Applications of Flat Geometry Remote Field Eddy Current Techniques in Aircraft Nondestructive Inspection

The Flat Geometry Remote Eddy Current (FG RFEC) Technique has its unique advantages in aircraft non-destructive inspection (NDI) including deep penetration, high sensitivity, portability, low price, etc. The technique has recently been recognized by aircraft NDI societies. Currently FG RFEC technique is capable of detecting deeply hidden cracks and corrosion flaws. This paper will introduce the recent progresses made in R&D of the FG RFEC technique including: 1. Accurate detection of inner layer fastener hole cracks in multiple layer aircraft structures using rotational FG RFEC probes and an auto-centering device; 2. Accurate detection of landing gear cross-bolt hole cracks through bushing; 3. Detection and calibration of deeply hidden corrosion in multiple layer aircraft structures. The Super-Sensitive Eddy Current (SSEC) system, used as the instrumentation tool for FG RFEC technique, will also be introduced. The project is currently supported by Federal Aviation Administration of United States of America. Introduction: Greater and greater impact comes from the aging of currently in-service airplanes. Two of the most important issues affecting possible extending the life of aging airplanes are cracking and corrosion. Therefore, a good non-destructive inspection (NDI) technique for aircraft corrosion and cracking inspection is essential to the safety of aging airplanes. Aircraft components often suffer from fatigue cracking and corrosion because their particular working environment. Conventional NDI techniques are incapable of detecting for inner layer fatigue cracks and corrosion from aircraft exterior; therefore, to inspect inaccessible areas of an aircraft component usually requires the removal of obstructions or disassembly prior to inspecting the desired component. An airplane is inspected every given flight-hours or given landing cycles. It is often that such an inspection is unnecessary since cracking or corrosion may not be present. Such an inspection is costly, because the direct cost spent on the inspection consists of only a couple of percent of the total cost. The costs of other related work, including the teardown, replacement, and parking costs, are dominating. Therefore, there is a demand for advanced NDI techniques for aircraft fatigue damage and corrosion detection. The major requirements to such a new NDI technique include: 1. Detect inner layer cracks/corrosion from outside aircraft skin through multi-layered structures. The number of layers can be up to 5. 2. Deep penetration to cover possible aircraft thickness which can be up to 5-10 centimetres of aluminium alloy, titanium alloy and/or composite structure. 3. Sensitivity to small-sized inner layer crack/corrosion with and without presence of fasteners of different materials. 4. Reliability of detection without human factors involved. 5. High-speed and large-area inspection considering all the above 1-4 requirements. 6. Discriminate noises from different factors, such as edge effect, thickness variations and possible sealant/gaps between layers, etc. 7. Low cost. 8. Portability and convenience in use. There are quite a number of aircraft NDI techniques including conventional and emerging techniques. However, until now there is no single one that meets all the above requirements. Examples include: 1. Ultrasonic techniques (UT), including guided wave UT, is incapable of penetrating through multiple layer structures and restricted to first layer and upper surface of second layer inspection; 2. X-ray techniques involve heavy equipment, protecting operators from radiation, is relatively expensive and of low sensitivity to small cracks. 3. Most eddy current (EC) techniques and its alternatives have limited potential for further increase of their penetration depth in metallic structures because of skin-depth effect; 4. One exception can be listed is Super-conducting Quantum Interface Device (SQUID). It is capable of penetrating relatively deep. However, the system is quite heavy, large, expensive and having high noise problems. These issues restrict its practical applications. Among current emerging aircraft NDI techniques FG RFEC & SSEC system has shown great promise in becoming a good candidate for expected aircraft NDI needs in the near future. FG RFEC and SSEC System [1-3] The original Remote Field Eddy Current (RFEC) has used in NDI of conducting tubing for years. The RFEC technique is characterized by its features of deep penetration and the linear relation of its signal phase to the total wall thickness under inspection. The signal phase to wall thickness relation is independent of probe lift-off and the location of a flaw in respect to the wall thickness. IMTT has expanded the applications of RFEC techniques to the inspections of conducting objects of flat or nearly flat geometries with the help of specially designed probes called FG RFEC probes, Figure 1. The probe blocks the direct coupling path. The electromagnetic energy released from the drive unit is forced by an FG RFEC probe to go along the indirect coupling path. Therefore, all signal received by the pickup unit has passed the wall twice and carries the entire information about the wall condition. The signal can be extremely weak, but is very clean without noise coming from the driving unit. Direct Coupling Path Drive Unit Pickup Unit Indirect Coupling Path Figure 1.Simplified drawing of an FG RFEC probe and the energy coupling paths. An SSEC system is a modified version of a conventional eddy-scope. An SSEC system has comparatively high gain and low noise level. It brings the weak pickup signal to a level that is readable on a computer screen. Current version of an SSEC system consists of a piece of software and a Printed Circuit Board (PCB) that are installed/inserted into a regular personal or industrial computer, see the left picture in Figure 2. It utilizes the fundamental features of a computer as the base of an SSEC system. Figure 2 right is the second version of the system where it becomes a small box and can be connected to a customer preferred computer through a universal serial port (USB). This version will be available soon. Figure 2. Current (left) and future ( right) versions of an SSEC system. Test Panel SSEC Board & Software Installed In a Book-Size PC Monitor