Abstract
To evaluate the depth of small fatigue cracks under a condition of no contact and without any coupling medium, a novel microwave technique was demonstrated. An open-ended coaxial line sensor was used to increase the spatial resolution and the ratio of signal to noise. Closed fatigue cracks were detected successfully and a W-shaped characteristic signal was obtained. A dual frequency technique was proposed to evaluate the depth of small fatigue cracks of which situation regarding crack closure is unknown. The evaluated results are shown to agree well with the actual values.
