CORROSION DETECTION IN AERONAUTICAL STRUCTURESUSING LAMB WAVES SYSTEM FOR SHM

A. K. F. Tamba∗, L. C. Vieira∗, G. O. C. Prado∗, F. Dotta∗, R. P. Rulli∗, P. A. da Silva∗, R. R.Cunha∗, J. P. Costa∗

∗Embraer S.A., Brazil

Keywords: Structural Health Monitoring, Probability of Detection, Lamb Waves, Corrosion

Abstract

Structural Health Monitoring (SHM) can modifythe current procedures of aircraft mainte-nance with new technologies, concepts andphilosophies. When compared to current NonDestructive Inspection (NDI) technologies, SHMcan reduce the amount of time and workloadof inspection tasks and consequently reducecosts because it has less complex and less timeconsuming processes. Reliable SHM systemsare able to automatically evaluate structuralconditions and inform the maintenance companyof the presence and the location of a structuralflaw.

For years Embraer has investigated differentSHM technologies for damage detection withpossible application scenarios. Embraer under-stands that SHM can provide without great effortthe structural diagnosis in restricted access areaswith early detection of structural damages andreduction of maintenance costs, besides mini-mizing the effects of “human-factors” during aninspection due to automated damage detection.

In aircraft maintenance sometimes it isnecessary to disassemble aircraft parts to reachthe inspection location, and after that process,the aircraft must be reassembled. All these stepscan be costly and favorable to induce flaws whenincorrect executed. Embraer has an incentive toexplore ways of reducing maintenance costs andincreasing efficiency of its services.

Aircraft in service are susceptible to cor-rosion and these structural damages can bedetected during a scheduled maintenance. NonDestructive Test (NDT) methods for rapid andreliable corrosion detection in complex metallicassemblies are an on-going challenge due topracticalities of inspection and geometric com-plexity. The Embraer’s R&D team has dedicatedstudies for corrosion damage detection usingLamb Waves (LW) Technology.

This work demonstrates further Lamb Wavesresults for detecting corrosion in aluminumalloys for aircraft structures. These studies wereperformed using Acellent Technologies LambWaves system with different types of specimenconfigurations.

1 Introduction

Embraer has been studying SHM technologiesand application scenarios for structural damagedetection. Technologies such as ComparativeVacuum Monitoring (CVM), Electro-MechanicalImpedance (EMI), Acoustic Emission (AE) andLW are under investigation [1]. For Embraer,SHM can provide facilitated damage detec-tion in areas with restricted access with earlydetection of structural damages and reductionof maintenance costs for current and futureaircrafts, besides minimizing the effects of“human-factors” during an inspection [2]. Typ-

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ically, corrosion damage are in difficult accessareas, hidden by the lavatory or galley and LWtechnology can assist in the implementation ofthese inspection tasks following the ideology ofSHM adopted by Embraer. This paper presentscomplementary studies based on some results ofthe tests performed by Embraer with the AcellentTechnologies Lamb Waves system for detectionof corrosion damage with the use of artificiallyinduced damage on test specimens.

Figures 1 and 2 shows examples of specimenconfigurations used in the tests and the AcellentTechnologies software screen.

Fig. 1 Test Specimens Example

Fig. 2 Damage Detection on Software

The corrosion damage was induced arti-

ficially with a series of thinning in the testspecimens. These lab tests provide data that canbe used for generating Probability of Detection(POD) curves for LW systems. The experimentalresults indicate that Lamb Waves technique isaccurate and it is shown as a promising tech-nology for the application of corrosion damagedetection.

1.1 Corrosion Damage

Corrosion is a natural and costly phenomenonand cannot be avoided, but aircraft manufacturersand operators spend time and money to keep itin control. According to (NACE), in the yearof 1996 the total annual direct cost of corrosionto the U.S. aircraft industry is estimated at $2.2billion, which includes the cost of design andmanufacturing ($0.2 billion), corrosion mainte-nance ($1.7 billion), and downtime ($0.3 billion)[3]. Corrosion protection in aircraft structures isa well-established technology, both to preventand to inhibit corrosion damage. However, thereis a high probability of failure of these protectionmethods on older aircraft, with long periods ofservice.

The surface corrosion in its early stagescan be detected visually through signs such asdiscoloration, faint markings of dust, bubblesand paint damage. On the other hand, it is verydifficult to detect hidden corrosion damage sincethe characteristics do not allow for identificationmethods or conventional NDT. In many cases,the damage reaches a very advanced stage beforeit is detected, requiring dismounting of the struc-ture, which leads to a very high maintenancecost. Therefore, it becomes interesting to usea sensor system that effectively monitors thecorrosion damage of the structure and allowsobtaining data regarding the problem severity. Apreliminary study was conducted to assess theuse of LW system to detect thickness reductionand the results were promising [4].

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CORROSION DETECTION IN AERONAUTICAL STRUCTURES USING LW SYSTEM FOR SHM

2 LW Approach for Damage Detection

The fundamental of this technique is based onthe assumption that structural damage changesthe physical dynamic response of the structure,such as natural frequencies, mode shapes anddamping, frequency response, etc. [4]. Lambwaves represent two-dimensional wave propa-gation in plates or shells, which are describedby known mathematical equations originallyformulated by Horace Lamb in 1917 [5]. Astructure with a damage like thickness reduction(corrosion) exhibits non-linear vibrations due tothe stiffness change under load variation [6].

The accuracy of relating changes in modalparameters to flaws such as cracks becomes quitepoor when the aspect ratio between the size ofthe structure and the size of the flaw is larger than10 [6, 7]. Lamb waves are two groups of waves,the symmetric waves and the anti-symmetricwaves, that satisfy the wave equation and theboundary conditions. The general solutions canthen be split into two modes: symmetric (S0)and anti-symmetric (A0) [8] as shown in Figure3. The Lamb wave modes are considered tobe sensitive to fatigue cracks and can be usedin metallic or composite material structures todetect a wide range of damages types.

Fig. 3 Wave modes: (a) Anti-symmetric, wherea peak at one surface corresponds to a troughat the other surface; (b) Symmetric, where thewave peaks or troughs occur simultaneously atthe same in-plane location

3 Probability of Detection

POD is a means of describing how well aninspection procedure can detect the requireddefects. Based on data statistic, the POD havea focus on measure the chance or probability ofdetecting a defect. By assigning POD valuesto a procedure, it is possible to compare theeffectiveness of different inspection techniques.Alternatively, POD values can be used as a targetwhich procedures have to be shown to be able toachieve.

Basically the POD is calculated using statisti-cal methods for analyzing these data to produce acurves that provides a quantitative and graphicalrelationship between probability of detection andthose factors that control it, such as target size.The Figure 4 exemplifies a POD curve.

Fig. 4 POD Curve Example

To uses a POD curve or values, it is impor-tant to understand how it was obtained, the pro-cedure used, kind of defects its applied and anyother assumptions have been made during the ex-perimental program. The POD is estimate basedon results with representative significant statisti-cal sampling.

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3.1 MIL-HDBK-1823: Nondestructive Eval-uation System Reliability Assessment

The MIL-HDBK-1823[9] is a document ofDepartment of Defense of United States providesNon Destructive Evaluation (NDE) proceduresfor inspecting flight propulsion system (gas tur-bine engines and rockets) components, airframecomponents, ground vehicle components, eithernew or in-service hardware. The NDE methodscan be Eddy Current (EC), Fluorescent Penetrant(PT), Ultrasonic (UT), Magnetic Particle (MT)testing, Radiographic testing, Holographictesting, and Shearographic testing, providedthey produce an output similar to those listedherein and provide either a quantitative signal,â, or a binary response, hit/miss. This documentprovides uniform guidance requirements forestablishing NDE procedures used to inspectnew or inservice hardware for which a measureof NDE reliability is required.

One of the focuses of this MIL Handbookis present a guidance (for planning, conducting,analyzing, and reporting reliability evaluations)for assessing the capability of an NDE systemin terms of the POD as a function of targetsize to quantitatively determining NDE systemcapability.

4 Corrosion Damage Detection Evaluation

This session presents the experimental programfor this study and the POD results. As previewsexplained in this study the POD approach usedwas the MIL-HDBK-1823[9].

4.1 Test Summary

The test were performed in three types of testspecimens with different shapes for verificationof corrosion damage detection with LW tech-nology as showing in Figure 5. All specimenswere manufactured with aerospace aluminumalloy 2000 series for plates and splices andthe aluminum alloy 7000 series are used for

stiffeners.

Fig. 5 Specimens: (a) SHM-C02: Stiffener, (b)SHM-C04: Reinforced Panel and (c) SHM-C05:Spliced Panel.

Four progressive stages of thickness reduc-tion were elaborate to simulate the corrosion.The first stage (a) starts the simulated corrosionon structure with a punching tool that reduces0.21 mm of thickness. The next stages wereperformed by machine with a milling cutter tool.In the second stage (b) a reduction of 0.08 mmof the thickness with a diameter of 6 mm isperformed, followed by the third stage (c) witha reduction of 0.16 mm of the thickness withthe same diameter from previous stage and thefourth stage (d) enlarges the hole diameter to13 mm and keeps the hole depth. All stagesdescribed above and each manufactured damagelocations are shown on Figure 6.

The Figure 7 shows the detail of machiningthickness reduction for specimen 5B for damageon position 03.

The SHMPatch software was used for dam-age detection analysis. The default softwareprocedure was used for damage detection.Figure 7 shows the damage detection resultsfrom Acellent software, which the damageswere detected on manufactured damages DF02,DF03 and DF04. No damage was detected formanufactured damage DF01.

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CORROSION DETECTION IN AERONAUTICAL STRUCTURES USING LW SYSTEM FOR SHM

Fig. 6 Thickness reduction stages: (a) DF01, (b)DF02, (c) DF03 and (d) DF04 and Damage Posi-tions: (e) SHM-C02, (f) SHM-C04 and (g) SHM-C05.

Fig. 7 Detail of the thickness reduction on thespecimen.

4.2 Test Results

Embraer conducted the test with three config-urations of specimen but for POD study willbe used only one configuration: ConfigurationSHM-C05.

For each coupon the damages are manu-

Fig. 8 Damage Detection on SHMPatch

factured and the data were collected using theAcellent Technologies System. All test resultsdata [10] are organized and the damage detectionfunctionality of SHM Patch Software was usedto diagnosis and evaluate if the system detect ornot detect the defect in each step of test.

For this specimen configuration were in-stalled 6 Lead Zirconate Titanate (PZT) sensorsand software was configured 2 frequencies(200KHz and 250KHz) to evaluate the specimenin analysis. The use of these two frequenciesresults in 96 interrogations of the system toanalyze if detect or not detect the damages in thespecimens. During the test one specimen has aproblem (05J) and it was eliminated from test setand reduced the total valid data points to 88.

For POD analysis the Hit/Miss approachcan be used because the test were done withfixed damage sizes steps and the system returnsdetection or no detection the damage in otherwords the test results in binary response. Thelarge number of interrogations is other factorthat contributed to choice this approach, justi-fying use of Hit/Miss methodology to POD study.

The Table 1 presents the test results forHit/Miss approach. In table are the frequencyused to run damage detection, the specimens

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identification and values for Hit (1) when thesystem detected the damage or Miss (0) whenthe not detected for each damage size.

Table 1 Configuration SHM-C05 Hit/Miss Data

The data in table is used as input data formh1823 POD software [11] to calculate andgenerate the POD curves for this test. Thesystem obtained a total of 50 detections and38 not detections. All valid data were inputin mh1823 software and the Acellent’s systemobtained value of 8.498mm3 of volume loss with90% of POD for this test.

For some NDE inspections included theaeronautical area is used the called POD 90/95that corresponds 90% of probability of detectionwith 95% of confidence in this probability.For this test the system obtained a value of21.79mm3 of volume loss with 90% of POD and95% of confidence. Figure 9 shows experimen-tal data POD’s curve from mh1823 software [11].

Fig. 9 POD Curve from mh1823 Software

5 Conclusion

Embraer have tested 12 specimen of Configu-ration named SHM-C05 (Spliced Panel) with 4damage sizes manufactured step by step. Theevaluate have done with Acellent Technologiessystem using 6 sensors installed on each speci-men.

The system was configured to evaluatethe sensor mash with 2 different frequencies(200KHz and 250KHz) resulting in 24 datapoints for each size manufactured and 96 datain total. During the test one specimen presentedproblem and was eliminated of the test andreduced the data points to 88 in total.

The Hit/Miss methodology was used in thistest to obtain the Probability of Detection forAcellent’s system. This POD study is valid onlyfor these configuration of specimen, sensor mashand system configurations to detect mass loss.

The results and POD study show the capa-bility of Acellent Technologies System to detectcorrosion damage based on loss mass. Thevalues of POD obtained in this study show quan-titatively the system sensitivity and reliability todetect corrosion. In the future, other tests will be

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CORROSION DETECTION IN AERONAUTICAL STRUCTURES USING LW SYSTEM FOR SHM

conducted by Embraer to evaluate the SHM sys-tems capability based on MIL-HDBK-1823[9]and others POD approaches.

6 Contact Author

For further information please contactandre.tamba@embraer.com.br.

References

[1] R. P. Rulli, P. A. Silva, Embraer Perspectivefor Maintenance Plan Improvements by Us-ing SHM, 3rd Asia-Pacific Workshop on SHM,2010.

[2] Schmidt, H.-J., et al., 2004. Application ofStructural Health Monitoring to Improve Ef-ficiency of Aircraft Structure. 2nd EuropeanWorkshop on SHM, July 2004, Munich, Ger-many.

[3] Corrosion Control - The Cost ofCorrosion and How to Mitigate Ithttp://www.corrosioncost.com/../index.htm[Acessed on Jun. 18, 2018]

[4] F. Dotta, F. S. da Silva, A. B. da Silva, EarlyResults of Lamb Waves Approach to AssessCorrosion Damage Using Direct Image Path inan Aeronautical Aluminum Alloy, 8th Interna-tional Workshop on SHM, 2011.

[5] A. Viktrov, Rayleigh and Lamb Waves: Phys-ical Theory and Applications, New York:Plenum (1967).

[6] Matveev V. V and Bovsunovsky A P 2002Vibration-based diagnostics of fatigue damageof beam-like structures J. Sound Vib. 249 2340.

[7] Ihn, J-B. and Chang, F-K. 2004. Detection andmonitoring of hidden fatigue crack growth usinga built-in piezoelectric sensor/actuator network:I. Diagnostics. Smart Materials and Structures(13), pp. 609âAS620.

[8] Giurgiutiu, V. 2005. Tuned lamb wave exci-tation and detection with piezoelectric waferactive sensors for structural health monitor-ing. Journal of Intelligent Material Systems andStructures, 16(2):291-305.

[9] Department of Defense. MIL-HDBK-1823A:

Nondestructive Evaluation System ReliabilityAssessment, 2009.

[10] A. K. F. TAMBA, F. DOTTA, G.O. PRADO, L.C. VIEIRA, R. P. RULLI 1, G. R. SILVA, J. M.COSTA. Further Results of Lamb Waves Ap-proach to Asses Corrosion Damage in an Aero-nautical Aluminum Alloy, 9th European Work-shop on Structural Health Monitoring (EW-SHM), 2016.

[11] mh1823 Software, Statistical Engineering,https://statistical-engineering.com/mh1823-../[Acessed on Jun. 18, 2018].

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The authors confirm that they, and/or their companyor organization, hold copyright on all of the origi-nal material included in this paper. The authors alsoconfirm that they have obtained permission, from thecopyright holder of any third party material includedin this paper, to publish it as part of their paper. Theauthors confirm that they give permission, or have ob-tained permission from the copyright holder of thispaper, for the publication and distribution of this pa-per as part of the ICAS proceedings or as individualoff-prints from the proceedings.

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