NDE of small 3-D surface fatigue cracks in metals by microwaves

So far, it had been hard to detect fatigue cracks, especially closed cracks, on a metal surface by using microwaves. Fatigue cracks are often closed under a no load condition, and this phenomenon makes nondestructive evaluation of the cracks difficult. Recently, a highly sensitive coaxial line sensor was developed and it became possible to detect fatigue cracks. Also more recently, a noteworthy technique, that is called microwave dual frequency technique, was developed for quantitative nondestructive evaluation of the cracks, irrespective of crack closure, and has been verified by using 2-D fatigue cracks. In the present paper, the technique was applied to evaluate a 3-D fatigue crack. The shape and the size of the crack and the distribution of the extent of crack closure along the crack line were evaluated