I have written a UMAT code in ABAQUS. This UMAT degrades the young modulus of an isotropic elastic material at a point when the VonMises stress at that point reaches a specific value. It can be seen as a simple progressive failure code. I have applied this UMAT code to a Notched plate under uniform tension.
The problem is that exactly before the damage initiation, the ABAQUS/Standard diverges. I am sure that there is a very simple mistake that I just can't see!
:)
I have attached the Input file of the model and the UMAT code. I will be so thankful if anyone could help.
Thanks in advance
@ Qingqing Zhao:
Yeah. It seems that the "Recomputation Part" must be included. (See the comments above). However, there is a numerical instability in this code due to the instant reduction of the elastic modulus which can be solved by doing a gradual reduction of the elastic modulus in 10 steps. I strongly recommend to read the following reference completely. There are valuable comments in it which may be helpful for you.
Knight, N. F., Jr., User-Defined Material Model for Progressive Failure Analysis, NASA CR-2006-214526,December 2006.
I have checked the code for simple cases and it is working properly. No covergency issues are seen; however, in the case of notched plated the code diverges exactly before damage initiation!
The error is :
"Time increment required is less than minimum specified."
The minimum time increment is: 1e-8
when I remove the recomputation of the stiffness matrix and stress after damage initiation check in the code, everything seems fine and abaqus/standard converges!
(Please take a look at the "3DIsotropicElasticityVonMisesDamage, ImplicitNone.for".
Here is the part I meant:
C RECOMPUTE COMPONENTS OF STIFFNESS TENSOR
E=E*(1.D0-D)
LAMBDA=E*NU/((1.D0+NU)*(1.D0-2.D0*NU))
MU=E/(2.D0*(1.D0+NU))
CALL STIFFMAT(LAMBDA,MU,NTENS,DDSDDE)
C RECOMPUTE THE STRESS TENSOR
DO I=1, NTENS
STRESS(I)=0.D0
DO J=1, NTENS
STRESS(I)=STRESS(I)+DDSDDE(I,J)*UPSTRAN(J)
END DO
END DO
when I remove this part everthing is working, but I know that the recomputation is required, as it has been mentioned in the following reference:
Knight, N. F., Jr., User-Defined Material Model for Progressive Failure Analysis, NASA CR-2006-214526,
December 2006.
What is the correct definition of DDSDDE for the damaged material? Is it equal to the damaged stiffness matrix?
Do we have any UMAT code in ABAQUS relating to Progressive Failure Analysis on the net?
I recommend you to see, Abaqus documentation==>Examples book, 1.4.6 Failure of blunt notched fiber metal laminates. in this example you can find a .FOR file that can help you.
If you mean the attached file to this message, I have seen that before! :)
The code is too long and cumbersome! Also the criterion used there, is based on strains not stresses! :)
@ Qingqing Zhao:
Yeah. It seems that the "Recomputation Part" must be included. (See the comments above). However, there is a numerical instability in this code due to the instant reduction of the elastic modulus which can be solved by doing a gradual reduction of the elastic modulus in 10 steps. I strongly recommend to read the following reference completely. There are valuable comments in it which may be helpful for you.
Knight, N. F., Jr., User-Defined Material Model for Progressive Failure Analysis, NASA CR-2006-214526,December 2006.
Hi Hosein, would you mind I ask you one question relating to UMAT validation. What validation tests have you performed? Just incorporated into your model and plot the curves to compare with experimental results or you first test on single element model. Thanks.
You should do them both! :)
At the first step you may run the code with single element model for axial tension and pure shear. For these cases it is easy to predict the response so you can compare the response from the code with what is in your mind. In the next step you can increase the number of elements in the aforementioned cases to be sure about the code working correctly. The last step is the model verification which is implemented in the code . It can be achieved by comparing with experimental results.
Hi Hosein,
you should recovery the stresses for previous increment and update them with the increment strain of current increment with degraded stiffness matrix.
REPLACE:
DO I=1, NTENS
STRESS(I)=0.D0
DO J=1, NTENS
STRESS(I)=STRESS(I)+DDSDDE(I,J)*UPSTRAN(J)
END DO
END DO
with:
DO K1=1,NTENS
DO K2=1,NTENS
STRESS(K2)= STRESS(K2)+DDSDDE(K2,K1)*DSTRAN(K1)
ENDDO
ENDDO
Regards
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