Week - 8 Mass Scaling

Aim :

Use mass scaling to reduce the runtime of a model and ensure the stability of mass scaling. The hard limit on mass scaling is 8% and stability should be completely intact.

Objective:

• The hard limit on mass scaling is 8% and stability should be completely intact.
• The DT and TSSFAC parameters should be optimized.
• Plot a histogram to compare the runtime with mass scaling trials.
• To run the same model using implicit solver with necessary changes and compare implicit and explicit runtime.

Procedure:

Case 1:- Explicit Analysis(With Mass-scale)

The given (.k) file of the model has been opened on ls-prepost.

• 

  The model has been checked for any errors and it's been saved and run using ls-program manager.

• Here the model was run without any mass scaling and the total time estimated is around 54hrs.

1

• The model was again opened in ls-prepost and then checked the*CONTROL_TIMESTEP card. The default values for TSSFAC and DT2MS were given 0.9 and -3.5e-05 respectively.
• Now DT2MS value has been incremented by a unit one keeping TSSFAC to a constant value (0.9).
• A trial and error method was followed and the process has been continued until it shows a percentage increase of around 8%.

2. TSSFAC= 0.9 and DT2MS= -4.5e-05

3. TSSFAC= 0.9 and DT2MS= -3.154e-4

4. TSSFAC= 0.9 and DT2MS= -1e-3

5. TSSFAC= 0.8 and DT2MS= -3.154e-4

6. TSSFAC= 0.7 and DT2MS= -6.5e-4

7. TSSFAC= 0.7 and DT2MS= 1.01e-4

8. TSSFAC= 0.7 and DT2MS= 1.02e-4

Histogram for the Above Mass Scaling Trails-

Sr. No.	DT2MS	TSSFAC	Mass Percentage	Computation Time
1	-3.5e-5	0.9	0	54hrs
2	-4.5e-5	0.9	0	43hrs
3	-3.154e-4	0.9	8.103	5hrs 49min
4	-1e-3	0.9	8.952	4hrs 1min
5	-3.154e-4	0.8	8.103	6hrs 36min
6	-6.5e-4	0.7	3.731	7hrs 49min
7	1.01e-4	0.7	1.138	16hrs 21min
8	1.02e-4	0.7	1.161	16hrs 11min

Case-2 Implicit Analysis:

Generally in LS pre-post, the solver practices an Explicit method for all the FE models. In order to perform an Implicit method, the solver requires the following keywords to switch from Explicit to Implicit analysis.

• *CONTROL_IMPLICIT_GENERAL: The keyword makes sure that the analysis will start in an implicit manner since the default analysis mode for ls Dyna is explicit. Here the IMFLAG is set to 1 to activate the implicit analysis.
• *CONTROL_IMPLICIT_AUTO: Used to change the time step for the given problem. In implicit the time step can be changed since its unconditionally stable but in explicit analysis, the time step is the condition beyond a certain value. Here IAUTO is kept as one.

• From the above image, there is no mass added during the computational time, also attached with CONTROL_TIMESTEP.
• In this method, the computation time is less then 1 seconds maximum for this setup case.

Conclusion

• The mass scaling for the given model has been performed as per the objective and compared the results.
• Its been found that when a mass scaling factor is applied, the mass of the elements with a time increment below a certain value is scaled. By adding a mass to these critical elements, the calculation time is decreased.