LS DYNA- Spotweld Beam and Solid Element comparision in Crash

Considering that the crash box welded pair collides with the rigid wall at velocity of 14m/s;

This has be done by using a planar rigid wall stationary at one end, while the welded crashbox drops towards the rigid wall at an initial velocity of 14m/s.

SpotWeld-Beam:

Beam elements are used to model spot-welds. Here the material card for the crash box is MAT_ELASTIC. The spotweld is modelled using MAT_SPOTWELD. The beam elemement used is tubular with diameter of 3, hence cross-sectional area of 7.065. The failure criterion used here is time based.

Each spotweld has 4 beam elements. These spotweld ends are fixed to the respective crashbox nodes. Therefore, the CONTACT_TIED SHELL EDGE TO SURFACE card becomes redundant. All other details are same as that of the crash box assignment.

The termination time of the simulation is 3. The time based failure condition has be given as 2. This time was decided based on the time at which maximum force was observed and hence probable failure (If at all it fails).

• Acceleration:
  A node near the spotweld is considered on the two crash box pieces that are welded. The graph of acceleration of the nodes with time has been plotted.
  
  The graph above has a lot of noise after the collision takes place. But its is to be expected that the CB that impacts the wall finally tends to accelerate away from the wall, while the CB behind the spotweld continues to accelerate toward the wall after the spotweld fails.
  
  
• Total energy:
  The total energy in the system remains constant in the course of the collision. A graph of the total energy vs time is plotted.
  
  
  
  The kinetic energy of the crash box is stored in the deformation of the crashbox as internal energy from the time at which collision starts, to its rebounce. The constant internal energy ios an indication of a converging solution.
  
  SPOTWELD BEAM ELEMENTS IN THE MODEL FOR FOLLOWING GRAPHS:
  
  
  
• Force:
  The force in the spotweld can be understood by observing the forces in an element in each spotweld. A graph of the force with time has been plotted.
  
  
  
  The force in each of the spotweld appears to be very similar. Thus approximating the same failure time for all the spot-welds can be justified. There is a slight difference in the force between different positions of spotweld.
  
  
• SW Length:
  The spotweld length will tend to increase after the impact takes place. After SW failure at t=2, the length is shown to decrease to zero in the graph. Here since adjacent nodes have been used to define the spotweld, the length is not the actual gap between the two surfaces.
  
  
  
  INFERENCE:
  The beam element should have be defined as a floating element in the gap between the two shell surfaces. The *CONTACT_SPOTWELD should have been used to define the relationship between the beam end node and the shell surface.

SpotWeld-Solid:

Solid elements are used to model spot-welds. Here the material card for the crash box is MAT_ELASTIC. The spotweld is modelled using MAT_SPOTWELD. The solid element for the spotweld has a width of 2.66, so as to have an area of 7.065 (same as that of the beam element spot weld). The failure criterion used here is time based.

Each spotweld has a solid element. These spotwelds are modelled to the gap between the respective crashbox segments. Here, the solid element is not fixed to the CB, thus the CONTACT_SPOTWELD card is used to specify the relation between them. All other details are same as that of the crash box assignment.

The termination time of the simulation is 3. The time based failure condition has be given as 2. This time was decided based on the time at which maximum force was observed and hence probable failure (If at all it fails).

• Acceleration:
  A node near the spotweld is considered on the two crash box pieces that are welded. The graph of acceleration of the nodes with time has been plotted. In this the noise is present from the beginning.
  
  
• Total energy:
  The total energy in the system increases until the spotweld fails. After which the energy is constant. A graph of the total energy vs time is plotted. This plot is similar to the beam element case, but the energy values differ slightly.
  
  SPOTWELD SOLID ELEMENTS IN THE MODEL FOR FOLLOWING GRAPHS:
  
  
  
• Force:
  The force in the spotweld can be understood by observing the forces in an element in each spotweld. A graph of the force with time has been plotted.
  
  
  The deviation in the force values can be due to the use of single solid element in contrast to four beam elements in the previous case.
  
  
• SW Length:
  
  The behaviour shown below is expected, but unlike the previous case, the solid spotweld element length is equal to the gap between the two shell surfaces. There is no increase in length in this case.
  
  
  
  INFERENCE:
  The contact CONTACT_TIED_SURFACE_TO_SURFACE is to be used when using solid spoweld elements. For simple crash models, *CONTACT_SPOTWELD can be used.
  
  
  DIFFERENCES BETWEEN SOLID AND BEAM SPOTWELD ANALYSIS:

1. The solid element considers the physical model for the crash simulation. The beam element only defines the connecting details without taking its physical model into account.

2. The beam element for spotweld runs faster than the solid element.

3. The K.E and P.E also vary by different amounts though the same material has been defined in both the cases. More potential energy is stored in the deformation when solid element is used for the spotweld.

4. *CONTACT_SPOTWELD interface is used for beam element spot welds and a CONTACT_TIED_SURFACE_TO_SURFACE interface for solid element spot welds.