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Johnson-Cook Material Model, Abaqus Explicit

I am currently running a Quasi-Static compression simulation in Abaqus Explicit using the Johnson-Cook Plasticity option. As the strain gets beyond the elastic range, the stress-strain response begins to oscillate +- 5 MPa around the expected curve. Does anyone know why this is happening and/or how to repair the problem?

Thanks, 

D. Barrett Hardin

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Hi,

I'm having a very similar problem using a vumat. The element behaviour is similar to that of a cohesive element: once the strength is reached the tractions are reduced linearly to zero. As long as my strains remain below 100/150% everything works fine, beyond 100/150% I also get oscillations of about +-10 MPa around the expected curve. 

So I was wondering if you had identified the origins of the oscillations in your model or a way to fix them? Any other comment is very welcome.

 

Many thanks,

Fiz

Oscillations can be a result of the objective rate being used in the algorithm.  If I recall correctly, Abaqus uses a Green-Naghdi rate, i.e., the stress and rate of deformation are rotated to the reference configuration before the constitutive relation is evaluated.  Everything is then rotated back to spatial coordinates.  You can use a hyperelastic-plastic material model to get rid of that sort of oscillation.  See Nonlinear Finite Elements by Belytschko, Liu, Moran p. 134-136 for a brief explanation.  There's a huge amount of literature on the subject.

-- Biswajit 

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Dear
All,

I would like to study the drop behaviour of Polypropylene at -30oC from a
height of 1.2 m. I have two questions in my mind:

Q 1: Is the Johnson/Cook model is the right choice for my kind of application?
I have seen many people using this model in their publications. But my concern
is that is it good model for Polypropylene as it is thermoplastics? I have
checked the material models from Abacus and in their matreial data base they
say it is good for metals only. I can see in this model that it takes into
consideration effects of strain rate, temperature and hardening.

Q2: What kind of tests I have to perfrom to get material data for my model:
tensile, compression etc and at what temperature these tests have to be
conducted? Is it possible that I perform my test at room temperature and at a
particular strain rate and then enter this data and my model will take into
consideration the variation of temperature and strain rates based upon my
boundary conditions etc. I reallly donot understand how it works?

I would be thankful if anyone could write me about my matter.

Thanks very much

Best Regards

Shahid

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