Week - 10 Hyperelastic Material Models
objective
to calculate mooney rivlin and ogdens material constants and compare both using stress-strain data from a dogbone specimen tensile test with 100 percent strain.
procedure
1. the given engineering stress-strain values are tabulated in excel and the values are plotted as shown in below.
2. the given godbone model is imported in the lsprepost.
3.the boundary conditions are explained in the figure.
for the above highlighted nodes all the degree of freedom are arrested except for translation along the y axis.
4.for the above highlighted node the translation along the y axis is only arrested.this is done to achieve realistic deformation on the model. for the other end of nodes a boundary prescribed motion set is defined to apply the displacement.
the displacement curve id is shown in below.
the scale factor for this displacement definition is given as 100mm.
this value is obtained from the calculation shown below. 100 percent strain equals a stretch ratio of 2.
5. the material curve id for the two materials is shown in the figure below. this is directly obtained from the tabulated excel values in step 1.
6. the material card definition for both the material are shown in the figure below.
the value of N is to be changed after every run.the two material models are to br simulated seperately. for validation the value of N must be kept o and the obtained constants have to be plugged in to get the results .
7. the section_shell card is defined with a thickness of 1.5mm.the secton and material card are assigned to the part card.
8. the control_implicit card are defined since this analysis can be considered as a static structural problem. the termination time is given as 1 ms. the card are shown in the image below.
9. the necessary output request are placed which include database_extent _binary and *database_ascii and binary_d3plot.
10. the model is saved with a suitable name with .k extension.the sumulation is done using ls-dyna manager.
result
1. the stress contours for different cases.
2. The following constant values are obtained for N=1, 2and 3 .
3. From the d3hsp file the values of true stress and strain are obtained and they are converted to the engineering stress and strain to compare with experimental data.
Validation and constants obtained
As Discussed in procedure in the experimental card N is kept as o and the obtained constaants are plugged in and the model is simulated. In this part only for N=1 constant validation procedure is explained.
The obtained values are tabulated in excel for all three cases and results are compared with the experimental data.
For ogden rubber
2. The following constant values are obtained for N=1, 2 and 3.
3. The d3hsp file the values of true stress and strain are obtained and they are converted to the engineering stress and strain to compare with the experimental data.
Validation of constants obtained
As discusses in the procedure in the material card N is kept as 0 and the obtained constants are plugged in and the model is simulated. In this part only for N=1 constant validation procedure is explained.
the plot obtained is shown below.
the obtained values are tabulated in excel for all three cases and results are compared with the experimental data
Conclusion
The validation result different from experimental data as shown in the cases above. This is because the constants are evaluated by keeping the N values as 1,2 and 3. But while validating the N value is kept as 0. This is can lead to difference in values.
The lower lpt x strain is obtained by activating the database extent binary option. This card writes out the strain tensors.
In some cases the simulation won't converge.
Since the loading curve is defined till a strain of 1 the true values cannot be computed to a strain of 1.
The element approximately at the center of the specimen must be taken for obtaining the values of stress and strain.
The accuracy of the material model is highly dependent on the experimental values obtained also more number of simulations using various parameters has to be carried out till consistent data is obtained from the material modal.
Glink
https://drive.google.com/file/d/1DJgfyCL8An28MyylxbtzfBxaIr9iKD6U/view?usp=sharing