Simulating the Effects of Improved Shell Element Properties on Parameters like Energy Error, Mass Error, Displacement, Velocity, Von Mises Stress and Hourglass Energy for a Beam Crash Simulation and comparing it to a Basic Simulation on Radioss.

Objective:

To run two simulations:

•  Basic simulation of a Beam crash:

  • Using the crash beam file from the previous assignment, change the run time to 55 ms.

  • Change the number of animation steps during simulation to a minimum of 25 and maximum of 60.

  • Run the base simulation without any modification to element properties.

  • At the end of the simulation, do the energy error and mass error checks and determine whether results would be acceptable.

  • If acceptable, Plot rigid wall forces, internal energy, hourglass energy, contact energy, total energy of the simulations.

•  Simulation with improved shell element properties:
  • Change all the shell elements properties to recommended properties as shown below, save this as separate, rad files and run the simulation.

Recommended element properties:

and to compare the results of the same.

Procedure:

CASE 1: Base Simulation

 1. Open Hyperworks and select the Radioss user profile.

2. Import the Solver Deck by going to File -> Import -> Solver Deck and choose the Element_Formulation-Shell-3_assignment_0000.rad file and click import.

3. We now need to assign the run time value as 55. This is done by going to Cards -> ENG_RUN and setting the Tstop value to 55 as shown below:

4. Next we need to set the number of animation steps. For this, set the value of Tstart at 0 and Tfreq as 1 such that there are 55 animation steps, which is in the range that is specified. This is shown below:

5. Make sure that the following values are enabled for the analysis: Status, EPSP, ENERGY, VONM, HOURG in the ENG_ANIM_ELEM Card.

6. Also make sure that Status, DISP, VEL are enabled in the ENG_ANIM_VECT card.

7. Now we can run the analysis in Radioss. Go to Analysis -> Radioss, set the file name as per required into the Local disk and set options as -nt 4.

8. The Hyperworks solver opens to show the following:

9. We notice that the value of error is 10.3% and mass error is 0.0001659. This is in the acceptable range and hence we see that this simulation is acceptable.

10. We then click on the results button to open the results in Hyperview. The simulations are shown below:

11. The following contour plots were recorded:

Displacement:

Velocity:

Von Mises:

The following graphs were plotted: 

CASE 2: Improved Shell element properties.

1. Follow the same steps (1 to 6) as the previous simulation.

2. Change the element properties in the property card as shown below:

3. Change the properties in Hat-Section as well as Close-Out Panel since these are the components to which these specific properties are assigned to.

4. The changes are shown below:

5. Run the simulation and record the errors, contours and graphs by following steps 7 to 11 in the previous simulation.

6. The errors were observed to be below the allowed limit and hence is considered to be okay.

7. Displacement contour:

8. Velocity Contour:

9. Von Mises Stress Contour:

10. Graph plots:

Results, Comparison and Conclusion:

S no	Parameter	Base Simulation	Improved Element Simulation
1	Energy Error	-10.3%	-0.1%
2	Mass Error	0.00016	0.00016
3	Displacement	Maximum Displacement: 7.611 E+2 at Node 169	Maximum Displacement: 8.580 E+02 at node 719
4	Velocity	Maximum Velocity: 2.509 E+1 at node 17	Maximum Velocity: 2.708 E+01 at node 527
5	Von Mises Stress	Maximum Stress: 3.479 E+2 at Node 476	Maximum Stress: 3.497 E+2 at Node 708
6	Hourglass Energy	Noticeable through the simulation in the graph	Not observed in the simulation or in the graph.

From the above observations we can conclude that:

1. The use of improved shell elements cause noticable change in the parameters which were analysed for.

2. The most prominent change is in the Hourglass energy. The change in properties for the second simulation showed an elimination of hourglass energy which is hugely beneficial. 

3. A decrease of Energy Error was observed from -10.3% to -0.1%.

4. An increase in the Maximum diplacement and Maximum Velocity was noticed in the second simulation. 

5. The total resultant force shows a sudden increase towards the end in the improved simulation whereas it is a much more gradual increase in the base simulation.

Hourglass Modes and Hourglass Energy:

Hourglass modes are nonphysical modes that occur in under-integrated elements and show deformation but produce no stress.

This is caused mainly caused due to:

1. Presence of only a single integration point in a Solid element.

2.  Presence of only a single integration point in a Shell or T-Shell component.

The reduced integration can lead to non physical zero energy modes called hourglass modes. The energy associated with these modes are called hourglass energy.

There are two methods to prevent formation of hourglassing:

Viscous Method: (ishell -1,2,3 or 4 (Q4))

In this method the hourglass detected with the relative motion of the nodes. A force is applied on the nodes to stabilize the deformation. This force is defined in accordance with the element stiffness. It introduces an artificial energy known as hourglass energy.

Stiffness Method: (24, QEPH)

Since hourglass energy depends on natural yield, material level changes are done to reduce hourglass energy. Hourglass loading stiffness is dependant on natural tangent modulus.

Overall, the improved shell elements showed promising results as compared to the base simulation, especially in terms of hourglass energy and Energy error.