Damage identification techniques based on the evaluation of the change of an Output signal wrt the reference: Response M

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Experimental investigation: mode correlation Results: experimental investigation 3/6 (LSCE-FDD) The proposed methods allow the damage identification by means of the estimate of modal parameters Approximated solution procedure: least square technique with not-unique solutions The effectiveness of the approach is based on the employments of critical points and on small structural variations The 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 perturbations The OO approach can be used for the (SHM) Structural Health Monitoring of Aerospace structures Looking for damage in stiffness distribution only OPEN QUESTION Looking for damage in both mass and stiffness Initial correlation GENERALIZED MASSES (DIAA, [2002]) Unknown changes of design parameters, , given for each i-th iteration step by: b) Mass related design parameters a) Stiffness related design parameters Convergence history Wrong stiffness-related design parameters identified Immaterial changes in the mass-related design parameters # 28,33,60 Lack of damage identification due to NON optimal selection of design parameters? (from numerical point of view) Theoretical Background 2/3 Theoretical Background 1/3 Theoretical Background 1/3 Heath monitoring with OO approach Theoretical Background 1/1 [Grafe, 1998] • Damage identification by iteratively solving a non-linear optimization procedure via least square algorithm • F.E. model represents the reference structure • A sensitivity matrix, and error vector built from the correlation of the FRF analytical and experimental • Localized and small damage entity required • Refined numerical model and high number of design parameters are needed • “by-step” enhancement is proposed for reduction of both computational time and computer memory amount • Numerical and experimental validation reported Damage identification techniques based on the evaluation of the change of an Output signal wrt the reference: Response Model Damage identification techniques based on the evaluation of the change of an Output signal wrt the reference: Modal Model, Response Model, Sound, Ultrasound Correlation functions: • Advantage of Output-Only technique for the estimate of the modal parameters • Only the output time responses of the structure are employed • Measurements of the input loads of the structure are not necessary • The used output data are those of the structure in operative conditions  save costs and time • The approach is particularly convenient whenever the input is unknown • Aerospace field: aeroelastic phenomena • Civil field: vibrations of builds and bridges X ,A= Experimental, Analytical • Various type of damage • identifiable from • the natural frequency shift • pattern modification of mode shapes Sensitivity of the system to changes in the design parameters: Uncertainties minimized through reduction of data handling and manipulation Component disembark required Health of the structure monitored evaluating changing in design parameter related to mass and stiffness distribution • Low sensitivity to damage level • Fine tuning of the F.E. model • Low accuracy of experimental data from estimating process Theoretical Background 2/3 Theoretical Background 3/3 Heath monitoring with OO approach Results: experimental investigation 1/6 Results: experimental investigation 2/6 • Localized and small entities of structural damage requires high number of • DOFs in FEM • Design parameters Experimental Analysis based on Output Only Differences between the reference and the actual strucure: • Experimental analysis: • Modal impact • Free-Free B.C. • Freq. Band: 0-640 Hz • 4096 Spectral lines • 81 DOFs (trasversal) Dynamic Response model Added masses 64 + 64 design parameters 12 Experimental Dofs considered • Damage identification process divided into consecutive steps: • For each iteration step, only the most sensitive design parameters to actual dynamic difference are retained • Those parameters could differ from one iteration to another • Small structural changes identified with acceptable computational costs Undamaged Structure Damaged Structure Reduction of 20% of thickness at the center of the plate (corresponding to the 28° element) Variation of global parameters Introducing the weighting matrices: 0.6%average change in fn No effects on damping ratios minimizing the functional: The method do NOT identify the correct parameter (# 28) The solution is given by: Damage Identification and therefore: Results: experimental investigation 6/6 Experimental investigation: OO test on the undamaged structure Results: experimental investigation 4/6 Results: experimental investigation 5/6 Localization process identified correct damaged region for mass distribution, not for stiffness 64 design parameters 12 Experimental Dofs considered Procedure 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 2 Actual damage involves mostly the stiffness characteristics, the mass changes are H.O.T. Convergency history of stiffness related design parameter- Step #4 Effects on global parameters Mass-related design parameters reduces the stability of the numerical algorithm (Least Square solution) • Two adjacent elements identified (# 28 and 36) corresponding to the actual damage location • reduction in the eigenfrequency shifts • increase in the correlation among the FRFs Experimental investigation: damaged structure Experimental investigation: comparison Concluding Remarks: • The procedure has been developed and successfully applied to an aluminum plate • Sensitivity-based approach (from structural updating discipline) enhanced with a “by-step” algorithm • Reduction of numerical instability, from “noisy data”, computer memory, and computational time achieved • Blind search is dangerous: • Design parameters MUST describe the topology of the actual damage • Analysis speed up with a suitable frequency band FREQUENCY SHIFT DAMAGED V.S. UNDAMAGED MODELS 0.88% Variation of thickness: 5% FRF COMPARISON ON THE FIRST MEASUREMENT POINTS