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

***WARNING: UNSTABLE HYPERELASTIC MATERIAL

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 ?

***.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 ?

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