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19 May 2005 Impedance based corrosion detection
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Corrosion begins as moisture penetrates the protective barrier of a surface, starting an electrochemical process which over time leads to surface pitting. The combined action of mechanical stresses and corrosion induced pitting reduces structural integrity as the pits enlarge to form nucleation sites for surface cracks, which propagate into through-the-thickness cracks. In most cases, the total mass loss due to corrosion within the structure is small; however, significant reductions in mechanical strength and fatigue life can occur in the corroded material leading to advanced crack growth rates or fast fracture. Since the structural damage due to localized corrosion pitting is small and the crack growth rates may be large, traditional inspections methods and "find it and fix it" maintenance approaches may lead to catastrophic mechanical failures. Therefore, precise structural health monitoring of pre-crack surface corrosion is paramount to understanding and predicting the effect corrosion has on the fatigue life and integrity of a structure. In this study, the impedance method was experimentally tested to detect and quantify the onset and growth of pre-crack surface corrosion in beam and plate like structures. Experimental results indicate the method is an effective detection and tracking tool for corrosion induced structural damage in plates and beams. Additionally, impedance based monitoring of the corrosive effect on the piezoelectric sensors may serve as a useful tool for predicting corrosion growth rates on the surface of structures.
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Garnett E. Simmers Jr., Henry A. Sodano, Gyuhae Park, and Daniel J. Inman "Impedance based corrosion detection", Proc. SPIE 5767, Nondestructive Evaluation and Health Monitoring of Aerospace Materials, Composites, and Civil Infrastructure IV, (19 May 2005);

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