Center of Contact Pressure, getting incorrect results, any ideas why?
Center of Contact Pressure, getting incorrect results, any ideas why?
(OP)
I am loading a model of a human foot onto a flat plate. There is frictional contact between the bottom surface of the foot's soft tissue and the plate. The foot is about 300mm long (oriented along x axis) and I would think center of pressure would be somewhere in the middle, maybe around 100mm. The back of the heel is located at approx x=0.
I'm using the history output to get 'center of the total force due to contact pressure', specifically, XN1. I get a value of 400. That's 100 forward of the front of the foot. I don't even understand how that's possible...
Any ideas why this is happening and how might I fix it?
Thanks,
Sam
I'm using the history output to get 'center of the total force due to contact pressure', specifically, XN1. I get a value of 400. That's 100 forward of the front of the foot. I don't even understand how that's possible...
Any ideas why this is happening and how might I fix it?
Thanks,
Sam





RE: Center of Contact Pressure, getting incorrect results, any ideas why?
During the application of load the resultant force direction must vary quite a bit. Perhaps, you could apply a dummy no-load step and, in this step, request XN output history variable.
http://www.eng-tips.com/faqs.cfm?fid=376
RE: Center of Contact Pressure, getting incorrect results, any ideas why?
Thanks,
Sam
RE: Center of Contact Pressure, getting incorrect results, any ideas why?
But, in any case, I meant adding a final step wherein no load is applied. No previous loads need to be propagated.
I am not sure if this "trick" will work at all but I guess if you simply have the previous equilibrium state maintained throughout the dummy step, the force vectors at the contact surface should not change their direction at all and then, I believe, the resultant force vector (for which resultant moment is minimal) should lie somewhere under the foot.
You could also try to request CF/CM (or, perhaps, SOF/SOM) output variables and see what/where the resultant is/applied. You will need to read the documentation for details.
http://www.eng-tips.com/faqs.cfm?fid=376
RE: Center of Contact Pressure, getting incorrect results, any ideas why?
Oddly enough, the center of pressure abaqus gives is more what I would expect in the middle of this dummy step (~200mm), though the foot is only in contact with the plate at the front, kinda confusing...
Do you think XN1 is really what I want? What do the X and N stand for?
I will look into the other things you mentioned.
Thanks,
Sam
RE: Center of Contact Pressure, getting incorrect results, any ideas why?
However, the location of the CoP may not show up where you expect it to be - precisely because of the quotation from the documentation (see my first response).
As far as I can tell, three output variables exist for coordinates of pressure (or, intuitively, normal stress), frictional (i.e., shear stress) and total stress. These are given the identifiers: XN, XS, and XT, respectively. X, as you might guess, is an identifier for coordinate. See section 34.4.1.
Also, why don't you try viewing CPRESS/CSHEAR/CNORMF/CSHEARF (and CPRESSERI/CSHEARERI)? In the Viewer, you could observe how the location of the max. CPRESS or CNORMF moves as the load is applied. In any case, CPRESS/CNORMF may be more relevant to the study.
I made a mistake last time: Section force/moment output requests (SOF/SOM) are not appropriate in this situation.
http://www.eng-tips.com/faqs.cfm?fid=376
RE: Center of Contact Pressure, getting incorrect results, any ideas why?
Sorry for the delayed post and thanks again for all the help,
Sam