Verifying Engineering Analysis
Verifying Engineering Analysis
(OP)
Does anybody know a method for verifying foreign complex engineering analysis like FEA-strength computation of a car, airplane etc.? Is there an established procedure to check foreign computational model, loads and results or is this up to the checking engineer how to do this?
Thanks leju
Thanks leju





RE: Verifying Engineering Analysis
corus
RE: Verifying Engineering Analysis
If we don't like what we see, they get asked to try again.
So far as correlating with real world data goes, that depends on the contract. Sometimes we do it, sometimes they do it. Usually we measure the real data, they fit to it.
Cheers
Greg Locock
RE: Verifying Engineering Analysis
RE: Verifying Engineering Analysis
Modeling errors refer to how you choose to simplify/represent the real world problem without regard to the computer/numerical issues. For example, taking a mult axial, time varying load and representing it with a single peak force. Or assuming that a press fit assembly is represented with "welded" or "ridgid" contact without accounting for the press-fit stresses. Proper selection of material properties is also included in this area. I will also add that you should have a good plan of how you will use the results before you get them. Will you be comparing to a previous design or to an engineering failure criteria such as yield stress or von Mises (I think that not having a plan for the results is one of the most common mistakes made in using FEA)? "Modeling" errors are where an experienced and knowledgeable client may be better suited than a consultant analyst at judging the fitness or acceptability of the assumptions. Your design team and analayst should discuss the modeling representation used and their appropriatness. Ideally, the analyst could show that assumptions (such as ignoring small off axis loadings) make very small contributions to the result.
Numberical errors refer to problems with descritizing the continuous model for computer analysis and numerical issues that may arise (especially in nonlinear analysis). For example, the number and type of elements used to represent the real world device, or the type of solver used in a nonlinear analysis as well as the time interval in transient analysis. These errors are more difficult for those not familiar with FE to check. As mentioned previously, a mesh density study will help show sufficient resolution. One should check for the continutity or smoothness of the depenedant variable (eg are the stresses/displacements "smooth"). Beware of results plots that are averaged because averaging can hide meshing problems as well as make judgements of stress continuity more difficult. Exagerated plots of the deformations are a good tool for this. Also, statistics of the element quality can be helpful, though poor element shape does not automatically mean the result is erroneous. As mentioned in modeling error section, in an ideal situation, your analyst could show the effect of descritization or averaging by demonstrating the effects of each parameter to the overall result (eg., that the difference in stress between 1000 and 10000 elements is negligable).
Unfortunately, its not so easy to look backwards at a result and know if errors are modeling or numerical related. Sometimes, the two errors can acutally cancel each other. Doing some manual calculations (combined with experience) is one of the best ways to validate a model and you should always include this. Trying picking some "easy" locations (the geometry is simple, or the loading components are simple) and see how close the FEA matches what you get. You can also try bounding the calculations to get a reasonable range that you expect the stresses should fall within. Real world testing is the gold stanadard, but that can have a whole other set of errors that sometimes make it less desireable than a hand check.
A final word of wisdom: I read from another FEA user (I think on Eng-Tips, I apologize to the individual for not providing credit) that his goal was to provide such detailed error checking that for someone to prove him wrong would be too expensive/timely to even try. This maybe the best advice I've seen for FEA.
www.probasci.com -
Implantable FEA for medical device manufacturers
RE: Verifying Engineering Analysis
Good Luck
Dave
RE: Verifying Engineering Analysis
In our consultancy, we do a combination of everything noted by the others in the posts above. In addition, we require the electronic files from the models to be submitted for our review. We then check everything to confrim, at least, there's nothing being "hidden". Sub-modeling of complex, high stress areas is also done sometimes to improve accuracy both for modeling and numerical accuracy (good description Probasic)
Manual calculations certianly help judge what's going on. Typically, we find the modeling errors Probasic speaks of most often in the boundary conditions, loads and supports.
In my opinion physical testing is the best check. In our line of work we require tests of structures large and small.
Batman2
RE: Verifying Engineering Analysis
The NAFEMS Analysis Management Working Group has recently been looking at the following ASME definitions (re ProbaSci's comments on Modelling/Numerical checking):
Verification
=========
The process of determining that a model implementation accurately represents the developer's conceptual description of the model and the solution to the model.
Validation
========
The process of determining the degree to which a model is an accurate representation of the real world from the perspective of the intended use of the model.
The more important thing than mesh accuracy can be the source/conservatism/physical sense of the loading regimes, constraint locations etc - ideally you should get a report justifying these choices, demonstrating reasonable conservatism etc; if not in writing then a verbal "grilling" should give you some confidence!
The more important the application, the more difficult testing can be (eg nuclear installations)...
RE: Verifying Engineering Analysis
Prior to obtaining test data, there is a good example of verification procedure for parts of a large model when it is developed by outside agencies. It's basically NASA Ames formalization of much of what is mentioned in the previous responses.
http://tec
RE: Verifying Engineering Analysis
EXCELLENT LINK. Thanks very much!!!
Batman2