes up and down as shown in the fig above. the top metal sheet bounces back due to the impact where as the lower metal sheet part connected with the spot weld continues to travel in the same direction due to momentum from the top part.

conclusion:

• The spot welds are modeled using beam and solid elements for same component 
• The material card and all other keywords are properly defined for simulation.
• The simulation carried out with time dependent failure and spot weld is failed at half of the termination time.
• The component bounces back after impacting on the rigid wall.
• The plot results and contour diagrams are compared.