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Nachiket Kadu

Energy error of around 40%

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Hi

I am trying to simulate a model wherein i am pushing steel ring over a rubber hose. I am getting little high energy error, i suspect it to be due to contact. I tried giving igap values but there is no effect of it. can anyone please suggest some inputs. Basic Contact inputs given :Istf: 4, Igap:2, Inacti:6, Iform2. There are no initial penetration or intersections in the model

 

 

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Hi Nachiket,

If the model undergoes high plastic deformation high energy error can result in the model.

But the contact energy seems to be high in the model.

The  checks for high contact energy are:

 • Set recommended parameter for the contact definition.

   Suggested parameters for Type 7 contact are 

          Istf=4

          Igap=2

          Fscale_Gap=0.8

          INACTI=6

          Gap_min=

          Fric = 0.1

          Iform=2

 • Set “physical” contact interface gap value. Slightly increase the Gapmin value also.

If friction value is provided in the interface, please remove the same, try running and review the contact energy results.

Please share the model if the issue persist.

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Hi George

I have another query, while i am plotting any graph be it energy or the force, the nature of graph is not smooth. It is have having lots of crests and trough. Can you suggest me something for smoothing i already have applied Time history O/P freq as 0.0001. I have attached a force curve for your reference. The black curve denotes what i am expecting.

 

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Hi Nachiket,

This is because of the dynamic effects in the model.

You can capture the /TFILE with a frequency .001. I have tried and the plot obtained is attached.

t01.PNG

 

 

Regarding the high contact energy, received your files and we will review the model and will update you very soon.

 

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Hi George

Glad that initial contact energy is reduced, but as u can see in rad file ,for pressure i gave tabled value from 0.05 sec as 0.21. The reason for giving this was to first eliminate the contact energy error exclusively. But now when i give pressure from initial point itself i.e. from 0, the animation will show you bizarre results. i have attached screenshot for changes made.

 

 

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Hi Nachiket,

Normally we ramp the pressure loads on t the surface in the prescribed time span. But as you told the model will behave different when the load is ramped at 0.

I feel it's an obvious behavior of the model.

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Hi George

It starts to behave abnormally after few steps not at start itself. Moreover i ran this case in my previous model it worked well and the only problem i was facing was high contact energy. So to first encounter the problem of contact energy i gave pressure value from 0.05 sec as initial pressure will increase internal energy and so the energy error.

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Hi Nachiket,

I feel this issue is because of the load curve. The contact energy in the model is very less, and so it is not causing the issue of high energy error.

Since the load is applied from time 0 the energy error is showing high as it causes large deformation in the model. And i think that causes the weird behavior of the model.

Normally  starting from a zero load, and zero solution, we gradually ramp up the load until we achieve the desired total load. This gradual ramping up the load from a value close to zero is often a more robust approach for solving nonlinear problems.

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Hi George

I got your point, i did some changes and now energy error as well as dynamic effect which we were talking has been neutralized. Thank you for your suggestions.

 

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Hi George

I have doubt regarding change in material law. For the same simulation if i give Mat law 69 instead of mat law 42, the simulation animation results shows weird behavior. With same contact condition if u change the MAT law why this behavior is observed.

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Hi Nachiket,

/MAT/LAW42 is used to model visco-hyperelastic materials in RADIOSS. /MAT/LAW69 replicates /MAT/LAW42 but there are few differences between these material laws.

Differences are 1. LAW69 can reproduce only pure hyperelastic part, no viscous behaviour will be there for LAW69 as in LAW42.

                            2. In LAW69 we do not input the materiel coefficients but from the stress/strain curve RADIOSS automatically calculates the material parameters. This                                                       can also make the difference in the results.

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Hi Nachiket,

As in the last reply LAW69 is a pure hyperelastic materiel card where LAW42 is a visco-hyperelastic material card which will have a viscous part also with hyperelastic behavior.

Any unclear or illogical data provided during material characterization will lead to uncertain results. Choosing the right material type in performing a simulation is very important.

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