New Methods of Detection and Characterization of Surface Flaws

A new approach to microwave eddy current testing for surface cracks in metals involves the use of ferromagnetic resonance in a small garnet crystal placed close to the surface to be tested. It is well known in this case that the induced eddy currents on the metal surface cause a strong displacement of the ferromagnetic resonance frequency. The presence of a crack on the surface produces, by disturbing the eddy current pattern, a perturbation of the resonant frequency as the garnet sa.ple passes over it. Theoretical detectability criteria for the garnet probe are developed on the basis of a magnetic perturbation relation and compared with calculations of Rayleigh wave and plate wave backward scattering and intermode scattering at a half-penny surface crack. Some experimental results are also given. Up to this point the main thrust of research on methods for quantitative NDE has been directed toward internal flaws in materials using primarily ultrasonic methods of testing which provide the most easy access to the interior of an opaque body. In the case of surface flaws, a very important and critical branch of NDE, the flaw is accessible to probing by any one of a number of forms of energy. The purpose of this project is to investigate and evaluate at a fundamental level the use of different forms of energy ultrasonic, electrical, magnetic, thermal, etc. -for the detection and characterization of surface flaws in materials, and then to explore in detail one or two of the more promising approaches. After an initial investigation it was determined that eddy current and ultrasound techniques were the best candidates for further study, in that they have high sensitivity and offer the best possibilities for the development of novel approaches to search and characterization procedures. In the domain of eddy currents, microwave techniques have been studied using a ferromagnetic resonator as a probe that provides high sensitivity and spatial resolution, as well as the capability for novel array scanning techniques. The aspect of ultrasonic detection of surface flaws that has been emphasized is the use of guided wave modes to localize the probe on the surface and to deduce additional information about the flaw from intermode scattering. Ferromagnetic Resonance in Eddy Current Testing The change in the frequency of a microwa.e cavity resonator perturbed by a small slot or crack in the enclosing wall according to the Slater formula is 6f 1 magnetic stored energy in crack 1' =-I magnetic stored energy in resonator .(l) This shows that the sensitivity of such a crack detection system is enhanced by reducing the VQlume of the resonator relative to the crack and by increasing the resonator Q. 108 The ultimate in this regard is a ferrimagnetic resonator, typically taking the form of a yttrium iro~ ga5net (YIG) sample with a volume less than 10in. and a resonance Q in the order of 1000. Such a resonator does not require an enclosing wall and is readily passed over the metal surface to be tested for cracks. A resonator of this kind may be used for crack detection by measuring the input impedance at the coupling loop and observing the frequency shift as it passes over the flaw. This method is sensitive to lift off and a better technique is to use a balanced system with two YIG-controlled oscillators. The frequency difference is perturbed by the presence of a crack under one resonator, but is unaffected by changes in lift off d. From the Slater formula the detection sensitivity of a system with typical parameters