Abaqus error in nonlinear modeling of hyperelastic materials

Hello everbody,

Im a young researcher in the field of nonlinear-hyperleastic materials (elastomers). I try to simulate in Abaqus 6.10 a simple uniaxial tensile test on a dumbbell-shaped cut form specimen. For that, i use experimental test data from uniaxial tests to evaluate the material in Abaqus (Material>Hyperelastic>Unknown>Test Data>Uniaxial Test Data). For evaluation i choose the following strain energy potential forms: Mooney Rivlin (N=1), Polynomial (N=2), Neo-Hookean (N=1), Ogden (N=2 and N=3), Arruda-Boyce, Yeoh (N=3) and Van Der Waals hyperelastic models. Abaqus fits this models with my test data and return the hyperelastic coefficients. In Material Parameters and Stability Limit Information window open after evaluate material some of the models are stable for all strains and some arent (unstable hyperelastic material). A few models fits good with my test data so i choose one, Mooney-Rivlin (MR), that is stable for all strains. The MR coefficients are: D1=1.116817074E-04, C10=8.826440899E-02, C01=9.081714157E-02. Further, i use these coefficients for simulation (Material>Hyperelastic>Mooney-Rivlin>Coefficients). For mesh i use: Element type>Standard>3D Stress>Linear>Hybrid (C3D8H-8-node linear brick, hybrid, constant pressure), in Mesh Controls>Element shapeAdvacing front, Global Seeds:aproximate global size:0.3...Verify mesh>Shape Metrics (Hex elements: 12624, Min angle Size Metrics (Elements: 12624, Geometric deviation factor > 0.2: 0 (0%), Average geometric deviation factor: 1.35e-004, Worst geometric deviation factor: 0.00584). In Step>Static General>NLgeom:ON>Time period=1>Incrementation:Automatic, Maximum number of increments:100000, Increment size: initial:1, Minimum: 1E-005, Maximum: 1). As Boundary conditions (BCs): the specimen is fixed (Encastrated: U1=U2=U3=UR1=UR2=UR3=0) on the top and the load (pressure) is applied at the bottom. When i run the job with a load of 0.01 MPa and with the above increment size, the job is complete. If i slightly increase the load to 0.05 MPa, the job is aborted with the error Too many attempts made for this increment. in Job Monitor>Warnings:Excessive distortion at a total of 14 integration points in solid (continuum) elements and FORCE equilibrium accepted using the alternate tolerance, Message file are a lot of warnings, i.e. WARNING: THE SYSTEM MATRIX HAS 1 NEGATIVE EIGENVALUES, WARNING: THE SYSTEM MATRIX HAS 2 NEGATIVE EIGENVALUES, ***NOTE: THE SOLUTION APPEARS TO BE DIVERGING. CONVERGENCE IS JUDGED UNLIKELY....

I try also a second-order (quadratic) element type C3D20H, but the error Too many attempts made for this increment still persist. Also, in Job Monitor>Warnings: Excessive distortion at a total of 400 integration points in solid (continuum) elements.

Can anyone help me?

Thanks

Can you check if some of the elements get distorted?

-Jorgen