Damage Imaging Identification of Honeycomb Sandwich Structures Based on Lamb Waves.

In the field of structural health monitoring, Lamb Wave has become one of the most widely used inspection tools due to its advantages of wide detection range and high sensitivity. In this paper, a new damage detection method for honeycomb sandwich structures based on frequency spectrum and Lamb Wave Tomography is proposed. By means of simulation and experiment, a certain number of sensors were placed on the honeycomb sandwich plate to stimulate and receive the signals in both undamaged and damaged cases. By Lamb Wave Tomography, the differences of signals before and after damage were compared, and the damage indexes were calculated. Furthermore, the probability of each sensor path containing damage was analyzed, and the damage image was finally realized. The technology does not require analysis of the complex multimode propagation properties of Lamb Wave, nor does it require understanding and modeling of the properties of materials or structures. In both simulation and experiment, the localization errors of the damage conform to the detection requirements, thus verifying that the method has certain feasibility in damage detection.

Microseismic P-Wave Travel Time Computation and 3D Localization Based on a 3D High-Order Fast Marching Method.

The travel time computation of microseismic waves in different directions (particularly, the diagonal direction) in three-dimensional space has been found to be inaccurate, which seriously affects the localization accuracy of three-dimensional microseismic sources. In order to solve this problem, this research study developed a method of calculating the P-wave travel time based on a 3D high-order fast marching method (3D_H_FMM). This study focused on designing a high-order finite-difference operator in order to realize the accurate calculation of the P-wave travel time in three-dimensional space. The method was validated using homogeneous velocity models and inhomogeneous layered media velocity models of different scales. The results showed that the overall mean absolute error (MAE) of the two homogenous models using 3D_H_FMM had been reduced by 88.335%, and 90.593% compared with the traditional 3D_FMM. On that basis, the three-dimensional localization of microseismic sources was carried out using a particle swarm optimization algorithm. The developed 3D_H_FMM was used to calculate the travel time, then to conduct the localization of the microseismic source in inhomogeneous models. The mean error of the localization results of the different positions in the three-dimensional space was determined to be 1.901 m, and the localization accuracy was found to be superior to that of the traditional 3D_FMM method (mean absolute localization error: 3.447 m) with the small-scaled inhomogeneous model.

Phase Demodulation Method for Fringe Projection Measurement Based on Improved Variable-Frequency Coded Patterns.

The phase-to-height imaging model, as a three-dimensional (3D) measurement technology, has been commonly applied in fringe projection to assist surface profile measurement, where the efficient and accurate calculation of phase plays a critical role in precise imaging. To deal with multiple extra coded patterns and 2π jump error caused to the existing absolute phase demodulation methods, a novel method of phase demodulation is proposed based on dual variable-frequency (VF) coded patterns. In this paper, the frequency of coded fringe is defined as the number of coded fringes within a single sinusoidal fringe period. First, the effective wrapped phase (EWP) as calculated using the four-step phase shifting method was split into the wrapped phase region with complete period and the wrapped phase region without complete period. Second, the fringe orders in wrapped phase region with complete period were decoded according to the frequency of the VF coded fringes and the continuous characteristic of the fringe order. Notably, the sampling frequency of fast Fourier transform (FFT) was determined by the length of the decoding interval and can be adjusted automatically with the variation in height of the object. Third, the fringe orders in wrapped phase region without complete period were decoded depending on the consistency of fringe orders in the connected region of wrapped phase. Last, phase demodulation was performed. The experimental results were obtained to confirm the effectiveness of the proposed method in the phase demodulation of both discontinuous objects and highly abrupt objects.

An In Situ Reflectance Spectroscopic Investigation to Monitor Two-Dimensional MoS2 Flakes on a Sapphire Substrate.

In this work, we demonstrate the application of differential reflectance spectroscopy (DRS) to monitor the growth of molybdenum disulfide (MoS2) using chemical vapor deposition (CVD). The growth process, optical properties, and structure evolution of MoS2 were recorded by in-situ DRS. Indeed, blue shifts of the characteristic peak B were discussed with the decrease of temperature. We also obtained the imaginary part of the MoS2 dielectric constant according to reflectance spectra. This method provides an approach for studying the change of two-dimensional (2D) materials' dielectric constant with temperature. More importantly, our work emphasizes that the DRS technique is a non-destructive and effective method for in-situ monitoring the growth of 2D materials, which is helpful in guiding the preparation of 2D materials.

High Precision Detection Method for Delamination Defects in Carbon Fiber Composite Laminates Based on Ultrasonic Technique and Signal Correlation Algorithm.

This paper presents a method based on signal correlation to detect delamination defects of widely used carbon fiber reinforced plastic with high precision and a convenient process. The objective of it consists in distinguishing defect and non-defect signals and presenting the depth and size of defects by image. A necessary reference signal is generated from the non-defect area by using autocorrelation theory firstly. Through the correlation calculation results, the defect signal and non-defect signal are distinguished by using Euclidean distance. In order to get more accurate time-of-flight, cubic spline interpolation is introduced. In practical automatic ultrasonic A-scan signal processing, signal correlation provide a new way to avoid problems such as signal peak tracking and complex gate setting. Finally, the detection results of a carbon fiber laminate with artificial delamination through ultrasonic phased array C-scan acquired from Olympus OmniScan MX2 and this proposed algorithm are compared, which showing that this proposed algorithm performs well in defect shape presentation and location calculation. The experiment shows that the defect size error is less than 4%, the depth error less than 3%. Compared with ultrasonic C-scan method, this proposed method needs less inspector's prior-knowledge, which can lead to advantages in automatic ultrasonic testing.

Damage Localization of Composites Based on Difference Signal and Lamb Wave Tomography.

In order to deal with the problem of composite damage location, an imaging technique based on differential signal and Lamb wave tomography was proposed. Firstly, the feasibility of the technique put forward was verified by simulation. In this process, the composite model was regularly set down by the circular sensor array, with each sensor acting as an actuator in sequence to generate Lamb waves. Apart from that, other sensors were used to collect response signals. With regard to the damage factor, it was mainly determined by the difference between the damage signal and the non-damage signal. The probabilistic imaging algorithm was employed to carry out damage location imaging. Then, experiments were carried out so as to study the selected composite plate. Finally, the tentative outcomes have illustrated that the maximum error of damage imaging position was 7.07 mm. The relative error was 1.6%. In addition, the method has the characteristics of simple calculation and high imaging efficiency. Therefore, it has large technical potential and wide applications in the damage location and damage recognition for composite material.

Improved unwrapped phase retrieval method of a fringe projection profilometry system based on fewer phase-coding patterns.

In this paper, based on two additional phase-coding patterns, an improved phase demodulation method is proposed. First, six equally spaced coding phases in the interval [$ - \pi $-π, $\pi $π] are embedded in different periods of the coded fringes following a certain sequence. Subsequently, since a group of phase orders can be uniquely determined by the four adjacent coding phases, the phase-order map of the object can be generated. To ensure the accuracy of decoding results, the interference coding numbers should be corrected in advance. In the meantime, the connected regions exhibiting the same orders are classified and then labeled for simplifying the decoding process. The simulation results verify the feasibility of the proposed method. By two groups of 3D imaging experiments, the applicability of this method to multiple objects and discontinuous objects is confirmed.

Direct Observation of Monolayer MoS2 Prepared by CVD Using In-Situ Differential Reflectance Spectroscopy.

The in-situ observation is of great significance to the study of the growth mechanism and controllability of two-dimensional transition metal dichalcogenides (TMDCs). Here, the differential reflectance spectroscopy (DRS) was performed to monitor the growth of molybdenum disulfide (MoS2) on a SiO2/Si substrate prepared by chemical vapor deposition (CVD). A home-built in-situ DRS setup was applied to monitor the growth of MoS2 in-situ. The formation and evolution of monolayer MoS2 are revealed by differential reflectance (DR) spectra. The morphology, vibration mode, absorption characteristics and thickness of monolayer MoS2 have been confirmed by optical microscopy, Raman spectroscopy, ex-situ DR spectra, and atomic force microscopy (AFM) respectively. The results demonstrated that DRS was a powerful tool for in-situ observations and has great potential for growth mechanism and controllability of TMDCs prepared by CVD. To the best of the authors' knowledge, it was the first report in which the CVD growth of two-dimensional TMDCs has been investigated in-situ by reflectance spectroscopy.

Design, Optimization and Improvement of FBG Flexible Sensor for Slope Displacement Profiles Measurement.

The accurate measurement of slope displacement profiles using a fiber Bragg grating flexible sensor is limited due to the influence of accumulative measurement errors. The measurement errors vary with the deformation forms of the sensor, which dramatically affects the measurement accuracy of the slope displacement profiles. To tackle the limitations and improve the measurement precision of displacement profiles, a segmental correction method based on strain increments clustering was proposed. A K-means clustering algorithm was used to automatically identify the deformation segments of a flexible sensor with different bending shapes. Then, the particle swarm optimization method was adopted to determine the correction coefficients corresponding to different deformation segments. Both finite element simulations and experiments were performed to validate the superiority of the proposed method. The experimental results indicated that the mean absolute errors (MAEs) percentages of the reconstructed displacements using the proposed method for six different bending shapes were 1.87%, 5.28%, 6.98%, 7.62%, 4.16% and 8.31%, respectively, which had improved the accuracy by 26.83%, 18.94%, 29.49%, 26.35%, 7.39%, and 19.65%, respectively. Therefore, it was confirmed that the proposed correction method was competent for effectively mitigating the measurement errors and improving the measurement accuracy of slope displacement profiles, and it presented a vital significance and application promotion value.

Fast determination of oxides content in cement raw meal using NIR-spectroscopy and backward interval PLS with genetic algorithm.

Determining oxides content in cement raw meal with near infrared (NIR) spectroscopy, associated with partial least square (PLS) regression, is fast and potential for cement industry to realize cement raw material proportioning control. However, it has hardly been studied. Backward interval PLS (biPLS) with genetic algorithm (GA-biPLS) were applied to select characteristic variables closely related to the concentration of oxide of interest to establish calibration model. The optimal GA-biPLS models showed that the determination coefficient (Rp2) and root mean square error of prediction (RMSEP) were 0.8857 and 0.0994 for CaO, 0.8718 and 0.1044 for SiO2, 0.7417 and 0.0693 for Al2O3, 0.5404 and 0.0387 for Fe2O3, correspondingly. These results indicate that GA-biPLS can select less variables with better prediction performance by comparison with PLS and biPLS, the NIR spectroscopy combined with GA-biPLS algorithm is a fast, accurate and reliable alternative method for determination of oxides content in cement raw meal.

Flexible calibration method of an FPP system based on a geometrical model and NLSM with fewer parameters.

Fringe projection profilometry (FPP) technology is an important method for 3D reconstruction. In this paper, we proposed a flexible calibration method of an FPP system based on the imaging principle and geometrical structure of the system. The target coordinates are only related to its pixel coordinates and phase. First, the fringe images are projected onto the calibration plate, and the phase can be calculated through the four-step phase-shifting method. Then, the pixel coordinates of the feature points can be located with the binarized fringe images and the centroid method. Finally, the calibration parameters are calculated by the nonlinear least-squares method (NLSM). The reconstructed experiment of 162 testing points was carried out, and the result shows that the maximum relative errors on coordinates X, Y, and h are 0.27%, 0.42%, and 0.59%, respectively. The other two surface reconstruction experiments also verify the feasibility of the calibration method.

Flexible calibration method of an FPP system based on a geometrical model and NLSM with fewer parameters: publisher's note.

This publisher's note amends the Funding section of Appl. Opt.58, A7 (2019)APOPAI0003-693510.1364/AO.58.0000A7.

Design and Optimization of FBG Implantable Flexible Morphological Sensor to Realize the Intellisense for Displacement.

The measurement accuracy of the intelligent flexible morphological sensor based on fiber Bragg grating (FBG) structure was limited in the application of geotechnical engineering and other fields. In order to improve the precision of intellisense for displacement, an FBG implantable flexible morphological sensor was designed in this study, and the classification morphological correction method based on conjugate gradient method and extreme learning machine (ELM) algorithm was proposed. This study utilized finite element simulations and experiments, in order to analyze the feasibility of the proposed method. Then, following the corrections, the results indicated that the maximum relative error percentages of the displacements at measuring points in different bending shapes were determined to be 6.39% (Type 1), 7.04% (Type 2), and 7.02% (Type 3), respectively. Therefore, it was confirmed that the proposed correction method was feasible, and could effectively improve the abilities of sensors for displacement intellisense. In this paper, the designed intelligent sensor was characterized by temperature self-compensation, bending shape self-classification, and displacement error self-correction, which could be used for real-time monitoring of deformation field in rock, subgrade, bridge, and other geotechnical engineering, presenting the vital significance and application promotion value.

Development of an FBG Sensor Array for Multi-Impact Source Localization on CFRP Structures.

We proposed and studied an impact detection system based on a fiber Bragg grating (FBG) sensor array and multiple signal classification (MUSIC) algorithm to determine the location and the number of low velocity impacts on a carbon fiber-reinforced polymer (CFRP) plate. A FBG linear array, consisting of seven FBG sensors, was used for detecting the ultrasonic signals from impacts. The edge-filter method was employed for signal demodulation. Shannon wavelet transform was used to extract narrow band signals from the impacts. The Gerschgorin disc theorem was used for estimating the number of impacts. We used the MUSIC algorithm to obtain the coordinates of multi-impacts. The impact detection system was tested on a 500 mm × 500 mm × 1.5 mm CFRP plate. The results show that the maximum error and average error of the multi-impacts' localization are 9.2 mm and 7.4 mm, respectively.

[Research on the Phase-Shifted Fiber Bragg Grating Spectra under Dynamic Strain Fields].

Fiber optic sensing technology has been widely used for acoustic emission (AE) measurement in aerospace and geotechnical engineering due to the advantages of immunity to electro-magnetic interference, high accuracy, and multiplexing capacity. As for the fiber Bragg grating based on AE measurement system, traditional efforts have been made to study the spectra of Fiber Bragg Grating under AE signal. While this paper focused on phase-shifted fiber Bragg grating (PS-FBG) which is a new type fiber Bragg grating, and investigated the spectra of under dynamic strain field generating with an AE signal. A dynamic strain sensing model of PS-FBG was built based on the transfer matrix theory, and a cosine exponential attenuation function was utilized to simulate the dynamic strain field. Then the effects of amplitude, sampling time, exponential attenuation coefficient, frequency and wavelength of AE signal on the spectra of PS-FBG were studied in detail using numerical simulation. The result demonstrate that the spectra of PS-FBG changes periodically with frequency and different sampling time, especially the peak wavelength of the transmission window in the PS-FBG spectrum shifts periodically; and the increase of amplitude mainly contribute to more harmonic peaks in the reflected spectrum. The attenuation coefficient affects the spectrum of PS-FBG within certain range. And the AE wavelength affect the spectrum of PS-FBG when it is between 0.1 to 2 L, beyond the range, the spectrum of PS-FBG has minor variation with the AE wavelength. Finally, dynamic strain experiments were conducted, and the spectrum of PS-FBG under continuously vibration signal with different amplitude and frequency were logged for analysis. The experimental results agree well with simulation, which indicate that under different dynamic strain fields generated by AE signal, the spectra of PS-FBG are different, while the variation of spectra follows certain laws. This paper provides theoretical support for the AE measurement system on the basis of PS-FBG.

[Research on the reflection spectrum of linear chirped fiber bragg grating under the impact of dynamic stress].

Dynastic stress field detection based on the linear chirped fiber Bragg grating (LCFBG) was proposed. Firstly, the reflectance spectra strain sensing model of LCFBG adopting the transfer matrix method was constructed, and attenuation sine function was used to simulate the dynamic stress field along LCFBG. In the simulation experiment, the responding character of LCFBG reflection spectrum to different amplitude, different attenuation coefficient and different propagation speed of dynamic stress was studied in detail. The simulation results show that the reflectivity, wavelength and spectral shape of LCFBG reflection spec- trum are related to the dynamic stress. However, the LCFBG reflection spectrum has different response to different parameters of dynamic stress. In a range, maximum reflectivity of LCFBG reflectance spectra increases when the amplitude and propagation speed of dynastic stress field becomes larger, but it decreases when the attenuation coefficient increases. Eventually, vibration sensor using LCFBG as the sensing element was designed, and then LCFBG dynamic stress fluctuations experiment platform was build. The data obtained from experiment agrees with the simulation results. Therefore, a novel detection method of dynamic stress field through real-time acquisition of LCFBG full spectral information is proposed in this article.

[Spectral characteristics of refractive index based on nanocoated optical fiber F-P sensor].

An optical fiber Fabry-Perot (F-P) interferometer end surface was modified using layer-by-layer assembly and chemical covalent cross linking method, and the refractive index (RI) response characteristics of coated optical fiber F-P sensor were experimentally studied. Poly diallyldimethylammonium chloride (PDDA) and sodium polystyrene sulfonate (PSS) were chosen as nano-film materials. With the numbers of layers increasing, the reflection spectral contrast of optical fiber F-P sensor presents from high to low, then to high regularity. And the reflection spectral contrast has good temperature stability. The reflection spectra of the optical F-P sensor coated with 20 bilayers for a series of concentration of sucrose and inorganic solution were measured. Experimental results show that the inflection point extends from 1.457 to 1.462 3, and the reflection spectral contrast sensitivity to low RI material and high RI material is 24.53 and 3.60 dB x RI(-1), respectively, with good linearity. The results demonstrate that the functional coated optical F-P sensor provides a new method for biology and chemical material test.

TiO2 nanoparticle thin film-coated optical fiber Fabry-Perot sensor.

In this paper, a novel TiO(2) nanoparticle thin film coated optical fiber Fabry-Perot (F-P) sensor had been developed for refractive index (RI) sensing by monitoring the shifts of the fringe contrast in the reflectance spectra. Using in situ liquid phase deposition approach, the TiO(2) nanoparticle thin film could be formed on the fiber surface in a controlled fashion. The optical properties of as-prepared F-P sensors were investigated both theoretically and experimentally. The results indicated that the RI sensitivity of F-P sensors could be effectively improved after the deposition of nanoparticle thin-films. It was about 69.38 dB/RIU, which was 2.6 times higher than that of uncoated one. The linear RI measurement range was also extended from 1.333~1.457 to 1.333~1.8423. More importantly, its optical properties exhibited the unique temperature-independent performance. Therefore, owing to these special optical properties, the TiO(2) nanoparticle thin film coated F-P sensors have great potentials in medical diagnostics, food quality testing, environmental monitoring, biohazard detection and homeland security, even at elevated temperature.

[Research on the phase-shifted fiber grating spectrum characteristics in the inhomogeneous strain fields].

Based on the transfer matrix method, the spectrum characteristics of phase-shifted fiber grating were analyzed systematically, and the reflection spectra of phase-shifted fiber grating under uniform strain, linear strain, secondary strain and cubic strain were simulated respectively. The regularity conclusion that the influence of different strain distribution functions on reflectivity, transmission bandwidth, wavelength of transmission window and spectral shape came out. In order to verify the simulation result, strain-tuning equipment was designed based on cantilever with different structures. Finite element method was carried out to simulate the strain distribution of phased shift fiber grating fixed on the surface of cantilever during the strain-tuning experiment. Experiment result demonstrated that the reflection spectra of phase-shifted fiber grating under uniform strain, linear strain and secondary strain exhibited a regular change, which is in agreement with simulation results.

[Research on gas detection based on spectrum scanning techniques of ultra-narrow-bandwidth laser].

Based on the basic principle of differential absorption detection and the spectrum scanning technology of ultra-narrow-bandwidth (UNB) laser, a kind of distributed carbon dioxide detection system with high precision was designed. An UNB laser was utilized as light source and a novel structure gas cell was also used in this system. By using the wavelength modulation technique and space division multiple access technique, the interference of other gas and dust was eliminated and the distributed detection of carbon dioxide was achieved. With the spectrum scanning technology of UNB laser, spectra of carbon dioxide near the region of 1 572.66 microm at different pressures were obtained which were also analyzed. Finally the experiments indicated that the minimum detectable carbon dioxide is 0.005% and the measurement relative error is less then 3%. The system dynamic response time is less than 8 s by filling a volume of carbon dioxide gas into the gas cell gradually.