Characterization of EMAT Guided Wave Reflectivity on Welded Structures for Use in Ranging

Guided wave ranging measurements offers an elegant method to localize an inspection robot relative to the geometric features, such as welds, of a structure under test. This paper characterizes the suitability of various EMAT generated guided wave modes when reflecting from butt welds for the purpose of choosing a low frequency mode suitable for accurate ranging. Wave modes were tested in 10mm mild steel plate in experiment and simulation, the method of data extraction is discussed as well as the determination of the wave mode best suited for weld ranging by means of comparison of the reflection coefficients. The authors conclude SH1 at a frequency-thickness product of 2 MHz.mm, is shown to be a highly suitable wave mode for gaining a large reflection from a weld, with an average reflection co-efficient of approximately 0.45 across four different sized weld crowns. A ranging over 1 meter experimentally was demonstrated to have a 2.65% error using our method. This work will enable simultaneous detailed mapping through ranging and inspection of large welded structures by mobile robotic inspection systems using EMAT's.