How to face complex structures
How to face complex structures
(OP)
Hello,
I am a mechanical engineer with 10 years of FEA experience. I have read a lot of theory about the subject, but didnt find any book/course/tutorial,which can help us complex structures, I mean, all show how to face singularities, hourglassing, etc, but I couldnt find any course which establishes a procedure to face structures like the showing one:


This structure, has been optimized to static environment, but the showed one, is an spectrum analysis, where the normal and shear stresses are very high.
If it where a simple structure, I would change the geometry, or add inertia/area, where the stresses are high, but a logical aproach of changing-proofing will take to much time, and probably wont give the optimal structure.
Anyone knows if there is a document of procedure to face this kind of complex-redundant structures.
Thank You
Regards
I am a mechanical engineer with 10 years of FEA experience. I have read a lot of theory about the subject, but didnt find any book/course/tutorial,which can help us complex structures, I mean, all show how to face singularities, hourglassing, etc, but I couldnt find any course which establishes a procedure to face structures like the showing one:



This structure, has been optimized to static environment, but the showed one, is an spectrum analysis, where the normal and shear stresses are very high.
If it where a simple structure, I would change the geometry, or add inertia/area, where the stresses are high, but a logical aproach of changing-proofing will take to much time, and probably wont give the optimal structure.
Anyone knows if there is a document of procedure to face this kind of complex-redundant structures.
Thank You
Regards
RE: How to face complex structures
- simplifying the geometry (including fewer details)
- using shell and beam elements wherever possible
- taking advantage of symmetry
- treating selected parts as rigid
- using bonded connections instead of nonlinear contact conditions
- using submodeling and substructuring (superelements)
RE: How to face complex structures
"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.
RE: How to face complex structures
Hourglassing should be avoidable by using proper plate/shell elements (e.g., MITC4 elements or whatever the state-of-the-art shear locking free element is these days) which are not integrated in a single point.
Singularities can be avoided by using common sense. At sharp corners with small element sizes or skewed elements (aspect ratio of triangles or rectangles too small to provide well-conditioned Jacobian for integration in reference element), there may be a large peak of stress. If it seems un-physical, it probably is, and if your material is steel, local plastification and stress redistribution often ensure that such stress peaks do not occur in the actual structure.
Singularities can also be caused by load application (RM plate elements, for example, do not handle point loads well in theory) or supports (supporting solids on a line or on a point is nonsensical). Surface loads and line or surface supports are often a safe bet for plates and shells.
RE: How to face complex structures
An FEA book isn't going to address this need. It's a mix of isolating the key question/quantity from "noise", your engineering intuition, and FEA skills that will have to come at play here. By noise, I mean questions that are hurled towards a modeler from non-FEA experts; some questions are feasible, most are not. In rare cases, however, you end up having to use a brute force approach by throwing a gazillion elements with the trade-off that your turnaround time takes a hit.
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RE: How to face complex structures
I didnt explain myself correctly, sorry.
My question is not related with this particular example, but for a procedure for a common aproach to redundant structures, where the elastic limit is far away from analysis results.
The example, has no hourglassing, no corner singularities, by definition of classification, no stress beyond elastic limit is allowed, and above all, there is a condition of not "changing too much the geometry", this problem requires a good aproach of how and where apply the thickness, and what i am asking is a condensed procedure.
For me, in this moment, the answer is kill the normal stresses as they are the bigger, giving inertia, increasing thickness on flanges, after that i would go to shear, with thickness on webs, but maybe, some of you could give another insight, from a previous experience.
Thank you all.
Regards
RE: How to face complex structures
RE: How to face complex structures
1) work in the industry for 10 to 20 years,
2) design a bunch of bad ones first ...
"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.
RE: How to face complex structures
However, the images make me curious: You are concerned that the random spectrum analysis is showing you very high stresses. How confident are you about the PSD that was fed as input? Is the PSD an accepted standard? Are those stresses 1/2/3-sigma stresses?
*********************************************************
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: How to face complex structures
"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.