ABAQUS/Explicit: Guideline on Hourglass control energy limit?
ABAQUS/Explicit: Guideline on Hourglass control energy limit?
(OP)
What is the guideline on Hourglass control energy limit?
Somewhere its suggested to keep it less than 1-2% of the Internal energy; somewhere else its less than 5% of Strain energy and somewhere else its said to be less than 10% of Total energy.
We need a definite guideline on this, a fixed parameter and a fixed value.
Somewhere its suggested to keep it less than 1-2% of the Internal energy; somewhere else its less than 5% of Strain energy and somewhere else its said to be less than 10% of Total energy.
We need a definite guideline on this, a fixed parameter and a fixed value.





RE: ABAQUS/Explicit: Guideline on Hourglass control energy limit?
This is a tough one because I think this is where prior experience and skill come in to play; some may call it voo-doo but I want to think of it as art. As long as the numerical juice (non-physical energies: hourglass, stabilization, .. ) makes a tiny fraction of the physical energies in the simulation, then I feel pretty good. So, depending on the problem, I may rely on high school/early undergraduate level physics, experience (my own, others', published work, etc.), and intuition for this one.
*********************************************************
Are you new to this forum? If so, please read these FAQs:
http://www.eng-tips.com/faqs.cfm?fid=376
http://www.eng-tips.com/faqs.cfm?fid=1083
RE: ABAQUS/Explicit: Guideline on Hourglass control energy limit?
"Verify that the artificial energy used to control hourglassing is small (<1%) relative to the internal energy. Use the energy output quantities ALLAE and ALLIE to verify this. The exception to this rule is the case of elastic bending with a coarse mesh when enhanced hourglass control is used."
Generally I would agree with the statement above though - if you have to introduce artificial energy in an analysis, make sure its a small fraction of the physical energies.
RE: ABAQUS/Explicit: Guideline on Hourglass control energy limit?
Until now I have used C3D8I to get rid of all types of element related shortcomings, but it has 8 times the number of integration points than C3D8R and it is taking ages to complete a simulation run (11 days to be precise). Using C3D8R will surely help me actually get something done other than staring at the screen for eternity.
RE: ABAQUS/Explicit: Guideline on Hourglass control energy limit?
RE: ABAQUS/Explicit: Guideline on Hourglass control energy limit?
RE: ABAQUS/Explicit: Guideline on Hourglass control energy limit?
In your paper you can state the following:
"The energies were monitored throughout the duration of the analysis and ALLAE was found to be X% of ALLIE which is above the recommended threshold of 1%. However when an identical analysis was completed using C3D8I/C3D8 elements, no difference was observed in the force-displacement response."
In my own experience in my field, this level of detail is rarely included in journal articles.
RE: ABAQUS/Explicit: Guideline on Hourglass control energy limit?
The Enhanced hourglass control is said to be winner allover, but I often find Relax stiffness to produce better result. I think Relax stiffness is the default for C3D8I/C3D8/C3D8R in Abaqus as well?
RE: ABAQUS/Explicit: Guideline on Hourglass control energy limit?
Hourglass control for C3D8R:
So it seems that Relax stiffness produces better result than Enhanced hourglass control. Any comments?? Also, is Relax stiffness the default hourglass control for C3D8R element?
RE: ABAQUS/Explicit: Guideline on Hourglass control energy limit?
Have you compared ALLAE and ALLIE for both analyses?
RE: ABAQUS/Explicit: Guideline on Hourglass control energy limit?
For left image -
% Relax Stiffness (ALLIE) = (ALLAE/ALLIE)*100%
% Relax Stiffness (ETOTAL) = (ALLAE/ETOTAL)*100%
Same calculation for the right image (% Enhanced).
Do you think ALLAE values are too much??
RE: ABAQUS/Explicit: Guideline on Hourglass control energy limit?
RE: ABAQUS/Explicit: Guideline on Hourglass control energy limit?
It will not match with experimental results in Continuum scale, needs multiscale analysis as the structure is at micron scale actually. :)
So for now my target is to use C3D8R elements instead of C3D8I, but to keep the mesh coarser and get the same load-displacement result. I selected a nominal element size of 0.075mm based on the below meshing parametric study -
Note that some parameters have been scaled (by multiplying with a fractional number) to fit the range between 0 to 10. Do you have any comment regarding this novel idea on element size selection? :)
Now with this 0.075mm element size:
Load-displacement: (C3D8I vs C3D8R)
Hourglass control for C3D8R:
RE: ABAQUS/Explicit: Guideline on Hourglass control energy limit?
"A good way to reduce hourglassing is to refine your mesh"
Also, it seems stiffness-based controls are prefereable for drop test analyses:
"As a general rule, stiffness-based algorithms are more effective for lower energy simulations like drop tests"
Finally, in your first chart, you do not include the variation of any field output or variable of interest? So its impossible to say whether your mesh is converged at 0.075mm?
RE: ABAQUS/Explicit: Guideline on Hourglass control energy limit?
Here is the latest meshing, does it look good enough? -
One thing to note that, Whatever hourglassing I'm using, the displacement hourglass scaling factor is set to 1. Should I change it?
RE: ABAQUS/Explicit: Guideline on Hourglass control energy limit?
RE: ABAQUS/Explicit: Guideline on Hourglass control energy limit?
Your model looks like one repeating structure that is mirrored/patterned multiple times. Maybe you could run your sensitivity analyses on this small section alone? Just to quickly determine the effect of mesh refinement, element type and controls on your results?
Also, have you checked the critical element in your mesh? Sometimes when copying/merging instances you can make small sliver elements which are almost impossible to detect by eye. In an explicit analysis, these sliver elements will torpedo your stable time increment.
Just some suggestions, not sure about the displacement scaling factor - never alter it myself.
RE: ABAQUS/Explicit: Guideline on Hourglass control energy limit?
So I varied the scaling factor and the results are below. Note that scaling factor less than 1 (default) causes softening effect, I imagine value greater than 1 will cause hardening effect but the hourglass control energy will increase too. I terminated the analysis for scaling factor 0.5 since its showing the same trend in both load-displacement and Hourglass control graph.
Load-displacement: (0.075mm element size)
Hourglass control energy for C3D8R: (0.075mm element size)
Should I try other Hourglass control methods? So far I have tried Enhanced and Default = Relax stiffness.
RE: ABAQUS/Explicit: Guideline on Hourglass control energy limit?
(This is assuming all your other modelling assumptions are justified)
RE: ABAQUS/Explicit: Guideline on Hourglass control energy limit?
Load-displacement:
Hourglass control energy for C3D8R:
Any comments on the above??
RE: ABAQUS/Explicit: Guideline on Hourglass control energy limit?
As I stated before, your mesh looks coarse. Although your assembly has 380,000 elements, your 0.075 mm mesh has ~4 elements through the strut diameter and ~8 elements around the strut perimenter (not sure how much better this is for the 0.06 mm mesh). This level of refinement may not be adequate. Particularly if the struts are subjected to bending. Additionally, the performance of C3D8I elements deteriorates considerably if the elements are not rectangular. When the elements are trapezoidal, for example, their performance is not much better than regular displacement elements. Looking at the elements through your strut cross-section, they appear to be trapezoidal.
You can find this info in the theory manual: "Section 3.2.5 Continuum elements with incompatible modes"
You need to establish what the actual force-displacement curve looks like and use that to determine an appropriate discretisation scheme.
RE: ABAQUS/Explicit: Guideline on Hourglass control energy limit?
For future reference -
1. The only parameter should be checked is (ALLAE/ALLIE)*100% (w.r.t Internal energy, not Strain energy or Total energy).
2. LS-DYNA website mentions 10% is okay, ABAQUS documentation says 1-2%. But practically it depends on the discretion of the analysts, as long as it is less than 10% of the Internal energy.
RE: ABAQUS/Explicit: Guideline on Hourglass control energy limit?
*********************************************************
Are you new to this forum? If so, please read these FAQs:
http://www.eng-tips.com/faqs.cfm?fid=376
http://www.eng-tips.com/faqs.cfm?fid=1083