Portable Instrument for Detection of Surface Flaws Using EMATs

The work reported here is the development of a first prototype portable ultrasonic inspection instrument based on EMAT (electromagnetic acoustic transducer) technology. The goal was to demonstrate EMAT inspection capabilities for small-size flaws in metal parts and to build a self-contained NDE unit that had a high degree of signal processing on-board so that human interpretation was minimized. The unit also served as a test bed, so that a number of new concepts could be evaluated. This instrument is viewed as an important step in the development of future NDE equipment. DESCRIPTION OF UNIT AND BASIC CAPABILITIES The EMAT flaw detector instrument (see Fig. 1) is capable of launching and detecting ultrasonic energy in metal parts using transducers that are physically se£a2ate from the specimen under investigation. • This operation eliminates the inconvenient and restrictive coupling medium (fluid) that is required by conventional piezoelectric ultrasonic instruments. The prototype is designed to inspect for surface flaws in both ferrous and nonferrous materials using a 1 MHz surface-acoustic-wave launched from a unidirectional EMAT transducer. This single transducer serves both as a generator and detector of straight crested surface waves. The unit is a radical departure from conventional equipment in that all information can be displayed in a digital format using the front panel digital LC (liquid crystal) display. Front panel ports are available if a display of reflected signal amplitude versus time (A-scan) is desired. The instrument has two modes of operation: Search Mode Ultrasonic signals are introduced into the part and reflections monitored automatically. Reflected signals larger than a threshold that is operator-set sound an audio alarm. The distance between transducer and flaw is automatically displayed on the digital meter. Stray reflections are excluded by a range-gate circuit (set according to the size of part under inspection) so that natural part boundaries will not trigger the unit. A number of closely spaced flaws can be separated (resolved) by a "signal select" circuit that allows monitoring of the first, second (etc.) reflections occurring in a given time interval. Inspection Mode The instrument measures the magnitude of the signal reflected from the flaw and compares it to a preset signal level. This preset level is derived from a "standard" used for calibration. The LC meter displays the ratio (in dB) of the signal from the flaw under investigation compared to the standard. Positive readings indicate flaws reflecting more signal than the standard and negative readings less signal than the standard.