ABSTRACT Detection of corrosion damage in aircraft wing skin structures is an ongoing NDT challenge. Ultrasonic methods are known and wellaccepted techniques, which are relatively simple to carry out in terms of setup, probes and instrumentation and operator training. owever, with conventional inspection from the top surface using a transducer at normal incidence incidence !"o to the normal to the surface# producing a visual picture in the form of a $-scan, it is very time consuming to carry out point-by point inspection of large aircraft wing skin areas. %n addition it is too difficult to detect disbonds in thin multilayered and fatigue cracks in the shadow region at fastener holes in airframe structures where water and humidity then are infiltrated to create corrosion and e&foliation around and under the rivets. Ultrasonic guided waves demonstrate potential as promising, global and fast inspection method. %t can be used to comp compli lime ment nt and and in some some case cases, s, be an alte altern rnat ativ ivee to conv conven enti tion onal al ultrasonic $-scan inspection method.
TABLE OF CONTENTS
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TITLE
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Introduction Non destructive test.
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CORROSION DETECTION WITH GUIDED WAVES.
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E"UIP#ENT AND INSTRU#ENTATION.
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INSPECTION RESULTS. *ingle layer corrosion. $orrosion detection in lap oint. $orrosion detection under fasteners of wing skin structure.
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ADVANTAGES ' DISADVANTAGES
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CONCLUSION.
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REFERENCES
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LIST OF FIGURES
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TITLE Transmission results from corroded and non corroded area. 0uided wave testing system. *ingle layer corrosion results. %nspected specimen for lap oint corrosion. *caning results for lap oint. Transmitted waves from corroded region. 1&foliation at fasteners. %nspection of fasteners with guided wave. 0uided wave scan result.
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1. INTRODUCTION The problem of detecting internal defects in composite materials have received great attention in recent years both for quality control during production phases and for inservice inspection during maintenance operation. The use of non destructive techniques is necessary for the analysis of internal properties of structure without causing damage to the materials. *ome of these NDT41 techniques are based on analysis of transmission of different signals such as ultrasonics. %n last decade ultrasonic techniques have shown to be very promising for non-destructive inspection and they are becoming an effective alternative to such traditional and well studied approaches as thermography, eddy current , and shearography.5(6 $orrosion is one of the serious problem affecting airforce and other aviation industries. %t affects the aircraft on its wings, surface, between oints and fasteners. The presences of corrosion underneath the paints of surface and between oints are not easy to be detected. The unnoticed presence of corrosion may cause the aircraft to crash leading to human and money loses. To detect the corrosion present on the metal surface, various methods and tests are used. These tests conducted should be such that it does not destroy
or disassemble the plane to parts or damage its surface. ence for the further use of the plane, Non-destructive tests !NDT# are carried out.
1.1 NON*DESTRUCTIVE TESTS
Non-destructive testing as the name suggests is testing procedure without any damage to the part being tested. The various non-destructive testing methods used are7 '# 8isual inspection +# 9-ray inspection # Die !liquid# penetration inspection (# :agnetic particle inspection )# 1ddy current inspection ;# Ultrasonic inspection Ultrasonic inspection is conventionally used for corrosion detection in aircraft wings.
ed for detection of particular types of defects. :ode optimi>ation can be done by selecting modes with ma&imum group velocities !minimum dispersion#, or analysis of their wave mode structures !particle displacements, stresses and power distributions#. 0uided ?amb modes have been used for long-range=large area corrosion detection and the evaluation of adhesively bonded structures. Ultrasonic guided waves are promising but require procedure development to
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ensure high sensitivity and reliable transducer coupling and to provide a mechanism to transport the probe!s# over the area to be scanned. This paper describes some practical inspection setups and procedures based on guided wave modes for corrosion damage detection in single and multilayered wing skin structures and e&foliation detection immediately adacent to fasteners in aircraft wing skin. %t describes the results of their application to detection of corrosion in simulated and real components of aircraft wing skin. Using a tone-burst system, the wave modes are selected, e&cited and tested in pulse-echo and pitch-catch setups. ?aunch angles were obtained from the calculated dispersion curves. Theoretical group velocities were compared to tested group velocities to confirm the e&cited modes at frequency-thickness product and launch angle. The simulated corrosion in single and multilayered wing skin structures and e&foliation located under several rivets was successfully detected. *ome guided ?amb modes proved to be more sensitive to corrosion type defects and produced better results.
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2. CORROSION DETECTION WITH GUIDED WAVES 0uided ?amb modes are dispersive waves and their velocities are a function of the frequency thickness product. Therefore, any material changes such as corrosion=e&foliation or lack of adhesion between two layers will affect the propagating mode +,-itud/0 /ocit0 3r/4u/nc spectrum and its time-of-flight. @A waveforms from guided modes going through a corroded area have a relatively low transmitted signal amplitude and time-of-flight shift, while noncorroded areas are associated with stable time-of-flight and high received signal amplitude. %nspection of lap splice oint with guided waves in a pitch-catch setup permits a selected guided wave mode to travel from the sender toward the receiver probe, producing relatively low amplitude @A signal when corrosion e&ists between the two bonded parts. Btherwise, if there is no corrosion, the e&cited mode will leak into the second oint producing relatively high amplitude @A signal !Aigure '#. %n a pulse echo setup, a low @A signal is obtained in the presence of corrosion and high @A signal is obtained for absence of corrosion. Aatigue cracks and e&foliation under the shadow of fastener heads in aircraft skin structures can be detected using ultrasonic guided waves. 0uided modes are selected and launched from outside the e&foliated and hidden area to interrogate the interested rivets. %n pulse-echo setup, the received mode
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associated with @A signals includes indications and reflections from e&foliation. 5'6
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!. E"UIP#ENT AND INSTRU#ENTATION Cs part of Tektrends research program on the study of ND% reliability and its implementation, a series of e&periments have been carried out using simulated and real specimens to determine the reliability of three ND1 techniques for disbond, corrosion and e&foliation detection in aircraft structures. $orrosion levels in the specimens then were characteri>ed by these multiple ND% techniques. *pecimens with real and simulated defects were provided to us by National @esearch $ouncil $anadas %nstitute for Cerospace @esearch !%C@=N@$# and C@%N$ %nc Bklahoma city as part of mutual evaluation and research work.
The system used in our e&perimentation is Tektrends ECNDCF 0uided Gave *ystem !Aigure +#. The new ECNDC F 0uided Gave *ystem unit is an advanced modular and portable automated scanning system. %t can be configured for conventional UT and 1T transducer positioning, providing $-scan images. The ECNDCF can be configured for guided wave inspection, providing cost effective, practical nondestructive evaluation. The ECNDCF Cutomated *canning *ystem is self-contained in a single unit in which all the electronic boards are mounted in the system computer workstation. %t offers advanced analysis and interpretation capabilities, where intelligent scans can be performed with a pre-designed intelligent classifier. The system contains tools to tag signals for e&port to an integrated pattern recognition package. The positioning control, ultrasonic control, data acquisition, displays and analysis software are all integrated into a single software package, ARIUS %8F.
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Fi5ur/ !.1. 0uided wave testing system
The 0uided Gave *ystem is hosted on fle&ible rail to allow scanning of curved surfaces and to enable complete automation of the ultrasonic field inspection. Cn adaptable spring-loaded piston design for holding transducers is mounted on the H-a&is scanning arm, which moves on the 9- a&is. The system is fitted to the inspected surface with a vacuum control system. The ECNDCF Crm can operate in vertical and hori>ontal orientations and scan contoured and edged surfaces. :easurement can be made in pulse-echo as well as pitch-catch modes with pie>oelectric transducer probes !optional with 1:CT probes# with "."") and ".""+ inch ma&imum scanning accuracy and resolution with a ma&imum scanning rate of ; inches=second at ma&imum resolution. The transducer probes are driven by a tone-burst pulser to e&cite narrow-band guided wave modes and to provide high power to launch the wave over long distances. Gith tone-burst e&citation, the operating frequency and the pulse characteristics of the transmitter can be controlled in a repeatable manner. 5+6
%. INSPECTION RESULTS
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Delectability of corrosion in aircraft wing skins was investigated for three cases. *ingle layer corrosion using controlled thinning areas, two layers corrosion detection in lap splice oints and corrosion detection under fasteners of wing skin structures. Tests were performed using three aluminum specimens with different types of simulated corrosion.
%.1 Sin5/ L+/r corro6ion u6in5 contro/d t7innin5 +r/+68 The first specimen represented (;"&(")& % mm aluminum plate with controlled thinning in designated areas. To demonstrate the sensibility of the e&cited wave modes, corrosions were induced in three places with different levels of thinning !'"I, ')I and +)I#. :easurements were made using the pitch-catch setup which consisted of two variable angle broadband transducers with central frequencies at .) :>, one of the transducers acts as transmitter used to generate the guided wave mode and the other one was used to receive the generated mode and its interaction with the corroded structure. The first set of tests demonstrates detectability of the open corrosion on the aluminum plate using the pitch catch setup with pie>o-composite transducers to generate the C' mode at +.+ :> with an incident angle of +"". Aigures b, c and d show the @A waveforms obtained with transducers positioned perpendicular to the corroded areas !three locations#, while Aigure a shows @A waveform obtained with transducers perpendicular to the noncorroded area. C' guided mode signals passing through the corroded area have a transmitted signal low amplitude and higher time-of-flight which is consistent with theoretically calculated group velocity dispersion curves, while signals from the noncorroded area are associated with stable time-of-flight and high received signal amplitude. Therefore, wave propagation behavior in corroded areas allows estimation of the percentage of the corrosion material loss. :ode selection and optimi>ation can improve the resolution of material loss estimation.
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Fi5ur/ %.1.1. a# noncorroded area b# corroded area c# corroded area d#
corroded area
%.2 Corro6ion d/t/ction in +- 9oint6 ?ap oints commonly found in aircraft industry can easily be tested with guided waves.
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0uided wave inspection was performed on the lap oint specimen and inspection results were evaluated in terms of the sensitivity and repeatability. *canning was carried out over the sample illustrated in Aigure ( along the 9-direction using two transducers in the pitch-catch setup to e&cite the * " mode at '.) :>. The corroded area between the second and the first aluminum layers, created a disbond and permitted bad transmission of the generated mode from the sender toward the receiver without any energy leakage in the additional bonded aluminum layer. %n the noncorroded area, there was a good bond between the second and the first layerK therefore, the transmitted signal amplitude was attenuated due to leakage of the transmitted energy into the second layer.
(.+. transmitted waves from corroded and non-corroded re ion '+
Aigure ) a shows single line modified $-scan results of this inspection and presents a series of signals in three-dimensional format. Transducer displacement
!9-direction#,
time-of-flight
!H-direction#
and
signal
amplitudes !L-direction#. The well-bonded !non-corroded# areas are characteri>ed by high amplitude signals !signals indicated by red colour#. Eoorly bonded areas !caused by corrosion# resulted in a reduction of amplitude of the received signals as shown by the low amplitude echoes at both ends of the specimen. The high and low amplitude signals are represented by the lighter and heavier colors, respectively. The interruptions between signals in Aigure )a-are due to the presence of rivets. To verify the guided wave results, these specimens were also inspected using, an eddy-current technique as well as an enhanced optical technique !D-*ight#. $orrosion was detected in the two ends of the specimen by both techniques as shown in Aigure )b and )c. The red and orange colors in the eddy current image show areas of severe corrosion while the green and blue represent areas having very light corrosion. %n the D-*ight image, the e&istence of corrosion is inferred by the presence of waviness !pillowing# between the rivets, which is $aused by the formation of corrosion products !aluminum o&ide and hydro&ide# at the interface between the two plates.5+656
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%.! Corro6ion d/t/ction und/r 3+6t/n/r6 o3 :in5 6;in 6tructur/6 The third series of tests were performed on fasteners of wing skin structures to detect corrosion damage immediately adacent to the fastener holes in airframe structures as shown in Aigure ;. Aatigue cracks commonly initiate at fasteners since high stresses around it are created. Gater and humidity then are infiltrated to create e&foliation and corrosion around and under the rivets. Cs the guided waves penetrate within and beyond the region of the fastener head, ultrasonic energy is reflected from discontinuities !corrosion, mechanical damage# present in the region of interrogation.
%n this test, once again, a linear manual guided wave scan was performed by moving a single transducer in a pulse-echo mode at .) :> with in incident angle of 2" along the specimen in the H-direction parallel to the fastener row at a distance varying from ".'M to ".)M from the line of holes !Aigure 2#. The displacement of the wedge=transducer assembly was performed using the ECNDCF automated scanner shown in Aigure which encoded position in both the & and y directions. Bne full @A waveform was acquired at every ".'+mm along the scan path. The @A waveform was digiti>ed at '"" :> and contained +"(/ points. The acquired signals were averaged and filtered and all the data for each scan were saved in a file for later retrieval and analysis.
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Aigure (..+ shows single line of scan results of this inspection. The image is color-coded according to the reflected amplitude !ultrasonic energy#K i.e., blue corresponds to minimum reflected energy and white to ma&imum !Aigure (..#. The time scale increases vertically from top to bottom and the hori>ontal scale corresponds to the scan displacement at an increment. The reflected energy front the left-hand $luster shows a trail of small reflections on both the left and right of the fastener. These regions are indicated in bo&es in Aigure /. These reflection trails are clearly distinguishable from the indication of a defect-free cluster shown on the right in Aigure /. %nterpretation of the fastener hole integrity is based on the presence of a trailing shadow below the fasteners on either side of the main reflection. Clthough the e&foliation reflectors are more diffuse than the discrete reflectors provided by crack-like defects, the indications are clear.
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Eerformance and repeatability tests were performed on similar specimens. The initial inspection and immediate interpretation provided (;I identification rate of all defects in the ')&'+&".+M specimens !;/I %f we include the possible defects# with two false calls.
ADVANTAGES
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'6 Aast and effective inspection method. +6 Cpplicable to any shaped surface. 6 $an detect large e&tend of corrosion. (6 as greater accuracy than other non destructive methods in determining the depth of internal flaws.
DISADVANTAGES
'6 @equires sensitive and reliable transducer which increases the cost. +6 $annot be applied to thicker surfaces as transmission does not take place properly. 6 @equires very skilled labour.
&. CONCLUSION
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C practical inspection procedure was demonstrated using guided waves for fast and effective inspection to detect and locate defects in layered aircraft structures. ?amb wave inspection can be carried out either by using two probes in pitch-catch or one probe in pulse-echo configurations. %t can detect corrosion in lap-splice oints in a single scan and the procedure setup is suitable for presentation of the results as an image relating the amplitude and time-of-flight to facilitate interpretation. %t has also the capability to detect the e&tend of corrosion. @esults from e&foliation under the shadow of fastener heads were detected using ultrasonic guided waves launched from outside the area with imaging to assist in interpretation. owever, results from thicker tapered wing skin specimens were not conclusiveK the guided wave technique did not seem to apply appropriately to these samples. %t appears that some bulk shear components dominated the scan results and provided e&tra reflection from the countersink and the e&foliation. %t also suffers from the drawback of the need of highly sensitive and reliable transducers.
). REFERENCES 5'6 @eview of %C@ ND% research in support of ageing aircraft. Jer>y p.komorowski, David *. forsyth, $harles 1 .chapman 5+6 NDT inspection techniques for corrosion detection of aircraft structures. :.<@C**C@D, C. $Cio, :.leo, C.Distante, 8.pianese
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5)6 Ultrasonic guided waves for ND1 of Cdhesively
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