Assignment 3

Questions:

Comparison of results with base simulation and improved shell element properties

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

2. Change the number of animation steps during simulation to a minimum of 25 and 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 4 and 5.

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.

Objective: The objective of this assignment is to compare two impact models with and without the recommended element formulation properties as described in Table 1.

Table-1: Recommended element formulation properties.

Parameters	Comment
Ishell =24	QEPH 4 nodes shells are the 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	The 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.

Solution:

1. The run time in the Engine file of the crash beam model was changed to 55 ms and has been illustrated in Figure-1 (Refer to the text highlighted in cyan).

2. To obtain 25 animation steps, the frequency for writing animation files (TFREQ) was updated to 2.2 ms (= 55/25 ms). Figure-1 illustrates the engine file with the updated frequency (Refer to text highlighted in yellow). Similarly, to obtain 60 animation steps, again TFREQ was updated to 0.916 ms (= 55/60 ms).

Figure-1: Engine file with the updated run time of 55 ms and frequency for writing animated files of 2.2 ms for 25 animations steps.

3. The simulation was run for the default values as described in Figure-2a. Figure-2b illustrates the problem setup showing the crash beam and the rigid wall.

Figure-2: (a) Property card showing the default values used in the simulation; (b) Problem Definition showing the crash beam and rigid wall

(a)	(b)

4. For the simulation with default element properties, at timestep 54.64 ms, the energy error of -10.2% and mass error of 0.1659E-03 was recorded from the FIRST_RUN_001.out As the energy is negative, it signifies that the energy has been dissipated. Also, as the energy error is more that -15%, the results are acceptable.

5. Figure-3 illustrates the variation of the internal energy, kinetic energy, contact energy, hourglass energy, total energy and rigid wall forces (normal and frictional) for case-1.

Figure-3: Variation of the internal energy, kinetic energy, contact energy, hourglass energy, total energy and rigid wall forces (normal and frictional) for case 1.

6 & 7. The simulation was run again by changing the default element properties values as described in Figure-2 to the recommended values as described in Table-1. In this case, an energy error of -0.1% and a mass error of 0.1659E-03 was observed. Figure-4 illustrates the variation of the internal energy, kinetic energy, contact energy, hourglass energy, total energy and rigid wall forces (normal and frictional) for case-1.

Figure-4: Variation of the internal energy, kinetic energy, contact energy, hourglass energy, total energy and rigid wall forces (normal and frictional) for case 2.

8 & 9. Video-1 shows the variation of von-misses stress contour plots using the simple averaging method for the crash beam with Q4 (Case 1) and QEPH (Case 2) element formulation. The difference in the deformation mechanism is evident from the video which is caused due to the difference in the element formulation. Here, A maximum von-misses stress of 349.9 MPa and 349.1 MPa for case-1 and case-2 models were observed, respectively.

Video-1: Comparison of von-misses stress control plots for the crash beam with Q4 (Case 1) and QEPH (Case 2) element formulation.

Figure-4a provides a comparative study between the internal energy, kinetic energy, hourglass energy and total energy recorded for both the cases. Similarly, Figure-4b provides a comparative study between the resultant normal and tangential force recorded for both the cases.

From Figure-4a, it is observed that the total energy is the sum of the internal energy, kinetic energy and hourglass energy. Comparing the curves for the hourglass energy for both the cases, it is observed that for case 1, the hourglass energy gets accumulated over the time step caused by the default element formulation (Q4). On the contrary, for case 2 almost zero hourglass energy is registered over the time caused by QEPH element formulation due to its fundamental ability to reduce the total accumulated hourglass deformation over the time step. Therefore, the total energy for case 2 remains constant over the entire time step as there is no energy lost in the hourglass effect. Comparing the internal energy, it is observed that the model in case 2 has relative higher internal energy compared to the model in case 1. Although, the total energy absorbed by both the models at the end of the time step is similar. Comparing the kinetic energy, it is observed that the rate of diminishing kinetic energy is similar for both the models for up to 25 ms. FRom 25 ms to 55 ms, the case-2 model has a lower rate of diminishing kinetic energy relative to the case-1 model. Hence, the model in case-2 has a higher velocity relative to the case-1 model at the end of the time step.

From Figure-4b, it is observed that both the models have similar normal resultant force up to 20 ms. From 20 ms to 50 ms, the case 2 model tends to have a lower resultant normal force because of higher energy absorption due to relatively larger deformation as seen in Video-1 (00:12 to 00:22 sec). But from 50 ms up to to 55 ms, there is a spike in the resultant normal force. This spike in force is caused by the impacting of the lower member of the crash beam into the rigid wall as seen in Video-1 (00:22 sec). This signifies that the case-2 model has lower impulse in the normal direction relative to the case-1 model. Also, relative to case-1 model, the case-2 model has lower resultant tangential force up to 50 ms and again a spike is registered from 50 ms to 55 ms due to the increase in contact area as caused by the impacting of the lower member of the crash beam into the rigid wall as observed in Video-1 (00:22 sec).

Figure-4: Comparison between (a)internal energy, kinetic energy, hourglass energy and total energy for both the cases; (b) resultant normal forces and resultant tangential force.

(a)

(b)

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