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Nominal strain at which yeoh material becomes unstable

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  • Nominal strain at which yeoh material becomes unstable

    I have modeled a cube of length 1 unit under uni-axial tension with hyper elastic yeoh material. The displacement force is deliberately chosen to be high enough, such that the material becomes unstable at some point during the analysis. For hyperelastic materials, Abaqus usually gives the norminal strains under which, the material is stable in *.dat file. For example, with my model the material stability calculated by Abaqus is as follows:

    ***WARNING: UNSTABLE HYPERELASTIC MATERIAL

    UNIAXIAL TENSION: UNSTABLE AT A NOMINAL STRAIN LARGER THAN 0.4300
    UNIAXIAL COMPRESSION: UNSTABLE AT A NOMINAL STRAIN LESS THAN -0.3170
    BIAXIAL TENSION: UNSTABLE AT A NOMINAL STRAIN LARGER THAN 0.2100
    BIAXIAL COMPRESSION: UNSTABLE AT A NOMINAL STRAIN LESS THAN -0.1638
    PLANAR TENSION: UNSTABLE AT A NOMINAL STRAIN LARGER THAN 0.3700
    PLANAR COMPRESSION: UNSTABLE AT A NOMINAL STRAIN LESS THAN -0.2701
    VOLUMETRIC TENSION: STABLE FOR ALL VOLUME RATIOS
    VOLUMETRIC COMPRESSION: STABLE FOR ALL VOLUME RATIOS

    But, when I plot the nominal strain NE.NE11 and stress S.S11 for the result to crosscheck how good their limit nominal strain value for uni-axial tension is, I have found out that the value at which the material becomes unstable (i.e S11 slope become negative) is 0.593437 (around) rather than the stipulated 0.4300 as shown in figure below. Why is this difference ?

    Click image for larger version

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  • #2
    What strain and stress values are you considering / plotting? The Drucker stability rule is typically based on the increment of logarithmic strain and the increment of Kirchoff stress.

    -Jorgen
    Jorgen Bergstrom, Ph.D. PolymerFEM Administrator

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