| ABSTRACT: | QUANTITATIVE MEASUREMENT OF MATERIAL DEGRADATION WITH SCANNING
EMATS
R.B. Alers1 , J.T. Boyle1, G.A. Alers2
1Sonic Sensors of EMAT Ultrasonics 2EMAT Consulting, San Luis Obispo, CA, USA
The non-contacting and couplant-free nature EMATs makes them particularly useful for inspections that
require moving a transmitter/receiver pair over large objects in a field environment. Examples include the
inspection of boiler tubes, buried pipelines, refinery piping and storage tanks. These inspections must
produce a quantitative measurement of the remaining wall thickness under the deepest corrosion pit or stress
corrosion crack. This implies a requirement for scanning the transducers over a considerable area to locate
the most severe damage. Because EMATs can be designed to excite and detect guided waves in plate or
pipe structures, large areas can be inspected at the speed of sound by monitoring reflections from defects
illuminated by a beam of acoustic energy directed from transducers operating in a pulse-echo mode. With
this mode of operation, the scan direction will provide one coordinate of a C-scan image while the time of
arrival of the echo provides the other coordinate. More important, however, several different guided wave
modes can be used to form the beam of acoustic energy and the information contained in these modes can be
combined to give more reliable information on the flaw dimensions. Examples will be presented to illustrate
how illumination of a given flaw with different guided wave modes can be used to determine its dimensions.
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