Analysis of a tube crushed into a rigid wall
Objective:
• Create a mesh for the bumper assembly with a mesh size of 6mm.
• Crash tube models with 6 different cases are compared.
• Plot RWALL forces, contact forces, internal energy and separate plots for each component.
• Study the effect of the notch on the crush tube model.
Case setup:
Bumper model:
• The bumper model is loaded into hypermesh workspace and free nodes are deleted.
• Some surfaces are edited accordingly so that the free edges are converted into shared edges. The middle surface is not converted into share edge as that is retained so that the elements can be connected at the middle surface.
• A mesh with quad elements having an element size of 6mm is created from the surfaces of the bumper model.
• Connectors are used between the components with suitable connections.
Crash tube:
A crash tube model is pressed against a rigid wall with an initial velocity of 13.3m/s in the Z-axis direction.
• Case 1:
The model is run as it is given.
• Case 2:
The existing Type 7 contact definition is modified as Inacti=6.
• Case 3:
In addition to the existing contacts, Type 11 is defined to account the edge-edge penetrations of the elements. Recommended values are used for the interfaces.
• Case 4:
The existing notches are deleted from the model my aligning the nodes of the components. The existing boundary condition at the rigid body is deleted.
• Case 5:
A notch is created in the middle by offsetting the elements by a distance of 3.19mm.
• Case 6:
One notch on each of the opposite side of the components is created.
Execution:
The model is executed under radioss solver and results are obtained.
Results and conclusions:
• Energy and mass errors are tabulated for all the above cases. Values are in the acceptable range and it is observed energy error is negative and decreasing in all the cases. At the end of each simulation, after the energy error is maximum, the error gets increased and this increasing trend can represent the rebound phase of the crush tube.
| Case No. | No.of Cycles | Time elapsed | Energy Error | Mass error |
| Case 1 | 83691 | 189.06 | [0,-4.6]% | 0(Constant) |
| Case 2 | 82077 | 224.42 | [0,-5.7]% | 0(Constant) |
| Case 3 | 82063 | 239.14 | [0,-5.7]% | 0(Constant) |
| Case 4 | 74785 | 213.80 | [0,-5.7]% | 0(Constant) |
| Case 5 | 87800 | 248.81 | [0,-6.2]% | 0(Constant) |
| Case 6 | 84953 | 301.41 | [0,-6.2]% | 0( Constant) |
• All 6 cases are animated and observed at different time intervals.
Initial stage:
Intermediate stage:
Final stage:
Maximum von-mises stress is observed in case 1 i.e. 0.69GPa.
• Rigid wall forces:
Increase in Rwall forces is due to the bounce of the crush tube and this happens at different times in each case.
Contact energy:
Maximum contact energy is observed in case 6 i.e 2970 J.
Internal energy:
Internal energies in all six case follow an increasing trend as this represents energy absorbed by the crash tube due to deformation. In the later stages of the simulation, the internal energies are found to be almost constant (during rebounce period) as we know at this stages the model doesn't undergo any deformation, thus internal energy being constant.
• Effect of Notches:
From the total energy plots of all parts i.e. components, location of notch affects the total energy. We can say the location of notch affects the internal energy and internal energy is measured from the deformation. So notch induces deformations very early and thus absorbs most of the energy in the parts that are having the notches.