Cheap optical transducers (CHOTs) are patterns on the surface of a component activated by lasers to generate and detect ultrasound. Excited optically, with minimal surface impact, and fully customizable, CHOTs provide a simple alternative to conventional piezoelectric transducers, offering wireless, remote operation. Of particular interest is application of CHOTs for in-situ ultrasonic inspection of hard-to reach and complex-geometry components such as those of aero-engines. A suitable fabrication method has been developed to allow in-situ application of CHOTs onto large size and curved components, as well as those already in service, challenging for current laboratory-based micro-patterning methods. This work describes the fabrication of a transferable g-CHOT for generation of ultrasound. The g- CHOT has been made on an SU8 carrier film using a sacrificial polystyrene layer, allowing the transducer to be transferred from the substrate and subsequently delivered and applied to the surface of the sample in-situ. The functionality of the fabricated transducer is demonstrated by detection of the Surface Acoustic Waves (SAW) generated by the g-CHOT transferred onto glass and aluminium samples.
New approaches for efficient NDT inspection of modern additively manufactured metallic components are required urgently to qualify and validate the next generation of metallic parts across a range of industries. Ultrasonic testing is a fundamental component of NDT for such additive manufacturing processes. This work studies the ultrasonic propagation characteristics of EBM manufactured sample coupons in Alloy 718 material. Fundamental longitudinal and shear wave velocity measurements are experimentally measured in 3 orthogonal build directions of the sample coupons. Results show a dependency of the ultrasonic velocities and the build direction. The measured velocities are further verified in a phased array measurement showing successful results that highlights the potential of continued studies with synthetic apertures techniques.
The recently proposed selective matrix capture approach used in inspections with laser induced phased-arrays has made this technology faster and more accurate. The process consists of two stages, one for rapid defect detection and another one for high fidelity defect characterisation. The phased-array configuration is optimally adapted to the current inspection scenario for each stage. This paper proposes a rigorous optimisation process for the defect detection stage for the case of a large component where the array is scanned along the length of the component. This requires an array configuration that achieves the required detection performance with the minimum number of generation and detection points per unit distance. An experiment is set-up to demonstrate the optimality of the resulting array in an aluminium specimen.
To acquire full matrix capture (FMC) data from an N element array, N individual transmissions are required. In cases where the acquisition process is lengthy and expensive (as in laser ultrasonics), it is often desirable to reduce the overall data acquisition time by decreasing the number of individual transmissions. Unfortunately, as the recording footprint of the transducer array is depleted, our ability to image with the collected data can be hindered. In this paper, we present a methodology to synthesize reflected data at points on the inspection surface not covered by an active array element, thus reconstructing the same data that would be recorded by a dense array from more sparsely distributed elements. To achieve this we apply matrix completion techniques to both the raw incomplete FMC data and incomplete scattering matrices. The matrix completed data is then compared to the complete dataset and to data reconstructed using a standard inpainting technique.
Additive manufacturing (AM) has been revolutionizing the manufacturing industry due to its ability to significantly reduce waste and produce components with intricate shapes. Laser Ultrasonics (LU) is a non-contact and couplant free method to generate and detect ultrasound. LU can accommodate complex component shapes; thus, it has the potential to provide a reliable in-process inspection method for AM components. In recent years the development of Laser Induced Phased Arrays (LIPAs) helped overcome the inherently low signal amplitudes of LU at the non-destructive, thermoelastic regime. In this paper, the Full Matrix Capture data acquisition method is used and a LIPA of 68 elements is synthesized in post processing. The Total Focusing Method imaging algorithm is applied for ultrasonic imaging. The technique is demonstrated on a highly scattering titanium alloy Wire Arc Additive Manufactured (WAAM) component producing high quality ultrasonic images, accurately imaging defects at depths up to 10mm below the inspection surface.
Laser irradiation may alter the colour of certain pigments used in artworks. Owing to the increasing use of lasers in artwork conservation, this problem becomes extremely crucial especially in the case of a polychromy covered by environmental encrustation. X-ray diffraction analysis and optical microscopy were used to study the crystalline structure of selected irradiated pigments in order to identify the changes that take place upon laser irradiation. Two representative pigments were studied, cinnabar (HgS) and lead white (Pb(OH)2 2PbCO3). The results revealed a change in the crystalline phase or chemical composition, respectively, which can be barely detected owing to its superficial character, which is of a sub-micrometer scale.
In-process inspection of the additive manufacturing process requires a technique that can provide reliable measurements given the extreme operating environments, the small size of the defects and the cyclic melting and heating of the material, caused by subsequently deposited layers. A remote and couplant-free ultrasonic inspection technique using bulk waves that can image near-surface defects could address these in-process inspection requirements. Laser induced phased arrays (LIPA) generate and detect ultrasound based on laser ultrasonics principles, while the array is synthesised in post-processing. However, when using LIPAs for inspection, the surface acoustic waves (SAWs) interfere with the bulk wave modes giving rise to crosstalk and artefacts, which makes near-surface defect imaging difficult. This work experimentally validates and compares five techniques for SAW suppression: amplitude thresholding, mean waveform subtraction, principal component subtraction, frequency-wavenumber filtering, and phase coherence imaging. SAW suppression is demonstrated in ultrasonic images of transverse waves based on 71-element LIPA data synthesised on a Ti-6Al-4V directed energy deposition-arc (DED-Arc/Ti6Al4V) sample with a ∼1 mm diameter side drilled hole, located at ∼4 mm below the inspected surface. The reported results show that the principal component subtraction approach achieved the highest 'signal-to-crosstalk ratio' improvement of 16 dB, while successfully suppressing the SAW.
'%3e%3cpath fill='%23012169' d='M0 0v30h60V0z'/%3e%3cpath stroke='%23fff' stroke-width='6' d='m0 0 60 30m0-30L0 30'/%3e%3cpath stroke='%23C8102E' stroke-width='4' d='m0 0 60 30m0-30L0 30' clip-path='url(%23b)'/%3e%3cpath stroke='%23fff' stroke-width='10' d='M30 0v30M0 15h60'/%3e%3cpath stroke='%23C8102E' stroke-width='6' d='M30 0v30M0 15h60'/%3e%3c/g%3e%3c/svg%3e)
