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This paper investigates the method for water ingress detection in an aluminum honeycomb sandwich structure by laser ultrasonics. To clarify the propagation of Scholte waves generated at the solid-liquid interface, the theoretical dispersion curves in the water-aluminum layered structure are calculated by the global matrix method. Using a laser ultrasonic visualizing inspector, the guided wave propagation in the wavefields has been visualized, which shows the mode conversion behaviors occurring in the water ingress area. The clarification of mode conversion is then investigated by the finite element method. The Scholte modes at the solid-fluid interface and the Lamb modes in the face sheet are interconverted because of the change in the waveguide structure. Wavenumber filter reconstruction and the resulting energy maps are applied to illustrate the mode conversion in the wavefields and to detect the water ingress area. Results show that the appearance of the water layer contributes to the mode conversion behaviors during the guided wave propagation in the honeycomb sandwich structure, which offers the potential for water ingress detection based on the mode conversion behaviors. [ABSTRACT FROM AUTHOR]

Detecting disbonds in honeycomb sandwich structures using ultrasonics is challenging due to the complex wave propagation. This paper investigates the method for disbond quantification in honeycomb sandwich structures using laser ultrasonics. In the experiments on an aluminum specimen, the detection frequency range was determined by the local defect resonance (LDR) in the disbond regions. The energy maps of bandpass wavefields identified the locations of disbonds. To improve the results, a frequency-wavenumber analysis of the wavefields was performed. The wavenumber spectra were reconstructed by wavenumber filtering to strengthen the outline components for quantifying disbonds. Local cross-correlation (LCC) was proposed to show the outlines of disbonds and honeycomb cells, whose results agreed well with the X-ray imaging. Moreover, LCC could also quantify a disbond in the composite honeycomb sandwich structure. Consequently, combining energy maps and LCC results can effectively detect the locations and sizes of disbonds in honeycomb sandwich structures. [ABSTRACT FROM AUTHOR]

Signal instability due to temperature fluctuations, sensor degradation, and debonding introduces additional amplitude loss in the detected signals during acousto-ultrasonic detection, which may be falsely attributed to defects in a structure. First, we determined that the amplitudes of both high-frequency and low-frequency Lamb waves decrease after propagation through a damaged area. Then, we found that the amplitude ratio of such waves not only exhibits a downward trend but is also immune to fluctuations in the input signals. A qualitative numerical expression was proposed to explain this phenomenon, and preliminary experiments were conducted to demonstrate that the amplitude ratio is an effective parameter for mitigating instability in signal detection. Particularly, the number of impacts on a composite laminate was evaluated with respect to changes in the input signal amplitude. Notably, this method can be further simplified by designing a dual-frequency input signal. After conclusively validating the performance of the novel method in a composite subjected to temperature fluctuations, we conclude that the proposed acousto-ultrasonic detection method is robust in mitigating signal instability, and that it yields reliable information for damage evaluation. [ABSTRACT FROM AUTHOR]

A method for determining the elastic constants of polyethylene terephthalate (PET) film is examined. Topics include the calculation of the PET film's acoustic reflection coefficient, and the use of an ultrasonic microspectrometer.

In this research, we attempt to establish a reliable structural health monitoring technique for composite materials by combining phase-shifted fiber-optic Bragg grating sensing with the laser ultrasonic visualization technology. In the first part of this article, a novel cross-adhesion configuration is designed to resolve the directionality problem of the phase-shifted fiber-optic Bragg grating ultrasonic sensing. In the adhesion configuration, Lamb waves are guided by an orthogonally bonded optical fiber from the adhesion point to the phase-shifted fiber-optic Bragg grating sensor. The analysis of the ultrasonic measurement results reveals that the proposed adhesion method enables us to use one sensor to receive Lamb waves in all in-plane directions with similar magnitude because two wave components propagating along with the two orthogonal directions are guided to the phase-shifted fiber-optic Bragg grating sensor and exhibit a linear superposition in the sensor. This simplified configuration gives our method an advantage over the existing approaches, such as the rosette configuration in which three or more phase-shifted fiber-optic Bragg grating sensors are required to relieve the sensing directionality. The phase-shifted fiber-optic Bragg grating ultrasonic sensor with the proposed adhesion configuration is then applied to visualize the propagation of ultrasonic waves in aluminum plates and carbon fiber–reinforced plastic laminates. Those verification experiments also show us that the new adhesion configuration is effective at protecting the phase-shifted fiber-optic Bragg grating ultrasonic measurement from the sensing directionality. Meanwhile, the broad bandwidth of the phase-shifted fiber-optic Bragg grating sensor enables us to visualize the propagation behavior of various Lamb wave modes over a broad frequency range. Finally, we also validate that the ultrasonic visualization technique merged with the phase-shifted fiber-optic Bragg grating ultrasonic sensing can be used to identify the hidden damage in the carbon fiber–reinforced plastic composite. [ABSTRACT FROM AUTHOR]

For the development of new types of lightweight sandwich structures, the compressive properties and strengthening mechanism of the middle-trabecular beetle elytron plate were investigated for various values of η (the ratio of the trabecular radius to the honeycomb wall length). The results are as follows: (1) When η = 0.1, the increases in the compressive strength and standard energy absorption capacity of the middle-trabecular beetle elytron plate compared with the honeycomb plate exceed those of the end-trabecular beetle elytron plate; with an increase to η = 0.15, the compressive strength remains nearly the same, the energy absorption capacity undergoes a significant further increase, and the trabeculae exhibit Φ-type failure. (2) The strengthening mechanism that gives rise to the compressive properties of the middle-trabecular beetle elytron plate is proposed as follows: the trabeculae are located at the center of the honeycomb walls, where the maximum deformations would otherwise occur; they constrain the deformation of the honeycomb walls; and the number of trabeculae in the middle-trabecular beetle elytron plate also exceeds that in the end-trabecular beetle elytron plate. (3) Middle-trabecular beetle elytron plates have the advantage of facile manufacturing, which will establish a basis for promoting the application of beetle elytron plates. [ABSTRACT FROM AUTHOR]

In the present study, the edgewise compressive mechanical properties of the biomimetic fully integrated honeycomb plates manufactured from short basalt fiber reinforced polymer are studied. The findings are as follows: (1) Fully integrated honeycomb plates are primarily characterized by compression failure of the non-perforated surface when bearing an edgewise compressive load. The fully integrated honeycomb plates clearly exhibit the features of plasticity. (2) A buckling failure mode of the upper or lower laminates is not observed in the fully integrated honeycomb plates, and the fully integrated honeycomb plates possess the ideal integrity. Additionally, the fully integrated honeycomb plates produced with 9-mm-long fibers have the greater shearing modulus of elasticity and good plastic deformation ability. These results are of significance for disaster prevention and safety in quakeproof applications. (3) The failure features, mechanical property parameters, and simple calculation formula for edgewise compression of fully integrated honeycomb plates obtained in this work provide a foundation for the prediction of edgewise compression and the design of fully integrated honeycomb plates in future engineering applications. [ABSTRACT FROM AUTHOR]

This paper investigates the mechanical properties of metal beetle elytron plates (a type of biomimetic sandwich structure) and the elementary unit of the trabecular-honeycomb core structure by compression experiments. The following conclusions are drawn: (1) after reaching the compressive strength, the stress–strain curve of the beetle elytron plate contains a yield plateau that advantageously exploits the material strength of metal and its plastic deformation capacity; (2) the elementary unit of the core structure in beetle elytron plates is proposed explicitly based on the trabecular characteristic; and (3) this elementary unit remains relatively unchanged on the constraint condition of the trabecular structure and has high independence and stability when placed under out-of-plane compression. Thus, the yield plateau and variation tendency of the compressive strength after buckling of this elementary unit are highly consistent with that of the entire structure, which provides the basis for further research on the equivalent parameters and equivalent model of the beetle elytron plate and its trabecular-honeycomb core structure. [ABSTRACT FROM AUTHOR]

Nonlinear ultrasonic technique that evaluates the growth of the barely visible fatigue crack in metal is difficult to be achieved by conventional optical fiber sensor because of the critical requirements for sensor sensitivity and bandwidth. In this paper, a high-performance optical fiber sensor using phase-shifted fiber Bragg grating and balanced demodulation system was applied to detect the fundamental and second harmonic Lamb waves propagating in an aluminum plate. After the nonlinearity was calculated from the modes of the Lamb waveform detected by the optical fiber sensor, its change due to the second harmonic generation shows an exponential trend in a fatigue test, which can be used to evaluate the crack growth with sufficient reliability. The experimental results from the optical fiber sensor were validated by comparing with results from the PZT sensor. This paper facilitates the development of optics-based nonlinear ultrasonic structural health monitoring for fatigue crack evaluation. [ABSTRACT FROM AUTHOR]

To increase the engineering applications of basalt fiber, the characteristics of the shear mechanical properties and the influence mechanisms of the fiber length (Lf) and volume fraction (Vf) for short basalt fiber reinforced epoxy resin polymer composites were investigated. The following results were obtained: (1) The first critical loads on all the shear load–displacement curves are close to the same value due to having the same interface bonding of the fiber–matrix; (2) the shear strength, deformation, and energy dissipation capacity all increase with increasing Vf ; however, when Lf is 6 or 9 mm, these increases are slower, whereas when Lf is 3 mm, these values increase at an approximately constant rate; and (3) the shear elastic modulus of the samples reaches its maximum value when Lf is 3 mm and Vf is 25%; however, it increases more quickly when Lf is 6 or 9 mm. The failure mode of the samples with the larger Lf values is mainly debonding between the fiber and matrix; when Vf is high, the fibers can easily bundle, which reduces the adhesion between the fibers and matrix. In addition, the main failure mode of the 3-mm-long fiber samples is fiber pull-out; when Vf is high, the short fibers that are not pulled out significantly increase the deformation ability. [ABSTRACT FROM AUTHOR]

Aluminum alloy honeycomb structures were designed based on origami technology, and the specimens were fabricated by a new fabrication technology (i.e. a press and folding process). In folding process, a new folding device was successfully developed to achieve automatic fabrication of honeycomb structure. To prove the practicability of developed device, the honeycomb cores with claws were fabricated by this device, which were used to compare the mechanical properties with that bonded by common adhesive. The deformation behaviors and mechanical properties of honeycomb structures were investigated by the flatwise compressive test and three-point bending test. The load–displacement curve obtained at the room temperature showed that the load increased to a peak value and then tended rapidly to a constant. Besides, the deformation process approximately categorized into three zones, namely linear-elastic zone, plastic-plateau zone, and densification zone. The experimental results suggested that regardless of specimen type, the bending stiffness and compressive strengths were approximately 0.32 KN·m2and 0.39 MPa, respectively; revealing the bonded method by aluminum claws did not dramatically affect the mechanical properties of honeycomb structure. Moreover, the elastic deformation of honeycomb structure was numerically studied by the finite element analysis. [ABSTRACT FROM AUTHOR]

In engineering structures, metal materials always endure fatigue cracks under long-term service. There has been a demand for developing a structural health monitoring method to evaluate micro-sized fatigue cracks, as cracking is considered as a precursor to structural failure. However, conventional linear-ultrasound-based technology is not sensitive to crack when it is barely visible in a metal medium. In this article, we present a nonlinear ultrasonic technology based on crack–wave interaction to investigate the growth of a fatigue crack. A breathing-crack model with a plastic zone around it was precisely established to reveal the change in the Lamb wave. The relative nonlinear parameter calculated from the fundamental and harmonic components of the Lamb wave showed linearly increasing with the growth of the fatigue crack. The relative nonlinearity was related to ultrasonic parameters, such as the cycle number and the excited frequency of the tone-burst signal. In addition, it was also related to the angle between the sensor and the crack rather than their distance. A set of experiments were conducted, demonstrating that the increasing trend of ultrasonic nonlinearity fits very well to the finite element analysis results. In conclusion, the nonlinear ultrasonic method that can be applied to the detection of micro fatigue cracks in metal plates is an effective structural health monitoring technique. [ABSTRACT FROM AUTHOR]

With the wide application of carbon fibre reinforced plastic (CFRP) structure, a structural health monitoring (SHM) system that can easily detect damages in CFRP structures is urgently needed. Since it is possible to detect damages effectively in a large area in CFRP by using the scattering of Lamb waves, in this research, we investigated the scattering of Lamb waves at an impact damaged area in quasi-isotropic (QI) CFRP laminate. By modelling the impact damage as a cylinder area with a uniform stiffness degradation, the scattering waves are calculated based on Mindlin plate theory, which is more precise than Kirchhoff plate theory. It is shown that in the low-frequency region, the calculation results based on Kirchhoff and Mindlin plate theory was similar, but in a higher frequency range, the results differed. The calculation results were compared with the simulation results, suggesting the validity of Mindlin plate theory within a practical frequency range. Moreover, we validated the analytical and simulation results experimentally using a QI CFRP laminate with a hole. We also conducted some parameter study based on the analytical calculation to clarify influential stiffness components. Since scattering behaviour of Lamb waves can be calculated in a fast and easy way using this method, this work is useful for the successful development of a scattering-based SHM system. [ABSTRACT FROM AUTHOR]

In this study, a phase-shifted fiber Bragg grating sensor was employed for acoustic emission detection in cross-ply carbon fiber–reinforced plastic laminates. First, we evaluated the response of the phase-shifted fiber Bragg grating sensor to a simulated acoustic emission source using a pencil lead breakage test. The existing modes of Lamb waves were observed, and their relations to the acoustic emission source orientation were investigated using a modal acoustic emission method for discriminating between acoustic emission signals generated by transverse cracking and delamination. As a result, those two types of damage were identified during a tensile test. In the time–frequency analysis of the waveforms detected by the phase-shifted fiber Bragg grating sensor, in addition to the S0 and A0 modes that are usually used in modal acoustic emissions, we found an A1 mode in the high-frequency range of the acoustic emission generated by a transverse crack. Then, we conducted a finite element analysis to examine the inherent mechanism of the A1 mode excitation. The results of the calculation almost agreed with the experimental results and indicated that a phase-shifted fiber Bragg grating sensor has a high sensitivity response to acoustic emission in the carbon fiber–reinforced plastic laminates over a broad bandwidth. Therefore, this novel fiber-optic sensor has been shown to have high potential for monitoring the ultrasonic structural health of composites. [ABSTRACT FROM AUTHOR]

Ultrasonic wave-sensing technology has been applied for the health monitoring of composite structures, using normal fiber Bragg grating (FBG) sensors with a high-speed wavelength interrogation system of arrayed waveguide grating (AWG) filters; however, researchers are required to average thousands of repeated measurements to distinguish significant signals. To resolve this bottleneck problem, this study established a signal-processing strategy that improves the signal-to-noise ratio for the one-time measured signal of ultrasonic waves, by application of parallel factor analysis (PARAFAC) technology that produces unique multiway decomposition without additional orthogonal or independent constraints. Through bandpass processing of the AWG filter and complex wavelet transforms, ultrasonic wave signals are preprocessed as time, phase, and frequency profiles, and then decomposed into a series of conceptual three-way atoms by PARAFAC. While an ultrasonic wave results in a Bragg wavelength shift, antiphase fluctuations can be observed at two adjacent AWG ports. Thereby, concentrating on antiphase features among the three-way atoms, a fitting atom can be chosen and then restored to three-way profiles as a final result. An experimental study has revealed that the final result is consistent with the conventional 1024-data averaging signal, and relative error evaluation has indicated that the signal-to-noise ratio of ultrasonic waves can be significantly improved. [ABSTRACT FROM AUTHOR]

An integrated in-fiber Mach–Zehnder interferometer (MZI) and fiber Bragg grating (FBG) sensor that can simultaneously detect localized temperature, strain, and ultrasonics is proposed and demonstrated. Using a 60-mm single-mode fiber with a 12-mm FBG spliced to two 3-mm multimode fibers and lead-in and lead-out single-mode fibers, the sensor can discriminate temperature and strain from the responses of the MZI and the FBG to the two static parameters. Ultrasonic signals are directly recorded as high-frequency voltage vibration by demodulation of Bragg wavelength shift using a corresponding sensing system. The overall system shows high practical potential for use in nondestructive testing. [ABSTRACT FROM PUBLISHER]

Utilizing optical fiber technique to structural health monitoring of composite materials is attracting attention recently due to the inherent advantages in optical fiber sensor. However, acoustic emission signals with low energy and high frequency from transverse cracks in carbon fiber–reinforced plastic laminates are difficult to be detected by normal optical techniques. In this work, we present a phase-shifted fiber Bragg grating balanced sensing system to detect acoustic emission signals generated in a carbon fiber–reinforced plastic laminate in a three-point bending test. We demonstrated that the phase-shifted fiber Bragg grating balanced sensing system with a broad bandwidth and high sensitivity can detect high-frequency, low-energy acoustic emission signals caused by transverse cracks. In our experiments, the transverse cracks were identified very well through the detected acoustic emission signals. The relationship between the strain and the acoustic emission signals, and the typical waveforms of the acoustic emission signals were investigated. Furthermore, accurate position identification using acoustic emission detection by the phase-shifted fiber Bragg grating sensor was demonstrated. All the results obtained from the phase-shifted fiber Bragg grating sensor were compared to the results obtained from commercial lead–zirconate–titanate acoustic emission sensors. We found that the phase-shifted fiber Bragg grating balanced sensing system performance was comparable to the commercial lead–zirconate–titanate acoustic emission sensors. [ABSTRACT FROM PUBLISHER]

A novel real-time acousto-ultrasonic sensor system using a phase-shifted fiber Bragg grating rather than a normal fiber Bragg grating was investigated. The spectrum of the phase-shifted fiber Bragg grating was simulated and analyzed, which indicates that a phase-shifted fiber Bragg grating has superior properties when compared to a normal fiber Bragg grating. Based on theoretical considerations, a novel real-time acousto-ultrasonic sensor system was proposed. Two identical 5-mm phase-shifted fiber Bragg gratings, with slightly different Bragg wavelengths, were used as the filter and the sensor. An amplified spontaneous emission light source and an apodized fiber Bragg grating were connected to illuminate the phase-shifted fiber Bragg gratings. The design allows the strain resulting from the ultrasonic wave to be precisely received and converted to the fluctuation of the output voltage. An ultrasonic wave generated in a carbon fiber–reinforced plastic plate using a macrofiber composite actuator was detected by both the phase-shifted fiber Bragg grating sensor system and a sensor system based on a normal fiber Bragg grating and arrayed waveguide grating. Comparison of the temporal and spectral responses of these two systems indicates that the phase-shifted fiber Bragg grating sensor system has a higher sensitivity than the fiber Bragg grating system. The results show that the phase-shifted fiber Bragg grating system does not require data averaging for noise reduction to measure the Lamb wave modes in a carbon fiber–reinforced plastic plate. [ABSTRACT FROM PUBLISHER]

An epoxidized lignophenol (ELP) was synthesized by a two-step reaction from a lignophenol (LP) as the starting material. The reaction of LP with epichlorohydrin was carried out in the presence of a phase transfer catalyst, tetrabutylammonium bromide, at 90°C without alkaline reagent, followed by epoxidation with aq. NaOH solution. Epoxy groups in ELP were found to react with phenolic OH groups of coniferyl alcohol as a model compound of LP, which was used as a curing agent in this study. Direct curing of ELP with LP resulted in heterogeneous cured resins, and therefore, petrochemical epoxy resins of bisphenol A diglycidyl ether (DGEBA) and 3,4-epoxycyclohexylmethyl 3',4'-epoxycyclohexanecarboxylate (ECEC) were used with ELP in order to improve the fluidity of the systems. Epoxy resins that mixed ELP with DGEBA or ECEC were cured with LP by heating up to 180°C in the presence of imidazole derivative (2E4MZ-CN); these afforded ELBL and ELCL as novel thermosetting resins. The ELBL and ELCL showed well-balanced thermal and mechanical properties. Especially, the glass transition temperature (Tg) of the ELCL (70), which included 30 wt% of ECEC as the epoxy resin where the total amount of LP element was 82 wt%, was 219°C. This value was 85°C higher than that of the conventional phenol novolac-cured DGEBA. These results demonstrated that LP was a promising biomass material for thermosetting resins. [ABSTRACT FROM AUTHOR]

When impact damage occurs in carbon fiber-reinforced plastic (CFRP) structures, it is barely visible but may cause significant degradation in the mechanical properties of the structure. Hence, a structural health monitoring (SHM) system that can be installed in CFRP mobility structures and is sensitive to impact damage is needed. In this study, we attempted to establish an SHM system based on ultrasonic guided waves, which are generated by inputting a broadband chirp signal into a film-like piezoelectric actuator. The relationship between impact damage size and maximum time-of-flight (ToF) delay was investigated for three types of CFRP plates: woven, non-woven, and hybrid laminates. As a result, it was found that the maximum ToF delay increased linearly with an increase in the damage size for all CFRP laminates. Moreover, the amplitude of the A0 mode was found to be significantly affected by the damage length in the wave propagation direction. Thus, this SHM method using chirp ultrasonic waves can quantitatively evaluate the size and extent of the impact damage in CFRP laminates. [ABSTRACT FROM AUTHOR]

In this research, optical fiber sensors and shape memory alloys (SMA) were incorporated into sandwich panels for development of a smart honeycomb sandwich structure with damage detection and shape recovery functions. First, small-diameter fiber Bragg grating (FBG) sensors were embedded in the adhesive layer between a CFRP face-sheet and an aluminum honeycomb core. From the change in the reflection spectrum of the FBG sensors, the debonding between the face-sheet and the core and the deformation of the face-sheet due to impact loading could be well detected. Then, the authors developed the SMA honeycomb core and bonded CFRP face-sheets to the core. When an impact load was applied to the panel, the cell walls of the core were buckled and the face-sheet was bent. However, after the panel was heated over the reverse transformation finish temperature of the SMA, the core buckling disappeared and the deflection of the face-sheet was relieved. Hence the bending stiffness of the panel could be recovered. [ABSTRACT FROM AUTHOR]

The authors focus on the construction of a structural health monitoring (SHM) system with an advanced grid structure (AGS) made of carbon-fiber reinforced plastic (CFRP). AGS is often applied to aerospace structures because the ribs carry only axial forces in the carbon fiber direction, making AGS structurally effective and lightweight, and because the repetition of many ribs in the AGS composition results in damage tolerance. The failure of a single rib hardly affects the fracture of the whole structure, making AGS a fail-safe structure. In this research, the authors have embedded multiplexed-fiber Bragg grating (FBG) sensors into an AGS rib in the longitudinal direction to measure mechanical strains of all ribs in order to detect the existence and regions of AGS rib fractures. Monitoring the change in riblongitudinal strains is the most effective SHM system for AGS. To confirm the proposal, the authors explore the following. First, various damage characteristics under low-velocity impact loading are investigated and it is verified that partial rib cracks are the most typical damage in AGS. An AGS is then fabricated with embedded FBG sensors and verified that the SHM system is able to measure all rib strains. Subsequently, it is analytically determined that the change in longitudinal-rib strains is the most appropriate mechanical feature for damage detection. Moreover, a statistical outlier analysis is introduced into the SHM system for automatic damage detection. Finally. AGS is established with the SHM system and verified experimentally. Results confirm that the existence of damage and its regions in AGS can be detected with the proposed SHM system. [ABSTRACT FROM AUTHOR]

The authors have developed a novel technique to detect debonding in honeycomb sandwich structures using small-diameter fiber Bragg grating (FBG) sensors. A small-diameter FBG sensor is embedded in the adhesive layer between the core and the facesheet during the curing process of the adhesive layer. After the curing process, the reflection spectrum from the small-diameter FBG sensor is distorted because the formation of fillets induces a non-uniform strain distribution in the adhesive layer. However, after debonding, the core with the fillets peels off the facesheet, and, as a result, the reflection spectrum recovers its original shape due to the release of the non-uniform strain. Debonding can thus be detected with high sensitivity in real time from the recovery in the shape of the reflection spectrum. In addition, a more reliable debonding detection technique using a small-diameter chirped FBG sensor is proposed, in which two indicators in the reflection spectrum have been successfully used for a more reliable judgment of debonding. [ABSTRACT FROM AUTHOR]