Microwave Detection of Fatigue Cracks in Specially Prepared Steel Specimens

About the report:

In the aging highway systems the problems of fatigue-induced damage and cracking in metal structures are very severe. Many such systems are operating even beyond their design lifetime, which requires more than the originally prescribed inspection cycles. In order to reduce the cost, increase testing efficiency and reliability, which subsequently improve the safety of steel bridge members, it is necessary to develop a highly reliable, fast and relatively inexpensive NDE technique. Currently, NDE methods for detecting fatigue cracks include visual inspection or the use of dye penetrant and magnetic particles. These techniques are labor-intensive, time-consuming and hazardous. Subsequently, other techniques are needed to detect and evaluate fatigue cracks in a bridge with minimal time for scanning and minimal safety precautions. These techniques should be able to detect cracks under paint well before any visible change (rust stain) appears on the paint surface. Additionally, these techniques should be able to detect surface breaking cracks on unpainted weathering steel as well as cracks filled with rust or paint. A solution to these problems is the use of different types of microwave sensors and testing methods for exposed, covered and filled crack detection. Microwave based NDE methods are capable of providing many practically useful features for this purpose. Open-ended coaxial line probes were used, at various frequencies, to detect the presence of cracks on weld joints and at critical locations. Many prescribed samples with known defects were obtained from the sponsor to conduct these microwave measurements. The detailed results of this investigation are presented in this report and the subsequent appendices.

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