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The detection of embedded cracks under fastener heads (CUF) in multilayer structures is a major challenge facing the aviation industry. Eddy current (EC) based techniques, such as Magneto-Optic Imager, Sliding probe and Self-nulling EC probes, and low frequency EC methods have been investigated. The prior work of authors presented an EC system using a uniform field excitation combined with giant magnetoresistive (GMR) sensors for imaging the normal component of magnetic flux density, has improved the effectiveness in detecting 2nd and 3rd layer CUFs with aluminum fasteners. However, detection of CUFs with ferromagnetic fasteners remains a major challenge. This paper presents the experimental study of automatic crack detection under steel fastener sites using low frequency EC with 3D GMR sensors that measure all 3 components of the induced magnetic flux density. A finite element model based study is used to optimize the system design and the experimental validation is presented. [ABSTRACT FROM AUTHOR]

Magneto-optic imaging (MOI) is a relatively new sensor application of an extension of bubble memory technology to NDT and produce easy-to-interpret, real time analog images. MOI systems use a magneto-optic (MO) sensor to produce analog images of magnetic flux leakage from surface and subsurface defects. The instrument’s capability in detecting the relatively weak magnetic fields associated with subsurface defects depends on the sensitivity of the magneto-optic sensor. The availability of a theoretical model that can simulate the MOI system performance is extremely important for optimization of the MOI sensor and hardware system. A nodal finite element model based on magnetic vector potential formulation has been developed for simulating MOI phenomenon. This model has been used for predicting the magnetic fields in simple test geometry with corrosion dome defects. In the case of test samples with multiple discontinuities, a more robust model using the magnetic vector potential A¯ and electrical scalar potential V is required. In this paper, a finite element model based on A-V formulation is developed to model complex circumferential crack under aluminum rivets in dimpled countersink. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Magneto-optic imaging is a relatively new sensor application of bubble memory technology to NDI. The Magneto-Optic Imager (MOI) uses a magneto-optic (MO) sensor to produce analog images of magnetic flux leakage from surface and subsurface defects. The flux leakage is produced by eddy current induction techniques in nonferrous metals and magnetic yokes are used in ferromagnetic materials. The technique has gained acceptance in the aircraft maintenance industry for use to detect surface-breaking cracks and corrosion. Until recently, much of the MOI development has been empirical in nature since the electromagnetic processes that produce images are rather complex. The availability of finite element techniques to numerically solve Maxwell’s equations, in conjunction with MOI observations, allows greater understanding of the capabilities of the instrument. In this paper, we present a systematic set of finite element calculations along with MOI measurements on specific defects to quantify the current capability of the MOI as well as its desired performance. Parametric studies including effects of liftoff and proximity of edges are also studied.—This material is based upon work supported by the Federal Aviation Administration under Contract #DTFA03-98-D-00008, Delivery Order #IA013 and performed at Iowa State University’s Center for NDE as part of the Center for Aviation Systems Reliability program. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]