ANALYSIS OF CRASH BEAM AS BASE SETUP AND IMPROVED SHELL ELEMENT PROPERTIES
Aim - The task was to compare the simulation results of the base setup and improved shell element properties for Crash Tube Analysis.
Description -
Initially, the simulation was conducted on crash tube with base setup i.e without any modification to element properties. Then, the recommended changes were applied to analysis provided by the Skill-Lync.
HOURGLASS MODE -
It occurs in under integrated shell elements because there is only one integration point at the element. When the element deforms which produces no strain, Hence no forces to resist, strain is the cause of stress then there will no stress at all. So that deformation is called Zero-Energy mode or HourGlass effect.
Energy error is negative represents that energy is dissipating.
The mass error is 0.165*10^ -3.
MODEL BASE SETUP -
- Import the model starter file (modified_0000.rad) using Import Solver Deck.
- Edit the run time as 55 ms in RUN card
- Set the T start as 25 ms and T freq as 60 ms
- Take the values of base simulation without changing the properties and Run by using Radioss Solver.
- Take the values of base simulation with recommended properties and Run by using Radioss Solver.
- All the values are Acceptable and Graphs are plotted using Hypergraph 2D.
2. The recommended properties that are provided -
The following properties were applied to property cards -
1. SIMULATION CARRIED OUT WITHOUT THE CHANGES PROVIDED
1. 4-NODED SHELL ELEMENT FORMULATION (Ishell)-
By Viscous method -
- When the hourglass deformed, Damping force applied Because of that damping that deformation tries to get stabilized but there was an Artificial Energy(Hourglass Effect) generated though its not stabilized and energy loss occured.
- Maximum Hourglass Energy generated = 1.298E+4.
- The Energy Error is Negative, so there is energy dissipation, Because of viscous method there is only one integration point on the element so the deformation of elements is not good.
- The hourglass energy is not counted in the energy balance so error occured.
- Finally the hourglass energy is not stabilized in viscous method.
2. SELECT 3 NODED SHELL ELEMENT FORMULATION (Ish3n)-
Ish3n = 2 (Default)
Description:
- No hourglass deformation.
3. SPECIFY SMALL AND LARGE STRAIN FORMULATION (Ismstr) -
Ismstr = 1 (Default)
Description: Small strain from time = 0
- Strains computed bassed on the initial element shape.
- It is usually for problems with small deformation.
4. METHOD FOR CALCULATING YIELD STRESS (Iplas) -
When the stress reaches the yield point Iplas gets activated.
Iplas = 2 (Default)
Description: Radial Return
- Radial return is done over plastic surface because it gives faster results.
- Througout the area the thickness will be constant and its not updated by strain.
- The results are not accurate.
- Maximum plastic strain = 0.954
5. THICKNESS CHANGE (Ithick) -
Ithick = 2 (Default)
Description: Thickness change is constant
- It gives faster calculation but thickness considerad as constant throughout the simulation
- Stress Integration is not computed for post processing.
- Results are not accurate.
2. SIMULATION DONE WITH THE CHANGES PROVIDED BY SKILL-LYNC
1.SELECT 4-NODED SHELL ELEMENT FORMULATION (Ishell) -
By Stiffness method -
- Maximum hourglass energy generated = 0.
- Due to adding stiffness on the material the hourglass is stablized physically.
After the simulation, the energy error were -
2. SELECT 3 NODED SHELL ELEMENT FORMULATION (Ish3n) -
Ish3n = 2 (Recommended)
Description:
- No hourglass deformation.
3. SPECIFY SMALL AND LARGE STRAIN FORMULATION (Ismstr) -
Ismstr = 2 (Recommended)
Description: Large strain with full geometric non-linearities
- It is for large problems with large deformations and strains.
- Strains computated based on the current element shape.
4. METHOD FOR CALCULATING YIELD STRESS (Iplas) -
Iplas = 1 (Recommended)
Description: Iterative projections with 3 newton iterations
- Computed the new thickness of the shell element.
- Because thickness is updated through strains.
- It gives accurate results.
- Maximum plastic strain = 0.907
5. THICKNESS CHANGE (Ithick) -
Ithick = 1 (recommended)
Description: Thickness change is taken into account
- It compute stress based on initial thickness and also thickness variation is taken onto account
- Various thicknes formulation are computed for stress integration for post processing.
- For necking or spring back , it produces accurate results.
##The reference material links are given below##
https://altairuniversity.com/wp-content/uploads/2012/04/HM_Analysis_Extract.pdf
https://forum.altair.com/topic/16509-how-to-define-thickness-for-shell-elements/