Modelling and characterisation ultrasonic phased array transducers for pipe inspections

Nuclear pipes require regular inspection to prevent critical damage and extend their safe working lifetime. Ultrasonic inspection using phased array transducers is a Non-Destructive Testing (NDT) method used to monitor the condition of pipes, providing internal integrity information using multiple small transducer elements. This grants a controllable ultrasonic beam that provides more detail than a conventional single element transducer. This paper presents a hybrid simulation model to characterise phased array ultrasonic transducers for pressure pipe inspections. The paper focuses on identifying defects on pipe surfaces, which have total propagation path lengths in the order of 170 ultrasound wavelengths. Simulating inspections with such large path lengths is challenging due to unreliable results from numerical dispersion errors in large finite element models. The model presented here combines the benefits of finite element modelling with analytical extrapolation to overcome the issue of large propagation distances. A practical experiment demonstrates and verifies that the hybrid simulation model matches real world inspections. Inspection accuracy was quantified using the hybrid simulation model over frequency (5, 7.5 and 10 MHz) and number of elements (8, 16 and 32) to inspect two defect shapes (rectangular and V-shape). The 10 MHz transducer showed the highest resolution for the depth measurement, while the 5 MHz transducer presented the strongest penetration to inspect the defects on the pipe's outer surface. The largest aperture transducer, 32 elements, brought about the most accurate defect size measurement. This work serves to inexpensively guide the selection and later deployment of phased array inspection tools in nuclear pipe defect detection and monitoring.