Laser generation of narrowband lamb waves for in-situ inspection of additively manufactured metal components
08 May 2019-Vol. 2102, Iss: 1, pp 070001
TL;DR: In this article, narrow band lamb wave modes were generated using a pulsed Nd-YAG laser system consisting of a spatial array illumination source, which enabled generation of specific Lamb wave modes for in-situ inspection of additively manufactured components.
Abstract: Recent developments in metal additive manufacturing (AM) has created a lot of interest in sectors including automotive, aerospace and biomedical engineering. It is imperative that the components manufactured additively be inspected for flaws, mechanical properties and dimensional accuracy. Several non-destructive testing (NDT) techniques such as X-ray computed tomography and conventional ultrasonic testing have been implemented to evaluate the quality of these components. Recently, research has been focused on techniques that can perform non-contact testing and carry out an online inspection layer by layer while the component is being fabricated. Laser based ultrasonic technique has been found to be a promising method owing to its non-contact nature and ability to operate in harsh environments. In our study, narrow band lamb waves were generated using a pulsed Nd-YAG laser system consisting of a spatial array illumination source. The generated wave modes were detected using a two-wave mixing based laser interferometer. The wavelength-matched method enabled generation of specific lamb wave modes for in-situ inspection of additively manufactured components.
Citations
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TL;DR: A brief review of 3D printing processes and evolution of CT technology is presented and documented data proved that CT is an appropriate non-contact technique for technical evaluation of various printed parts.
Abstract: Technical advantages of additive manufacturing (AM) have drawn great attention over the past few years. This cost-effective manufacturing process proved its potential applications in a wide range of fields. Although AM techniques (known as 3D printing) are able to fabricate geometrically complex components, it is necessary to evaluate internal and external dimensions of the printed parts. In this context, x-ray computed tomography (CT) as a nondestructive evaluation technique has been utilized. Indeed, CT can be used for geometric analysis, defects detection, quantitative comparison, structural quantification and porosity analysis. In the current study, we present a brief review of 3D printing processes and evolution of CT technology. Moreover, applications of CT in assessment of 3D-printed components are explained in detail. Although CT has been used in academic and industrial researches, abilities of this inspection method are not yet fully documented for precision engineering applications. In this work, usage of this technique in study of printed components are categorized in four subdomains and discussed. The documented data proved that CT is an appropriate non-contact technique for technical evaluation of various printed parts. As usage of CT in assessment of printed parts is still evolving, the limitations, challenges and future perspective are outlined.
21 citations
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TL;DR: In this paper , a phased array ultrasonic testing (PAUT) is suggested to inspect thick LPBF components, while guided waves are explored for thin curved ones, and the results are compared to defect images predicted by finite element simulations.
Abstract: Additive manufacturing of alloys enables low-volume production of functional metallic components with complex geometries. Ultrasonic testing can ensure the quality of these components and detect typical defects generated during laser powder bed fusion (LPBF). However, it is difficult to find a single ultrasonic inspection technique that can detect defects in the large variety of geometries generated using LPBF. In this work, phased array ultrasonic testing (PAUT) is suggested to inspect thick LPBF components, while guided waves are explored for thin curved ones. PAUT is used to detect cylindrical lack of fusion defects in thick LPBF rectangular parts. Practical defects are generated by reducing the laser power at prespecified locations in the samples. The defects’ shape and density are verified using optical microscopy and X-ray computed tomography. Partially fused defects down to 0.25 mm in diameter are experimentally detected using a 10 MHz PAUT probe with the total focusing method post-processing. The experimental results are compared to defect images predicted by finite element simulations. For thin components with curved geometry, guided waves are used to detect powder-filled cylindrical defects. The waves are generated using piezoelectric transducers, and the spatiotemporal wavefield is measured using a scanning laser Doppler vibrometer. Using root-mean-square imaging of the wavefield, defects down to 1 mm are clearly detected despite the complex internal features in the samples.
4 citations
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TL;DR: In this article, the authors proposed a method for generating narrowband ultrasonic guided waves using an additively manufactured slit mask that is integrated onto the component during selective laser melting (SLM) process.
Abstract: Guided ultrasonic waves are attractive for inspection of additively manufactured plate-like components. Illumination of a slit mask by a pulsed laser is one method by which guided ultrasonic waves can be generated. This work proposes a method for generating narrowband ultrasonic guided waves using an additively manufactured slit mask that is integrated onto the component during selective laser melting (SLM) process. Multiple guided wave modes with a dominant wavelength but with different frequencies were generated using the slit mask fabricated using AlSi12 material. The generated modes were identified using the time frequency response of the received signals and dispersion plots. Identifying the modes and its characteristics (frequency, wavelength, phase and group velocity) beforehand facilitates material and defect characterization. A multiphysics numerical model was developed to simulate laser generation of ultrasound and the model was validated using experimental results. The numerical model developed aided in understanding the physics of line arrayed laser ultrasonic generation and was used as a tool to optimize laser parameters. The developed model was used to study the effect of pulse width of the laser on Lamb wave mode generation. It was observed that a pulse width of 100 ns reduced the overall ultrasonic bandwidth to 4.5 MHz thereby limiting the modes to the fundamental modes A0 and S0 for the given wavelength of 0.8 mm. Rayleigh wave studies using a slit mask showed that the rate of decay of the fundamental frequency component was steeper than the rate of decay of the second harmonic component.
1 citations
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TL;DR: In this article , the characteristics of Lamb wave detection (LW) are analyzed in terms of laser ultrasound and hardware configuration, and four combined methods that balance detection efficiency and accuracy are summarized.
Abstract: Thin-walled structures, like aircraft skins and ship shells, are often several meters in size but only a few millimeters thick. By utilizing the laser ultrasonic Lamb wave detection method (LU-LDM), signals can be detected over long distances without physical contact. Additionally, this technology offers excellent flexibility in designing the measurement point distribution. The characteristics of LU-LDM are first analyzed in this review, specifically in terms of laser ultrasound and hardware configuration. Next, the methods are categorized based on three criteria: the quantity of collected wavefield data, the spectral domain, and the distribution of measurement points. The advantages and disadvantages of multiple methods are compared, and the suitable conditions for each method are summarized. Thirdly, we summarize four combined methods that balance detection efficiency and accuracy. Finally, several future development trends are suggested, and the current gaps and shortcomings in LU-LDM are highlighted. This review builds a comprehensive framework for LU-LDM for the first time, which is expected to serve as a technical reference for applying this technology in large, thin-walled structures.
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TL;DR: In this article, the formation of residual wavefield is investigated and an optimization method of laser scanning path is developed to improve the damage visualization using the residual guided waves, which is intensively used in guided wave-based non-destructive testing due to its noncontact scanning, automated scanning path and full wavefield measurement.
Abstract: Laser ultrasonic scanning is intensively used in guided wave-based non-destructive testing due to its non-contact scanning, automated scanning path and full wavefield measurement. However, in practical applications, the selected repetition rate of laser generation is limited due to the existence of residual guided waves generated by consecutive pulsed laser impingements, leading to a low scanning speed. To overcome such limitations, the formation of residual wavefield is investigated and an optimization method of laser scanning path is developed to improve the damage visualization using the residual guided waves. First, the energy partition of residual wavefield is investigated based on diffuse wavefield approximation and transient statistical energy analysis. Then, a virtual superposition method is proposed to obtain the synthetic wavefields with different laser repetition rates. The influence of laser repetition rate on the wavefield directivity and energy distribution is analyzed. Moreover, according to the ultrasonic attenuation property of the test specimen, an optimized sub-block raster scanning path is proposed to minimize the spatial misalignment of the energy distribution in residual wavefield. Experiments are conducted on an aluminum alloy plate with multiple artificial notches using a 1-kHz-repetition-rate scanning laser ultrasonic system. Based on the residual wavefield signal energy, damage visualization results before and after laser scanning path optimization are compared. Results show that the proposed method is not only more efficient during the laser scanning process, but also more capable of distinguishing damaged areas from the intact structure.
References
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01 Jan 1990
TL;DR: In this paper, the authors describe the characteristics of laser light for ultrasonics, including the acousto-optic effect, the measurement of ultrasonic fields Bragg diffraction, and the interaction of light with surface waves.
Abstract: INTRODUCTION Ultrasonics Lasers Main characteristics of laser light Lasers for ultrasonics ACOUSTO-OPTIC INTERACTIONS The acousto-optic effect The measurement of ultrasonic fields Bragg diffraction Acousto-optic devices Applications of acousto-optic devices The interaction of light with surface waves LASER INTERFEROMETRY Principles of laser interferometry Theory of interference between light beams Reference beam interferometry with rough surfaces Light detection and signal amplification Effects of laser mode structure Signal processing Stabilized interferometers Optical frequency shifting Quadrature interferometers Long path difference and interferometry Fabry-Perot interferometers APPLICATINOS OF LASER INTERFEROMETRY TO ULTRASONIC DISPLACEMENT MEASUREMENT The measurement of acoustic fields Scanned laser interferometry Full field visualisation of surface displacement The measurement of surface waves Calibration of transducers Acoustic emission The measurement of transverse and vector displacement Sensitivity comparison with other ultrasonic detection techniques ULTRASONIC GENERATION BY LASER Absorbtion of electromagnetic radiation Temperature distributions Thermoelastic stresses Other effects Constrained surfaces Ultrasonic wave propagation in unbounded solids Propagation in bounded solids Radiation patterns for laser-ultrasonic sources Ultrasonic bulk waveforms in plates Ultrasonic surface and guided waves Lasers for ultrasonic generation APPLICATIONS USING LASER GENERATION OF ULTRASOUND Applications to flaw detection Applications to materials property measurement APPLICATIONS TO ACOUSTIC MICROSCOPY Calibration Wave propagation studies Medical applications CONCLUSIONS AND FUTURE PROSPECTS FOR LASER TECHNIQUES IN ULTRASONICS Summary and conclusions Future research and development Future prospects for applications REFERENCES INDEX
949 citations
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TL;DR: In this paper, an array of ten pulsed Nd: YAG lasers was constructed in order to study the effects of generating ultrasonic signals from a single laser source in the thermo-elastic regime.
Abstract: An array of ten pulsed Nd: YAG lasers was constructed in order to study the effects of generating ultrasound with an array of laser sources. The laser system permitted the spatial and temporal control of the firing of the individual lasers in the array necessary for the production of both narrow-band ultrasonic signals and phased array single pulses. The increase in sensitivity of a laser ultrasonic system associated with the generation of narrow-band and phased array acoustic waves is discussed theoretically and verified experimentally for surface and bulk wave generation. The ultrasonic signals were generated in aluminium samples of various thicknesses and with source laser power densities consistent with generation in the thermoelastic regime, thus causing no damage to the surface of the specimens. The signals were detected using a path stabilized Michelson interferometer. In the narrow-band case, the waveforms were digitally filtered in order to take advantage of the reduced spectral range of the generated acoustic energy. A significant increase in the sensitivity of the laser ultrasonic system, consistent with theoretical predictions, was observed in both the narrow-band and phased array cases.
83 citations
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TL;DR: In this article, the impulse directivity patterns of laser-generated longitudinal acoustic waves have been computed for duraluminum samples in the thermoelastic regime and steel sample in the ablation regime.
Abstract: Focused ultrasonicwaves have been generated in a solid by irradiating its surface with a multiple beam‐pulsed YAG laser. A set of 16 rectilinear sources is used, equivalent to a phased array of ultrasonic transducers. Longitudinal waves are focused in the sample by introducing an appropriate time delay between each laser pulse. The elastic waves are detected either by a broadband optical heterodyne probe to analyze the wide ultrasonic signal spectrum (0–20 MHz), or by a narrow‐band piezoelectric transducer to achieve the sectorial acoustic beam scanning of the sample. Neglecting heat diffusion in the solid and considering the source as a surface center of expansion, the impulse directivity patterns of laser‐generated longitudinal acoustic waves have been computed. Experiments performed on duraluminum samples in the thermoelastic regime and steel samples in the ablation regime are presented and compared with this analysis. It is shown that a high focusing and a significant improvement of the signal sensitivity for longitudinal waves can be achieved with this technique.
66 citations
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TL;DR: In this paper, a spatially resolved acoustic spectroscopy (SRAS) technique is applied to prepare additively manufactured material to measure the material properties and identify defects, and the number of pore size remains the same for 140 W to 190 W melting power (mean: 115-119 μm optical and 134-137 μm velocity).
Abstract: Additive manufacturing (AM) is a manufacturing technique that typically builds parts layer by layer, for example, in the case of selective laser melted (SLM) material by fusing layers of metal powder. This allows the construction of complex geometry parts, which, in some cases cannot be made by traditional manufacturing routes. Complex parts can be difficult to inspect for material conformity and defects which are limiting widespread adoption especially in high performance arenas. Spatially resolved acoustic spectroscopy (SRAS) is a technique for material characterisation based on robustly measuring the surface acoustic wave velocity. Here the SRAS technique is applied to prepare additively manufactured material to measure the material properties and identify defects. Results are presented tracking the increase in the measured velocity with the build power of the selective laser melting machine. Surface and subsurface defect measurements (to a depth of ∼24 μm) are compared to electron microscopy and X-ray computed tomography. It has been found that pore size remains the same for 140 W to 190 W melting power (mean: 115–119 μm optical and 134–137 μm velocity) but the number of pores increase significantly (70–126 optical, 95–182 velocity) with lower melting power, reducing overall material density.
65 citations
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TL;DR: Experimental outcomes prove that typical micro-defects due to the layer-by-layer deposition process, such as near-surface and surface flaws in a single layer deposit, can be detected.
Abstract: Laser powder deposition (LPD) is a rapid additive manufacturing process to produce, layer upon layer, 3D geometries or to repair high-value components. Currently there is no nondestructive technique that can guarantee absence of flaws in LPD products during manufacturing. In this paper a laser ultrasonic technique for in-line inspection of LPD components is proposed. Reference samples were manufactured from Inconel and machined flaws were created to establish the sensitivity of the technique. Numerical models of laser-generated ultrasonic waves have been created to gain a deeper understanding of physics, to optimize the set-up and to verify the experimental measurements. Results obtained on two sets of reference samples are shown. A proof-of-concept prototype has been demonstrated on some specific deposition samples with induced flaws, that were confirmed by an ultra-high sensitivity X-ray technique. Experimental outcomes prove that typical micro-defects due to the layer-by-layer deposition process, such as near-surface and surface flaws in a single layer deposit, can be detected.
63 citations
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