Dark-field magneto-elastic nondestructive testing technique

Abstract

In the field of nondestructive testing, great advantages can be obtained if a dark-field technique can be achieved. A magneto-elastic technique generating sound waves only on the presence of a defect and from the defect is described (no defect—no sound waves in the material under test). The magneto-elastic technique generates three-dimensional information about the defect location, dimensions, and characteristics, unlike conventional ultrasonic NDE techniques where the defect is used as a scatterer of incident ultrasonic waves. Eddy currents are generated in the material under test with a current carrying conductor. A magnet is placed such that its magnetic field is parallel to the current carrying conductor. Elastic waves are generated in the conductive material under test only at the crack edges from the interaction of the magnet with transverse eddy currents flowing around the defect. The generated elastic waves are detected with an ultrasonic receiver transducer coupled to the material. An array of receivers may be used to triangulate on the defect and also to characterize the properties of the defect from the 3-D information. Conventional EMAT (electromagnetic acoustic transducers) are used to generate elastic waves in the materials even when no defects are present; the magneto-elastic technique depends on the presence of a defect to create the elastic waves. Preliminary experimental results are presented together with other potential application of the new concept.