Presented at: NASA/FAA/DoD Conference on Aging Aircraft Kissimmee, Florida; Sept. 10-13, 2001
High-Resolution Eddy Current Sensor Arrays for Detection of Hidden Damage including Corrosion and Fatigue Cracks Tom Yentzer WR-ALC/TIEDM Robins AFB, GA 31098 Phone: 912-926-4489; e-mail:
[email protected] Steven Kramer Raytheon Aircraft Integration Systems, Greenville, TX e-mail:
[email protected] Neil Goldfine, Mike Fisher, Vladimir Zilberstein, Darrell Schlicker, Mark Windoloski, Tim Lovett, David Grundy, Yanko Sheiretov JENTEK Sensors, Inc., 110-1 Clematis Avenue, Waltham, MA Phone: 781-642-9666; e-mail:
[email protected] ABSTRACT Life extension and airworthiness assurance efforts have taken advantage of advances in nondestructive evaluation (NDE) through damage tolerance methods. In damage tolerance methods, periodic inspections are implemented to ensure that cracks, large enough to propagate to a critical size before the next inspection, do not exist. Recent efforts have focused on development of damage tolerance methodologies for corrosion management, and propulsion system programs are beginning to look at subsurface anomalies such as cracks that initiate below the surface of a shot peened or coated part. Also, improved detection thresholds for second and third layer cracks are needed for standard damage tolerance practices to remain practical for aging commercial aircraft. Thus, there is a need for improved imaging technologies to provide reliable detection and quantitative measures of hidden damage. This paper defines NDE problems related to detection and imaging of hidden damage. New imaging methods and recent results using deep penetration Meandering Winding Magnetometer® (MWM) eddy current array imaging technology with inductive and magnetoresistive sensors are also described. Introduction With both new and aging fleets requiring increasingly efficient management in the face of severe readiness demands and reduced procurement budgets, the capability to assess and predict the state of individual components and address fleet wide conditions is increasingly important. Examples of the conditions of concern include: • Hidden cracks and corrosion in structures. • Cracks and thermal damage under coatings. • Variations in residual stress patterns (e.g., in landing gear after hard landings). • Surface and subsurface deleterious microstructure features introduced by processing. Each of these issues requires the capability not only to detect and locate the flawed condition, but also to prioritize detected damage or poor quality for rejection, rework/repair, replacement or scheduling of inspection. This prioritization requires high quality, quantitative information. Reliable, robust sensing of absolute properties and high-resolution imaging can provide part of the information needed for such Page 1 of 13
prioritization. This high resolution imaging could be performed at OEMs, depot rework facilities and in the field. The MWM and MWM-Array provides a tool for imaging, detection, and tracking of changes in absolute properties. This paper presents results of preliminary studies for detection of hidden cracks and corrosion using MWM-Arrays and a new sensor that uses a giant magnetoresistive (GMR) sensing element within a distributed shaped field inductive drive.
MWM-Array and Grid Measurement Methods Overview The Meandering Winding Magnetometer (MWM) sensor is an inductive, eddy current based sensor that has been designed specifically for absolute material property imaging [Melcher 1991; Goldfine 19941999]. The sensor consists of a meandering primary winding for creating the magnetic field, and meandering secondary windings located on opposite sides of the primary for sensing the response. The windings are typically mounted on a thin and flexible substrate, producing a conformable sensor. Microfabrication techniques are employed to produce the sensors, resulting in highly reliable and highly repeatable (i.e., essentially identical) sensors. The sensors are designed to produce a spatially periodic magnetic field in the material under test, which permits the sensor response to be accurately modeled and reduces calibration requirements. For example, in some situations an "air calibration" can be used to measure an absolute electrical conductivity without calibration standards. Scanning with these sensors (or sensor arrays) provides the capability for imaging of material properties and detection of both surface defects (e.g., cracks, lack of fusion, and alpha case in titanium) and subsurface defects (such as hard alpha inclusions, hidden corrosion, or cracks). The use of micro-fabrication and flexible circuit technologies for sensor manufacture has inherent advantages over the coils used in conventional eddy current sensors. As indicated by Auld and Moulder, for conventional eddy current sensors “nominally identical probes have been found to give signals that differ by as much as 35%, even though the probe inductances were identical to better than 2%” [Auld, 1999]. In contrast, duplicate MWM sensor tips have nearly identical magnetic field distributions around the windings as standard micro-fabrication (etching) techniques have both high spatial reproducibility and resolution. The MWM sensor response is converted into material or geometric properties using JENTEK's GridStation® Measurement System. Measurement Grid Methods are typically used to map the magnitude and phase of the sensor impedance into the properties to be determined and provide for a real-time measurement capability. The measurement grids are two-dimensional databases that can be visualized as “grids” that relate two measured parameters to two unknowns, such as conductivity and lift-off (where lift-off is defined as the proximity of the material-under-test surface to the plane of the MWM windings). For the characterization of coatings or surface layer properties, three-dimensional versions of the measurement grids called grid lattices can be used. Alternatively, the surface layer parameters can be determined from numerical algorithms that minimize the least-squares error between the measurements and the predicted responses from the sensor. The GridStation software environment controls the data acquisition instrumentation and provides a graphical user interface that permits immediate display of inspection results with minimal user interpretation. For the detection and imaging of defects and flaws, scanning MWM-Arrays provide images of material property variations. These sensor arrays use novel winding geometries that promote accurate modeling of the response and eliminate many of the undesired differences in the response of the sensing elements in existing eddy current arrays, such as cross-coupling between individual array elements. Figure 1 shows a schematic of two MWM-Arrays. The MWM-Array has a single primary winding and multiple secondary or sensing elements, which provide property images when scanned over a surface. There are numerous different MWM-Array formats in use. The sensing elements provide absolute measurements of the material response. In an array, current flow through the primary winding creates a Page 2 of 13
spatially periodic magnetic field that can be accurately modeled. The voltage induced in the secondary elements by the magnetic field is related to the physical properties and proximity to the material under test. The spatial wavelength, λ, shown in Figure 1 determines the maximum penetration depth of the applied magnetic fields. In the standard format, a single sensing element is located within each meander of the primary winding and each grouping of sensing elements provides an image pixel. Scanning of the array then provides an image of the material properties. The sensing elements are offset to provide an overlap and complete coverage when the array is scanned. Using multiple sensing elements within the array with parallel channels provides improved data rates and real-time imaging capabilities. The use of multiple sensing elements with one meandering drive permits high image resolution and sensitivity with relatively deep penetration. λ
(a) λ
(b) Figure 1. (a) Schematic and photograph of a relatively deep penetration MWM-Array, and (b) Schematic and photograph of a relatively shallow penetration MWM-Array. Page 3 of 13
Detection of Hidden Cracks
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In a recent capability demonstration for detection of hidden cracks, four specimens containing EDM notches were used with and without cracks off the notches. These specimens were provided by Raytheon Aircraft Integration Systems. They included: • An aluminum plate with a 3/16-in. hole without EDM notches [Figure 2a)]. • A fatigue specimen with a 3/16-in. hole in the center and EDM notches off the hole in the transverse plane. This specimen [Figure 2(b)] has notches with a concave front and short fatigue cracks not reaching the edges, i.e., the cracks do not appear to extend beyond the notches at either of the specimen surfaces. • Two fatigue specimens with a 3/16-in. hole, two diametrically opposed EDM notches off the hole in the transverse plane, and cracks off the notches. One of these specimens [Figure 2(c)] has a 0.07-in. long crack at each EDM notch on one side of the specimen and 0.01-in. and 0.07-in. long cracks on the other side of the specimen. The other specimen [Figure 2(d)] contains 0.15-in. and 0.20-in. long fatigue cracks at the EDM notches. In both specimens, the crack length is measured from the tip of an EDM notch to the crack tip.
3/16” hole
EDM Notch
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Figure 2. Specimens supplied by Raytheon. (a) An aluminum plate with a 3/16-in. diameter hole without EDM notches; (b) a fatigue specimen with a 3/16-in. hole in the center and EDM notches off the hole in the transverse plane; (c) specimen with a 3/16-in. hole, EDM notches off the hole in the transverse plane, and 0.07-in. long cracks at one surface and 0.01-in. and 0.07-in. long cracks at the other surface; (d) specimen containing 0.15-in. and 0.20-in. long fatigue cracks that extend beyond the EDM notches; (e) schematic of the hole with EDM notches and cracks; (f) cross-section showing the thin MWM-Array placed on a 0.06-in. thick aluminum plate for detection of hidden cracks in scanning mode
Crack detection with an MWM sensor scanned directly over the area containing cracks in the fatigue specimen shown in Figure 2(d) is illustrated in Figure 3. The specimen was scanned in the direction parallel to the axis of the specimen with a single sensing element MWM sensor that had a ¼ by ¼-in. Page 4 of 13
footprint. In the scans with the MWM sensor, the “inboard” edge of the sensor was offset so that the sensor would scan over the crack and would not cross the notch. A typical conductivity curve obtained with this MWM sensor (Figure 3) shows that the crack is readily detected with this “general use” sensor. Scanning high-resolution MWM-Arrays and permanently mounted linear MWM-Arrays as well as MWM-Rosettes permanently mounted around fasteners can detect significantly shorter “exposed” cracks, i.e., significantly smaller than 0.03 in. The feasibility of detecting cracks with a permanently mounted MWM-Rosette has been previously presented at the 2001 SPIE conference [Goldfine, 2001]. In more recent fatigue tests, MWM-Rosettes were mounted between two layers of aluminum. These tests indicated capability to detect 0.01-in. to 0.03-in. long cracks when the first channel is located close to the fastener hole. One long-term goal is to develop MWM Rosettes that can detect cracks in configurations shown in Figure 4. As shown in Figure 4(a), for one configuration, cracks must be detected through a ≥ 0.06-in. thick layer. As a first step, detection of hidden cracks was demonstrated in tests with an MWM-Array scanning through two layers of aluminum alloy plates with a combined thickness of 0.06 in. placed over the specimens. All the cracks with length ≥0.07-in. were detected. Typical results for scanning through two layers of aluminum are presented in Figures 5 and 6. Figure 5(a) shows an image obtained from a specimen with EDM notches and cracks (both specimens with notches and cracks produced similar images). Figure 5(b) shows images obtained from the specimen with notches and no significant cracks. Figure 5(c) shows an image obtained from the specimen shown in Figure 2(a), i.e., the specimen with no notches at the hole. To determine if a crack was present (i.e., not just an EDM notch), signatures of the sensor response to the hole with the EDM notches were derived. Then a quantitative measure of the difference between the sensor response and the response without cracks was computed. A threshold on this response difference was used to indicate “a crack” or “no crack” condition. The white ellipse with an “X” in the Figure indicates the presence of a crack. Note that the detection threshold can be adjusted to exclude the contribution of the hole. This is illustrated in Figure 6 where images shown in Figures 6 (a), (b), and (c) correspond to the same scans that are presented in Figures 5 (a), (b), and (c), respectively. However, a different filter was used in this case and the rejection threshold for Figures 6 (a), (b), and (c) was different to minimize the contribution of the hole. Thus, the MWM-Array can readily detect cracks at fastener holes through a 0.06-inch thick aluminum. More advanced versions of deep-penetration MWM-Arrays are being used in an ongoing FAA-funded effort for detection of cracks in the second and third layers.
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MWM Conductivity, %IACS
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Crack Detection
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Scan Location
Figure 3. Detection of a fatigue crack at one of the notches in the specimen shown in Figure 2(d). In this case, the crack was detected by scanning an MWM sensor with a ¼-inch by ¼-inch footprint directly over the specimen.
(a) (b) Figure 4. Schematic of mounting an MWM-Rosette over a patch repair for monitoring cracks under the repair (a). When possible, an MWM-Rosette can be mounted within a smart washer on the side opposite to the repair as shown schematically in (b).
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(a)
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(c) Figure 5. Images generated by the MWM-Array during scanning of the specimens (shown in Figure 2) covered with 0.06-inch thick set of two aluminum alloy plates. These images illustrate the MWM-Array’s capability, with scanning through 0.06-inch aluminum, to discriminate between (a) a hole with EDM notches and fatigue cracks, (b) a hole with EDM notches, and (c) a 3/16-inch hole without EDM notches or cracks.
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(a)
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(c) Figure 6. Images obtained in the same scan as the images shown in Figure 5 but with a different filter so that the hole with EDM notches (but without significant cracks) is also identified as rejectable. Rejection is highlighted by the white ellipses with an “X”.
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Detection of Hidden Corrosion Earlier work [Goldfine, 2000] described MWM detection of metal loss between two layers (0.04-in. thick each) in a two-layer lapjoint specimen with Alclad on both sides of each layer. The lapjoint specimen was scanned with an MWM sensor that had a single secondary and a 0.5-in. by 0.5-in. footprint. The corroded image is reproduced here in Figure 7.
(a)
(b) Sensor footprint
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Figure 7. (a) Schematic representation of the gap between alclad layers used to generate thickness lift-off (∆ ∆-h) grids for GridStation; (b) Cross-section of a two-layer alclad sample with corrosion between the two layers; (c) and (d) MWM/Gridstation measurement of the corrosion gap thickness, ∆, in mils (1 mil = 0.001-in.).
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Recently, initial examinations of aluminum panels from a C-130 flight deck chine plate inside the fuselage were performed using the same deep-penetration MWM-Array as used for detection of hidden cracks described in the preceding section. Figure 8 shows one of these panels containing corrosion on the back side not accessible for inspection without a major disassembly. It is critical to inspect the chine plate for signs of corrosion on the back side without disassembly. Figure 9 shows an image depicting the extent of metal loss in the chine plate panel. The remaining wall thickness at the deepest metal loss point in this region was about 0.03 in., i.e., the metal loss due to corrosion was up to 25 percent of the ≥0.040-in. thick section of the panel. Thus, the corrosion was detected and imaged through 0.03 to
