The Role Of Grain Noise In Eddy Current Inspection Of Titanium Alloys
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As a result, from an elastic point of view, cubic crystals are also anisotropic just like other crystallographic classes. In nondestructive materials characterization electrical conductivity is usually measured by the non-contacting eddy current method. Neighbor was the first to extend the eddy current method to electrically anisotropic materials and showed theoretically that one can obtain the full conductance tensor from such measurements. 9 Special eddy current coil configurations that allow the simultaneous measurement of electrical conductivity in two principal directions have been developed for texture assessment in plates. 10,11 Just like in the case of elastic anisotropy, the source of electrical anisotropy can be either (i) intrinsic crystallographic anisotropy in single crystals and textured polycrystals or (ii) structural anisotropy caused by oriented reinforcement in composite
materials. The latter can be exploited for eddy current assessment of constituent volume fractions in metal matrix composites. 12,13 Grain boundary contributions to the electrical resistivity 14 can cause additional electrical anisotropy in polycrystalline materials with elongated grains aligned in preferred orientation due to thermal or mechanical treatment. It should be emphasized that, in contrast with elastic properties, the electric conductivity is completely isotropic in cubic crystals which constitute the overwhelming majority of polycrystalline metals; therefore, the role of intrinsic crystallographic anisotropy in eddy current testing has not been investigated in detail. However, less common materials of hexagonal symmetry can exhibit strong electrical anisotropy with significant difference in conductivity between the basal plane and normal to it. Titanium is one of the few structural metals of practical importance, especially in aerospace applications, which preferentially crystallizes in hexagonal symmetry and therefore exhibits strong electrical anisotropy. There are two areas where this electrical anisotropy becomes very relevant from the point of view of mechanical fatigue in titanium alloys. First, eddy current inspection is probably the most commonly used nondestructive testing technique for fatigue crack detection in airframe structures and engine components and electrical grain noise presents the same problem in eddy current crack detection as acoustic grain noise does in ultrasonic flaw detection. Second, proper microstructure is absolutely essential for assuring good fatigue tolerance in the material 15 and the electrical grain noise can be exploited for nondestructive characterization of the microstructure by eddy current inspection in the same way as acoustic grain noise is used in ultrasonic characterization of the microstructure. EXPERIMENT Single crystal materials generally behave anisotropically in response to a given stimulus such as heat, electricity, or force due the symmetry conditions of the atomic lattice structure. In contrast, polycrystalline materials tend to behave essentially isotropically du
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