Comparing simulations with basic and improved shell element properties to analyse and understand their effect on end results.
Objective: To compare Results for shell element with base and improved element properties for the same inputs.
Case 1: Impact analysis of crumble box with a rigid wall at Initial velocity of 15.6 m/s. The element formulations are kept to default values. Analysis end time of 55 milliseconds.
Case 2:Impact analysis of the same crumble box for the exact boundary conditions. Shell elements have improved formulation properties for more accuracy.
As stated a direct comparison of the results for two different property elements with the exact conditions of analysis.
Case 1:
Impact analysis of crumble box with a rigid wall at Initial velocity of 15.6 m/s. The element formulations are kept to default values. Analysis end time of 55 milliseconds.
As seen from the screenshot here, The energy error at the final time step is -10.3% for default element properties which is within the 15% acceptable limit due to hour-glassing. The K. E. Of the system is linearly decreasing whereas the I.E. Is going up linearly which is how it is supposed to go. The mass error doesn’t deviate from the initial value which is negligible. Hence, the results should be acceptable.
Results:
1.As asked in the question following is the plot of Rigid wall force against time for the given impact analysis. As it can be seen in the first 10ms time, a lot of force and hence energy is absorbed by the component in the crumbling action away from the end of the box in contact with the rigid wall, hence the force exerted on it goes down almost to zero. After that the force goes up with fluctuating nature with a number of peaks and valleys which portrays the non-linear nature of the component.
2. Energy plot: As required following is the energy plot against time fo the given case. Here, As predicted, the internal energy and kinetic energy plots are mirror image of each other since the acceleration due to forces induced produces strain which contributes to internal energy. Contact energy is almost zero, the hourglass energy introduced goes up across the analysis as the elements have default properties and hence do not resist any unusual deformation.
- The Von Mises Stress plot shows a peak 349.9MPa. The nature of the deformation on impact is not very natural for a non-linear analysis. The crumbling of the box isn’t as expected for ex. the box doesn’t crumble much at the end closest to the wall, unnatural kinks etc.
Case 2:
Impact analysis of the same crumble box is to be carried out for the exact conditions. The difference being the element formulation properties only. In this case the shell elements are given properties for more accurate analysis an hence results.
Parameters Comment
Ishell = 24 QEPH 4 nodes shells are best combination of cost and accuracy.
Ismstr=2 Full geometric nonlinearities (default)with possible small strain formulation activation in RADIOSS Engine.
Ish3n=2 Standard 3 noded shell (C0)with modification for large rotation(default)
N=5 Number of integration points set to 5 for accuracy bending.
Ithick=1 Thickness change is taken into account for accuracy.
Iplas=1 Iterative plasticity for good accuracy.
As seen from the screenshot here, The energy error at the final time step is -0.1% for default element properties which is within the 5% acceptable limit as there is no hour-glassing. The K. E. Of the system is linearly decreasing whereas the I.E. Is going up linearly which is how it is supposed to go. The mass error doesn’t deviate from the initial value which is negligible. Hence, the results should be acceptable.
Results:
1.In this case the Drop in Rigid wall force due to crumbling and hence absorption of the impact energy isn’t abrupt and drops almost till 12ms. There is a Secondary peak after 50ms mark as the farther end of the box makes contact with the wall as the box is crumbled in the half. The force dies down after peaking at roughly 53ms as the end part also crumbles and absorbs the energy.
2.Energy plot: As required following is the energy plot against time for case 2. Here, As predicted, the internal energy and kinetic energy plots are mirror image of each other and as stated in previous plot the 2nd impact causes a peak. The Contact energy is almost zero, the hourglass energy here is also zero as due to stiffening added to the elements at material level using QEPH shell property/element.
3.As seen from the screenshot, the peak Von Mises stress value comes out 349.1MPa which is a bit lower than case 1. Due to the use on non-linear property for the shell elements, the deformation looks very natural. The crumbling is as expected and the flow of stress wave is continuous.
Conclusion:
- From the comparison between the cases, it can be deduced that the extra time required in the 2nd case can be justified as the results are way more accurate for a small amount of time.
- Hence, it is advisable to use the following element properties in case of an non-linear impact analysis consisting of 2D shell elements.
Parameters Comment
Ishell =24 QEPH 4 nodes shells are best combination of cost and accuracy.
Ismstr=2 Full geometric nonlinearities (default)with possible small strain formulation activation in RADIOSS Engine.
Ish3n=2 Standard 3 noded shell (C0)with modification for large rotation(default)
N=5 Number of integration points set to 5 for accuracy bending.
Ithick=1 Thickness change is taken into account for accuracy.
Iplas=1 Iterative plasticity for good accuracy.
Please go through the ppt with comparison slides:
https://drive.google.com/open?id=1HdUUmFNW9AzgQkg2J5ESpdOVq3Z8FlCN
Link to the folder with the animation, output and other files and ppt:
https://drive.google.com/open?id=18FTssfXcyv3HwUWE7NdVzUC4d6GT9HGh