Mohr-Coulomb - associative problem

[Elisabet82]

Hello
I want to start by saying that I am new to ABAQUS.

I am trying to set up a passive earth pressure problem in ABAQUS. I am using Mohr-Coulomb plasticity in my model as the material properties. I want the model to be associative, (that is friction angle = dilation angle). But I always get an error message that states.

" INCREMENT 27 STARTS. ATTEMPT NUMBER 3, TIME INCREMENT 1.000E-10

***WARNING: THE PLASTICITY/CREEP/CONNECTOR FRICTION ALGORITHM DID NOT CONVERGE
AT 1 POINTS

***NOTE: MATERIAL CALCULATIONS FAILED TO CONVERGE OR WERE NOT ATTEMPTED AT ONE
OR MORE POINTS. CONVERGENCE IS JUDGED UNLIKELY.

***ERROR: TIME INCREMENT REQUIRED IS LESS THAN THE MINIMUM SPECIFIED"

If I make the model non-associtative, it runs fine.
Is there something I am forgetting or doing wrong?

Really hope someone can help me.

Thanks
Elisabet

 

[xerf]

The convergence of the solution depends on the non-linearity of the material response.The fact that the analysis is convergent for one constitutive response has nothing to do with not being convergent when using another constitutive response since one constituve response can be more non-linear than another.

-You could try even smaller time increments: i.e. if you use automatic incrementation allow for smaller minimum and initial time increments. (though it is already quite small 1.e-10).

-Set NLGEOM=ON, if the structure undergoes significant deformation.

-Make sure the load is ramped, not instantaneous.

-If the structure deforms fully plastically then it is not unusual to have convergence difficulties in implicit codes.

-If no luck, you should consider using ABAQUS/Explicit

 

[Elisabet82]

Thanks xerf
This helped a lot. Now I can get the test to run but after up to 6 hours it crashes, because the dilation angle is equal to the friction angle.
The warning message says: "A dilation angle of 38.700 may result in decreasing plastic work or unstable material behavior at high confining stress states. Set the dilation angle less than 36.298 to ensure stable material behavior under all loading conditions"

I want the material to have the friction angle and dilation angle equal 38.7°.
Does anyone know if this is possible or not in ABAQUS?

Thanks
Elisabet

 

[Cansand]

You are trying to use associative flow rule (AFR). May I ask you what materials do you have.. cohesive or noncohesive and under what drainage condition you are talking about . Associative flow rule is sometimes hard to converge due to the accessive (irrelaistic) volume change shown at failure that this rule results in. Loosely speaking Associative rule is only realistic when having cohesive materials (clay) subject to undrained condition.
Why not try to use a Drucker Prager model; e.g, linear version, you can easily transfer the strength parameters.
ALso we should not forget that Mohr Coloumb Yield function (in the deviatoric plane) exhibits vertices that results in divergence.