Femap_Training_Lesson 14 - Finite Element Model Debugging

CT 1690 – Student Guide for Femap 101 - v10.2
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Lesson 14
FEA Modeling Debugging
Finite Element Mesh Sizing
Depending on the desired accuracy of the model, a course mesh (low number of larger elements) or a fine mesh (high number of smaller elements) must be created. For instance, a more complicated structure requires a finer mesh in order to produce accurate answers.
Fine Model
Accuracy for complicated geometry is improved Less distorted elements improves results
Higher number of Elements More Degrees of Freedom Increased Solve Time
CT 1690 – Student Guide for Femap 101 - v10.2
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Lesson 14
FEA Modeling Debugging
Checking Results
It is always a good idea to do a “reality check” with the results of any Finite Element Analysis. Using engineering knowledge and common sense will allow a user to determine if the results make sense for the applied boundary conditions. For instance, an under under-constrained or “unconnected” (nodes not merged where they should be) model may exhibit much larger displacements than expected a displacement in the opposite direction of the applied load during a linear cantilever beam analysis. Some Good Practices: • • • • Always visually plot the models elements if possible for verification Make sure responses correspond with applied boundary conditions Check input loads with reaction forces: ΣF = 0 Hand calculations are always a great idea whenever possible
Pros
Cons
Lower number of Elements Coarse Model Less Degrees of Freedom Reduced Solve Time
Less accuracy for complicated geometry Distorted elements can be too stiff and misrepresent response
CT 1690 – Student Guide for Femap 101 - v10.2
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Lesson 14
FEA Modeling Debugging
Model Debugging
Recommended minimum checks on input • Stiffness matrix checks • • • At the G-size After MPC Processing After All Processing
Lesson 14 Finite Modeling Debugging
Purpose: This lesson is an overview of methods to check and debug Finite Element Models. Topics: Finite Element Modeling Mesh Sizing Results Checking Model Debugging Common Errors Recommended Model Checks
Mass Checks • • • Grid point weight generator output Rigid-body mass checks Assembly mass checks
Loading checks
CT 1690 – Student Guide for Femap 101 - v10.2
CT 1690 – Student Guide for Femap 101 - v10.2
ቤተ መጻሕፍቲ ባይዱ
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Lesson 14
FEA Modeling Debugging
Common Types of Errors (continued)
Beam Orientation Beam Releases Loading (Make sure that the model is loaded accurately) Finite Element Error Round-off Error (Can cause serious, serious problems) off Program Bugs (Please Report them to UGS Solutions via GTAC) • A list of known errors is maintained and distributed Plates not lining up (zipper effect)
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Lesson 14
FEA Modeling Debugging
Model Debugging (continued)
Structural plots are useful primarily to visually verify model geometry. Other tools must be used to assess the numerical accuracy of a finite element model. These tools include many automatic error checks performed by NX Nastran and user-supplied diagnostic requests in the form of DIAG, supplied PARAM, DMAP Alters, and Case Control requests. NX Nastran performs many error checks during an analysis to ensure that all input data is in the proper format and usable. If an error is detected during data processing, an error message is generated. If the error is fatal, the analysis terminates. Many times NX Nastran errors have a number and a short description of the error which shows up in the .f06 file. Many times the same error number can represent a variety of different issues. In cases where the error is not obvious, consult the Help->Analysis menu and choose the error >Analysis message segment where the Error Message number appears. Go to the error number for a broader description.
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Use of consistent units is of utmost importance!!! Always use unique IDs – NX Nastran sometimes allows for duplicate element IDs, but not always. Duplicates can and do lead to problems, especially during data recovery
CT 1690 – Student Guide for Femap 101 - v10.2
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Lesson 14
FEA Modeling Debugging
Common Types of Errors
Mistakes in engineering judgment Approximations to physical behavior Engineering Theory Finite Element Theory Finite Element Implementation Modeling • Bolted connection • Welded connection • Corners • Transitions Connections • Beam to Plate • Beam to solid • Plate to solid