Ultrasonic phased array imaging is an invaluable tool in Non-Destructive Testing, allowing for the assessment of quality and internal integrity of components. The golden standard in ultrasonic imaging is currently considered to be the Total Focusing Method (TFM), where a Full Matrix Capture (FMC) dataset is used to synthesise the array’s focusing and steering in post processing. However, the FMC data acquisition method is based on concepts for transducer-based phased arrays that assume a fixed geometry of instrumentation and capture the Full Matrix irrespective of the usefulness and contribution of the signals to the final image. This is a major limitation when data acquisition speed and data volume is important. In order to overcome this obstacle a fundamental change in the data acquisition methodology for ultrasonic imaging is required. The work presented here introduces a novel data acquisition method, the Selective Matrix Capture (SMC), that can adapt the array geometry during data acquisition, to the demands of the inspected structure, such as the defects encountered. The adaptive data acquisition method is enabled by the use of Laser Induced Phased Arrays (LIPAs).LIPAs use laser ultrasonics to generate and detect ultrasound by means of lasers. In a LIPA, the array is synthesised in post processing by recording laser ultrasound data from multiple generation and detection points on the surface of a component. The focusing and steering of the ultrasonic beam is done in post-processing by an&nbsp;imaging algorithm. The advantage is that laser ultrasonics is a remote and couplant-free technique addressing the current NDE challenges of inspection of complex structures and under extreme environments. In addition, LIPAs provide superior ultrasonic imaging performance over conventional laser ultrasonic techniques because of the beam forming that is done in post processing, while operating in the non-ablative, non-destructive regime. LIPAs have elements made of light and they are highly reconfigurable with respect to element number and position, unlike their transducer-based counterparts. We have previously demonstrated high-resolution ultrasonic images of the interior of components using FMC and TFM. However, capturing the FMC requires long synthesis time due to signal averaging and mechanical laser scanning, compromising the application potential of LIPAs. Given that most components are defect free, significant time savings can be obtained by only acquiring high-fidelity data when a defect is indicated. The paper presents the Selective Matrix Capture that captures data more efficiently without a priori knowledge of the location of the defects, while still achieving the superior imaging quality provided by an FMC dataset. This is achieved by taking into consideration the generation and detection characteristics of laser ultrasonics and utilising the high flexibility of LIPAs. The SMC data acquisition method proposed here, consists of two stages, starting with a rapid, iterative, inspection, establishing the number of defects and their locations, while providing low imaging quality. Once the location of the defects is known, the second, focusing stage follows, with extra element locations and finer element pitch, enabling higher frequency imaging, suitable for defect characterisation. By acquiring data only where meaningful information is captured, data acquisition time and size of dataset are significantly reduced, while data processing speed is increased.The SMC data acquisition method is demonstrated by synthesising a LIPA on an aluminium sample. The quality of ultrasonic images obtained remotely by processing the FMC and SMC datasets is compared. Data acquisition time and data size are reduced by an order of magnitude, without compromising signal-to-noise ratio. This translated to a reduction of scan time from 42 minutes for conventional FMC to &lt;4 minutes using SMC, for the case of an aluminium test object with one side-drilled hole of 1 mm diameter, located at 15 mm depth below the inspection surface.

Adaptive Data Acquisition for Fast Ultrasonic Imaging Using Laser Induced Phased Arrays

Paper Proceedings: https://asmedigitalcollection.asme.org/qnde/QNDE2021/volume/85529

ASME 2021 48th Annual Review of Progress in Quantitative Nondestructive Evaluation (QNDE2021)

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