Presentazione sul tema: "Results: experimental investigation 1/6"— Transcript della presentazione:
1 Results: experimental investigation 1/6 Theoretical Background 1/3Theoretical Background 2/3Theoretical Background 1/3Heath monitoring with OO approachTheoretical Background /1[Grafe, 1998]Damage identification by iteratively solving a non-linear optimization procedure via least square algorithmF.E. model represents the reference structureA sensitivity matrix, and error vector built from the correlation of the FRF analytical and experimentalLocalized and small damage entity requiredRefined numerical model and high number of design parameters are needed“by-step” enhancement is proposed for reduction of both computational time and computer memory amountNumerical and experimental validation reportedDamage identification techniques based on the evaluation ofthe change of an Output signal wrt the reference:Modal Model, Response Model, Sound, UltrasoundDamage identification techniques based on the evaluation of the change of an Output signal wrt the reference:Response ModelCorrelation functions:Advantage of Output-Only technique for the estimate of the modal parametersOnly the output time responses of the structure are employedMeasurements of the input loads of the structure are not necessaryThe used output data are those of the structure in operative conditions save costs and timeThe approach is particularly convenient whenever the input is unknownAerospace field: aeroelastic phenomenaCivil field: vibrations of builds and bridgesX ,A = Experimental, AnalyticalVarious type of damageidentifiable fromthe natural frequency shiftpattern modification of mode shapesSensitivity of the system to changes in the design parameters:Uncertainties minimized through reductionof data handling and manipulationComponent disembarkrequiredHealth of the structure monitored evaluating changing in design parameter related to mass and stiffness distributionLow sensitivity to damage levelFine tuning of the F.E. modelLow accuracy of experimental data from estimating processTheoretical Background 2/3Theoretical Background 3/3Heath monitoring with OO approachResults: experimental investigation 1/6Results: experimental investigation 2/6Localized and small entities of structural damage requires high number ofDOFs in FEMDesign parametersExperimental Analysis based on Output OnlyInitial correlationDifferences between the reference and the actual strucure:Looking for damage in both mass and stiffnessExperimental analysis:Modal impactFree-Free B.C.Freq. Band: Hz4096 Spectral lines81 DOFs (trasversal)DynamicResponse modelAddedmassesdesign parameters12 Experimental Dofs consideredDamage identification process divided into consecutive steps:For each iteration step, only the most sensitive design parameters to actual dynamic difference are retainedThose parameters could differ from one iteration to anotherSmall structural changes identified with acceptable computational costsUnknown changes of design parameters,, given for each i-th iteration step by:UndamagedStructureDamagedStructureReduction of 20% of thickness at the center of the plate (corresponding to the 28° element)Variation ofglobalparametersIntroducing the weighting matrices:minimizing the functional:0.6% average change in fnNo effects on damping ratiosThe method do NOT identify the correct parameter (# 28)The solution is given by:and therefore:DamageIdentificationResults: experimental investigation 3/6a) Stiffness related design parametersb) Mass related design parametersConvergence historyWrong stiffness-related design parameters identifiedImmaterial changes in the mass-related design parameters # 28,33,60Results: experimental investigation 4/6Results: experimental investigation 5/6Results: experimental investigation 6/6Experimental investigation: OO test on the undamaged structureOPEN QUESTIONLooking for damage in stiffness distribution onlyLocalization process identified correct damaged region for mass distribution, not for stiffness64 design parameters12 Experimental Dofs consideredProcedure speed up using a sensitivity matrix built on a frequency band of [0,300] Hz (final dimensions: 2450 £ 64)After 4 steps, the number of design parameters useful to describe the damage condition reduces to 2Actual damage involves mostly the stiffness characteristics, the mass changes are H.O.T.Convergency history of stiffness related design parameter- Step #4Mass-related design parameters reduces the stability of the numerical algorithm(Least Square solution)Effects on global parametersTwo adjacent elements identified (# 28 and 36) corresponding to the actual damage locationreduction in the eigenfrequency shiftsincrease in the correlation among the FRFsLack of damage identification due to NON optimal selection of design parameters?(from numerical point of view)(LSCE-FDD)GENERALIZED MASSES (DIAA, )Experimental investigation: mode correlationExperimental investigation: damaged structureExperimental investigation: comparisonConcluding Remarks:The proposed methods allow the damage identification by means of the estimate of modal parametersApproximated solution procedure: least square technique with not-unique solutionsThe effectiveness of the approach is based on the employments of critical points and on small structural variationsThe OO technique (e.g., based on the use of the strain-gage) allows to estimate the variation of the modal parameteres also for small structure perturbationsThe OO approach can be used for the (SHM) Structural Health Monitoring of Aerospace structuresThe procedure has been developed and successfully applied to an aluminum plateSensitivity-based approach (from structural updating discipline) enhanced with a “by-step” algorithmReduction of numerical instability, from “noisy data”, computer memory, and computational time achievedBlind search is dangerous:Design parameters MUST describe the topology of the actual damageAnalysis speed up with a suitable frequency bandFREQUENCY SHIFT DAMAGED V.S. UNDAMAGED MODELS0.88%Variation of thickness: 5%FRF COMPARISON ON THE FIRST MEASUREMENT POINTS
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