Error in FAIL_JOHNSON

Altair Forum User

Hi

I am trying to simulate ship collision with Failure criteria. For that I tried with simple plate impact simulation.

 

I used Johnson Failure prop. Difficult to input the value D1, D2, D3.... Anyhow managed to input it from example given. But don't know the significance of these inputs.

 

Than the ERR shows very high as attached image below.

 

The animation also shows some unusual behavior.

 

I tried with Ixfem=1 (as i want to see the proper cracks formation), but it shows error, so goes with Ixfem=0.

 

Attaching the deck also. Please check any error it it and suggest your valuable comments and any modification needed.

 

 

Thanking You.

Unable to find an attachment - read this blog

Altair Forum User

Hi,

Regarding the damage parameters (D1, D2 and D3), for the Johnson-Cook failure model, there are three parts of material tests based on which these 3 values are calculated.

 

<?xml version="1.0" encoding="UTF-8"?>

 

In a simple case, only the first part, triaxiality influence is considered. Then failure strain is:  <?xml version="1.0" encoding="UTF-8"?>

 

I just checked your model. You can run with Ixfem=1, and I think the error was due to Ifail_so option. You have activated that option but there are no solid elements in the model as this option is a solid failure flag. You can deactivate that option. Use a Dadv value of 0.5, which is used for crack advancement. 

 

Unfortunately in your model these damage parameters are not causing failure and moreover high deformation is happening which leads to time step drop. 

Try with a different type of failure parameters and/or material parameters. Else you can try with basic rupture criteria (Eps_max) in material card.

 

I'm suggesting you a video tutorial on damage evolution in RADIOSS which you can access at https://altairuniversity.com/conceptual-design-of-a-3-wheeler-motorbike/conceptual-design-of-a-3-wheeler-motorbike-stone-impact-simulation/

 

I slightly modified some values (both material and damage values) and cracking has initialized.

 

<?xml version="1.0" encoding="UTF-8"?>

 

Altair Forum User

Hi George,

 

Thanks for your prompt reply.

 

But still the crack doesn't look realistic and still material deletion is there.

 

How to estimate the Parameters in failure criteria.

 

As you said:

Altair Forum User said:

Unfortunately in your model these damage parameters are not causing failure and moreover high deformation is happening which leads to time step drop. 

 I am clueless about this and how to rectify this. I just taken random valued to set the methodology. If possible modify the values as per recommendation and set the methodology.

 

meanwhile I will go through the video tutorial.

 

thanking You.

Altair Forum User

Hi,

The crack will be initiated and finally the element will be deleted once the damage values reach the one integration point of the element. That is the failed elements are removed.

The realistic crack propagation actually depends on the damage parameters we provide and users has to find these parameters manually. Many open web resources are available on Johnson-Cook damage parameters and please go through the same which will give you some idea on these parameters. 

In the video tutorial a different approach is taken where the property used is sandwich shell (where multi-layer XFEM will be applied) and /FAIL/FLD is used, where the user has to get the forming limit diagram after experimental tests and based on the same damage is initialized in the material model.

Altair Forum User

Thanks George,

 

In my simulation I am using normal steel of standard grade.

Can you please guide me what should be the inputs (may be in limit) for LAW2 parameters (a, b, n..) and failure parameters.

 

It will be a great help for my reference.

 

Thanking You.

Altair Forum User

Hi,

For /MAT/LAW2 , KEEP Iflag=1 which is the simplified input flag. With this flag active enter Yield Stress, Ultimate Tensile Stress and Engineering strain at UTS.

Internally RADIOSS will calculate the a, b and n values based on these.

 

 

You can input the failure plastic strain (Eps_max) so that once the material reaches this plastic strain value the same will be deleted. As a trial you can try with small Eps_max (as an example 0.1) and can check the rupture happening.

Altair Forum User

Thanks George,

Do we need to input Sig_max also?

Is Eps_max and Sig_max is the value of strain & stress respectively where material ruptures? (I mean the last point of stress strain curve)

Altair Forum User

Hi,

Sig_max is not mandatory for failure modelling. If you input a value for this option then during the analysis the maximum stress developed for that material will be your input.

Altair Forum User

Hi George,

 

Just for Information, the value of a, b & n is automatically generated by RADIOSS when we used LAW 2 with Iflag=1.

In some other discussion you attached a file (named: b&n_Law2.xlsx) to calculate a, b & n. But when I am using that file to calculate the values it is not matching with the value auto calculated by RADIOSS.

 

This is the out file generatid with a, b & n:

Altair Forum User

Hi,

That excel sheet was used long back before this Iflag option was introduced. And much developments also happened with RADIOSS as they optimized many parameters for better simulation and results. We don't recommend to use the excel sheet now as we have the simplified input available.