Tensile Test Correlation
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Tensile Test Correlation Objectives: To benchmark the material cards used in ISCAE model through tensile test correlation. Confirm stress-strain curve used in the model
Overall Conclusions: Through the tensile test correlations, material cards used in the model have been understood. Definitive steps to extract true stress strain curve for MAT24 have been established. MAT24 can be used in plastic component simulation using recommended method of extracting the true stress strain
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Tensile Test Correlation  X250 IP is made from PP_GLF30 plastic, which is brittle with only around 3% failure strain. Tensile tests of the material have been carried out and typical test curves are shown below. Force-Elongation
True Stress-Strain
3.5
0.09
0.08
3
0.07 2.5
0.06
True Stress (GPa)
Force (KN)
2
1.5
1
0.05
0.04
0.03
0.02 0.5
0.01 0 0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0 0
-0.5
0.005
0.01
0.015
-0.01
Displacement
True Strain
Elongation
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0.02
0.025
0.03
Tensile Test Correlation  MAT24 is used for the material card *MAT_PIECEWISE_LINEAR_PLASTICITY $$ E Yield 101.1800E-09 6800.0 0.44 22.6 $$ True stress-strain curve 0.0 0.0 201 0 1.0
0.0
0.65
0.0
 Stress-strain curve for MAT24: LS-Dyna recommend standard method to cut and shift the test true stress-strain curve from the yield point Stress-strain curve for MAT24
Test stress-strain curve Yield
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Tensile Test Correlation  A simple CAE model of the test sample (dog-bone) is created using shell elements. Prescribed motion is defined at one end of the sample and fixed at another. The strain gauge is simulated as a spring. The stress-strain curve generated from the LSDyna method is input. Fix at one end
Strain gauge spring
Simple CAE Model
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Prescribe motion
Tensile Test Correlation With input test data, output force-displacement doesn’t match the test curve. (green vs blue curves) Mesh sizes, solid elements, integration points and loading speed have been studied. There is no significant effect on the result.
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Tensile Test Correlation Alternative MAT89 has been studied. MAT89: MAT_Plasticity_Polymer requires FULL test true stressstrain curve without “cut” and “shift”. output force-displacement with this material card gives full correlation. *MAT_PLASTICITY_POLYMER_TITLE MATL89_1 1 1.1E-9 2381.0 0.0 0.0 252022 0.0 0.0 0.0
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0.4 0 0
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Tensile Test Correlation Understanding MAT89: MAT 89 plasticity works by treating the elastic strain under plasticity separately. The elastic strain is subtracted from the true strain at any stress using the Young’s modulus (E) defined in the material cards or the local slope of the stress-strain curve, whichever is the greater. For the X250 IP substrate, E = 6800 MPa.
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Tensile Test Correlation Recommended Method of Extracting Stress Strain Curve for MAT24: Subtracting the elastic strain (green curve) from the total true strain (red curve) arrives at the true plastic strain (blue curve). Note: the negative strains on the plastic curve show the elastic response of the material and can also be used to derive the material yield stress (σy) We can now apply this methodology to MAT24. This method is referred as ISCAE Method
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Tensile Test Correlation Recommended Method of Extracting Stress Strain Curve for MAT24: Plot below shows the stress-strain curves for MAT24 derived from ISCAE method and Ls-Dyna method The effect of the elastic strain is particularly strong for high strength plastics with a low failure strain. In this condition the elastic strain has a large influence on the total true strain.
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Tensile Test Correlation Re-correlation with MAT24: With the true plastic strain curve extracted by the ISCAE method then good correlation can be achieved.
Elongation
MAT24 Ls-Dyna Method MAT24 ISCAE Method
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Tensile Test Correlation  Tensile Test Correlation For ABS Material:  This theory is applied to the tensile test correlation for X250 door trim Lustran H604 ABS, which is a much more ductile material than IP substrate
Force-Elongation
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True Stress-Strain
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Tensile Test Correlation  MAT89:  Full true stress strain from the test has input in the model. Again, good correlation can be achieved.
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Tensile Test Correlation MAT24: True stress strain curves for MAT24 are obtained by using both LS-Dyna (old) and ISCAE method (new). The results show that with a small portion of elastic strain in this material, both “old” and “new” are close to the test data. Clearly the “new” is more accurate.
Ls-Dyna Method
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Tensile Test Correlation Conclusion: MAT89 shows good correlation of tensile tests for both brittle and ductile plastic material. However, MAT89 is only available for shell elements not solid. This limits its application for Interior System CAE, as some components have to be modelled as solid elements. MAT24 is originally developed for metals, which are ductile materials with large plastic strain and high E values. This study shows that the true stress strain for MAT24 derived from the LS-Dyna method does not provide good correlation for brittle materials. The definitive extraction method or ISCAE method allows MAT24 be used accurately in both ductile and brittle materials, particularly, for plastics. MAT24 can be used either in shell or solid elements and therefore fulfils the requirement of ISCAE applications. ISCAE has generated an excel script to automatically produce the stressstrain curves from the test true stress-strain data for MAT24.
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