zero deformation passed into UMAT :( no updates.

[rm447]

Hi there, I had posted a similar question before but I stoped working on this problem and now I'm back to it and stuck with the same issue!! I hope some one can help with this.

I have written a UMAT for a time/stress adaptive material. In my UMAT I derive the elasticity tensor, stress, etc. from a strain energy function that I have picked, along with its material constants, also using the deformation gradient that must be passed in by ABAQUS. In order to make sure first that apart from the adaptive part, DDSDDE and STRESS are formulated correctly, I used a strain energy function of an isotropic linear elastic material, giving it the Young's modulus and Poisson's ratio in the input file as the constants. Well, the problem is that neither DFGRD0 nor DFGRD1 update! They remain equal to the identity matrix. As the test element, I used a CPE4 with its two lower nodes pinned, and the upper two constrained in x direction. I then tried to compress the square once using a moving BC:

*NODE,NSET=NALL
1,0.,0.,0.
2,2.,0.,0.
3,2.,4.,0.
4,0.,4.,0.
*ELEMENT,TYPE=CPE4,ELSET=umat
1,1,2,3,4
*SOLID SECTION,ELSET=umat,MATERIAL=umat
**----------------------------------------------------
*MATERIAL,NAME=umat
*USER MATERIAL, CONSTANTS=2
.15e3, 0.3
**----------------------------------------------------
*BOUNDARY
1,PINNED
2,PINNED
3,1
4,1
**-----------------------------------------------------
*Step, name=LOAD, AMPLITUDE=ramp
*STATIC
0.01, 0.10
*BOUNDARY
3,2,,-.3
4,2,,-.3

I calculated STRESS in the umat in two different ways:
1) Using DSTRAN provided by ABAQUS:
DO K1=1,NTENS
DO K2=1,NTENS
STRESS(K2)=STRESS(K2)+DDSDDE(K2,K1)*DSTRAN(K1)
END DO
END DO

2) Using a slid mechanics equation giving the stress using stress invariants and the left Cauchy-Green deformation tensor.

In both cases, DFGRD0 and DFGRD1 do remain 1, while strains are correct. In case 1, stress values are also correct since they were obtained using STRAN, but in the second case stresses are zero, since they were obtained using DFGRD. The second formulation is what I eventually need, since I will need to calculate my stresses using some modified variable obtained from DFGRD. In either case, DFGRD doesn't update and I don't understand why. Can some body provide some input? I'll appreciate it! Thanks

Here is a sample of the output to the log file:

1) First case:

1.00000000000000 0.000000000000000E+000 0.000000000000000E+000
0.000000000000000E+000 1.00000000000000 0.000000000000000E+000
0.000000000000000E+000 0.000000000000000E+000 1.00000000000000
END DEF. GRADIENT (DFGRD1)
1.00000000000000 0.000000000000000E+000 0.000000000000000E+000
0.000000000000000E+000 1.00000000000000 0.000000000000000E+000
0.000000000000000E+000 0.000000000000000E+000 1.00000000000000
CURRENT STRESSES
-6.45000000000000 -15.0000000000000 -6.45000000000000
0.000000000000000E+000
CURRENT STRAINS (STRAN)
0.000000000000000E+000 -6.843749999999998E-002 0.000000000000000E+000
0.000000000000000E+000
5/12/2009 4:52:13 PM
End ABAQUS/Standard Analysis
Begin Extrapolator
5/12/2009 4:52:13 PM
Run Extrapolator.exe
5/12/2009 4:52:13 PM
End Extrapolator
ABAQUS JOB DISP COMPLETED

2) Second case:
INI. DEF. GRADIENT (DFGRD0)
1.00000000000000 0.000000000000000E+000 0.000000000000000E+000
0.000000000000000E+000 1.00000000000000 0.000000000000000E+000
0.000000000000000E+000 0.000000000000000E+000 1.00000000000000
END DEF. GRADIENT (DFGRD1)
1.00000000000000 0.000000000000000E+000 0.000000000000000E+000
0.000000000000000E+000 1.00000000000000 0.000000000000000E+000
0.000000000000000E+000 0.000000000000000E+000 1.00000000000000
CURRENT STRESSES
0.000000000000000E+000 0.000000000000000E+000 0.000000000000000E+000
0.000000000000000E+000
CURRENT STRAINS (STRAN)
0.000000000000000E+000 -6.843749999999998E-002 0.000000000000000E+000
0.000000000000000E+000
5/12/2009 5:01:22 PM
End ABAQUS/Standard Analysis
Begin Extrapolator
5/12/2009 5:01:22 PM
Run Extrapolator.exe
5/12/2009 5:01:22 PM
End Extrapolator
ABAQUS JOB DISP COMPLETED

 

[xerf]

Have you tried to turn the NLGEOM=YES ?