View Full Version : Simple hydrastatic pressure sensitive plasticity model
Hello,
I am trying to model the stress field in short glass fiber reinforced Nylon. I need to model the influcence of hydrastatic pressure. My guess is that the triaxial stress state near the fiber would suppress the plasticity.
My question is how good it the internal ones by abaqus such as the extended Drucker-Prager? Anyone has had experience with it?
thank you.
Hansong
Hello Hansong,
The Drucker-Prager model might be sufficient for your needs. If it will or not will depend on what you are trying to accomplish. For example, what stresses and strains are you planning on applying? What is the maximum strain in the vicinity of the fiber ends? Are you planning on developing a micromechanical model that explicitly includes the fibers?
A hydrostatic pressure will certainly increase the deviatoric stress at which plastic flow is initiated. If the plastic flow becomes totally supressed or not will depend on the specifics of your material, loading state, etc.
Best of luck,
Jorgen
Hello Hansong,
The Drucker-Prager model might be sufficient for your needs. If it will or not will depend on what you are trying to accomplish. For example, what stresses and strains are you planning on applying? What is the maximum strain in the vicinity of the fiber ends? Are you planning on developing a micromechanical model that explicitly includes the fibers?
A hydrostatic pressure will certainly increase the deviatoric stress at which plastic flow is initiated. If the plastic flow becomes totally supressed or not will depend on the specifics of your material, loading state, etc.
Best of luck,
Jorgen
thank you Jorgen. I went out of country and did not have chance to see your reply till today. You have always been extremely helpful. I wish many other faculties/researchers offer their experties to help struggling others like you do. I certainly will follow your lead in the future.
I will develop a micromechanics model that models fiber explicitly. I am running linear elastic right now. The stress state in fiber end is very complex and triaxial. it by itself can be a paper as none has been done in true 3D. It is hard to say how it looks like, beside there is significent hydrastatic pressure so plasticity might be voiding dominating rather than shearing. I will modify some damage models that predicts the initiation of crack in matrix and put the new cohesive element in abaqus 6.5 in use for the interface. My task is to see how crack evolves using existing and simple material models, rather to spend a lot of time developing accurate ones. also an acurate plasticity model is not necessary I believe as the damage evolves pretty fast, the crack initiation and decohesive will pervade before significent plastic deformation as observed in experiments. It is also observed in continuous fiber composite materials that the high hydrastatic stress state and stress gradient near fiber favors crack much more than the plastic deformation. I would need to caputre this process in principal. at current initial stage, as long as I can show the it is the case, it is fine, accuracy is not an issue.
I have been searching constitutive model for polymer, PA and PBT specifically, but any polymer anyway, that is coded and free or very reasonally priced at least for academic purpose. But I haven't found even one except what I found here. Jorgen, would you recommend your models for my purpose? or point me to some places where I can obtain one?
The project that you are describing sounds quite interesting, and the approach that you outlined is fine. I am currently working on developing a damage model for UHMWPE and I am curious about what approaches you are considering for your work.
About good constitutive models for PA or PBT, that is certainly a sticky topic. As you know, ABAQUS is often slow at implementing new and exciting material models that are suitable for polymers. That is one of the reasons that I have developed my own set of UMAT/VUMAT subroutines (http://www.polymerfem.com/modules.php?name=User_Subroutines).
From what I know there are no free material models available. Hence, you have three options:
(1) Use one of the available models in ABAQUS (e.g. isotropic plastiticy)
(2) Purchase a license for one commercial UMAT/VUMAT subroutine
(3) Develop your own user material model from scratch.
Which of these alternative is best for you will depend on the specifics of the situation you are in and the project that you are working on.
- Jorgen
From experimental observation, the plastic deformation is not significant in fiber composites, and even it is, the dominant failure mechanisms are still the cracking, either at interface or in matrix. Therefore, I will be focusing on the crack formation and propagation instead of the polymer constitutive behavior as long as a model can provide roughly right stress field, say a model does give a positive stress at the point where there should be positive stress, but whether it is 30% over or under real stress is not a big issue.
crack propagation is relative straightforward, use max G or K. but the initiation is a problem. It can be resulted from large plastic deformation or brittle cavitation. both can happen in my opinion although experimental evidence are needed to support. I do have several possible criteria for both cases. after initiation, i will just switch to FRANC code to see how the crack propagates.
Therefore, a qualitatively correct plasticity field at crack initiation is important, as it tells me the location and the initial direction of the crack.
As to the the subroutines, would you mind point me to good resources where they have subroutines for sale for reasonable price. I don't mind paying for them if they are within my budget limit.
right now, I am using the Drucker-Prager model in Abaqus. The linear model is kind of unrealistic in the high hydrostatic tensile stress region.
unexpectly, the expoent model is very hard to converge. I don't know if it is because of my mesh is too large (several thousand elements and high stress concentration) or it is not well implemented in abaqus. I tried all kinds of control methods in abaqus, but just can't get it to converge after some point. I will try a simpler model to test the exponent model out next week.
It sounds like you have most aspects of your project figured out very well :)
About user material subroutines that are for sale - I only know of one or two places that provide subroutines, and the only one I can recommend is this site (http://usermaterial.polymerFEM.com).
You mentioned that you have convergence problems with the exponent model, what do you mean when you say "exponent model"?
Jorgen
It sounds like you have most aspects of your project figured out very well :)
About user material subroutines that are for sale - I only know of one or two places that provide subroutines, and the only one I can recommend is this site (http://usermaterial.polymerFEM.com).
You mentioned that you have convergence problems with the exponent model, what do you mean when you say "exponent model"?
Jorgen
the yield surface aq^b-p=pf as the exponent drucker-prager in abaqus.
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