STATUS AND FUTURE OUTLOOK OF CORRELATION AND MODEL UPDATING

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STATUS AND FUTURE OUTLOOKOFCORRELATION AND
MODEL UPDATING

• Christopher C. Flanigan
• Quartus Engineering Incorporated
• San Diego, California USA
• 16th International Modal Analysis Conference
• Santa Barbara, California USA
• February 2-5, 1998

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AGENDA

• Introduction
• Terminology
• Motive, means, and opportunity
• Current methodology
• Commercial s/w programs
• Future possibilities
• Conclusion

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TERMINOLOGYMANY METHODS FOR CORRELATION

• "Correlation" covers three areas
• Comparison of test and analysis results
• Orthogonality, x-ortho, MAC, COMAC, CORTHOG
• System identification
• Extract K, M, C matrices (plant)
• Model updating
• How to update FEM to match test results
• This presentation emphasizes last topic

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MODEL CORRELATIONIS DETECTIVE WORK
All knowledge comes usefulto the detective.
Sherlock Holmes Consulting Detective 221b Baker
Street
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INTRODUCTIONMOTIVE, MEANS, AND OPPORTUNITY

• Motive
• Why do correlation?
• Why is it important?
• Means
• What methods are used?
• What s/w is available?
• Opportunity
• When is correlation performed?
• How much time is available?

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MOTIVEMODELS ARE NOT REALITY!

• Computers models are not perfect (even today!)
• Modeling assumptions
• Joints, fasteners, material properties,
  effectiveness
• Mass distribution, secondary packages, etc.
• Model fidelity limitations
• Element formulation, FEM size, run times
• Communication
• Model ? drawings, structure ? drawings
• Pilot error

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MOTIVEMODELS NEED REALITY CHECK

• While models are typically much better today than
  ten or twenty years ago...
• Software capabilities
• Computer speed
• FEM resolution
• Testing still required to provide "reality check
• Data from actual hardware
• Needed to validate the models
• Unlikely to change for some time!

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TEST IS NOT REALITY EITHER!

• Test article instead of flight article
• Mass simulators, missing items, boundary
  conditions
• Excitation limitations
• Load level, spectrum (dont break it!)
• Nonlinearities
• Testing limitations
• Sensor accuracy and calibration
• Data processing
• But its the best reality check available

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MOTIVECORRELATION REDUCES RISK

• Must certify that structure will operate
  correctly
• Usually cannot test flight structure to
  operational loads
• Use test-correlated FEM to verify correct
  operation of flight structure to flight loads

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OPPORTUNITYCORRELATION MUST BE FAST!

• Very limited opportunity to do correlation
• After structural testing and data processing
  complete
• Before operational use of model
• First flight of airplane
• Verification load cycle of spacecraft
• Need methods that are fast!
• Maximum insight
• Accurate

There are always some lunatics about. It would
be a dull world without them.
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CHALLENGENOT A UNIQUE SOLUTION

• More unknowns than knowns
• Knowns
• Test data (FRF, frequencies, shapes at test DOF)
• Measured global/subsystem weights
• Unknowns
• FEM stiffness and mass (FEM DOF)
• No unique solution
• Seek best reasonable solution

When you have eliminated the impossible,
whatever remains, however improbable, must be the
truth.
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MEANSMUCH RESEARCH IN CORRELATION

• Much work done in this field
• Selected topics and authors
• Apologies to authors not mentioned!
• Recommended reading
• IMAC proceedings (CD-ROM)
• Finite Element Model Updating in Structural
  Dynamics,Friswell and Mottershead, Kluwer
  Academic Publishers

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MEANSMANY CORRELATION METHODS

• Trial-and-error
• Stop doing this! It's almostthe next millenium!
• Too slow for fast-paced projects
• Not sufficiently insightful for complex systems
• FEM matrix updating
• FEM property updating
• Error localization

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MATRIX UPDATE METHODSADJUST FEM K AND M ELEMENTS

• Objective
• Identify changes to FEM K and M so that analysis
  matches test
• Baruch and Bar-Itzhack (1978, 1982)
• Berman (1971, 1984)
• Kabe (1985)
• Kammer (1987)
• Smith and Beattie (1991)
• and many others

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MATRIX UPDATE METHODSHAVE LIMITATIONS

• Lack of physical insight
• What do changes in K, M coefficients mean?
• Lack of physical plausibility
• Baruch/Berman method doesn't enforce connectivity
• Limitations for large problems
• Great for small demo models, but ...
• Smearing" caused by Guyan reduction/expansion
• Lack of usability if test article different than
  flight vehicle
• Requires very precise mode shapes (unrealistic)

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PROPERTY UPDATE METHODSADJUST MATERIALS AND
ELEMENTS

• Objective
• Identify changes to element and material
  properties so that FEM matches test
• Hasselman (1974)
• Chen (1980)
• Flanigan (1987, 1991)
• Blelloch (1992)
• Smith (1995)
• and many others

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PROPERTY UPDATE METHODSBETTER FOR LARGE MODELS

• Design sensitivity methods
• Computationally efficient
• Can examine many parameters
• Modal approach
• Match frequencies, shapes, x-ortho
• Better if modes available
• FRF approach
• Match resonances, anti-resonances
• Better if mode extraction difficult

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MEANSCOMMERCIAL S/W FOR CORRELATION

• Correlation not a large market
• Commercial s/w slow to develop
• Main reason for trial-and-error
• Significant improvement recently
• Design sensitivity and optimization
• Major vendors
• SDRC, LMS, MSC, UAI
• Specialty companies
• DDS

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MEANSCOMMERCIAL S/W FOR CORRELATION

• SDRC
• I-DEAS Correlation (MAC, ortho, x-ortho, mapping)
• CORDS (property adjustment using modes and DSA)
• FRFCORR (property adjustment using FRF)
• LMS
• CADA LINK (parameter updating, Bayesian
  estimation)
• Close ties with MSC/NASTRAN
• MSC
• SOL 200 design optimization (modes, FRF)

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MEANSCOMMERCIAL S/W FOR CORRELATION

• UAI/NASTRAN
• Analysis-Test Correlation (ATC) in next release
• Sensor placement, error localization, parameter
  updating, mode tracking
• Dynamic Design Solutions (DDS)
• FEMtools (follow-on to Systune)
• Wide range of pretest, correlation, and model
  updating features
• Others (Sandia/ACTA SSID, etc.)
• Happy to see you at vendor exhibit!

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FUTURE OUTLOOK FORANALYSIS AND TEST

• Major analysis trends
• Larger models (105-107 DOF)
• Faster computers
• More accuracy? More insight?
• Major test trends
• More accels (102-103 DOF)
• More modes
• More reliance on analysis
• Fewer major tests

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FUTURE OUTLOOK FORCORRELATION AND MODEL UPDATING

• Many opportunities for further development
• Genetic algorithms
• Integration
• Modal and FRF methods
• Frequencies, shapes, ortho, x-ortho
• Process improvements
• Analysis - test - analysis
• Commercial software
• Critical for widespread use of advanced
  correlation

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WHAT ABOUTVIRTUAL TESTING?
There is nothing like first-hand evidence.

• Virtual testing
• FEM represents the hardware
• Simulated noise, nonlinearities
• Helpful to optimize a test
• Training and shakedown
• Sensor and exciter placement
• Does not replace a test

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THE ROAD AHEADCORRELATION IN 2001

• Analysis
• FEM with 10,000,000 DOF
• COTS s/w to assist test planning
• Accelerometer and exciter placement
• Test
• 1,000 accelerometers, 100 target modes
• Correlation
• Seamless integration of analysis and test
• COTS s/w for property updates

2 0 0 1
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CONCLUSIONCORRELATION IS AN IMPORTANT STEP IN
PRODUCT DEVELOPMENT

• Correlation helps analysis match reality
• Within limits of test hardware and test accuracy
  limits
• Correlation can be done quickly with correct
  tools
• Identify best changes to model to match test
  results
• Correlation reduces risk
• Use verified model to predict operating responses
• Correlation can be a powerful step that maximizes
  the combined benefits of analysis and testing

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MAKE THE MOST OFYOUR TIME AT IMAC-XVI
Come, Watson, come! The game is afoot!

• Put on your detective hats
• Learn about the latest
• Test techniques
• Structural dynamics
• Correlation methods
• Become a Sherlock Holmesof Model Updating and
  Correlation!

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BE A SHERLOCK HOLMES OFCORRELATION AND MODEL
UPDATING
Amazing, Holmes! How did you correlate that
model so quickly?
Elementary, My Dear Watson. I went to IMAC-XVI.