Comparision between results of crashing of a rail against a rigid wall with base shell properties & upgraded shell properties.  

Objective:- To compare results of crashing of a rail against a rigid wall with base shell properties & upgraded shell properties.

1.Using the crash beam file from the previous assignment, change the runtime to 55 ms.

2. Change the number of animation steps during simulation to a minimum of 25 and a maximum of 60.
3. Run the base simulation without any modification to element properties.
4. At the end of the simulation, do the energy error and mass error checks and determine whether results would be acceptable.
5. If acceptable, Plot rigid wall forces, internal energy, hourglass energy, contact energy, the total energy of the simulations.
6. Change all the shell elements properties to recommended properties, save this as separate, rad files, and run the simulation.
7. Follow steps 5 and 6.
8. Create a brief report comparing all the results from steps 5 and 6 for both simulations.
9. Comment whether there is any change in the results and why there is any change in the result.

Solution:-

Base shell element properties

Sr No	Property	Description
1.	Ishell =0	4 noded shell formulation flag
2.	Ismstr=0	Small strain formulation flag
3.	Ish3n=0	3 noded shell formulation flag
4.	N=5	The number of integration points within the shell thickness.
5.	Ithick=1	The number of integration points within the shell thickness.
6.	Iplas=1	The number of integration points within the shell thickness.

CASE 1:- Rail crash against a rigid wall with base shell element properties.

• Import the solver deck(0000.rad file) into HyperWorks (Make sure the Radioss user profile is active). Change the runtime to 55ms (Cards<ENG_RUN<Tstop=55ms).

• Set the animation steps to 25 steps (Cards<ENG_ANIM_DT<Tstart=25ms<Tfreq=0.1).

• To run the simulation( Analysis< Radioss<options=-nt 4). -nt 4 is the number of cores allotted to run the solving process. It is limited to 4 for the student version.

• To check the mass and energy errors open the 0001.out file. The energy error & mass error for base shell properties is -10.3% and 0.1659E-03 respectively.

• For the animation of results open the h3d file in hyper view. To add displacement, velocity,von-mises contours click on the contour tab in hyper view. The animation will be based on the parameters mentioned in the ENG_ANIM_DT card.

Von-Mises Stress Contour Plot.

Displacement Contour Plot.

• For 2D plots for rigid wall forces, internal energy, etc open theT01 file in hypergraph 2D. Select the parameters for X and Y axes as needed. The units will be the same as mentioned in the starter input file(0000.rad).

Energy Plot.

Rigid wall forces plot.

Upgraded shell element properties.

Sr No	Property	Description
1.	Ishell= 24	QEPH shell formulation with hourglass stabilization.
2.	Ismstr=2	Full geometric nonlinearities with possible small strain formulation activation.
3.	Ish3n=2	Standard triangle (C0) with modification for large rotation.
4.	N=5	The Number of integration points within the shell thickness.
5.	Ithick=1	The Number of integration points within the shell thickness.
6.	Iplas=1	The Number of integration points within the shell thickness.

CASE 2:- Rail crash against a rigid wall with upgraded shell element properties.

• Import the solver deck(0000.rad file) into HyperWorks (Make sure the Radioss user profile is active). Change the runtime to 55ms (Cards<ENG_RUN<Tstop=55ms).
• Set the animation steps to 25 steps (Cards<ENG_ANIM_DT<Tstart=25ms<Tfreq=0.1).
• Insert the above-updated properties in the property card.

• To run the simulation( Analysis< Radioss<options=-nt 4). -nt 4 is the number of cores allotted to run the solving process. It is limited to 4 for the student version.
• To check the mass and energy errors open the 0001.out file. The energy error & mass error for base shell properties is -0.1% and 0.1659E-03 respectively.

• For the animation of results open the h3d file in hyper view. To add displacement, velocity,von-mises contours click on the contour tab in hyper view. The animation will be based on the parameters mentioned in the ENG_ANIM_DT card.

Von Mises Stress Contour Plot.

Displacement Contour Plot.

• For 2D plots for rigid wall forces, internal energy, etc open theT01 file in hypergraph 2D. Select the parameters for X and Y axes as needed. The units will be the same as mentioned in the starter input file(0000.rad).

Energy Plot.

Rigid wall forces plot.

Results And Conclusion:-

1. Due to hourglass stabilization in upgraded elements, the displacement contour plot of upgraded elements seems to be more realistic than plot of base elements and in the upgraded element contour plot, the rigid wall the trailing part remains intact with the rail and the node coming in contact with the rigid wall adheres to the wall throughout the animation.

2. In von-misses stress contour plot for upgraded element properties bending & twisting phenomenon is captured unlike in base element properties.

3. In the energy plot, it is seen that in case 2 the hourglass energy is zero due to hourglass stabilization in QEPH elements. In case 1 it is seen that total energy is decreasing which contradicts the law of conservation of energy. In case 2 total energy is constant.

Note:- Case 1 on L.H.S and case 2 on R.H.S

4. In the rigid wall forces plot for case1, it is seen that the total resultant force tends to zero at a few instances which are unacceptable. In case 2 this problem is solved.

5. In case 1 the energy error is found out to be -10.3% and in case 2 the energy error reduced to -0.1%. This is due to hourglass stabilization in case 2.

6. The number of timesteps in case2 is more due to QEPH elements (increased stiffness to prevent hourglass phenomenon).

What is hourglass energy?

A basic 4noded shell element consists of:-

1. 12 translational modes.
2. 6 deformation modes.
3. 3 rigid body modes
4. 3 hourglass modes.

Hourglass energy is work done by the forces calculated to resist hourglass modes. Hourglass modes take away from the physical energy of the system. Severe hourglassing may be apparent without scaling displacements. Viscous forms generate hourglass forces proportional to components of nodal velocity contributing to hourglass modes. These modes give deformation but zero strain energy since there is no strain energy recorded these nodes make the system unstable so we should get rid of these nodes. This phenomenon is relevant in coarse mesh and absent in tria elements. Hourglass energy is directly proportional to system stability.

This phenomenon can be removed by:-

1. Viscous/Perbutation method:-When there is relative motion the hourglass deformation is detected, once hourglass deformation is detected a damping force is acted to resist the deformation. This force depends upon the element properties such as stiffness, etc.
2. Physical method/Stiffness method:- In this method, the stiffness of every element is increased to resist the hourglass deformation. This method is more efficient than the viscous method.