| Reliable inspection of hidden cracks in multilayer objects, such as the airframe structures still poses a major challenge in NDE. Conventional electromagnetic methods are limited in their abilities to detect 2nd and 3rd layer cracks in riveted structures. Cracks are normally found at fastener sites and the presence of steel fastener (high permeability) produces a strong signal that masks any indications from a crack. An eddy current (EC) system with sheet current source for uniform field generation and magnetoresistive (MR) sensing is studied in this dissertation. The contributions of this research are: 1) A study of three-component measurements using MR sensors and the use of information in these components to enhance crack detection at steel fasteners. 2) Experimentally validated model-based study of Pulsed EC excitation and low frequency EC excitation systems for the inspection of buried cracks at fastener sites. 3) Rotating current excitation designed to enhance the system capability to detect cracks of all orientations at fastener sites. 4) A statistical signal processing scheme developed for automated analysis of measured data. 5) Development of a numerical model for simulating the phenomenon in time and frequency domains. Model based study is used to assist the overall system and algorithm design. Results of implementing the techniques on simulation and measurement fully validate the proposed methods. |
