Week-6 Calculate the Stretch Ratio by comparing the ELFORM (-2,-1,1,2) with Ogden_Material Model.
CALCULATING THE STRETCH RATIO AND COMPARING THE ELFORM-2,-1,1,2) WITH OGDEN MATERIAL MODEL USING LS-DYNA
AIM:
i. To Create a block of 10 mmx10mmx10mm dimension with 10 elements for each
direction and use the material card attached (Ogden_Material.k).
ii. To Use appropriate boundary conditions to simulate uniaxial tensile behaviour for the
model.
iii. Compare the results of the simulation with ELFORM = 1, 2,-1,-2 by plotting Engineering
Stress vs Stretch Ratio up to stretch ratio 5.
iv. To use a proper solver (Implicit/Explicit).
PROCEDURE:
The given LS-Dyna keyword file is opened in LS-PrePost using option File>Open>LS-Dyna Keyword File
imported keyword file
The imported keyword file consists of only Ogden material card. The FE model of a solid block of 10mmx10mmx10mm dimension with 10 elements for each direction is created as
FE model of solid block with required dimensio
PART DEFINITION:
Section properties:
Selection Solid
The section properties of the FE model created is assigned as solid element with different ELFORM such as,
Case (1). ELFORM = 1, constant stress solid element.
Case (2). ELFORM = -1, Fully integrated S/R solid intended for elements with poor aspect ratio, efficient formulation.
Case (3). ELFORM = 2, Fully integrated S/R solid
Case (4). ELFORM =-2, Fully integrated S/R solid intended for elements with poor aspect ratio, accurate formulation.
Note: The simulation has to be executed for each case separately.
Material properties:Keyword manager>MAT>077_O-OGDEN_RUBBER
Ogden material
The above material card is already provided as a keyword file and it is assigned to the solid block part
Note: The unit of u in the material definition is in MPa and a is a dimensionless quantity.
Part definition:
Part Definition
BOUNDARY CONDITIONS:
To perform uniaxial tensile test on the specimen, the nodes of one of the faces is fixed as shown in the following figures
Nodes fixed in X-direction
Middle nodes along Z-direction fixed in Y-direction
Middle nodes along Y-direction fixed in Z-direction
Nodes in the moving face for applying load
The minimum load to be applied on the moving surface to attain a stretch ratio of 5, is calculated by using following relation,
Engineering strain, €e=δl/l
Stretch ratio, λ=1+€e
5=1+δl/10
δl = 40mm,
Hence, minimum value of 50mm is taken to define the load curve.
Load Curve
Boundary prescribed motion set card
BOUNDARY_PRESCRIBED_MOTION_SET card is used to apply the boundary condition on the moving surface.
CONTROL FUNCTION:
Control implicit auto
CONTROL_IMPLICIT_AUTO card is used for automatic time step control during implicit analysis. Input value for Automatic time step control flag = IAUTO = 1.
Control implicit general
CONTROL_IMPLICIT_GENERAL card is used to activate implicit analysis and define associated control parameters. This keyword is required for all implicit analyses.
Input value for IMFLAG=Implicit/Explicit switching flag = 1 (implicit analysis)
DTO=Initial time step size for implicit analysis = 0.01.
Control implicit solution
CONTROL_IMPLICIT SOLUTION card is used to specify whether a linear or nonlinear solution is desired. The default values are set as it is in the card.
Control implicit solver
CONTROL_IMPLICIT_SOLVER card is used for implicit calculation.
Control Termination
The control termination function is enabled to specify the end time of simulation. The termination time is set for 1ms.
DATABASE OPTION:
Database binary_D3plot
The time step value of 0.01 ms is given for the BINARY_D3PLOT and DATABASE_ASCII option for GLSAT and ELOUT.
Database Binary Extent
DATABASE_EXTENT_BINARY card with STRFLG =1, is used to compute the elastic strain in the model.
Databse history solid
DATABASE_HISTORY_SOLID card is used to compute the stress/strain of a 550 node in the model.
The keyword file created is checked for errors using the option keyword manager model check. The keyword file is saved using 'k' extension and is made to run in the solver by getting normal termination message for different cases of ELFORM.
RESULTS: The D3plot output file is opened in LS-PrePost using option File>open>LS-Dyna binary plot.
1. Von-Mises stress countour is shown below
Case(1) : Elform 1
Case(2) Elform-1
Case (3) : Elform 2
Case (4) : Elform-2
2. Effective plastic strain contour is shown below
Case(1) : Elform 1
Case(2) : Elform-1
Case(3) : Elform 2
Case(4) : Elform-2
It is observed from the stress contour that the maximum value of X-stress for case 1, 2, 3 and 4 are 5.025 MPa, 5.0239 MPa, 5.0312 MPa, 5.0239 MPa respectively. The maximum value of X-strain for case 1, 2, 3 and 4 are 1.7885, 1.7878, 1.789, 1.7879 respectively. There is a small variation in the stress value but the strain value is almost same for different cases.
3. Calculation of Engineering stress/strain and stretch ratio.
In LS-DYNA the output of stress and strains from Post Fringe Comp Stress/Strain is given as true stress and strains. To find the engineering stress/strain and stretch ratio following relations are used,
We know that, True stress,σt =σe (1 + €e)
Engineering stress,σe = (1 + €e)
Similarly, True strain,€t = ln(1 + €e)
Engineering strain,€e = e€t - 1
Stretch ratio, λ = 1+€e
Engg.Stress Vs Stretch Ratio plot for Elform 1
Engg.Stress Vs Stretch Ratio plot for Elform-1
Engg.Stress Vs Stretch Ratio plot for Elform 2
Engg.Stress Vs Stretch Ratio plot for Elform-2
From the graphs plotted for Engg. Stress vs Stretch Ratio for different cases, the Engg. stress value corresponding to a stretch ratio of 5 is 0.8 MPa approximately. The output from different cases of elforms gives approximately same results since the model and loading conditions are simple.
Nominal Stress Vs Stretch Ratio plot for Ogden material
The actual plot for nominal stress vs stretch ratio for ogden material is shown in fig. 19. In the actual plot the value of nominal stress for a stretch value of 5 is approximately 1.8 MPa but from the simulation result the value of stress is 0.8 MPa. It is observed that a variation of 1 MPa is caused in the simulation for uniaxial tension test for ogden material because of nonlinearity of the material and relatable boundary condition applied for the model.
CONCLUSION:
i. A FE model of solid block was created with ogden material to simulate uniaxial tension test for different cases of ELFORM (1,-1, 2,-2).
ii. The Engineering stress/strain and stretch ratio was calculated to plot a graph of Engg.stress vs stretch ratio up to stretch ratio 5.
iii. The Engineering stress value for a stretch value 5 from the simulation for different
ELFORM was approximately equal to 0.8 MPa, but from the actual graph it was equal to 1.8 MPa with a difference of 1 MPa.
iv. The uniaxial tension test was solved using implicit solver since the analysis is similar to a quasi-static analysis on a hyperelastic non-linear material with slow steady loading conditions.