Week - 9 Material modeling from raw data

MATERIAL MODELING FROM RAW DATA USING LS-DYNA

AIM: To create a material model using the data extracted from the given true stress-strain curve of graphite iron casting and validate it using the dogbone specimen.

Note: The unit system used is kg-mm-ms.

PROCEDURE:

1. Data Extraction:

The given image file of true stress-strain graph of graphite iron casting is opened in data digitizer software to extract the data and export it as excel file.

Fig.1 Data extraction of curve 2 using data digitizer.

The curve 2 is chosen to extract data of stress-strain of graphite iron casting as shown in fig.1.

Note: Ensure the data points picked is evenly spaced so as to get smooth curve and better convergence.

Fig.2 Plot of true stress vs true strain of curve 2.

The stress-strain data file obtained from data digitizer is opened in excel to plot a graph of true stress, (GPa) vs true strain as shown in fig.2. From the graph the yield stress value is taken as 0.151 GPa and the region beyond is considered as plastic region.

2. Part definition:

Fig.3.1 Material property.

The material card chosen is MAT_24_PIECEWISE_LINEAR_PLASTICITY to define the behaviour of an elasto-plastic material with an arbitrary stress versus strain curve and arbitrary strain rate dependency. With this material model it is possible to consider the effect of the strain rate.

The values of density and Poisson’s ratio is taken as generic values.

The given value of young’s modulus = 20.9E+06 psi = 144.1004 GPa.

The yield stress value from the graph = 0.151 GPa.

Fig.3.2 LCSS curve definition for EPS vs ES

The plastic behaviour of the material is defined using LCSS curve option in the material card. The curve is defined by inputting the values of effective plastic strain and effective stress (true stress).

The Effective Plastic Strain = True Strain – (True Stress/Young’s Modulus).

Note: To define LCSS curve the stress values beyond the yield stress values are considered.

Fig.4 Section property.

The section property of dogbone specimen is assigned as shell element with 2 mm thickness and ELFORM=16.

Fig.5 Part definition.

3. Boundary Condition:

Fig.6 The nodes at the fixed end constrained in X and Z direction.

The nodes at the fixed end is constrained in X and Z direction only and Y direction is not constrained because of lateral expansion during tensile test.

Fig.7 The nodes at the middle constrained in Y direction.

The nodes at the middle is constrained in Y direction since the neutral axis passes through the middle of the specimen in X direction.

Fig.8.1 Boundary prescribed motion in the pulling end of specimen.

The nodes of the pulling end are assigned with a boundary prescribed motion in X direction using displacement load curve as shown in fig. 8.2.

Fig.8.2 Displacement load curve.

4. Database Options:

Fig.9 Database options for requested output of the results.

The time step value for the BINARY_D3PLOT and DATABASE_ASCII option for GLSAT and ELOUT is given as 0.01 ms.

DATABASE_EXTENT_BINARY card with STRFLG =1, is used to compute the elastic strain in the model.

5. Control Functions:

Fig.10 Control functions for implicit analysis.

The tensile test of given dogbone specimen is considered as quasi-static analysis, hence implicit analysis is carried out with necessary control implicit cards as shown in fig.10. The Initial time step size for implicit analysis, dt0 =0.025.

Note: The value of dt0 should be chosen wisely for better convergence.

RESULTS AND DISCUSSIONS:

1. The animation of Von-Mises stress contour.

The animation of stress and strain contour resemble realistic simulation of tensile test. The maximum v-m stress value is 0.27085 GPa which is higher than the yield stress value of 0.151 GPa. Hence, plastic deformation is occurring in the specimen. The maximum stress is developed in the middle region of specimen but necking is not observed till the end of simulation.

2. The animation of Effective Plastic Strain contour.

The maximum effective plastic strain observed is 0.00756. Due to plastic strain the cross section of specimen is decreasing at the middle of specimen.

3. X-Stress plot.

Fig.11 X-Stress on element 330 of dogbone specimen.

4. X-Strain plot.

Fig.12 X-Strain on element 330 of dogbone specimen.

5. True Stress vs True Strain plot.

Fig.13 True Stress vs True Strain plot of tensile test on dogbone specimen.

The true stress vs true strain plot obtained from simulation of tensile test on dogbone specimen is as shown in fig.13 which resembles the given true stress vs true strain plot.

6. Validation of Results:

Fig.14 Validation of simulation results with raw data.

From the graph, the curve of raw true stress vs true strain plot exactly fits with simulation result curve of true stress vs true strain plot with dt0 = 0.0025. There is a deviation for simulation results with dt0 equal to 0.001 and 0.005 due poor convergence within the given initial time step size.

CONCLUSION:

1. The stress-strain data was extracted from the given diagram using data digitizer.
2. The extracted stress-strain data is used to model material using MAT_24 material card.
3. The material model is validated using tensile test on dogbone specimen by implicit analysis.
4. Learned to model material from raw data and validated it.

Google Drive Link: MATERIAL MODELING FROM RAW DATA USING LS-DYNA