Dual mode inspection using guided waves and phased array ultrasonics from a single transducer

Asset inspection of large structures such as storage tanks in the oil, gas and petrochemical industry is a challenging and time consuming process. Compared to the state-of-the-art inspection methods of these structures using ultrasonic bulk wave phased array techniques, guided waves provide a mechanism for monitoring inaccessible areas and increasing the testing range to speed up the inspection process. Combining these two approaches, an efficient inspection scheme can be achieved. For this purpose, in this work, the possibility of exciting a single higher order guided wave mode at a low dispersion region is examined, using a conventional linear phased array on a 10 mm thick sample. An analytical model based on modal analysis is derived. Then, the time delays across each element of the array are optimally selected and a necessary condition on the pitch is provided, to enhance the purity of the desired mode. The results are promising, illustrating that single mode excitation is possible at high frequency-thickness products, greater or equal than 15 MHz·mm. Furthermore, the phased array transducer allows for dynamic mode selection capabilities; that is, feeding the appropriate excitation signals on each channel, different modes can be excited, without having to physically alter the transducer configuration.