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Assignment 3

Objective The aim of the project is to analyze the crash beam file by setting up different property conditions for the file and report on the results. Task The baseline of the file is run and then the property parameters are set and the analysis is run. The energy error and mass error checks and results are compared. …

Aravind Subramanian
updated on 14 Jul 2020

Objective

The aim of the project is to analyze the crash beam file by setting up different property conditions for the file and report on the results.

Task

The baseline of the file is run and then the property parameters are set and the analysis is run.
The energy error and mass error checks and results are compared.

Case 1

The First run_000 model is imported and the model is analyzed using radioss.

Model

Analysis --> Radioss --> path of the file is saved --> radioss. This option is used to run the analysis.

In this case, the Q4 element is used for analysis it is the reduced integration and uses 1 integration point. The error is due to the hourglass energy.

In this method, the hourglass error is not considered since it cant be calculated using the 1 integration point. The hourglass error account for 10 - 15% in the analysis. The error values are around 11% so the results are at the acceptable levels.

Case 2

The case the property of the shell is altered & the analysis is done it uses full integration method.

Parameters		Comment
I_shell=24		QEPH 4 nodes shells are the combination of cost and accuracy.
I_smstr=2		Full geometric nonlinearities (default)with possible small strain formulation activation in RADIOSS Engine.
I_sh3n=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.
I_thick=1		Thickness change is taken into account for accuracy.
I_plas=1		Iterative plasticity for good accuracy.

In this case, the QEPH method is used for analysis, and the hourglass is considered so the error value are very less. The mass increase is constant and variation is also less.

Hourglass

Hourglass modes are element distortions that have zero strain energy. Thus, no stresses are created within the element. They produce linear strain modes, which cannot be accounted for using a standard one-point integration scheme.

Energy error

Where,

E_{k}

Translational kinetic energy at current time

E_{k}^{r}

Rotational kinetic energy at current time

E_{i}

Internal energy

E_{w k}

Work of external forces (energy brought to the system)

E_{, 1}

Energy at beginning of the RUN (not at time t=0)

The Energy Error computed by Radioss is a percentage.

If the error is negative, it means that some energy has been dissipated.
In the case of under integrated elements (Belytschko shells, solids with 1 integration point), the Hourglass energy can also explain a negative Energy Error since it is not counted in the energy balance. The normal amount of Hourglass energy is about -10% to -15%.
If the error is positive, there is an energy creation.

In the case of using QEPH shell formulation (I_shell=24) or fully integrated elements, the Energy Error can be slightly positive since there is no Hourglass energy and the computation is much more accurate. An error of +1% or +2% is acceptable.

Mass error

In this case, the added mass is totally made at time t=0. It can also be due to options for a constant time step.
It is also important to post-process this added mass in order to check that it is not too large locally since this could mean false results

Results

Displacement Contour

Case 1 Case 2

The node of the maximum & minimum value is the same but the magnitude is less in case 2 & the analysis of case 2 is accurate.

Hourglass energy

Case 1 Case 2

The hourglass energy in case 2 is zero this is type of the element used since in case 2, 5 integration points are used to calculate the deviation produced by the hourglass & it is considered in the analysis so the value becomes zero at the end. In case 1, 1 integration point is used for the calculation so the deviation produced by the hourglass is not considered in the analysis.

Total energy

Case -1

Case - 2

The total energy of the system must be constant but in case 1 since the hourglass energy is not considered so there is a deviation in the total energy. In the case all the parameters are considered so the total energy of the system becomes constant.

Rigid wall forces