Title: Laser/EMAT measurement systems for materials evaluation
Abstract: This paper describes two applications of laser/ electromagnetic acoustic transducer (EMAT) systems. The first application uses laser-generated ultrasound for the characterisation of rear surface artificial defects (vertical slots) in metal samples. An EMAT sensitive to in-plane motion was used to detect these ultrasonic waves. B-scan images were used to visualise any changes of interrogating waves due to defects. These images were generated as the sensor head was moved along the material's surface, forming a 2D intensity profile that revealed changes in the presence of a defect. The defects produced distinctive parabolic features in the images from the time-of-flight diffraction by shear waves. The paper presents a new method based on a generalized Radon transform (GRT) to automatically determine the depth and location of back-surface artificial defects from B-scan images. Experimental evidence validated predicted results for the case of 1.5-3.5 mm-deep defects. The second application uses Rayleigh waves to quantify inhomogeneities in metal alloys using a transient Rayleigh pulse detected with an eight-element EMAT-array. The array spacing established sensor separation, so that velocity measurements were performed independent of the source-to-detector separation. Eliminating this distance, which would normally lead to systematic errors, has produced a measurement system capable of measuring velocities in metal with a precision of 99.9 %. As an example, variations in the Rayleigh wave velocity have characterised the inhomogeneities in rolled bars of aluminium.