Eddy current testing is widely used for nondestructive evaluation of metallic structures in characterizing numerous types of defects occurring in various locations. It offers remarkable advantages over other nondestructive techniques because of its ease of implementation. This paper presents a technical review of Eddy current technique in various scope of defect detection. The first part presents Eddy current evaluation on various defects location and orientation such as steam generator tubes, stress crack corrosion, and fatigue cracks. The next section analyzes the use of pulsed Eddy current and pulsed Eddy current thermography as an alternative method for monitoring the growth of cracks with the aid of computational techniques for postsignal analysis.
The effect of compressive loading on stress corrosion cracking of aluminium alloy 7075(W) was studied using specimens containing simulated fastener holes and the results were compared with the behaviour of double cantilever beam specimens with tensile loading. Crack lengths were measured with an eddy current bore probe and confirmed by optical metallography. Intergranular stress corrosion was observed in both tensile and compressive loading. Cracks initiated readily in 3?5%NaCl solution with tensile loading and the alloy was much less susceptible in compression, with cracking being detected only after 14 day exposure to EXCO solution. No cracking was observed in unstressed specimens, even after 56 day continuous exposure to EXCO solution. In both cases, SCC was explained by an active path mechanism due to the galvanic interaction between grain boundary precipitates and adjacent precipitate free zones. Compressive loading produced a positive strain at the crack tip, which ruptured the protective corrosion product film, maintaining the path of preferential corrosion.
The double cantilever beam has been widely used in the past and has proved one of the most popular designs for measuring the growth rate of stress corrosion cracks in materials. In this study, the double cantilever beam specimens were used to study the effect of tensile loading on stress corrosion cracking behaviour in aluminium alloy 7075(W). Cracks initiated readily in 3.5%NaCl solution with tensile loading conditions. Stress Corrosion Cracking (SCC) development was found to follow an intergranular path, which strongly depended on microstructure of material. Tests also were carried out to measure the threshold stress intensity, KISCC, which SCC would not occur. The SCC test was explained by an active path mechanism due to the galvanic interaction between grain boundary precipitates and adjacent precipitate-free zones. Crack lengths were measured with an eddy current bore probe and confirmed by optical metallography. The data from the eddy current tests on real stress corrosion cracks were used to construct an eddy current calibration curve for predicting stress corrosion crack lengths of aluminium alloy 7075(W).
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