Explicit quasi-static contact simulation 1

[depierie]

Hey everybody,
I just joined the forum. I am a bioengineer focusing on joint implants and I am experiencing a problem with Abaqus when using the Explicit solver for a quasi-static contact simulation. My model is composed of three components: a hemisphere, a middle thick disk with a cavity (perfectly congruent to the sphere) on one side and a flat surface on the other, where it is in contact with another flat disk. My simulation is composed of two step, a initial (small) vertical displacement of the hemisphere to engage the two surfaces and then the application of a compressive load. During the load step I deactivated the displacement boundary condition but the sphere seems to push through and break the middle disk before the simulation fails. If instead of deactivating the displacement during the second step I set it to zero, the simulation gets solved but (obviously) the information on the contact pressure are lost. I think the problem might be related to the dynamics of the problem, therefore I tried to play around with inertia, springs, damping factors and contact options but it was unsuccessful so far. Is there anyone who experienced something similar before and could help me?
Thank you very very much.
Cheers,
Enrico

 

[IceBreakerSours]

What error/warning messages do you see in the .dat/.msg files?

Are you new to this forum? If so, please read these FAQ:

http://www.eng-tips.com/faqs.cfm?fid=376

http://www.eng-tips.com/faqs.cfm?fid=1083

 

[depierie]

Hi IceBreakerSours,
thank you very much for your quick reply and giving me those tips.. and sorry my late re-reply (I was abroad without a proper connection).
The warning I get in the .dat file is:
"THE OPTION *BOUNDARY,TYPE=DISPLACEMENT HAS BEEN USED; CHECK STATUS
FILE BETWEEN STEPS FOR WARNINGS ON ANY JUMPS PRESCRIBED ACROSS THE
STEPS IN DISPLACEMENT VALUES OF TRANSLATIONAL DOF. FOR ROTATIONAL
DOF MAKE SURE THAT THERE ARE NO SUCH JUMPS. ALL JUMPS IN
DISPLACEMENTS ACROSS STEPS ARE IGNORED"

I have different boundary conditions but the only one I change between one step and the following is the vertical displacement.
I applied a small vertical displacement with a smooth step amplitude in a 1st step to ensure proper engagement between the surfaces. At the moment I am working with easy and congruent surfaces but my final goal is to move to more complex interactions, therefore I need to implement this step properly.
I then deactivated this vertical displacement in a second step, as I apply compressive and shear forces and I am interested in the contact pressure pattern and how the components mutually re-position themselves. Keeping a certain fixed displacement (even if 0) active also during the second step would alter my readings of the contact pressure.

Later on during the simulation I get a second warning before failure:
"The ratio of deformation speed to wave speed in the elements in element set WarnElemDeformRateExceedsRatio-Step2 exceed the warning ratio. Refer to the status file for further details. This message is printed during the first applicable increment, but will not be printed during subsequent increments for the remainder of the step."
But this is a clear consequence of the first problem and the fact that the bottom spherical rigid component of my model act as a "bullet" and breaks through the middle component deforming it excessively.


I tried to change many parameters but now I set them back to the original model as follow:
Dynamic step1: time period=1 , bulk viscosity= default
Interactions: surface to surface explicit with weighing factor=1, penalty contact method and a penalty friction behaviour with coef=0.2
smooth step amplitude: 0:0 ; 1;1


I hope this is clear and detailed enough.

Thank you soo much for the help!
Cheers,
Enrico