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Ultrasonic measurement model-based non-destructive detection method for curved components using an immersion spherically focused transducer

ABSTRACT The ultrasonic measurement model (UMM) has been theoretically studied over the past few decades, but there is little published work on its practical application for guiding defect detection. In this work, we present a model-based ultrasonic method to detect flaws in curved components using immersion of spherically focused transducers. The UMM of focused transducers is further investigated theoretically and verified experimentally, and some key issues in the model with focused transducers are addressed, including the necessity of correctly using effective geometrical parameters and the corresponding transfer function. The UMM is employed to guide the inspection of a magnesium alloy shell component which consists of several curved surfaces. The alignment between the transducer and the shell is conducted by comparing the measured and theoretically predicted interface-echo signals, the water path is selected by simulating the effective detection region, and the system gain is set by quantitatively analysing the amplitudes of flaw signals and noises. When the inspection conditions are determined, a variable threshold based on the predicted flaw signals is introduced to distinguish noise and flaw signals. The ultrasonic C-scan images of the shell are compared with the digital radiography (DR) images, and the comparison shows that the flaws of interest can be detected in the C-scan images, which verifies the effectiveness of the UMM-based detection method.