Assignment 3

Assignment 3

Objective :

The aim of this exercise is to run the simulation of the given crash beam.

The base simulation was run and various parameters such as the energy ( hourglass, internal, kinetic etc) were taken into note and their variation was observed carefully.

The same model is simulated with the recommended property changes and variation of similar parameters was studied and the contrast among them is studied.

Procedure :

• Initially the base simulation is run with 55ms with 25 animation step.(case 1)

• The base simulation is run again after changing the animation steps to 60.(case 1a)

• Case 1 is modified by changing the recommended properties and the simulation is run again. (case 2)
• Case 1a is modified by changing the recommended properties and the simulation is run. ( case 2a).

Case 1:

• The input files the ….0000.rad and the …0001.rad(engine) file.
• In the engine file the total time is set to 55ms and the animation steps are 25, the frequency is set accordingly.
• The 0000.rad file is opened in the radioss option which is present in the analysis option.

• We have to just hit the radioss button and the simulation starts, once the simulation is done the results are generated.
• We can now view the animation and the results in the hyper view option.

• As we can see the animation we can even have different contours for different parameters Ex: vonmises stress.
• If we can go to the 0001.OUT (output) file and check the energy error it will be negative and it will varying from 0% to -10%.
• The negative value indicates there is some energy dissipated from the system.
• The dissipated energy is the hourglass energy, a nonphysical energy. It is dissipated because it is not consumed anywhere in the system.
• The hourglass energy generally will be untill -15%.As the error is less than than -15% it is acceptable.
• As we are using under integrated elements and we are not using any mitigation measures to reduce the hour glassing it is evident in the output file.

• Next we will try to plot different energies and how they vary with time using hyper graph.
• We have to now switch in to the hyper graph option to plot the graphs.
• After switching into hypergraphs we can plot the graphs of internalenergy, Kinetic energy, hourglass energy, total transilational energy and contact energy.
• The hourglass energy is increasing as the time progresses, it is due to the fact that the beam is crushing and there are large deformations in the elements.
• The kinetic energy keeps on decreasing as the velocity reduces over the course of the simulation.
• The internal energy increases simultaneously to keep the energy balance.
• The difference in the energies is the dissipated energy which is the hourglass energy.
• The hour glass energy is however is not considered in the energy balance so the total energy keeps decresing.
• Similarly we can plot the rigid wall forces at the beam, resultant normal as well as the tangent force are plotted vs time.

Case 1a

• The input files the ….0000.rad and the …0001.rad(engine) file.
• In the engine file the total time is set to 55ms and the animation steps are 60, the frequency is set accordingly.
• The 0000.rad file is opened in the radioss option which is present in the analysis option.
• Similar steps are followed and the plots are generated for the similar parameters.

• The plots show similar trends as we have not changed anything concerning the hour glassing.

Case 2

• In this case we will go through the same case 1, but we will change the properties as per the recommendations.
• As we open the model in Hyper mesh we can see a property card available.
• The properties such are modified according to the given inputs. This will allow us to reduce the hourglass effect.

• As shown in the above image the modificationsa re done and the same procedure mentioned above is repeated and the simulation is performed.
• The hyper view animation looks different when compared to the one where no method is used to reduce the hour glassing effect.

• The hyper graph shows the variation of different energies of the with respect to time.
• The hourglass energy remains zero as we see the graph this is due to the element formulation we used.
• And the total energy remains constant and there is no dissipation of energy.
• Similarly the wall forces are also plotted w r to time. The resultant normal force as well as the resultant tangential force is plotted.

• The energy error is also at its minimum it is btw 0%-0.1%. This shows that no hour glass energy is created.

Case2a

• Case 2a is modified version of 1a, the simulation is carried out after changing the properties according to the recommended.
• The hourglass energy is a flat line in the plot and animation is different to that of case2.
• The energy error is at its minimum and the value can be seen in the output file.
• The graphs and the output file images are attached below.

Conclusion:

Hour glassing:

Hour glassing is a nonphysical deformation, that arises due to the under integrated elements. There is a warping up in the elements as the number of integration or Gaussian points are less than the required. The hourglasss effect can be seen in the energy error in the output file. The negative energy error in the output file indicates there is a significant amount of hourglass deformation happening. To reduce the hourglass deformation in this exercise we are using a method in which we are adding stiffness to the component before hand. That is why in case 2,2a we will have QEPH shell formulation. This will add stiffness but not the Gaussian points.

This will not increase the computation time and cost, it reduces the hourglass effect whichis evident in the energy error as well as the plots.