In current rail inspection processes, following a detection of a suspected internal defect, an additional secondary detailed inspection is required to (1) confirm the presence of the flaw and (2) determine the severity of the flaw to allow for optimal post-detection rail maintenance planning. Current ultrasonic devices in this secondary inspection efforts heavily rely the expertise and experience of the test personnel’s judgement to confirm the rail flaw and to characterize the internal defect by analyzing reflected waveforms. To eliminate the uncertainties in this secondary inspection process and to provide the testing operators with better defect characterization such as the size and location of the flaw, a defect ultrasonic imaging device utilizing synthetic aperture focusing (SAF) techniques is proposed in this paper. These imaging techniques have been successfully demonstrated in medical imaging, providing quantitative characterization of internal components, allowing for a better prognosis. Ultimately, having a quantitative evaluation of the internal flaw can lead to an increase in the safety of train operations by preventing derailments. Thus, in this paper, a preliminary portable rail defect imaging concept is proposed by the University of California, San Diego, to provide three dimensional images of internal defects in the rail. The prototype reconstructs a three-dimensional volumetric image of the rail, utilizing multiple two-dimensional planar ultrasonic images. Improvements to the conventional tomographic imaging algorithms have been made by utilizing a mode-selective image reconstruction scheme that exploits the specific displacement field, respectively, of the longitudinal wave modes and the shear wave modes, both propagating simultaneously in the test volume. The specific mode structure is exploited by an adaptive weight assignment to the ultrasonic tomographic array. Such adaptive weighting forces the imaging array to look at a specific scan direction and better focus the imaging onto the actual flaw (ultrasound reflector). This preliminary study shows that the usages of the adaptive weights based on wave structure improves image dynamic range and spatial resolution, when compared to a conventional ultrasonic imaging technique such as Delay-And-Sum (DAS). Results will be shown both from numerical models and experimental tests of internal flaws in rails.

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