crash box simulation using LS-DYNA
Objective:
In this challenge, we will run the crash box simulation for the different thicknesses and compare the result with the help of LS-Dyna.
Crash Box:
The above figure shows the model provided by the support engineer for this analysis. The K file initially consists of elements, nodes, and parts.
Model Build Up:
Once we have the nodes and elements, we need to create the material and section.
Since it is a non-linear analysis, so we are using the Mat 024. Here in section the given model contain shell element. So we create the section as shell and give the thickness as 1.2 mm and 1.5 mm for comparing the results.
Once we create both of this we need to assign in the parts.
Since this box is going to crash by hit on the rigidwall. To create the rigidwall we use the Rigidwall keyword and create the planar rigidwall which is shown in the below image.
Boundary Condition:
Since we create the rigid wall as a fixed wall not moving in space. So the rigid wall is fixed and crash box going to hit the wall with a velocity of 50 kmph (13.88 mm/ms)
Here we have only one surface. So we are going to use the single surface contact and assign the slave id as crash box and master id is zero.
Output Assigning:
We need the output such as glstat,d3plot, etc we use the database keyword and assign it. Since we are using the single surface contact we are unable to calculate the sliding energy or friction energy even we specify the value of Fs and Fd in the parameter.
For that, we need to use the control contact and force transducer penalty under the keyword contact to find the force if it is possible to calculate.
we need to enable the control energy to calculate the Hourglass energy.
To calculate the strain we need to enable the database binary extent. Nodout for acceleration and velocity.
Since we are doing the explicit analysis there is no need to specify the memory and run the simulation.
Result Interpretation:
The above animation shows the simulation of the crash box hit the rigid wall. Here I did the crash box case such as it is not going to buckle and folding up. But it is going to hit the wall and rebounces completely. In realtime scenario it is not happed whenever the object hit it is going to deform. But here we just try to understand the simulation process how to do in the ls dyna.
Comparison of crash box 1.2 mm with 1.5mm shell thickness:
Stress at the time when it hit the wall:(x-axis)
1.2 mm Shell Thickness:
1.5 mm Shell Thickness:
Y-axis stress:
1.2 mm Shell Thickness:
1.2 mm Shell Thickness:
We know that thickness will help to increase the stiffness. More stiffness will result to lower stress value. But for this type of analysis, we are looking at how the failure will happen and how the deformation will happen. The figure shows that max and min stress. For more thickness, the area where it hit the wall is very low compared to less thickness but more stress appeared in the center. From the crash point of view more thickness more rigidity or stiffness present and from the result based on the stress 1.5mm thickness is better than 1.2mm thickness cashbox.
Strain in X :
1.2 mm Shell Thickness:
1.5 mm shell Thickness:
Y strain:
1.2 mm Shell Thickness:
1.2 mm Shell Thickness:
Strain represent the deformation of the body. From the boundary condition we know where the deformation is more and the result also give the same .
Kinetic Energy:
1.2 mm Shell Thickness:
1.5 mm Shell thickness:
The above two plot shows the kinetic energy of shell thickness 1.2mm and 1.5mm respectively. Initially, we have kinetic enrgy which is calculated from the mass and velocity. At 0.73 s the crash happening and the kinetic energy is reduced. Since thickness variation is only 0.3 mm so there is not much energy differences.
Internal Energy:
1.2 mm Shell Thickness:
1.5 mm Shell Thickness:
During the impact the kinetic energy is predominant transform into the deformation of the structual response for which the internal energy is calculated.
Total Energy:
1.2 mm Shell Thickness:
1.2 mm Shell Thickness:
Total energy is a combination of the Kinetic and internal energy.
Hourglass Energy:
1.2 mm Shell Thickness:
1.5 mm Shell Thickness:
The above two plot shows the hourglass energy. The HG energy must be less than the 10% of the total energy. If it is more then the results are not accurate we need to include more ipt. For this we are using elform 2 which is acceptable for this simulation.
Sliding Energy:
1.2 mm Shell Thickness:
1.5 mm Shell Thickness:
Interface Force:
1.2 mm Shell Thickness:
1.5 mm Shell Thicknes:
Rigid Wall Force
1.2 mm Shell thickness:
1.5 mm Shell Thickness:
From the above plot of rigidwall force, sliding energy and interface energy all are zero, here i did the crash just hit and rebounces and it not fold up or slides. Because of this reason we get the value zero.
Acceleration along its length of the crashBox
1.2 mm Shell Thickness
Resultant Acceleration:
1.5mm Shell Thickness:
Resultant Acceleration:
The above figure shows the acceleration of the object. We know that if the velocity is constant there is no acceleration. Upto 0.7 we have zero acceleration. After the impact the velocity ic changed and we get the acceleration value. Here the plot for 1.2mm and 1.5mm are mire or less same because of only 0.3mm differences.
Stress Plot Comparision:
1.2 mm Shell Thickness:
1.5 mm Shell Thickness:
Strain Plot Comparison:
1.2 mm Shell Thickness:
1.5mm Shell Thickness:
Similarly the above plot shows the stress and strain value of the centre element along its length.
Cross Sectional Force:
1.2mm Shell Thickness:
1.5mm shell Thickness:
The above figure shows the resultant force of the cross-sectional area along its length.