Studying the effect of Inacti flag 6 using the example of a crash tube simulation with some initial penetrations using RADIOSS SOLVER.

OBJECTIVE: To study the effect of the Inacti flag 6 using the example of a crash tube simulation with some initial penetrations in Radioss.

PROCEDURE:

1. Import the given solver deck in Hypercrash. The model consists of 4 components and a rigid body connected to one end of the crash tube.

   Fig 1. The imported model

2. The 4 components are defined using 2 properties. The lower two components are 2 mm thick and the upper two components are 3 mm thick and they are connected using spot welds.

   Fig 2. The property cards

3. Now let us have a look at the Interface type7 defined for the impact simulation in case 1 and case 2

   Fig 3. Type 7 interface defined in case 1 and 2 respectively

   Note: Penetrations result if a slave node is closer to the master segment than the calculated gap. Here, we have kept the value of Igap as 2. For Igap = 2, The Variable Gap = max {Gapmin, min [Fscalegap*(gs + gm), Gapmax]} where: • Gapmin is the minimum gap for impact activation • gm = tm/2, where tm is the thickness of master shell – or zero for master brick elements • gs = ts/2, where ts is the largest thickness of shell connected to the slave node So in this case, the Variable gap = max{Gapmin, min[1*((3/2)+(3/2)), Gapmax] } = max{1, 3} = 3 ; for pshell_3 property =max{Gapmin, min[1*((2/2)+(2/2)), Gapmax] } = max{1, 2} = 2 ; for pshell_2 property The distance between the overlapping plates is measured to be 2.1 mm. Thus, there will be no initial penetration for pshell_2 property components. The pshell_3 property components will have some initial penetrations as the variable gap calculated in their case is greater than 2.1 mm. In case 2, the Inacti value has been changed to 6. This will remove the initial penetrations in case 2.

4. Let us check for initial penetration in both the cases. In both the cases, a total of 300 penetrations were detected.

   Fig 4. Initial penetrations detected in both cases

5. Export the starter input files for both the cases and run analysis on the Radioss solver. On completion of the run, check for energy errors.
6. Generate the necessary plots and contour animations for comparing the two cases.

RESULT & DISCUSSION:

1. Though in both cases, the initial penetrations are the same, the use of the Inacti flag 6 seems to have removed the effect of the penetrations in the 2nd case. In case 1, the impact is already activated at zero milliseconds ( even before the actual physical impact ) due to the presence of initial penetrations and thus repulsive forces are present at the beginning of the simulation itself. These initial repulsion forces give an error in the energy balance due to the interface work. This is clearly evident from the enormous energy error of 99.9 % in case 1 compared to a much lower -5.7%(within acceptable limits), in case 2(Figure 5). The presence of the initial contact force generated due to these penetrations can also be seen in the contact forces contour animation shown in fig 6. A contact force of nearly 3240 kN is already experienced by the components even before the actual physical impact has occurred.

   Fig 5. Energy error check

   Fig 6. Initial Contact forces contour for cases 1 and 2 resp.

   Fig 7. Contact forces contour for cases 1 and 2 resp.

2. On detection of initial penetrations in the model, the Inacti flag 6 in case 2, has basically reduced the variable gap value such that all the penetrated nodes lie outside this newly defined gap region and thus removing all initial penetrations and its effects.
3. In case 1, repulsive contact forces present on the components deform the penetrated region as soon as the simulation begins. As a result, there is a large jump in the Internal energy for case 1 as compared to the nearly steady increase for case 2. The presence of initial contact energy arising due to the contact forces discussed in (i) can also be seen clearly in the Time vs Energy plots.

   Fig 8. Time vs Energy plots.

4. The lack of appropriate treatment for the initial penetrations in case 1 has resulted in notches being less effective in reducing the transfer of forces to the Rigid wall. The notches are present to absorb a large chunk of impact energy by crushing the elements near to the notch regions but in case 1, due to the initial penetrations and the resultant deformations, the energy absorbed by these notches has reduced and a higher amount of forces have gotten transferred to the rigid wall. This can be observed clearly in figure 8. The crushing ability of the crash tube has been significantly reduced in case 1.

   Fig 9. Time vs Energy plots.

   Fig 10. Displacement contour animation for cases 1 and 2 reps.

CONCLUSION:

1. It is recommended to not have any initial penetrations in the model. This gives rise to a simulation with sliding energy/ contact energy present at the initial conditions which is physically wrong and hence penetration checks are to be conducted and the penetrations should be removed before simulation.
2. In case the penetrations are not removed in the pre-processing stage, appropriate options should be chosen at the time of interface creation so that the solver can make necessary changes to avoid these errors. It is always recommended to keep the Inacti flag as 6 to do so.