Continuous monitoring with a permanently installed high-resolution ultrasonic phased array

High-resolution defect detection is an integral part of non-destructive testing (NDT). Ultrasonic array transducers have been developed to respond to this need and provide high-resolution images for such applications. At present, high-resolution imaging is achieved by ultrasonic array design, such as enhanced transducer materials, and optimised element geometry to fulfil inspection requirements and miniaturise imaging artefacts, etc. This paper presents a novel ultrasonic array and controller system design strategy that can miniaturise the array and reduce manufacturing cost of the entire array system. This then enables the permanent or semi-permanent installation of arrays onto structures and long-term continuous structural health monitoring (SHM). The array geometry is designed as a compromise between the required imaging performance for the desired application and the fabrication cost for permanent installation. A prototype array transducer is constructed from piezoceramic elements bonded to a flexible printed circuit (FPC) substrate and then permanently installed on example structures to demonstrate long-term monitoring. Improved long-term sensitivity to defect growth is achieved by applying a compensation strategy for velocity and phase changes with temperature. Results show that a 5 MHz, 18-element prototype array can detect changes as small as 0.1 mm in the diameter of a side-drilled hole (SDH) defect after 3 months of continuous monitoring.