07-01-2011, 11:00 PM
ABSTRACT
Detection of corrosion damage in aircraft wing skin structures is an ongoing NDT challenge. Ultrasonic methods are known and well-accepted techniques, which are relatively simple to carry out in terms of setup, probes and instrumentation and operator training. However, with conventional inspection from the top surface using a transducer at normal incidence (0o to the normal to the surface) producing a visual picture in the form of a C-scan, it is very tie consuming to point-by-point inspects large aircraft wing skin areas. In 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 exfoliation around and under the rivets. Ultrasonic guided waves demonstrate potential as promising, global and fast inspection method. It can be used to compliment and in some cases, be an alternative to conventional ultrasonic C-scan inspection method.
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TABLE OF CONTENTS
(1) INTRODUCTION
(i) NON-DESTRUCTIVE TESTS
(2) CORROSION DETECTION WITH GUIDED WAVES
(3) EQUIPMENT AND INSTRUMENTATION
(4) INSPECTION RESULTS
(i) LAYER CORROSION USING CONTROLLED THINNING
AREAS
(ii) CORROSION DETECTION IN LAP SPLICE JOINTS
(iii) CORROSION DETECTION UNDER FASTENERS OF
WING SKIN STRUCTURES
(5) CONCLUSION
(6) REFERENCES
INTRODUCTION
Corrosion is one of the serious problem affecting airforce and other aviation industries. It affects the aircraft on its wings, surface, between joints and fasteners. The presences of corrosion underneath the paints of surface and between joints 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. Hence for the further use of the plane, Non-destructive tests (NDT) are carried out.
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 are:
1) Visual inspection
2) X-ray inspection
3) Die (liquid) penetration inspection
4) Magnetic particle inspection
5) Eddy current inspection
6) Ultrasonic inspection
Ultrasonic inspection is conventionally used for corrosion detection in aircraft wings. But the conventional inspection method carries with it certain defects like:
(i) It scans perpendicular to the surface and hence rate of scanning (from point to point) is less and hence highly time consuming.
(ii) Conventional method is not capable of detecting disbonds between layers and cracks at fastener holes.
These defects are over come by a newly developed inspection method using guided ultrasonic waves.
Guided waves demonstrate an attractive solution where conventional ultrasonic inspection techniques are less sensitive to defects such as corrosion/disbonds in thin multilayered wing skin structures and hidden exfoliation under wing skin fasteners. Moreover, with their multimode character, selection of guided wave modes can be optimized for detection of particular types of defects. Mode optimization can be done by selecting modes with maximum group velocities (minimum dispersion), or analysis of their wave mode structures (particle displacements, stresses and power distributions). Guided Lamb 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 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 exfoliation detection immediately adjacent to fasteners in aircraft wing skin. It 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, excited and tested in pulse echo and pitch catch setups. Launch angles were obtained from the calculated dispersion curves. Theoretical group velocities were compared to tested group velocities to confirm the excited modes at frequency thickness product and launch angle. The simulated corrosion in single and multilayered wing skin structures and exfoliation located under several rivets was successfully detected. Some guided Lamb modes proved to be more sensitive to corrosion type defects and produced better results.
CORROSION DETECTION WITH GUIDED WAVES
Guided Lamb modes are dispersive waves and their velocities are a function of the frequency thickness product. Therefore, any material changes such as corrosion/exfoliation or lack of adhesion between two layers will affect the propagating mode amplitude, velocity, frequency spectrum and its time of flight.
RF 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.
Inspection of lap splice joint 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 RF signal when corrosion exists between the two bonded parts. Otherwise, if there is no corrosion, the excited mode will leak into the second joint producing relatively high amplitude RF signal (Figure 1). In a pulse echo setup, a low RF signal is obtained in the presence of corrosion and high RF signal is obtained for absence of corrosion.
Fatigue cracks and exfoliation under the shadow of fastener heads in aircraft skin structures can be detected using ultrasonic guided waves. Guided modes are selected and launched from outside the exfoliated and hidden area to interrogate the interested rivets. In pulse echo setup, the received mode associated with RF signals include indications and reflections from exfoliation.
EQUIPMENT AND INSTRUMENTATION
The system used in our experimentation is Tektrend's PANDA® Guided Wave System (Figure 2). The new PANDA® Guided Wave System unit is an advanced modular and portable automated scanning system. It can be configured for conventional UT and ET transducer positioning, providing C scan images. The PANDA® can be configured for guided wave inspection, providing cost effective, practical nondestructive evaluation.
Figure 2. Guided wave testing system
The PANDA® Automated Scanning System is self contained in a single unit in which all the electronic boards are mounted in the system computer workstation. It 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 export to an integrated pattern recognition package. The positioning control, ultrasonic control, data acquisition, display and analysis software are all integrated into a single software package, ARIUS IV®.
The Guided Wave System is hosted on flexible rail to allow scanning of curved surfaces and to enable complete automation of the ultrasonic field inspection. An adaptable spring loaded piston design for holding transducers is mounted on the Y axis scanning arm, which moves on the X¬- axis. The system is fitted to the inspected surface with a vacuum control system. The PANDA® Arm can operate in vertical and horizontal orientations and scan contoured and edged surfaces. Measurement can be made in pulse echo as well as pitch catch modes with piezoelectric transducer probes (optional with EMAT probes) with 0.005 and 0.002 inch maximum scanning accuracy and resolution with a maximum scanning rate of 6 inches/second at maximum resolution. The transducer probes are driven by a tone burst pulser to excite narrow band guided wave modes and to provide high power to launch the wave over long distances. With tone burst excitation, the operating frequency and the pulse characteristics of the transmitter can be controlled in a repeatable manner.
INSPECTION RESULTS
Detectability of corrosion in aircraft wing skins was investigated for three cases. One layer corrosion using controlled thinning areas, two layers corrosion detection in lap splice joints and corrosion detection under fasteners of wing skin structures. Tests were performed using three aluminum specimens with different types of simulated corrosion.
(i) Layer corrosion using controlled thinning areas:
The first specimen represented 460x405x I mm aluminum plate with controlled thinning in designated areas. To demonstrate the sensibility of the excited wave modes, corrosions were induced in three places with different levels of thinning (10%, 15% and 25%). Measurements were made using the pitch catch setup which consisted of two variable angle broadband transducers with central frequencies at 3.5 MHz, 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 piezo composite transducers to generate the A1 mode at 2.2 MHz with an incident angle of 200. Figures 3 b, 3 c and 3 d show the RF waveforms obtained with transducers positioned perpendicular to the corroded areas (three locations), while Figure 3a shows RF waveform obtained with transducers perpendicular to the noncorroded area.
A1 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. Mode selection and optimization can improve the resolution of material loss estimation.
Figure 3. a) noncorroded area b) corroded area c) corroded area d) corroded area
(ii) Corrosion detection in lap splice joints
Tests were also carried out on 406x322x1.0 mm lap joint (Figure 4). The width of the bonded area was 68.5 mm. The lap Joint was assembled and subjected to accelerated corrosion in a salt fog chamber.
Guided wave inspection was performed on the lap joint specimen and inspection results were evaluated in terms of the sensitivity and repeatability. Scanning was carried out over the sample illustrated in Figure 4 along the X direction using two transducers in the pitch catch setup to excite the S0 mode at 1.5 MHz.
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. In the noncorroded area, there was a good bond between the second and the first layer; therefore, the transmitted signal amplitude was attenuated due to leakage of the transmitted energy into the second layer.
Figure 5 a shows single line modified C scan results of this inspection and presents a series of signals in three dimensional format. Transducer displacement (X-¬direction), time of flight (Y direction) and signal amplitudes (Z direction). The well bonded (non corroded) areas are characterized by high amplitude signals (signals indicated by red colour). Poorly 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 Figure 5a 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 Sight). Corrosion was detected in the two ends of the specimen by both techniques as shown in Figure 5b and 5c. 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. In the D Sight image, the existence of corrosion is inferred by the presence of waviness (pillowing) between the rivets, which is Caused by the formation of corrosion products (aluminum oxide and hydroxide) at the interface between the two plates.
(iii) Corrosion detection under fasteners of wing skin structures
The third series of tests were performed on fasteners of wing skin structures to detect corrosion damage immediately adjacent to the fastener holes in airframe structures as shown in Figure 6. Fatigue cracks commonly initiate at fasteners since high stresses around it are created. Water and humidity then are infiltrated to create exfoliation and corrosion around and under the rivets. As 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.
In this test, once again, a linear manual guided wave scan was performed by moving a single transducer in a pulse-echo mode at 3.5 MHz with in incident angle of 370 along the specimen in the Y direction parallel to the fastener row at a distance varying from 0.1” to 0.5” from the line of holes (Figure 7). The displacement of the wedge/transducer assembly was performed using the PANDA® automated scanner shown in Figure 3 which encoded position in both the x and y directions. One full RF waveform was acquired at every 0.12mm along the scan path. The RF waveform was digitized at 100 MHz and contained 2048 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.
Figure 8 shows single line of scan results of this inspection. The image is color coded according to the reflected amplitude (ultrasonic energy); i.e., blue corresponds to minimum reflected energy and white to maximum (Figure 8). The time scale increases vertically from top to bottom and the horizontal scale corresponds to the scan displacement at an increment.
The reflected energy front the left hand Cluster shows a trail of small reflections on both the left and right of the fastener. These regions are indicated in boxes in Figure 8. These reflection trails are clearly distinguishable from the indication of a defect free cluster shown on the right in Figure 8. Interpretation of the fastener hole integrity is based on the presence of a trailing shadow below the fasteners on either side of the main reflection. Although the exfoliation reflectors are more diffuse than the discrete reflectors provided by crack-like defects, the indications are clear.
Performance and repeatability tests were performed on similar specimens. The initial inspection and immediate interpretation provided 46% identification rate of all defects in the 15"x12"x0.2” specimens (68% If we include the possible defects) with two false calls. Based on the experience obtained during Inspection Sessions, a subsequent interpretation session gave a detectability score of 90% with 5 false calls. The false calls were subsequently attributed to coupling inconsistencies and possibly stray signals produced by the presence of the stringer attached to the specimen.
CONCLUSION
A practical inspection procedure was demonstrated using guided waves for fast and effective inspection to detect and locate defects in layered aircraft structures. Lamb wave inspection can be carried out either by using two probes in pitch catch or one probe in pulse echo configurations. It can detect corrosion in lap-splice joints 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. It has also the capability to detect the extend of corrosion.
Results form exfoliation under the shadow of fastener heads was detected using ultrasonic guided waves launched from outside the area with imaging to assist in interpretation. However, results form thicker tapered wing skin specimens were not conclusive, the guided wave technique did not seem to apply appropriately to these samples. It appears that some bulk shear components dominated the scan results and provided extra reflection from the countersink and the exfoliation. It also suffers from the drawback of the need of highly sensitive and reliable transducers.
REFERENCES:
(1) Ultrasonic guided waves for NDE of Adhesively Bonded Joints in Aging Aircraft
– by J.L.Rose, K.Rajana
(2) Production technology – by R.K.Gain,
(3) Production technology – by O.P.khanna
(4) schehl[at]udri.udayton.edu
(5) tisec.com
