AMMTIAC Quarterly, Vol. 2, No. 2 - Advanced Materials ...

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Amplitude 150 0 -150 0 50 100 150 time(µs) 150 Figure 2. Waveforms for a Three-Layer Structure, Aluminum- Adhesive-Aluminum, (a) Non-Corroded Region, (b) Corroded Region at the Bottom, (c) Corroded Region at the Top. (Reprinted with permission from[6] © 2000 American Institute of Physics.) Guided ultrasonic waves have been applied for inspecting pipes by using an array of transducers around a pipe to focus energy at particular positions on the pipe circumference at chosen axial locations.[5] Although good results were obtained in a laboratory environment, this technique was not as successful in actual shipboard tests of piping where pipe elbows, hangers, flanges, valves, welds, etc., add significant complexity. However, this approach still has the potential to be applied to more complex structures if coherent noise can be controlled and algorithms can be developed to interpret waveforms. Prior work has shown that guided waves also have some potential for inspection of multilayer aircraft structures for hidden corrosion and cracking.[6] Figure 2 (a, b, and c) shows guided wave waveforms from a three-layer aircraft structure of aluminum-adhesivealuminum. The first signal is from a non-corroded region (Figure 2a), the second is from a corroded region in the bottom aluminum plate (Figure 2b), and the third is from a corroded region in the top aluminum plate (Figure 2c). Non-contact air-coupled transducers can be used to apply guided waves to the inspection of thinning in aluminum plates.[7] In this application, a pair of micromachined gas (air)-coupled capacitive transducers is used for the generation and detection of guided plate modes. Features in the dispersive behavior of selected guided wave modes were used for the detection of plate thinning. Mode cutoff measurements provided a qualitative detection of plate thinning, while frequency shift measurements were able to provide a quantitative measure of plate thinning. The experimental setup with aircoupled transducers is shown in Figure 3. Non-contacting electromagnetic acoustic transducers (EMATs) can also be used to generate and detect shear horizontal (SH)-guide waves for inspection and mapping of corrosion in pipe walls and plates.[8] The SH waves have a pure shear-motion parallel to the surfaces and perpendicular to the plane of incidence. SH-guided waves have a unique feature in contrast to guided waves with inplane polarization; the lowest order mode has no dispersion and the dispersion of the higher order modes is much weaker than modes Receiving Module RITEC RAM-10000 Gated Amplifier Amplitude (a) 0 -150 Charge Amplifier Simulated Hidden Corrision Test Plate AC/DC Decoupler 200V Bias Transmitter Receiver Control/Acquisition Computer Monitor Signals Oscilloscope Figure 3. Experimental Setup for Gas (Air) – Coupled Ultrasonic Guided Wave Detection of Thinning Defects in Aluminum Plates.[7] Amplitude (c) 150 0 50 100 150 time(µs) 0 -150 (b) 0 50 100 150 time(µs) with polarization in the plane of incidence. As a result, SH-guided waves could be economically and reliably used to detect and map corrosion in plates and pipes. Couplant free excitation and the resultant simplified waveforms add to the versatility and usefulness of the technique. A shorter range wave technique that utilizes creeping (or lateral waves) and head waves in parallel or near-parallel walled metal structures has been commercialized in the United Kingdom.[9] The technique utilizes a transducer at the critical angle to generate creeping and head waves, and a second receiving transducer is placed up to one meter away. The unique way in which the waves propagate provides complete isonification of the plate or pipe with little attenuation. This allows the probes to be well separated compared to traditional creeping wave inspection. The technique is also applicable to corrosion in pipes under pipe supports, and the instrumentation has been field proven. Ultrasonics Confidence in ultrasonic inspection to detect and quantify corrosion in field applications has often required the disassembly of systems and testing in water baths. Results of various tests have shown that the detection of hidden corrosion on various aluminum alloys of varying thickness was useful above 10% metal loss, but the technique was not applicable for metal loss below 10%. To improve the ability for detecting hidden corrosion, there have been continued efforts to apply the dripless bubbler ultrasonic scanner, which is an ultrasonic technique that does not require a water bath and disassembly.[10] This technique was selected as a primary candidate by the Air Force Logistics Center in Oklahoma City (OC- ALC) for the detection and quantification of intergranular corrosion 100 90 80 70 60 50 40 30 20 10 0 Eddy Current A Eddy Current B Ultrasonic Testing Dripless Bubbler (Sierra Matrix) Dripless Bubbler (ISU) Thermal A Hit % – Within Shadow Hit % – Beyond Shadow False Call Rate Thermal (Wayne State Univ.) Figure 4. Corrosion Detection around Wing-Skin Fasteners – Chemically Induced Intergranular Corrosion Trial Results.[10]* prior to the onset of exfoliation around wing skin fasteners. This is a major inspection problem for aging aircraft. Figure 4 shows the results of tests for corrosion detection around wing-skin fasteners. In a similar NDE study, a novel ultrasonic pulse echo technique was developed to detect intergranular corrosion around fastener holes in aluminum wing skins before the exfoliation stage.[11] In this case, a focused transducer with a special fixture was used to overcome the typical problems: not enough couplant, transducer not perpendicular to the part, and varying transducer pressure. In general, there was good agreement between the ultrasonic results and the results from the mechanical rework of the wing skin and dissection of the fastener hole. Ultrasonics can also be applied without a water bath by using laserultrasonics.[12] Laser ultrasonics have been applied for the inspection of painted metal skin, aircraft lap joints. When lap joint corrosion reaches a specific level, normally 10% of the nominal skin thickness, the section of the lap joint must be replaced. Visual inspection of the pillowing of the surface has been used to detect this type of corrosion, but it cannot supply quantitative information. By using spectral 4 The AMMTIAC Quarterly, Volume 2, Number 2
analysis of the laser-ultrasonic waveforms, the residual metal skin thickness of the top skin of the lap joint could be determined. Results from standard samples with flatbottom holes showed that the technique could detect metal loss below 1% of the nominal thickness of the metal skin. Comparison of the laser-ultrasonic measurements to X-ray images showed that the laser-ultrasonics has the same accuracy as the X-ray imaging (metal loss below 1%). However, laser-ultrasonics as compared to X-ray imaging does not require disassembly of the structure, and the inspection could be carried out during routine maintenance. Other ultrasonic approaches for detecting hidden corrosion involve ultrasonic imaging using dry-coupled probes.[13] In addition, a commercial instrument that essentially operates as an “ultrasonic camcorder” can produce C-scan images from ultrasound signals, which are introduced into the sample with a large, unfocused commercial transducer.[14] The implementation can be either in transmission or reflection. A water squirter or ultrasonic couplant on the surface is required for the application of this technique. By combining obliquely backscattered ultrasonic signals with the sensor array real-time charge-coupled imaging system used in the ultrasonic camcorder, a rapid technique for detecting corrosion in aircraft skins without the need for paint removal has been developed.[15] The Transducer Incident Beam system can produce 30 frames a second and the unit can be programmed to examine time-of-flight bounds and thus produce 3D images of material slices. Figure 5 shows a schematic of the technique and Figure 6 shows a pulse-echo image of a corroded area in an aluminum plate. The combination of obliquely backscattered ultrasonic signals and the charged-coupled ultrasonic imager produces a viable aircraft corrosion inspection technique. Eddy Current Eddy current NDE is a prime method for detecting hidden corrosion in electrically conducting materials. The method is based on generating a localized alternating current field in the sample using a probe coil. The same probe coil or another detector then measures the material’s response to the induced eddy currents. Defects cause a perturbation in the eddy currents, and this can be detected by a change in impedance or phase variation in the detecting circuit. In the past, eddy current NDE was limited in detecting hidden corrosion to very shallow depths by the lack of penetrating capability of the eddy currents. However, over the last several years there have been several notable advances in eddy current NDE stemming from research to detect hidden corrosion and other types of defects. When combined with magnetoresistive sensors, eddy currents can be used to detect corrosion in second and third layers in aircraft lap joints.[16] High performance magnetoresistive sensors are much more sensitive than standard pick up coils used in conventional eddy current NDE, and they operate at much lower frequencies. Their low frequency range and linear response to frequency allows for much greater depth of inspection than with conventional eddy current NDE that operate at higher frequencies. Figure 5. A Schematic View of the Wavefield in a Painted 2-Layer Structure.[15] Figure 6. A Pulse-Echo Image of Corroded Area in an Aluminum Plate Visualized by the Current Sensor Array Real Time Imaging (SARTI) System. The Depth Data Represent the Information at the Crosshair Cursor Location.[15] (Reprinted with permission from SPIE) Excitational Coil Specular Reflection Non-Specular Reflection Corrosion Figure 7. Two Energy Coupling Paths in a Remote Field Eddy Current Probe for Pipe Testing.[19] Pulsed eddy current NDE technology has also made recent advances.[17] Its robustness to variations in geometry, paint thickness, rivet heads, surface warping, and sensor liftoff are allowing this technology to become a major method for detection of hidden corrosion. Conventional eddy current instruments measure the impedance and reactance of the detection coil, while the pulsed eddy current instrument measures the transient voltage signal with a frequency spectral content from direct current to 100 KHz or higher. This broadband characteristic provides the ability to conduct digital signal analysis and to extract subtle differences in waveforms associated with small geometry variations. Hence, it provides the ability to quantify a large number of parameters. For example, material loss at or near the surface will have more of an effect on the early-time portion of the total transient interval than on the late-time portion. The opposite will be true for very deep defects. Remote field eddy current is another variation of conventional eddy current that has found widespread use in detecting hidden corrosion. Its capabilities have recently been expanded and automated.[18] The remote field eddy current technique is based on the measurement of the voltage induced in a pickup coil by flux which has passed twice through the test piece as shown in Figure 7. Measurement of the phase change caused by a discontinuity has a linear relationship with changes in thickness of the pipe wall. Remote field eddy current is not sensitive to liftoff, but an ultra sensitive eddy current system is required to handle the very low amplitude signals obtained from remote field eddy current probes. Magneto-optical imaging of eddy currents is another innovative advance in eddy currents applied to detection of hidden corrosion.[20] Magneto-optical imaging of the eddy currents generates real-time images of large surface areas to quickly detect subsurface corrosion and cracking. Although eddy current techniques do not have as much sensitivity and spatial resolution as ultrasonic NDE techniques, the ability to detect corrosion in multiple layer structures and at greater depths are advantageous. Thermography Thermography provides a fast, non-contact technique for detection of subsurface corrosion. Thermography is based on using the thermal differences between a material and material defects to image discontinuities such as corrosion. It has demonstrated ability to detect corrosion under paint without paint removal. In addition, advances in commercial instrumentation and algorithms, particularly for thermal wave techniques[21], have substantially enhanced field applications. Moving linear heat sources have also enhanced thermographic capability. Thermal wave systems operate by sending a pulse of heat from the surface into the material, where it undergoes thermal reflection at either the rear surface or at thermal impedance changes (e.g. corrosion). The effect of these thermal reflections is that it modifies the local cooling rate at the surface. The cooling rate, in turn, is monitored through its effect on the IR radiation from the surface. The IR http://ammtiac.alionscience.com The AMMTIAC Quarterly, Volume 2, Number 2 5 Paint Skin Primer Bond Substructure (Stiffener, T-Cap, Stringer, etc.) Indirect Energy Coupling Path Direct Energy Coupling Path Indirect Energy Coupling Path Pick-up Coil
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