Question about DDSDDE and STRESS variables in UMAT - Constitutive Models
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Wed, 08 Nov 2006 20:19:35 +0000It sounds like perhaps there is something wrong with your UMAT implementation...- Jorgen]]>Constitutive ModelsJorgenhttps://polymerfem.com/community/constitutive-models/question-about-ddsdde-and-stress-variables-in-umat/paged/2/#post-4529
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Tue, 07 Nov 2006 10:44:58 +0000I am just started writing umat for abaqus/standard. I checked with one ply such as either 0 deg. or 90 deg., my umat gives exactly the same answer as that normal abaqus simulation (without umat) using S4 shell element with *shell section, elset=XY, composite, orientation=AB ply thickness, no. of int. pt, mat. name, ply orientation (layer 1)ply thickness, no. of int. pt, mat. name, ply orientation (layer 2)etc. etc.But the moment I put two layer with 0/90, with my umat, simulation diverges out from the first increment. I checked with one layer +45 deg., results from abaqus (no umat) and same analysis with my umat differs greatly.I dont know what is happening inside my umat. I know that stresses and strains are coming inside umat are in local ply coordinate, so transformation is NOT necessary. DDSDDE is my ply stiffness matrix (or Qij matrix in terms of composite) of 3x3 dimension which involvesQ11=E11/(1.0-pr12*pr21)Q12=-pr12*E22/(1.0-pr12*pr21)Q22=E22/(1.0-pr12*pr21)Q66=g12it is a symmetric matrix.so, stress increment --> dstres=DDSDDE*dstran (dstran is strain increment)stress update is:--> stress=stress+dstressSince I did not put any damage still, so I am doing the above calculation for each gauss point in each ply.I do not know where I am still making mistakes. This umat is not running for multilayered composite plate or angle ply with S4 elements with keyword composite to include more than one plane stress layers.Can anybody provide some suggestion regarding my problem and hopefully solution of it?Thanks.]]>Constitutive Modelslesleychttps://polymerfem.com/community/constitutive-models/question-about-ddsdde-and-stress-variables-in-umat/paged/2/#post-4520
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Sun, 05 Nov 2006 17:39:15 +0000- Jorgen]]>Constitutive ModelsJorgenhttps://polymerfem.com/community/constitutive-models/question-about-ddsdde-and-stress-variables-in-umat/paged/2/#post-4512Abaqus standard - Umat for layered Composites
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Fri, 03 Nov 2006 10:50:18 +0000Hallo,I have just started writing Umat for Abaqus, with no previous experience. My aim is to write a umat for multilayered composite with progressive damage using some bilinear softenning law. I am wrting This Umat for shell element S4 available in abaqus.I think I am wrong in DDSDDE and updating STRESS. When even I am running all plies 0 deg. (4 plies total), with uniaxial tension along 0 deg. plies, it is running perfect. I have checked with abaqus reaction results with and without Umat, it is exactly the same. But whenever I am placing plies in 0/90/90/0 symm fashion, my umat does not run, from the first iteration it has got convergence problem. The loading is same uniaxial along the 0 deg plies.I am writing Umat for shell element S4 (or S4R) in abaqus. For mu umat with no damage, I am passing all ply properties in plane stress and inside umat, I am generating ply stiffness matrix, which is DDSDDE (constant), I am getting strain increment dstran, so get stress increment dstress, finally updating stress as --> stress=stress+dstressCan anyone give some tips what could be the error? Please let me know if I am not clear about the problem. Also any suggestions to check further my umat for simple cases.Thanks,]]>Constitutive Modelslesleychttps://polymerfem.com/community/constitutive-models/question-about-ddsdde-and-stress-variables-in-umat/paged/2/#post-4511
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Thu, 28 Sep 2006 21:15:41 +0000You can use either the deformation gradient or the strain tensor as the deformation measure for large deformation analysis. Typically, however, most constitutive theories are formulated in terms of the deformation gradient.- Jorgen]]>Constitutive ModelsJorgenhttps://polymerfem.com/community/constitutive-models/question-about-ddsdde-and-stress-variables-in-umat/paged/2/#post-4450
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Wed, 27 Sep 2006 15:12:16 +0000Constitutive ModelsPERhttps://polymerfem.com/community/constitutive-models/question-about-ddsdde-and-stress-variables-in-umat/paged/2/#post-4449
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Sun, 17 Sep 2006 12:44:12 +0000DDSDDE = \frac{1}{J}\frac{\partial\Delta(J\sigma)}{\partial\Delta\epsilon}or in words: DDSDDE is the partial derivative of the increment in Kirchhoff stress with respect to the increment in logarithmic strain.I believe that LS-DYNA and MSC.Marc use the same definition. Other FE software packages might use other stress and strain definitions.- JorgenAre you sure about the strain increment? Because if you look at DSTRAN, it does not contain the incremental log strains, but rather the linear strain increments...]]>Constitutive Modelszhaiyshttps://polymerfem.com/community/constitutive-models/question-about-ddsdde-and-stress-variables-in-umat/#post-4435
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Mon, 11 Sep 2006 21:01:34 +0000DDSDDE = \frac{1}{J}\frac{\partial\Delta(J\sigma)}{\partial\Delta\epsilon}or in words: DDSDDE is the partial derivative of the increment in Kirchhoff stress with respect to the increment in logarithmic strain.I believe that LS-DYNA and MSC.Marc use the same definition. Other FE software packages might use other stress and strain definitions.- Jorgen]]>Constitutive ModelsJorgenhttps://polymerfem.com/community/constitutive-models/question-about-ddsdde-and-stress-variables-in-umat/#post-4424
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Mon, 11 Sep 2006 17:19:25 +0000STRESS = active contraction stress (a function of current stretch ratio of fiber)DDSDDE = 0.Is this ok? Will convergence problem occur due to zero DDSDDE? Or, since STRESS is exactly given, I just set arbitrary non-zero DDSDDE to ensure convergence?The stress calculation has NOTHING to do with the tangent stiffness (Jacobian). The only thing the Jacobian is needed for (in an implicit analysis) is to ensure that the global equation solver produces a good guess for the next global iteration. If Newton-Raphson iterations are used and the Jacobian is consistent, this means that the solution will converge quadratically (as long as we are not too too far from the solution). In some cases, the effort in calculating a consistent Jacobian (and by consistent I mean consistent with the particular time integration procedure for the stress) is numerically involved. Therefore, people cheat and use a Jacobian that is much faster to calculate, but then at the expense of convergence rate in the global iterations. In general, I personally think this is a bad idea, because the robustness is compromised --you are fine in many cases, but all of a sudden, you are not. The best thing is to come up with a good Jacobian.A side note: I think the ABAQUS manuals explain this really poorly, if at all. In large deformation analysis, what is DDSDDE? Whats the stress and whats the strain? I am not sure. Anyone?]]>Constitutive Modelszhaiyshttps://polymerfem.com/community/constitutive-models/question-about-ddsdde-and-stress-variables-in-umat/#post-4420
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Mon, 04 Sep 2006 18:59:14 +0000- Jorgen]]>Constitutive ModelsJorgenhttps://polymerfem.com/community/constitutive-models/question-about-ddsdde-and-stress-variables-in-umat/#post-4404