A nonlinear acousto-ultrasonic based structural health monitoring system for aerospace applications Michele Meo

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1 A nonlinear acousto-ultrasonic based structural health monitoring system for aerospace applications Michele Meo epartment of Mechanical Engineering, University of Bath, Claverton own, Bath, UK The objective of this work was to demonstrate the feasibility of nonlinear vibration/acoustic/ultrasonic diagnostic tools to be implemented in a structural health monitoring (SHM) system for detecting, locating and imaging damage in aerospace s. Examples of the capability and limitations of the proposed SHM process to detect and image damages and flaws in aerospace structures such as corrosion and barely visible impact damage (BI) are presented for various metallic and composite structures. The results showed that the proposed nonlinear acousto-ultrasound methods is highly accurate in assessing the presence and magnitude of damage with very promising future NT and structural health monitoring applications in a real industrial aerospace environment.

2 Courtesy of Airbus SHM of Airbus Structures October Page 5 Outline Airframe oading SHM and NT&E Nonlinear Acoustic-Ultrasonic based Structural Health Monitoring/NT system for Aerospace and Energy applications Prof. Michele Meo University of Bath, Bath, UK m.meo@bath.ac.uk o Background o Introduction Why SHM and NT? o Non-linear Acoustic-Ultrasounic o Case Studies ongitudinal stress Static/residual strength Crack growth Hoop stress and longitudinal stress Upper skin: Compression/stability ower skin: Tension/crack growth Shear stress Bending and Torsion Impact Bending Impact Shear stress due to transverse shear and torsion st BUE Annual Conference & Exhibition (BUE ACE) Sustainable ital Technologies in Engineering and Informatics Conference & Renewable Energy Exhibition o Conclusions Impact Compression due to bending, Hoop stability, static strength, High local stress Impact corrosion resistance loads SHM and NT is needed in order to ensure the integrity of the airframe. Courtesy of Airbus SHM and NT Challenges o Aircraft/Wind turbine are maintained and regularly inspected for damage/flaws o Background amage can grow to critical level o Repairs are carried out regularly or as necessary o Some materials are difficult to inspect: BI, kissing bond, bolt loosening etc High Number of critical parts arge areas to monitor What is SHM iscrete or Continuous monitoring of structural parameters, stress/strain, environmental and flight parameters by means of permanently attached or embedded transducers to estimate structural health. Possibilities for NI. isual Inspection (I) More than 95% of all NI inspection are done visually. Non-estructive Testing (NT) Performed where I is not sensitive enough or damages are not visible on the surface. Structural Health Monitoring/Management Sensor permanently attached / embedded in the structure Information on structural events or states to arbitrary times available Automated assessment and prognostic of the health of aircraft structures The optimum solution for structural inspections are a function of a number of variables Why SHM and NT? amage are initiated by: Overloaded structures A-6 of American Airline

3 P. Johnson 996 Materials World ol. 7. No. 9, pp. 5-6 Sept 999 Why SHM and NT? Why SHM and NT? SHM and NT Challenges amage are initiated by: Missing loading case P Orion originated from US Airforce C from CIA o The structures are getting bigger but the maintenance windows are getting shorter for increased profitability o SHM could reduce structural check times by factor 5 at least Step- etection of anomalies Is it damaged or undamaged? Step- ocalisation Where is the damage located? Step- Crack size assessment What is the extent of damage? Step- Nature of anomalies What type of damage is it? Step5-Estimation of residual fatigue life Remaining useful life of the system Material Nonlinear-Elastic Wave Behaviour inear and Non-inear Ultrasound Sandstone Stress s Slate Traditional Modulus s dσ K dε dσ NEWS K( e...) dε s K() e () e OERIEW OF NONINEAR ACOUSTICS Concrete Strain e Titanium Alloy If damage increases the modulus K decreases Attenuation increase If damage increases, increases, much more than K decreases Nonlinear Elastic Wave Spectroscopy (NEWS) ε ε τ τ ε ε freq freq Undamaged amaged Material with hysteresis s t s Non-unique Stress-Strain relation e Area Area Max Strain Max Strain Modulus reduction for larger amplitude excitation Increased attenuation as function of amplitude Non-inear elastic wave propagation o amage introduces a significant change in the Increasing rive Amplitude material nonlinear elastic wave behaviour. (a ) Undamaged o This behaviour is manifest in different ways when an excitation is applied to the structure under investigation. o Under resonance conditions when the excitation is increased, a resonance frequency shift is observed. amaged

4 Nonlinear Elastic Wave Spectroscopy (NEWS) Nonlinear parameters emonstration of Nonlinear Elastic Waves o When exciting a sample under simultaneously with continuous waves of two separate frequencies, frequency-mixing spectral components are generated such as harmonics and the sum and difference of the excitation frequency waves (sidebands). Undamaged Amplitude Amplitude f f Hz Input load f Hz β A A γ A A Top view Self-moving laser beam reflects from sample surface. Possible to capture and visualize surface behaviour whilst excited by an ultrasonic wave. Sample Scanned Area aser ibrometer inear (intact ) structural response A (Fundamental) Amplitude Amplitude Nonlinear modulation effects f Input load f Hz amaged Hz f f f f -f f -f f f +f f +f Nonlinear structural response A ( nd Harmonic) A (r d Harmonic) Transmitting Sensor ISO view aser ibrometer Collected points in the scanned area Fatigue crack Scanning vibrometer software Aluminium sample Aluminium plate damaged with a fatigue crack in the central area Application ive ibrometer and Acquisition Results Beta Post-Process Representation Nonlinearity due to crack Transmitting and receiving piezoelectric sensors BOTE JOINT MONITORING USING NONINEAR IBRATION/ACOUSTO/UTRASOUN TECHNIQUES SHM of bolted structures o Nonlinear waves used to detect bolt loosening/separation of parts in contact (nonlinear features) o Single and multiwave excitation Bolted joint monitoring using nonlinear methods o Single excitation o Pitch and catch o Applied torque T > T T Multiwave excitation T Without Modulation Input Signals Output Signals With Modulation T T Applied torque T > T T >T

5 Nonlinear Modulation Parameter Bolted joint monitoring using nonlinear methods o o o A theoretical model was developed where the bolt torque load or bolt loosening can be detected at any time by measuring different nonlinear parameters. alidated on a number of structures and bolt size Based on this analytical expression the bolt torque load or bolt loosening can be detected at any time by measuring a nonlinear parameter. Tight bolt Wind-Turbine Bolted Structure etermine loosened state of bolted wind turbine structures Use piezoelectric sensor for evaluation of bolt loosened state etermine which bolts have become loose using a novel modal analysis nonlinear ultrasound method WIN TURBINE BOT OOSENING ESTIMATION oosened bolt Nonlinear Ultrasound Correlation of loosening with the presence of additional frequency components in the output signal Ultrasound signals excite particular mechanisms that give rise to particular features. Estimating Bolt oosening Estimating Bolt oosening Excitation frequency.85khz isplacement FFT elocity RMS By determining which frequencies give rise to the largest modal displacements for a given bolt these frequencies can be used to excite individual bolts. Bolts Surface Bolts Surface Experiments were broken down into 5 different cases: CASE = All bolts tight CASE = Bolt loose, All other bolts tight CASE = Bolt loose, All other bolts tight CASE = Bolt loose, All other bolts tight CASE = Bolt loose, All other bolts tight Bolt - Results Frequency 985Hz Composite Angled Panel TTU NT Inspection o TTU 5MHz Single Element o C-scan Inspection nd Harmonic g o Poor defect detection due to RA position COMPOSITE STRUCTURES o Method cannot provide TOF

6 j, k j j, F, k FP j, t Composite Angled Panel TTU S spectrogram t, STFTt, Composite Angled Panel TTU - Nonlinear Results amage Nonlinear Contact and Non-contact Methods Spectrogram determined for each element along the array inear and nonlinear information extracted from spectrogram B-Scans for individual frequency responses evaluated using spectrogram Fundamental (f,b, 5MHz), second (f,b, MHz), third (f,b, 5MHz) and nonlinear parameters (beta (β) and gamma (γ)) NACU khz transmission Nonlinear laser scanning vibrometry B-Scan image generated for each harmonic response in time Possible to generate B- Scan of Beta and Gamma responses in time Clear responses and damage identification using the fundamental, second and beta responses 8A A A k a A A 8 A g A k a A Composite T-Panel NT Inspection: o TTU 5MHz Single Element - C-scan Inspection o Inclusion not detected due to RA position, Composite T-Panel Composite T-Panel Sum of all excitation positions eft Right Side Side P P P P P P P P f k t f k t When considering multiple locations for j with a constant position k, a solution to get the maximum information in the receiving element k is to match the phase of f k and f t to obtain a constructive interference (when exciting from multiple (j) positions at the same time). F, k f, k t f, k t t t is the time step required to maximise FP, k FP f P, k t t P amage 8A A A 8 A g A A k a A k a A A A e A5 A A A / Half A t P is the last time step shift required to maximise Nonlinear Contact and Non-contact Methods Stepped Thickness Panel TTU inear & Nonlinear laser scanning vibrometry Single-sided khz NACU Nonlinear laser vibrometry NT Inspection o TTU 5MHz Single Element o C-scan Inspection o Inclusion not detected o Method cannot provide TOF o Noise at panel edge caused by water splashing Harmonic 5

7 5 6, N, N, N 5, N, N, N N N N N N 6 N 5 6 ocal efect Resonance o amage will have its own resonance frequencies o Exciting this frequency will enhance nonlinear behaviour ocal efect Resonance Composite Plate Frequency Sweep: to determine frequencies that produced the largest harmonic responses ( nd, rd etc). Excite around that frequency BAREY ISIBE IMPACT AMAGE- STIFFENE PANE Scanned Area ive linear wave propagation with no damage evidence Arbitray Excitation o emonstrated strong coupling and modulation between fundamental excitation f and local defect resonance f d ocal efect Resonance Transmitting ultrasonic vacuum sensor BAREY ISIBE IMPACT AMAGE- STIFFENE PANE BAREY ISIBE IMPACT AMAGE- STIFFENE PANE Nonlinear Parameter amage Nonlinear postprocess shows in a very clear way the presence of damage Nonlinear Parameter g amage Nonlinear postprocess shows in a very clear way the presence of damage Nonlinear amage ocalisation Algorithm Nonlinear amage ocalisation Algorithm eveloped a damage detection and localisation technique Unknown damage location and material properties Use a direct propagation path between a particular set of transducer location Hot spot locations Nonlinear amage ocalisation Algorithm First Step Second Step If S is sending a burst, we have x x y y t t x x y y t t x x y y x x y y t, t, x x y y x x y y t t t, If S is sending a burst, we have x x y y x x y y t t t, x x y y x x y y t t t, x x y y x x y y t t t, Nonlinear amage ocalisation Algorithm Subtracting and filtering at the second harmonic using Phase Symmetry Analysis (PSA), yields: y( t) ( ) t ( ) y t y PSA t t5 t x x y y x x y y N N x x6 y y6 t 6 N 6 The optimal sensors placement allows assuming: N N 5 N N N 6 N x x5 y y5 x x y y N N 5 x x y y x x y y N N x x y y N 6

8 A & AS β & βs Experimental Results ocalization Results Nonlinear Wave Modulation.5 khz f f Estimation of the Residual Fatigue ife using a Baseline-free Modulated Nonlinear Ultrasound Method f f f f f +f Crack ocation Aluminium amaged Sample Composite amaged Sample efence Science & Technology aboratory (ST-MAST Project CE 76) Fatigue Failure The problem - Fatigue detection Nonlinear Parameters derived The Fatigue failure process can be divided into five stages:. Cyclic plastic deformation prior to Fatigue crack initiation;. Initiation of one or more microcracks;. Propagation or coalescence of microcracks to form one or more macrocracks;. Propagation of one or more macrocracks; Tornado Aircraft Rib Known Fatigue crack growth Known high stress regions rivet holes Specimen designed to replicate crack growth Second-Order Nonlinearity Parameter g 8A A k a Third-Order Nonlinearity Parameter A 8 A k a Second-order harmonic parts forand frequency modulated and response of dual frequency A k A k x cosk x ct x cosk x ct 8 Amplitude nd Harmonic Amplitude st Harmonic Amplitude rd Harmonic istance between sensors Wavenumber k=f/c 5. Final failure. Sum and ifference Frequency A Ak k A Akk xsink k x ct xsink k x ct Nonlinear Acoustic Parameters Output the fundamental frequencies (f and f) amplitudes will decrease as damage f increases (this is a linear effect and is subsequently used in linear ultrasound f methods to detect cracks) Fatigue life and nonlinear parameters G N 5, S N G 7 Fatigue Crack propagation f-f f+f A A S S Af Af k f k f x Af Af k f k As: Increase Af & Af ecrease f x Increases the amplitudes of the modulated response (sidebands/harmonics, A and A S) should increase as damage increases due to the increase in clapping/rubbing generated from the cracks The dual effect of the described decrease in the fundamental frequencies amplitudes and the increase in the harmonics produced by the cracks results in nonlinear parameters that should increase as fatigue increases. R f E h R s s E h R s s m R f m R hs d hs d m R f is the fatigue life percentage m R Fatigue life prediction equation determined by combining the Nazarov-Sutin and Paris- Erdogan theories of crack behaviour and crack growth Cycles 8 Cycles Cycles Cycles 7

9 β & βs Fatigue life and Nonlinear Parameters Nonlinear Parameter and Residual ife Estimation Initial Fatigue ife prediction f Cycles f 8 Cycles f & 585 khz & 5 khz f f-f f+f f-f f+f Crack starts to propagate f Cycles f Cycles As: f+f Af-f & Af+f increase & 585 khz 9 & 5 khz f-f f+f f-f Af & Af ecrease Increases Fatigue ife estimation Procedure Nonlinear Ultrasonic Stimulated Thermography IR camera Ultrasonic horn High/low energy ultrasound is injected into the part. Infrared energy is edicated software emitted at the sample identifies subsurface surface near the crack features and removes location, and detected unwanted signals from the by the IR camera. background or surface. COUPING NEWS TO THERMA WAE IMAGING The ultrasonic excitation causes frictional heating as the faces of the crack surface rub against each other. Single Wave Excitation Heating and Strain Energy Stiffened Panel Nonlinear Ultrasound Stimulated Thermography (NUST) T rise Power dissipated elaminated area Resonance modes have high vibration amplitude frequency Resonance modes induces Clapping/rubbing (delaminations) From the definition of loss factor: P = crack f strain energy Clapping/rubbing creates acoustic nonlinearities Step - Nonlinear Ultrasound Clapping/rubbing of crack faces generates heat Step - Thermal Camera Power dissipated ( crack, f, ) amplitude f. E a ρ σ T rise (hence detectability) = function of (f, amplitude ) for a crack of known size/damping Stiffeners C-Scan results of tested regions using various methods (6 element array) ocation and test setup Three excitation points BI regions P = crack f T = function of (f, amplitude ) 8

10 NUST Testing Strategy ocal efect Resonance Heating Sweep Thermal Results o Comparison between aser ibrometer and Nonlinear Ultrasonic stimulated thermography A frequency sweep was conducted from khz to khz in order to determine the highest nonlinear harmonic responses amage A (khz) amage B (khz) amage C (khz) o Heating is due to a combination of excitation by the fundamental and second harmonic Conclusions Acknowledgement Nonlinear acousto-ultrasound principles and procedures were presented Application of nonlinear acoustic-ultrasound methods for aerospace and renewable energy structures showed clear promising results Part of this work presented herein has been performed as part of AAMSA project supported and funded by the European Union (EU) in the 7 th framework programme (FP7) for research and technological development. Collaborators: GP Fierro, F Amerini, F Ciampa, I Solodov, S Boccardi, Calla This methods are superior to linear ultrasound for specific type of damages isbonding elaminations Bolts Contact-type flaws 9