Hello All,
I am trying to develop a feel for the behaviour of Creo3.0's finite friction.
So starting with a simple model
Text book concludes no sliding.
Reality tells us that the pressure distribution is not like the text book Amongst other things,the load pulling the green block acts a distance above the surface and not in the surface. It's this estimate of reality we want.
We know that the blocks are elastic (even though they are steel) and therefore one should expect a high (theoretically enormous) stress around the perimeter of the green block ... and maybe in part where problems begin.
The values chosen should ensure no sliding.
I used brick elements (tets give poor pressure plots and bricks provide a more 'stable' solution)
The problem is very 'sensitive' to the initial time step.
It will not solve using the default 0-1 ramp function with much oscillation of the residual norm, contact area reducing to zero followed by bisection after bisection then failing. The load factor is reduced to
Load Factor: 0.000195313
*** A fatal error has occurred. ***
There are 3 directions to explore
1. Load step refinement
2. Mesh refinement
3. This is a load rather than displacement controlled problem
LOADSTEP REFINEMENT:
With 10% timesteps, bisection still occurs but I get a solution and the results don't look right. Unrealistic displacement of the green block and a tangential force measure magnitude of 0.00757N where the applied load is 0.1N. The load factor is reduced by the software:
** Warning:
Excessive motion detected at contact regions.
Cutting load step size.
Load Factor: 0.025 (presumably of my 10% step)
The interface force is 7.48e-2 whereas the normal force applied is 2000N (presumed: the interface force measure is normal to the surface)
All the movement occurs in the first time step.
All this suggests poor mesh/loadstep selection.
With 1% loadsteps there is some similar behaviour, just many more graph points (not all shown) and a lot more time (14658.75 secs vs 2246.68 secs)
There is a much smaller movement, with nearly all of it happening in the first loadstep with a very small increase in displacement each step after the first time step.
but now the tangential force measure doesn't stack up. The block is being pulled with 0.1N. The integral of tangential friction force across the contacting face cannot exceed the applied load.
Interface1_any_slippage: 2.456282e-01
Interface1_area: 2.000001e+03
Interface1_average_slippage: -8.653680e-02
Interface1_complete_slippage: -2.126610e-01
Interface1_force: 3.672802e+00
Interface1_max_tang_traction: 4.273336e-01
Interface1_tang_force: 4.515394e-01
the applied load pulling the block was 0.1N, the tang force is 0.45N
The resultant pressure load applied was 2000N, interface force is 3.67N
I should (and forgot) to create a ground constraint measure. Grrr.
So this is a mesh refinement thing? Should we have a finer mesh at the loaded edge? Should rounds be applied?
MESH REFINEMENT
Try tets? smaller bricks? Mesh is everything in Non-linear.
DISPLACEMENT CONTROL
A lot of problems have to be turned 'upside down'. Rather than applying the load an enforced displacement constraint is used such that a run-away condition is cannot be encountered; the model is always fully constrained. The reaction at the constraint is the load that would have otherwise been applied. This is how the examples posted on the learning connector (and others) work.
The problem with this is that the potential movements are so small. The green steel block is 50mmx40mmx10mm and if it's entire length 50mm length was involved in the stretching, it would have a stiffness of 10x40x200,000/50=1.6e6 N/mm. When the full load of 0.1N is applied then the extension would be 6.35e-8mm. The actual extension will be less than this as is seems reasonable that slip would only occur local to the loaded face initially with the rest of the contact interface effectively isolated and not moving. Surely therefore the enforced displacement constraint must have steps in the order of 10e-9mm or lower. I then have to question the mesh resolution again even though I know there are polynomials underneath.
So the fuzzy question is, There will have been much testing on simple models such as this. Can we have some guideline re mesh, loadsteps and where it is not appropriate to apply finite friction, benchmark models?
Could my meanderings above be unpicked for a better understanding? and apologies in advance for any glaring mistakes, it has been a long (interesting) week.
En passant, what is the practical difference between asking for full results at user defined steps on the output tab and having a load that is factored by a function?
Thanks for getting this far
bfn