Inspection of Hidden Defects in Metal-Metal Joints of Aircraft Structures Using Eddy Current Technique with GMR Sensor Array

To realise increased inspection depths in eddy current (EC) technique, giant magnetoresistive (GMR) magnetometer can be used as receiver element for an EC sensor. This paper reports on the efficient GMR sensor array for ET inspection of hidden defects in metal-metal joints of aircraft structures. This new developed sensor array is a result of the collaboration of Airbus Deutschland (Bremen) and Fraunhofer Institute for Non-destructive Testing (Fraunhofer IZFP, Saarbrucken). Usage of new materials and assembling technologies in modern aircraft design allows reaching high long-time quality and efficiency but also requires the application of qualified inspection techniques during the whole aircraft lifetime. The inspection of new materials with stronger requirements becomes possible using new technologies of non-destructive testing (NDT). In addition, the application of improved NDT techniques can reduce the maintenance costs for currently used as well as for future aircraft designs. The most frequently used inspection techniques for metallic components of aircrafts are ultrasonic testing (UT) and eddy current testing (ET). For metallic structures ET is the preferred technique for the detection of surface-breaking flaws as well as for hidden flaws in regions with specific geometry (e.g. joints, respectively riveted lap joints) as well as in areas which are difficult to access with UT. Especially the complex inspection situations generate the demand for improved ET techniques with increased inspection depth. The higher inspection depths are only possible at low EC frequencies because of the electromagnetic skin effect. Use of the sensitive magnetic field sensors such as SQUID's or magnetoresistive sensors can increase the efficiency of the EC measurement since this type of sensors compared with coils indicates frequency-independent sensitivity beginning with 0 Hz. Magnetic field sensors based on the GMR-effect are available on the market (1) and