Comparision of Beam and Solid weldspot by Crash Test
Objective:
Modelling the beam and solid spotweld for the plates and compare the result at different scenario.
Spotweld criteria:
When we create a weld, sometime it is difficult to tell the exact force required to break the weld might not get generated in the CAE analysis or maybe we already know where the weld fails so we interest to look the behaviour of bracket in car crash.
Basically two scenario for the spotweld:
Scenario 1:
We don\'t know when the weld will break. So we want to know when it will break. This is like a original simulation in that case we model the weld in good details.
Scenario 2:
In case of correlation studies, already the test is done, now we are trying to correlate the CAE model to the test to get better CAE model, in that case we know what time weld will fail.
Method to create Spotweld:
The use of the spotweld is to attach the two sheet metal by using the spotweld method. In ls dyna we have different method to model the weld. They are,
1. Using beam element and define contact as Tied.
2.Using Constrained Spotweld
3.Using Constrained generalized Spotweld
4. Using Solid element and define contact as Spotweld.
Here we are using the beam and solid element to define the spotweld and compare the result for those two scenario.
Cad Model:
The above figure shows the cad model which given by them containing element and nodes.
It contains two parts such as upper and lower plate. They are just placed and not connected with each other. In real scenario these sheet metal plate are connected by using spotweld. Before modelling we need to specify the material and section. For this plate we are define MAT 00 Elastic and section as SHELL type.
Creation of Beam Element:
The above figure shows the beam element is created between the two sheet metal. Once we create the beam element we need to define the material and section. Since this beam element we are consider as the spotweld so we are using the MAT 100 Spotweld keyword to define a material and section as beam.
Once we create the beam we need to give the contact to know the solver how it is connected between the part. Here we are using the Contact tied shell edge to surface. (Tied represent it is connected like glue).( For tied contact part is the master and the spotweld is the slave)
For the scenario 2 to make sure that after spot weld break, two parts come into contact and they should not penetrate with each other. For that we are creating another contact automatic single surface ( both parts are slave)
Creation of Solid Element:
The above figure shows the solid connection between the parts instead of beam.
To make proper representation of the weld it has to be a solid element. Because spotweld in real scenario looks like thick coin.
We are using the same material card MAT 100 and section as solid.
Here we are using the contact as spot weld.
Boundary Condition:
To run the impact test, we need to create the rigidwall and specify the velocity. Here we specify the for upper plate is 13.88 mm/ms.
To get the result we need to activate the database and control keyword.
The setup for both the method is same. Only the difference is element and its contact.
Scenario 1:
We don\'t know when the weld will break. So we want to know when it will break. For that we use these value for the MAT 100:
The above keyword values is used for both the method for the scenario 1 to see the result.
The keyword we used for this simulation are :
The above figure show the keyword filr to run this simulation. The result are:
Beam Element:
Since i am using termination time is 6. The above video shows that the beam element are not fail and help us to find the maximum axial and shear force by using the plot.
Here i plot the axial and shear force for the beam element. The maximum axial force and maximum shear force are around 6000 N and 4000 N respectively. In the MAT 100 spotweld we specify the axial and shear force are 10000 N. So the beam is not failed.
Solid Element:
The above video shows the model which is connected by solid element.
The above two plot shows the maximum axial and shear force are around 2500 N and 7000 N respectively.
For the scenario 1 now we have the failure force of axial and shear for the both beam and solid element.
From the above known values we can run the simulation for the weld failure for that force to find which time it will fail. But their requirement mentioned in the above statement said that the best simulation should have failure at the half of the termination time.
So we will modify the MAT 100 Tfail is half of the termination time and we will calculate the swforce and compare with beam and solid element.
Scenario 2:
Tfail of the material is half of the termination time.
Beam Element:
The above video shows the spotweld will fail at T=2.5. As a good CAE engineer we need to see how the effect after the spotweld fail. When the impact is happening and the spotweld is failed the upper part collide with the lower part and the crash is happening.
The above figure shows the Kinetic energy, internal energy and the total energy of the simulation.
Total energy is the sum of the kinetic and internal energy and it is always constant.
At the start of the simulation since we provide the velocity from the formula (mv^2)/2, the graph shows the kinetic energy around 70 and internal energy is zero.
When the plate is moving the kinetic energy is reduced and the internal energy is increased at the time of crash.
From the above figure we will get to know that the energy ratio is almost 1. It means the energy balance of the simulation is perfect.
Hourglass energy is zero because we are using the element formulation for the shell element is 9 it means it is fully integrated. So the results are more accurated and the element deformation is not happening.
Sliding energy is used to calculate the contact. When the two parts are moving there is a chance of penetration. If penetration is happening we will get the negative value. It means the contact we defined and the simulation are wrong. Here upto 4 we get zero, we can there is a contact happening on the rigid wall during the crash but sliding energy is not calculated the rigidwall force. It is the force generate between the part when come into contact. After the spotweld failure the two parts come into contact and penetration is not happening, because we are getting the positive sliding energy value.
The above figure shows the resultant interface force. The force generated during the contact. We know that slave id is spotweld and master id is part. At 2.4 the crash happening and the interface force is shown in the above plot which is increasing.
The above plot shows the rigidwal force. Since the part is moving in x direction, the rigid wall force generated only is the x while y and z are zero. During the contact the force we get around 70000 N at the crash.
The above plot for the node id middle shown the acceleration on x y and z direction.After the impact is happened and the spotweld is failure, the acceleration is increasing which is clearly shown in the plot.
The above figure shows the cross sectional force. Here i define the cross section on the upper plate near to the spot weld to see the force and its effect. The graph make sense and we get the result which is applicable.
The above two plot are most important result for the spotweld analysis. The axial and shear force define the durability of the weld. Here we can see that the maximum and minimum force of the axial and shear force. Since we give the spotweld failure time is 2.5 . From the plot we can see after the 2.5 s there is no data enter in the plot.
Maximum axial force is around 5000 N and maximum shear force is 4000 N.
Solid Element:
From the above video it shows that after the spotweld failure the part moves and it deforms which help us to know where we give more importance in the case of safety and durability.
The above plot shows the energy of the simulation. As already mention total energy is always constant and it is the sum of the internal and kinetic energy. We will see how the energy transforming happening from the plot.
The energy ratio is almost one which means the energy balance of the system is perfect and the simulation run perfectly.
Hourglass energy shows the huge variation compare to the beam spotweld. Because we are using the element formulation 2 for the solid weldspot which is not fully integrated. Because of this, these much of energy is necessary to avoid the deformation of the element.
The above plot shows the sliding energy of the analysis. We know arounf 2.5 s there is impact on the wall but there is no sliding energy occur, because rigidwall just absorb the energy and it rebounds. But after 5 s there is a sliding energy due to the two parts come in contact with each and slide it.
The above figure shows the contact resultant force generate between the slave and master region.
The above plot shows the rigidwall force. At 2.5 impact happened on the rigidwall. The kinetic energy of the object transform when it hit the wall and the force values are calculated.
The above plot shows the cross sectional force. Here cs area is created near the weldspot region.
The above two figure shows the axial and the shear force. This two force are more important force which define the failure of the weldspot. The maximum axial and shear force are around 2500 N and 7000 N respectively.
Comparision of axial and shear force for the beam and solid weldspot:
Type Axial Shear
Beam weld 5000 4000
Solid weld 2500 7000
Conclusion:
From the above comparision of the axial and shear force of the beam and solid weldspot we can conclude that,
Since the beam element can take load only in one direction and the values we are getting for the beam is lower than the solid weldspot.
Solidweld spot represent the proper representation of the weldspot and the values are proper accurate. Since axial force is very less it is better to go with beam to avoid the failure and make an improvement to increase the axial.
So it is always prefer to choose the solid weld spot to check the durability and the failure of the spotweld if the computational time is not a matter. Suppose we are in a need of urgent then we can reduce computational time by using the beam but we need a proper verification and validation of the result.