Rigid body not moving... 1

[Mohcine]

Hi,
Can anyone please advise me to why the rigid body in my simulation is not moving?
I believe I have set up everything correctly, yet it seems to stay still .

Thank you

 

[Mohcine]

I believe the problem is due to this :
"***WARNING: THE ANALYSIS NEEDS AN EXTREMELY LARGE NUMBER OF INCREMENTS (MORE
THAN 2,000,000). CHECK THE PROPERTIES OF THE MOST CRITICAL
ELEMENTS AND THE DATA FILE FOR WARNING MESSAGES.


***WARNING: THE ANALYSIS MAY NEED A LARGE NUMBER OF INCREMENTS (MORE THAN
300000), AND IT MAY BE AFFECTED BY ROUND-OFF ERRORS. RUNNING THE
DOUBLE PRECISION EXECUTABLE IS RECOMMENDED."

How can I solve it? I trying messing around with the step time and the increment time but no luck.

 

[akabarten]

Hi,

Did you ever finish the calculation?
I took a look for the inputdeck and boundary conditions and initial velocity looks ok.

I think you are right about very small time increment.
What system unit do you use in your model?

Small time step usually come from:
1. small elements in the model
2. not consistent unit system in the model.

Regards,
Bartosz

 

[Mohcine]

Thanks akabarten,
You are right, it is due to the chosen units.Since I'm using mm as my unit length, I inputted the density as 1000 E-12 (tonnes/mm^3) and Young modulus in GPa (which is wrong...it should be like stress, in Mpa).

It seems working now......However, I have a question about the Time increment and the accuracy of the results.
As a an approach to solve the problem earlier, I used the mass scaling, to get a Time increment of 0.0001 to speed up the simulation, If i choose a smaller value for time increment, would the results be more reliable?

And to answer your question, the simulation never finished, it took ages to get some numbers out, and the rigid impactor hardly moved-which made me think I set up the velocity and boundary conditions wrong.

Also, I would like to check that the units used for Mass for the rigid body is assumed in tonnes.
So to get 5kg, I have to input 0.005 in the inertia/point mass. similarly for the shear rigidity I need to input it in Mpa as the stress, right?

Sorry i'm asking too many question.

Thanks for your help.

 

[akabarten]

Hi,

To set consistent system unit You need to choose units for length/mass/time and other units will be related.
You mentioned you have mm for length, tonnes for mass and what about time?
I usually use following three system units:
1. length [mm], mass [ton], time , force [N], stress [MPa]
2. length [mm], mass [kg], time [ms], force [kN], stress [GPa]
3. length [m], mass [kg], time , force [N], stress [Pa]

So to get 5kg, I have to input 0.005 in the inertia/point mass. similarly for the shear rigidity I need to input it in Mpa as the stress, right?

You are right.

If i choose a smaller value for time increment, would the results be more reliable?

In the simplest case yes, but if you have very small time step you have a lot number of increments and then you might have wrong results due to numerical errors.
The best is to use time step which Abaqus calculate at the beginning of the analysis, it depends on the mesh and material data.
If you cannot or do not want to use bigger mesh to speed up your model you can use mass scaling.

Regards,
Bartosz

 

[Mohcine]

For the time I assumed it is in seconds....so for time period I inputted 1 second.

I have reduced this time however to a lower value, and because the body is moving very fast (4000 mm/s), the impact took place and in a total CPU of less than 20 min. which is ok, in comparison with how it was previously.
The time increment was reduced.

For the mass scaling, unless you know to use it well, it is a dangerous parameter to use . It could results in unrealistic results, so I stayed away from it .

Initially I was scaling the whole model, which is not ideal, as you only meant to scale the critical elements ( my understanding).


I have only one fear now, which is : when I mesh the part finer near the region where the impactor and the plate make contact, the element size will be smaller and the time will likely to increase.


Thank you for your help again, I really appreciated. Words can not express my gratitude.
Have an awesome new year