Interaction of Surface Cracks in Metals with Open Ended Coaxial Probes at Microwave Frequencies

Detection and evaluation of stress induced surface cracks in metals is an important issue in many critical environments. There exist several conventional nondestructive testing techniques for detecting such cracks, although each has its limitations and advantages. Recent developments in using microwave based techniques have also shown great promise. One of these microwave nondestructive testing techniques makes effective use of open ended coaxial probes for detecting and evaluating tight surface cracks. The potential and practical attributes of this technique are presented in this paper. The basic idea behind using this probe is presented, as well as several experimental results, laying the foundation for an electromagnetic modeling effort. The modeling approach is based on a mode matching technique and the conservation of complex power at the junction between a coaxial probe aperture and a narrow rectangular waveguide representing a tight crack. It is also shown that the phase of reflection coefficient, referenced at this junction, is an effective parameter for obtaining the coaxial crack characteristic signal, which indicates the presence of the crack as well as some of the crack dimensional information. Comparison between the measured and calculated results is also shown. A discussion regarding the influence of crack depth on coaxial crack characteristic signals is also given. The potential for crack width determination as well as the influence of the frequency of operation is also discussed.