Ultrasonic arrays for monitoring cracks in an industrial plant at high temperatures

A piezoelectric linear array structure has been designed to operate at temp eratures up to 400 degrees C for nondestructive testing of steel components of a hot industrial plant. It is intended that these arrays be fixed perma nently to the test subject so that known defects can be monitored by compar ing measurements taken over a period of time without needing to shut down t he plant. The arrays are used in pairs: the transmitter is a phased array p roducing a variable angle steered beam, and a second array is used for rece iving. The defect can be identified from a series of scans collected from i ndividual elements of the second array. A simple monolithic array structure was used, based on a single crystal of lithium niobate and operating in th e frequency range 3 to 5 MHz. Prototype devices have 64 elements on a 0.5 m m pitch. Simulated defects in steel blocks have been scanned at high temper atures to illustrate the arrays' capability for nondestructive testing. The results suggest an accuracy better than 1 mm in finding the location of cr ack tips.