Airframe structures typically are composed of multiple layers of aluminum that riveted together using thousands of aluminum, titanium or steel fasteners. These fastener sites are areas of high stress where cracks originate in radial directions. Inspection of multi-layered riveted structures and detection of subsurface cracks under fastener head is a major challenge in aviation industry. Some of the difficulties include: i) defects embedded deep in the structure, ii) arbitrary and unknown orientation of defects, iii) remanence magnetic of steel fasteners which shifts the operation point of the giant magnetoresistance (GMR) sensor out of linear range, iv) detection of small amplitude defect indications in the presence of dominant response signals from fasteners, v) effect of material properties of steel fasteners such as anisotropy, hysteresis, etc. vi) effect of earth’s magnetic field on weak magnetic field measurements using GMR sensors.This dissertation presents an in-depth analysis of these issues and a novel probe design for potentially addressing these problems. The operating principles of the probe is based on inducing eddy currents in the conducting test sample and measuring the perturbations in induced magnetic fields associated with the eddy currents. The sensor system utilizes a very low frequency rotating current excitation that is sensitive to deep embedded cracks of all orientations. An array of GMR sensors are used to measure the induced fields. The contributions of this research are the following: 1) orthogonal coil design for generating rotation excitation current, 2) novel differential sensor array scheme that eliminates the ambient/background field at sensors, 3) a new optimized non-uniform multilayer coil design for ensuring uniform excitation field; 4) analysis of effect of magnetic susceptibility and anisotropy of steel fastener; 5) magnetic balance measurement scheme using high sensitivity GMR sensor for steel rivets; 6) design and develop a prototype probe with rotating excitation and GMR array sensors 7) evaluate the probe performance for experimental validation of the overall hypothesis.The second application considered in this research is development of electrically rotating field/current probes for inspection of cylindrical geometry. Two novel probes are studied. The first design is a transceiver probe based on generating rotating fields using three phase windings. The key features of this design are high inspection speed, sensitivity to cracks of arbitrary orientation and simplicity of design. The second probe is based on rotating current excitation using orthogonal coils in axial and circumferential directions. The radial component of the magnetic field is picked up by a linear array of GMR sensors located along the circumferential direction. This probe can detect both circumferential and axial defects, offers high sensitivity over a wide range of frequencies and can potentially provide extremely high spatial resolution.
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