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Electromagnetic wave (EMW)-absorbing materials, manufactured with composites of magnetic particles, are essential for maintaining a high complex permeability and modulated permittivity for impedance matching. However, commonly available EMW-absorbing materials are unsatisfactory owing to their low complex permeability in the high-frequency band. Herein, we report a thin, flexible EMW-absorbing membrane comprising shape-modulated FeCo nanobelts/boron nitride nanoparticles, which enables enhanced complex permeability in the S, C, and X bands (2-12 GHz). The boron nitride nanoparticles that are introduced to the FeCo nanobelts demonstrate control of the complex permittivity, leading to an effective impedance matching close to 1, consequently resulting in a high reflection loss value of -42.2 dB at 12.0 GHz with only 1.6 mm thickness. In addition, the incorporation of boron nitride nanoparticles improves the thermal conductivity for the heat dissipation of the absorbed electromagnetic wave energy. Overall, the comprehensive study of nanomaterial preparation and shape modulation technologies can lead to the fabrication of an excellent EMW-absorbing flexible composite membrane.

A 5G EMI shielding composite flexible film with near-zero reflection is proposed with remarkably low reflection with thin thickness compared to the previous studies. The resonant frequency can be controlled via tailored design factors of the film.

We report a simple and continuous spray-drying method to synthesize high-purity epsilon iron oxide (ε-Fe2O3). The strategy enables ferromagnetic resonance tuning, making it potentially usable in millimeter-wave absorbers.

High-precision artificial synaptic devices compatible with existing CMOS technology are essential for realizing robust neuromorphic hardware systems with reliable parallel analogue computation beyond the von Neumann serial digital computing architecture. However, critical issues related to reliability and variability, such as nonlinearity and asymmetric weight updates, have been great challenges in the implementation of artificial synaptic devices in practical neuromorphic hardware systems. Herein, a robust three-terminal two-dimensional (2D) MoS

Herein, we introduce novel 1-dimensional nano-chained FeCo particles with unusually-high permeability prepared by a highly-productive thermal plasma synthesis and demonstrate an electromagnetic wave absorber with exceptionally low reflection loss in the high-frequency regime (1–26 GHz). During the thermal plasma synthesis, spherical FeCo nanoparticles are first formed through the nucleation and growth processes; then, the high temperature zone of the thermal plasma accelerates the diffusion of constituent elements, leading to surface-consolidation between the particles at the moment of collision, and 1-dimensional nano-chained particles are successfully fabricated without the need for templates or a complex directional growth process. Systematic control over the composition and magnetic properties of FexCo1−x nano-chained particles also has been accomplished by changing the mixing ratio of the Fe-to-Co precursors, i.e. from 7 : 3 to 3 : 7, leading to a remarkably high saturation magnetization of 151–227 emu g−1. In addition, a precisely-controlled and uniform surface SiO2 coating on the FeCo nano-chained particles was found to effectively modulate complex permittivity. Consequently, a composite electromagnetic wave absorber comprising Fe0.6Co0.4 nano-chained particles with 2.00 nm-thick SiO2 surface insulation exhibits dramatically intensified permeability, thereby improving electromagnetic absorption performance with the lowest reflection loss of −43.49 dB and −10 dB (90% absorbance) bandwidth of 9.28 GHz, with a minimum thickness of 0.85 mm.

For decades, porous ceramics have received much scientific and industrial interests because they possess better thermal and chemical stabilities than their counterparts, including porous polymers and porous metals. Among the numerous kinds of porous ceramics, reticulated porous ceramics have recently been prepared for various application fields. However, as far as the authors’ knowledge, the radar-absorbing properties of reticulated porous ceramics remain largely unknown, until now. Therefore, the feasibility of reticulated porous ceramics prepared using barium titanate, which has an inherently high dielectric constant, as a potential platform that could be used without additional radar-absorbing material coating has been discussed. From the results obtained in this study, it was discussed whether reticulated porous barium titanate could be fabricated with an acceptable mechanical strength and radar-absorbing properties. Quantitatively, the measured reflection loss (RL) of the 2.08-mm-thick reticulated porous barium titanate, approached − 16 dB (97.49% absorption of the radar wave) at 9.2 GHz. Meanwhile, the calculated RL of the as-prepared 2.50-mm-thick reticulated porous barium titanate approached − 21 dB (99.21% absorption of the radar wave) at 9.2 GHz.

Herein, we introduce novel 1-dimensional nano-chained FeCo particles with unusually-high permeability prepared by a highly-productive thermal plasma synthesis and demonstrate an electromagnetic wave absorber with exceptionally low reflection loss in the high-frequency regime (1-26 GHz). During the thermal plasma synthesis, spherical FeCo nanoparticles are first formed through the nucleation and growth processes; then, the high temperature zone of the thermal plasma accelerates the diffusion of constituent elements, leading to surface-consolidation between the particles at the moment of collision, and 1-dimensional nano-chained particles are successfully fabricated without the need for templates or a complex directional growth process. Systematic control over the composition and magnetic properties of FexCo1-x nano-chained particles also has been accomplished by changing the mixing ratio of the Fe-to-Co precursors, i.e. from 7 : 3 to 3 : 7, leading to a remarkably high saturation magnetization of 151-227 emu g-1. In addition, a precisely-controlled and uniform surface SiO2 coating on the FeCo nano-chained particles was found to effectively modulate complex permittivity. Consequently, a composite electromagnetic wave absorber comprising Fe0.6Co0.4 nano-chained particles with 2.00 nm-thick SiO2 surface insulation exhibits dramatically intensified permeability, thereby improving electromagnetic absorption performance with the lowest reflection loss of -43.49 dB and -10 dB (90% absorbance) bandwidth of 9.28 GHz, with a minimum thickness of 0.85 mm.

We investigated the electroless plating of magnetic metal nanoparticles on graphene to achieve nanoparticles with a tunable size and structure, a degree of exposure of the graphene surface, and magnetic properties of the hybrid material. FeCoNi ternary alloy nanoparticles were grown on the surface of graphene with various ratios of FeCoNi to graphene using two different electroless plating methods. The small size of the FeCoNi nanoparticles synthesized by conventional electroless plating resulted in a large amount of oxide and a low saturation magnetization of the hybrids. In contrast, the FeCoNi nanoparticles synthesized by dropwise electroless plating exhibited a large particle size and a high saturation magnetization. When the ratio of FeCoNi to graphene was increased, the saturation magnetization of both hybrids increased, but the size and structure of the nanoparticles were still different. These results imply that it is possible to synthesize tailored magnetic metal@graphene hybrid materials using an electroless plating method. This work provides guidelines for the preparation of metal nanoparticle@nanocarbon hybrid materials based on an electroless plating method.

Stretchable conductive fibers have attracted significant attention due to their ability to be directly woven into or stitched onto fabrics, making them ideal for use in the design of integrated wearable strain sensors. Here, we report on a highly stretchable multi-walled carbon nanotube (MWCNT)/Thermoplastic Polyurethane (TPU) fiber produced via a wet spinning process. The effects of MWCNT content and alignment on the structural, mechanical, electrical and strain-sensing properties of the composite fibers were investigated. The highest conductivity (6.77 S cm−1), tensile strength (28 MPa) and maximum elongation at break (565%) were obtained by controlling the MWCNT content. Gauge factor (GF) values were also affected by the content and MWCNT alignment in the composite fibers, as these parameters determine the change in the effective contact area and number of conductive paths available during stretching. The well-aligned MWCNT/TPU fiber showed a high GF value of 5200. Wearable strain sensors capable of obtaining real-time mechanical feedback for various human motion detections with different GFs and working strain ranges could be realized by controlling the MWCNT concentrations in the TPU matrix.

Magnetic metal and graphene hybrids possess the high dielectric properties of graphene and the high magnetic properties of the metal, which render them suitable as electromagnetic (EM) wave-absorbing materials. In this research, we fabricated an EM wave-absorbing film with excellent performance (reflection loss of −68 dB) based on a FeCoNi@graphene hybrid. We developed a new electroless plating method for fabricating magnetic metal@graphene hybrids without any heat treatment, and the new hybrid material showed a higher saturation magnetization (120.4 emu/g) than that of a hybrid material prepared by conventional electroless plating (56.8 emu/g). The sizes, structures, and oxidation ratios of the FeCoNi nanoparticles on graphene were different depending on the synthesis method, and Ni played an important role in the growth of the FeCoNi nanoparticles. The FeCoNi@graphene hybrid was well dispersed in tetrahydrofuran, and thus, could be fabricated into free-standing and flexible composite films by mixing with thermoplastic polyurethane. The high permeability of the FeCoNi@graphene film resulted in a high EM wave-absorbing performance due to impedance matching. These results provide an important advancement toward not only the commercialization of EM wave-absorbing films for healthcare, electronic reliability, and tactical security applications but also the development of magnetic metal and graphene hybrid materials using a scalable process.

In this paper, the effect of ablation on the shape of elastic waves generated by laser excitation is studied numerically and experimentally. Laser-induced ultrasound has been widely used in the nondestructive testing (NDT) field because it has the advantage that the sensor does not have to be directly attached to the target structure. In the safety assessment process, low energy excitation is used, and thus the structure is not damaged. Most studies related to laser ultrasound have focused on the method of detecting cracks within the elastic range, and there have been few studies on the effect of ablation. This research consists of experiments and numerical analyses. In experiments, elastic waves were generated in an aluminum plate by projecting laser pulses with different energy intensities. The velocities in the thickness direction were measured using a Laser Doppler Velocimeter (LDV) at a point 135 mm away from the excitation point. In the numerical study, two numerical simulations were carried out using heat flux and normal stress input to mimic laser pulse excitation. A thermo-mechanical simulation by heat flux was conducted to simulate thermal expansion by the laser pulse, and the normal stress was applied to reflect the effect of radiation pressure by ablation, respectively. Waveforms were synthesized by using different magnitude ratios of the obtained numerical responses and were compared with the experiment results. It is found that the effect of radiation pressure should not be neglected if the energy intensity is large although the effect of radiation pressure decreases as the energy intensity decreases. At the energy intensity with which ablation occurs, the effects of thermal expansion and radiation pressure exist simultaneously, and the contribution to the response depends on the energy intensity.

Carbon nanotube fiber (CNTF), prepared by the direct-spinning method, has several nanopores, and the infiltration behavior of resins into these nanopores could influence the mechanical properties of CNTF-based composites. In this work, we investigated the infiltration behavior of resin into the nanopores of the CNTFs and mechanical properties of the CNTF-based single-fiber composites using six epoxy resins with varying viscosities. Epoxy resins can be easily infiltrated into the nanopores of the CNTF, however, pores appear when a resin with significantly high or low viscosity is used in the preparation process of the composites. All the composite fibers exhibit lower load-at-break value compared to as-densified CNTF, which is an unexpected phenomenon. It is speculated that the bundle structure of the CNTF can undergo changes due to the high affinity between the epoxy and CNTF. As composite fibers containing pores exhibit an even lower load-at-break value, the removal of pores by the defoaming process is essential to enhance the mechanical properties of the composite fibers.

Recent advances in fifth generation (5G) mobile communication have allowed the development of high data rates, energy efficiency, and system capacity. The demand for millimeter-wave (mmWave) frequency electromagnetic wave shielding materials is also increasing simultaneously. However, current electromagnetic interference (EMI) shielding materials, including metals and composites with dielectric or conductive fillers, typically have a high absorption shielding effectiveness (SEA) with a low reflection shielding effectiveness (SER) at mmWave frequencies. To address this inherent issue of EMI shielding materials, we introduce a metal-mesh-supported polymer composite consisting of a reflection layer and an absorbing layer. The proposed composite shielding material has a SER of only 2 dB and a SEA of 46 dB. The permittivity, permeability, and tangent loss of the absorption layer, as well as the effects of thickness and type of mesh of the mesh-supported polymer composite are discussed. The mechanism of additional absorption capacity due to destructive interference is also discussed in comparison with theoretical calculations. Its shielding efficiency is compared with previously reported values, and a superior absorbance with a thinner thickness of the proposed mesh-supported polymer composite system is observed, which is very proper for 5G EMI shielding applications.

Laser ultrasonic scanning is attractive for damage detection due to its noncontact nature, sensitivity to local damage, and high spatial resolution. However, its practicality is limited because scanning at a high spatial resolution demands a prohibitively long scanning time. Inspired by binary search and compressed sensing, an accelerated laser scanning technique is developed to localize and visualize damage with reduced scanning points and scanning time. First, the approximate damage location is identified by examining the interactions between the ultrasonic waves and damage at the sparse scanning points that are selected by the binary search algorithm. Here, a time-domain laser ultrasonic response is transformed into a spatial ultrasonic domain using a basis pursuit approach so that the interactions between the ultrasonic waves and damage, such as reflections and transmissions, can be better identified in the spatial ultrasonic domain. Second, wavefield images around the damage are reconstructed from the previously selected scanning points using compressed sensing. The performance of the proposed accelerated laser scanning technique is validated using a numerical simulation performed on an aluminum plate with a notch and experiments performed on an aluminum plate with a crack and a carbon fiber-reinforced plastic plate with delamination. The number of scanning points that is necessary for damage localization and visualization is dramatically reduced from N · M to 2 log 2 N · log 2 M . N and M represent the number of equally spaced scanning points in the x and y directions, respectively, which are required to obtain full-field wave propagation images of the target inspection region. For example, the number of scanning points in the composite plate experiment is reduced by 97.1% (from 2601 points to 75 points).

Here, we report a novel highly sensitive wearable strain sensor based on a highly stretchable multi-walled carbon nanotube (MWCNT)/Thermoplastic Polyurethane (TPU) fiber obtained via a wet spinning process. The MWCNT/TPU fiber showed the highest tensile strength and ultra-high sensitivity with a gauge factor (GF) of approximately 2800 in the strain range of 5–100%. Due to its high strain sensitivity of conductivity, this CNT-reinforced composite fiber was able to be used to monitor the weight and shape of an object based on the 2D mapping of resistance changes. Moreover, the composite fiber was able to be stitched onto a highly stretchable elastic bandage using a sewing machine to produce a wearable strain sensor for the detection of diverse human motions. We also demonstrated the detection of finger motion by fabricating a smart glove at the joints. Due to its scalable production process, high stretchability and ultrasensitivity, the MWCNT/TPU fiber may open a new avenue for the fabrication of next-generation stretchable textile-based strain sensors.

In this study, different volume fractions of silicon-carbide-reinforced AA2024 matrix composites were successfully fabricated using stir-casting (SC) and die-casting (DC) processes. The microstructural difference and physical properties of the composites during the manufacturing process were investigated in detail. The microstructural analysis found that the composite produced by the SC process had some reinforcement clusters and pores, however, defects and clusters significantly decreased after the DC process. In particular, the degree of reinforcement dispersion was quantitatively analyzed and compared before and after the DC process using the dispersion-analysis method. As a result of quantitative evaluation, the degree of dispersion was improved 2.5, 4.6, and 4.0 times with 3 vol.%, 6 vol.%, and 9 vol.% SiC-reinforced composite after the DC process, respectively. The electron backscatter diffraction (EBSD) analysis showed that the grain size of the 9 vol.% SiC-reinforced DC composite (17.67 &mu, m) was 75% smaller than that of the SC composite (68.06 &mu, m). The average tensile strength and hardness of the 9 vol.% SiC-reinforced DC composite were 2 times higher than those of the AA2024 matrix. The superior mechanical properties of the DC-processed composite can be attributed to the increase in dispersivity of the SiC particles and to decreases in defects and grain size during the DC process.

In this study, a two-level scanning strategy for a noncontact laser ultrasonic measurement system is proposed to expedite the inspection of a wind turbine blade. First, coarse scanning of the entire blade is performed with a low spatial resolution for initial delamination localisation. Then, dense scanning with a high spatial resolution is performed only within the identified delaminated region for delamination visualization. This study especially focuses on the initial delamination localisation using adaptive coarse scanning. Laser ultrasonic responses from two pitch-catch paths, names inspection pairs, are obtained within a specified coarse scanning grid. Then, potential delamination locations within the given grid are estimated through time-of-flight analysis of delamination reflected waves. Once potential delamination locations are estimated, new inspection pairs are placed near the potential locations for precise localisation. These steps are repeated for every coarse scanning grids on the ta...

Many stretchable conductive composite fibers exhibit high elongation at break, but most of them do not have a large workable strain range towing to the low conductivity when used as strain sensors. In this paper, we fabricated a highly conductive silver nanowire (Ag NW)/multi-walled carbon nanotube (MWCNT)/thermoplastic polyurethane (TPU) fiber via a wet-spinning process to improve the workable strain range of composite fibers. TPU was used as a matrix material to introduce superior stretchability. MWCNTs act as sensing elements and Ag NWs were used to increase conductivity. We investigated the effect of Ag NW content on the mechanical, electrical, and strain-sensing performance of the fiber-type strain sensors. The optimal content of Ag NWs extended the workable strain range as higher as 254% with an electrical conductivity of 0.803 S/cm. A weight-to-strain cloth sensor was assembled by writing Ag NW/MWCNT/TPU fibers in the coagulation solution. Furthermore, such composite fiber can be free-written into any designed pattern, which can be used to prepare fiber-based devices.

This study proposes an automated quantification technique for reinforcement dispersion in metal matrix composites. While a fine and homogeneous dispersion of reinforcements is necessary to achieve an optimum reinforcing effect in metal matrix composites, there have been few studies on the quantitative measurement of reinforcement dispersion. The proposed technique extracts reinforcement information from a given microscopic image through image analysis with minimized human interruption and quantifies the reinforcement dispersion automatically using a statistical approach. The feasibility of the proposed technique is shown by analyzing the effect of the hot rolling process on the reinforcement dispersion in a B4C/Al metal matrix composite and comparing it with the as-cast state.

Fatigue crack is a progressive and localized structural damage that occurs when a structure is subjected to cyclic loading. It is a critical concern for in-service metallic structural components for the failures caused by fatigue crack, which constitute nearly 90% of the total failures in metallic structures. Additionally, a fatigue crack only becomes conspicuous after the crack approaches approximately 80% of the structural fatigue life. For laminated composite structural components, delamination is a critical failure mechanism in the form of separated layers caused by cyclic loading or impact because the composite laminates do not provide reinforcement through the thickness. Damages like fatigue crack and delamination are often invisible or barely visible but may compromise the integrity of structural components and cause catastrophic failures.

The practicality of laser ultrasonic scanning is limited because scanning at a high spatial resolution demands a prohibitively long scanning time. Inspired by binary search, an accelerated defect visualization technique is developed to visualize defect with a reduced scanning time. The pitch-catch distance between the excitation point and the sensing point is also fixed during scanning to maintain a high signal-to-noise ratio of measured ultrasonic responses. The approximate defect boundary is identified by examining the interactions between ultrasonic waves and defect observed at the scanning points that are sparsely selected by a binary search algorithm. Here, a time-domain laser ultrasonic response is transformed into a spatial ultrasonic domain response using a basis pursuit approach so that the interactions between ultrasonic waves and defect can be better identified in the spatial ultrasonic domain. Then, the area inside the identified defect boundary is visualized as defect. The performance of the proposed defect visualization technique is validated through an experiment on a semiconductor chip. The proposed defect visualization technique accelerates the defect visualization process in three aspects: (1) The number of measurements that is necessary for defect visualization is dramatically reduced by a binary search algorithm; (2) The number of averaging that is necessary to achieve a high signal-to-noise ratio is reduced by maintaining the wave propagation distance short; and (3) With the proposed technique, defect can be identified with a lower spatial resolution than the spatial resolution required by full-field wave propagation imaging.

In this paper, using Fourier transform infrared (FTIR) spectroscopy, ion conductivity measurements and first-principle density functional theory (DFT) calculations, we study intermolecular interactions between three molecules (methyl tetrahydrophthalic anhydride (MeTHPA), succinonitrile (SN) plastic crystal, and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt) constituting the lithium-ion battery electrolyte. The C–O stretching band position in MeTHPA shifts to a lower frequency in the order of MeTHPA–SN < MeTHPA < MeTHPA–LiTFSI/SN < MeTHPA–LiTFSI; the average C–O bond length in MeTHPA increases in the same order, which reveals the linear correlation between the vibration frequency shift and bond length change. Furthermore, the lithium ionic conductivities of MeTHPA–LiTFSI/SN and MeTHPA–LiTFSI are consistent with this linear relationship, which confirms that the bond length, vibration frequency and lithium-ion transport are strongly influenced by molecular-level interactions. Our results provide fundamental insights valuable for the understanding of the effect of intermolecular interactions on molecular geometry and physical quantities in different electrolytes, and could be utilized to guide the design of high-performance electrolyte materials.

This paper presents a complete noncontact fatigue crack visualization technique based on nonlinear ultrasonic wave modulation and investigates the main source of nonlinear modulation generation. Two distinctive frequency input signals are created by two air-coupled transducers and the corresponding ultrasonic responses are scanned using a 3D laser Doppler vibrometer. The effectiveness of the proposed technique is tested using aluminum plates with different stages of fatigue crack formation such as micro and macro-cracks. Furthermore, the main source of nonlinear modulation is discussed based on the visualization results and the microscopic images.

Laser ultrasonic scanning is attractive for damage detection due to its noncontact nature, sensitivity to local damage, and high spatial resolution. However, its practicality is limited because scanning at a high spatial resolution demands a prohibitively long scanning time. Recently, compressed sensing (CS) and super-resolution (SR) are gaining popularity in the image recovery field. CS estimates unmeasured ultrasonic responses from measured responses, and SR recovers high spatial frequency information from low resolution images. Inspired by these techniques, a laser ultrasonic wavefield reconstruction technique is developed to localize and visualize damage with a reduced number of ultrasonic measurements. First, a low spatial resolution ultrasonic wavefield image for a given inspection region is reconstructed from reduced number of ultrasonic measurements using CS. Here, the ultrasonic waves are generated using a pulsed laser, and measured at a fixed sensing point using a laser Doppler vibrometer (LDV)....

This study proposes a complete noncontact laser ultrasonic wavefield imaging technique to automatically detect and visualize hidden delamination and debonding in composite structures. First, ultrasonic wavefield is obtained from a target structure by scanning a Nd:YAG pulse laser beam for ultrasonic wave generation and measuring the corresponding ultrasonic responses using a laser Doppler vibrometer. Then, hidden damages are identified and visualized through adoption of a standing wave filter, which can isolate damage-induced standing waves from the obtained wavefield. The proposed technique has following advantages over the existing techniques: (1) it does not require any sensor installation; (2) it is noninvasive, rapidly deployable and applicable to harsh environments; and (3) it can visualize damage with high spatial resolution without any baseline data, which enables automated and intuitive damage diagnosis. The feasibility of the proposed technique is demonstrated by visualizing a debonding in a carbon fiber reinforced plastic aircraft wing and a delamination in a glass fiber reinforced plastic wind turbine blade. Furthermore, the effects of temperature and static loading variations on the proposed technique are also examined.

In this study, a noncontact fatigue crack detection technique for rotating steel shafts is developed using air-coupled transducers (ACTs). Two ACTs are used for ultrasonic wave generation at two distinctive frequencies, and a third ACT is used for sensing. The proposed technique is based on the premise that nonlinear ultrasonic modulation appears at the sum and difference of two input frequencies in the presence of any crack due to crack opening and closing. First, the spectral correlation (SC) between ultrasonic modulation components at the sum and difference of two input frequencies is computed. Next, an outlier analysis is performed on spectral correlations obtained from different input frequency combinations for automated crack diagnosis. To the best of the authors’ knowledge, this is the first study in which a sub-millimeter fatigue crack is detected in a rotating steel shaft using noncontact transducers. In this study, real fatigue cracks on half-scale and full-scale steel shafts used in automobile assembly lines are produced through cyclic loading tests, and the performance of the proposed technique is experimentally validated using ultrasonic response data obtained from the half-scale and full-scale steel shaft specimens. The formation of fatigue cracks is confirmed using fluorescent magnetic particle inspection (MPI), fluorescent penetrant inspection (FPI), and metallographic analysis.

This study presents a laser ultrasonic imaging and damage detection technique that creates images of ultrasonic waves propagating on a rotating structure and identifies damage. Laser ultrasonics is attractive for nondestructive testing mainly because of two reasons: (1) ultrasonic waves can be generated and/or measured in a noncontact manner and (2) even a small defect can be detected when laser ultrasonic scanning produces ultrasonic images with high spatial resolution. However, when it comes to a moving target, it becomes challenging to create reliable ultrasonic images. In this study, ultrasonic wave propagation images are obtained from a rotating blade using a pulse laser beam for ultrasonic generation, a galvanometer for laser scanning, and an embedded piezoelectric sensor for ultrasonic measurement. To properly estimate the laser excitation points during the scanning process rather than to precisely control the excitation points, a simple but rather effective localization technique is developed so that ultrasonic images can be constructed even from a moving target. Once the ultrasonic wave propagation images are created, damage on the target structure is visualized using a specially designed standing wave filter.

This study presents a new impact localization technique that can pinpoint the location of an impact event within a complex structure using a time-reversal concept, surface-mounted piezoelectric transducers, and a scanning laser Doppler vibrometer. First, an impulse response function between an impact location and a piezoelectric transducer is approximated by exciting the piezoelectric transducer instead and measuring the response at the impact location using scanning laser Doppler vibrometer. Then, training impulse response functions are assembled by repeating this process for various potential impact locations and piezoelectric transducers. Once an actual impact event occurs, the impact response is recorded by the piezoelectric transducers and compared with the training impulse response functions. The correlations between the impact response and the impulse response functions in the training data are computed using a unique concept of time reversal. Finally, the training impulse response function, which gives the maximum correlation, is chosen from the training data set and the impact location is identified. The proposed impact localization technique has the following advantages over the existing techniques: (a) it can be applied to isotropic/anisotropic plate structures with additional complex features such as stringers, stiffeners, spars, and rivet connections; (b) only simple correlation calculation based on time reversal is required, making it attractive for real-time automated monitoring; and (c) training is conducted using noncontact scanning laser Doppler vibrometer and the existing piezoelectric transducers that may already be installed for other structural health–monitoring applications. Impact events on an actual composite aircraft wing and an actual aluminum fuselage are successfully identified using the proposed technique.

Rotating shafts in drop lifts of manufacturing facilities are susceptible to fatigue cracks as they are under repetitive heavy loading and high speed spins. However, it is challenging to use conventional contact transducers to monitor these shafts as they are continuously spinning with a high speed. In this study, a noncontact crack detection technique for a rotating shaft is proposed using air-coupled transducers (ACTs). (1) Low frequency (LF) and high frequency (HF) sinusoidal inputs are simultaneously applied to a shaft using two ACTs, respectively. A fatigue crack can provide a mechanism for nonlinear ultrasonic modulation and create spectral sidebands at the modulation frequencies, which are the sum and difference of the two input frequencies Then LF and HF inputs are independently applied to the shaft using each ACT. These three ultrasonic responses are measured using another ACT. (2) The damage index (DI) is defined as the energy of the first sideband components, which corresponding to the frequency sum and difference between HF and LF inputs. (3) Steps 1 and 2 are repeated with various combinations of HF and LF inputs. Crack existence is detected through an outlier analysis of the DIs. The effectiveness of the proposed technique is investigated using a steel shaft with a real fatigue crack.

Adhesive bonded structures have been widely used in aerospace, automobile, and marine industries. Due to the complex nature of the failure mechanisms of bonded structures, cost-effective and reliable damage detection is crucial for these industries. Most of the common damage detection methods are not adequately sensitive to the presence of weakened bonding. This paper presents an experimental and analytical method for the in-situ detection of damage in adhesive-bonded structures. The method is fully non-contact, using air-coupled ultrasonic transducers (ACT) for ultrasonic wave generation and sensing. The uniqueness of the proposed method relies on accurate detection and localization of weakened bonding in complex adhesive bonded structures. The specimens tested in this study are parts of real-world structures with critical and complex damage types, provided by Hyundai Heavy Industries® and IKTS Fraunhofer®. Various transmitter and receiver configurations, including through transmission, pitch-catch scanning, and probe holder angles, were attempted, and the obtained results were analyzed. The method examines the time-of-flight of the ultrasonic waves over a target inspection area, and the spatial variation of the time-of-flight information was examined to visualize and locate damage. The proposed method works without relying on reference data obtained from the pristine condition of the target specimen. Aluminum bonded plates and triplex adhesive layers with debonding and weakened bonding were used to examine the effectiveness of the method.

Laser ultrasonic scanning, especially full-field wave propagation imaging, is attractive for damage visualization thanks to its noncontact nature, sensitivity to local damage, and high spatial resolution. However, its practicality is limited because scanning at a high spatial resolution demands a prohibitively long scanning time. Inspired by binary search, an accelerated damage visualization technique is developed to visualize damage with a reduced scanning time. The pitch-catch distance between the excitation point and the sensing point is also fixed during scanning to maintain a high signal-to-noise ratio (SNR) of measured ultrasonic responses. The approximate damage boundary is identified by examining the interactions between ultrasonic waves and damage observed at the scanning points that are sparsely selected by a binary search algorithm. Here, a time-domain laser ultrasonic response is transformed into a spatial ultrasonic domain response using a basis pursuit approach so that the interactions between ultrasonic waves and damage, such as reflections and transmissions, can be better identified in the spatial ultrasonic domain. Then, the area inside the identified damage boundary is visualized as damage. The performance of the proposed damage visualization technique is validated excusing a numerical simulation performed on an aluminum plate with a notch and experiments performed on an aluminum plate with a crack and a wind turbine blade with delamination. The proposed damage visualization technique accelerates the damage visualization process in three aspects: (1) the number of measurements that is necessary for damage visualization is dramatically reduced by a binary search algorithm; (2) the number of averaging that is necessary to achieve a high SNR is reduced by maintaining the wave propagation distance short; and (3) with the proposed technique, the same damage can be identified with a lower spatial resolution than the spatial resolution required by full-field wave propagation imaging.

This paper presents a fatigue crack detection technique based on visualization of nonlinear ultrasonic wave modulation produced by a fatigue crack. When distinctive low frequency (LF) and high frequency (HF) inputs are generated and applied to a structure, the presence of a fatigue crack can provide a mechanism for nonlinear ultrasonic modulation and create spectral sidebands around the frequency of the HF signal. In this study, the two input signals are created by two air-coupled transducers (ACT), and the corresponding ultrasonic responses are scanned over a target specimen using a 3D laser Doppler vibrometer (LDV). The crack-induced spectral sidebands are isolated using a combination of linear response subtraction (LRS), and continuous wavelet transform (CWT) filtering. Then, the extracted spectral sideband components are visualized near the fatigue crack. The effectiveness of the proposed non-contact scanning technique is tested using an aluminum plate with a real fatigue crack.

Although there are many laser ultrasonic imaging techniques developed so far, it still remains challenging to create such images from a rotating object. In this study, an advanced laser ultrasonic imaging technique is developed so that wavefield images can be constructed from a rotating blade using an embedded piezoelectric sensor and a scanning excitation laser system. Here, the biggest challenge is to precisely estimate and control the exact excitation point when the wind blade is rotating with additional ambient vibration and having complex shapes. In this study, the laser excitation point is precisely estimated by computing the correlation values between the measured response signal and the ones in the training data sets. First, training ultrasonic signals are measured at the fixed sensing point by scanning the excitation laser over the target surface of the blade when the blade is in a stationary condition. Once the training is complete, an ultrasonic signal is generated for the rotating blade using the excitation laser and measured by the sensor. The correlation between the measured response and a training response is maximized when they correspond to the same excitation point. Finally, ultrasonic images are generated by scanning the excitation laser over the target surface of the blade. The effectiveness of the proposed imaging technique is investigated through experimental tests performed on a rotating blade specimen.

In this study, a new standing wave energy (SWE) imaging technique is developed so that a crack in a metallic structure can be detection from ultrasonic wavefield image obtained by a laser scanning system. First, a non-contact laser ultrasonic scanning system is developed so that ultrasonic waves on a target structure can be generated and measured using two independent laser sources. Using this scanning system, an ultrasonic wavefield image with high spatial and temporal resolutions is constructed. The interaction of propagating ultrasonic waves with a crack is theoretically studied, and the creation of standing waves due to crack formation is confirmed. Then, the crack-induced SWE is isolated from the constructed wavefield image using the proposed signal processing technique, allowing the automated identification and localization of the crack. The effectiveness of the proposed technique is experimentally investigated using data obtained from a plate-like structure.