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The service life and structural safety of components are affected by the thermal ageing of materials; therefore, a non-destructive evaluation of materials is extremely essential and crucial. In this work, heat-resistant steel Cr12MoV was selected as a specimen, and the embrittlement period of heat-resistant steel Cr12MoV was obtained by accelerated thermal ageing experiments. The effects of service temperature and service time on the mechanical properties of heat-resistant steel were analysed. The non-destructive testing of heat-resistant steel was carried out by using magneto-acoustic compound detection techniques based on electromagnetic acoustic transducers. In the process, magneto-acoustic compound detection signals were decoupled. The electromagnetic characteristic parameters and ultrasonic characteristic parameters of the detection signals were extracted to evaluate the mechanical properties and thermal ageing of heat-resistant steel. The quantitative evaluation ability of thermal ageing brittleness of heat-resistant steel was improved by using the magneto-acoustic compound detection techniques compared with single eddy current detection method or single ultrasonic detection method. [ABSTRACT FROM AUTHOR]

Magnetostriction-based electromagnetic acoustic transducer (M-EMAT) is a promising tool for generating fundamental shear-horizontal (SH0) mode guided waves in plate. Though the M-EMAT possesses high-order harmonics excitation ability in nature, its conventional way can only generate a fundamental component of SH0 mode. The analytical modeling was performed in this study to releasing the ability of M-EMAT in generating odd harmonics of SH0 mode. First, the inaccurate conditions of magnetic fields for relative permeability calculation in the existing model are corrected in the proposed model. Second, the error of the existing model in predicting the spectrum of the SH0 mode generated in Nickel plate is estimated. Third, numerical solutions are provided by the improved model to examine the key factors governing the ability of M-EMAT in generating odd harmonics of SH0 mode. The conclusions demonstrated that the intensity ratio (HS/HD) of static and dynamic magnetic fields is adjusted below 1 and the ability of M-EMAT in generating odd harmonics of SH0 mode can be activated. The normalized amplitude of 3rd harmonics decays exponentially with the increase of HS/HD, which is independent of geometrical parameters of M-EMAT. The improved model can effectively optimize excitation parameters of M-EMAT and generate odd harmonics of SH0 mode in plate. [ABSTRACT FROM AUTHOR]

In this research, to solve the problem of the ultrasonic guided wave inspection of a U-shaped boom, an improved periodic permanent magnet electromagnetic acoustic transducer (PPM EMAT) is proposed for defect detection for a U-shaped boom. The acoustic field distribution range of a traditional PPM EMAT is approximately ±20°, and the energy distribution is relatively concentrated. By adjusting the tilt angle between adjacent magnets and coils of a traditional PPM EMAT, an improved PPM EMAT is proposed. The acoustic field distribution range of the improved PPM EMAT is approximately ±30°, and the energy distribution in this range is relatively uniform. The simulation and experimental results show that the improved PPM EMAT has superior frequency response characteristics and acoustic field directivity, which is suitable for detecting large and complex structures. Furthermore, based on the traditional PPM EMAT and the improved PPM EMAT array, combined with the reconstruction algorithm for the probabilistic inspection of damage (RAPID), the damage locations of the double-hole defects of a U-shaped boom are studied. It is shown that the imaging results for defect location using the improved PPM EMATs are better than those found using the traditional PPM EMATs. [ABSTRACT FROM AUTHOR]

In this study, a cylindrical test specimen with a 3D through-hole defect was processed, and the reflected echo data of the defect at different cross-sections were obtained by an ultrasonic testing detection system. On this basis, two data processing methods were designed to obtain two types of 3D reconstruction images of defects, and the reconstruction effects of two methods were compared using the real defects. In general, this study achieved a relatively accurate 3D reconstruction of through-hole defects at a low cost. Our methods provided lower cost than current state-of-the-art approaches. [ABSTRACT FROM AUTHOR]

Since magnetic Barkhausen noise is very sensitive to the change of the stress or residual stress level in components, it is feasible to detect the stress or residual stress profile along the depth direction by the magnetic Barkhausen noise method. An engineering approach was used to validate a theoretical model describing the relationship between magnetic Barkhausen noise and stress magnitude. To generate stress gradients in the specimen of Q235 steel under different deflections, the four-point bending device was used. Then the detected signals obtained in the experiment were divided into different sub-bands to evaluate the stress at different depths. The testing depth was set to be around 100 μm due to the rapid exponential attenuation of the noise signals. In the experiments, the sub-band energy density showed sensitive as the loading displacement increasing. The experimental results showed that the signal energy was also sensitive to the plastic deformation of the material. Within the depth of 100 μm, the stress change was very small. The final experimental results showed that even under such small stress variations, it is sensitive and feasible to use the magnetic Barkhausen noise energy to characterise the change of stress gradients. [ABSTRACT FROM AUTHOR]

In the study, the three-dimensional (3D) morphology of defects in thin steel strips with a thickness of 0.8 mm was determined with the imaging method of magnetic field distortion (MFD). First, MFD imaging of through-wall defects and wall-thinning defects in the samples collected from inline products was performed in our laboratory. The actual profiles of the defects were measured with a digital microscope. Second, MFD imaging results were compared with the actual shapes of the defects in order to develop a proper 3D reconstruction method of complex defects. Finally, the analysis results demonstrated that the high reconstruction accuracy of opening contours of complex defects could be realized with the carefully selected threshold value of MFD-induced voltage amplitude. The good linear dependency of MFD-induced voltage amplitude on the depth of complex wall-thinning defects was confirmed. Therefore, MFD imaging method is a promising method for accurately reconstructing 3D shape of complex wall-thinning defects in thin steel strips. [ABSTRACT FROM AUTHOR]

Crack detection is important in the quality control of silicon-based solar cells (SSC). Silicon wafers are semi-conductive and cracks disturb the local conductivity and even induce anisotropy in conductivity of SSC. Eddy current method, which can detect the change of conductivity in the material, is a potential tool for detecting cracks in SSC. In this study, an experimental set-up is proposed to conduct both eddy current O-scan and C-scan imaging to the SSC. The eddy current O-scan was firstly used to investigate the anisotropy in the conductivity of SSC. Through comparing the inherent and crack-induced anisotropy in impedance (amplitude ratio and phase lag), macrocracks could be detected. The changes in the measured directional impedance could roughly reflect the shape of the cross-type macrocrack. In the C-scan mode, multiple slit defects were prepared to simulate macrocracks. Though the lateral resolution of the sensor was limited, the crack detection ability of the sensor could be evaluated with the C-scan imaging results. A slit defect with a width of 0.5 mm and a length of 1 mm was detected. A new criteria, the distance between feature zero-crossing points in the first derivative of the phase lag curve accurately reflected the actual length of the slit. [ABSTRACT FROM AUTHOR]

The magnetic hysteresis behaviours of ferromagnetic materials vary with the heat treatment-induced micro-structural changes. In the study, the minor hysteresis loop measurement technique was used to quantitatively characterise the case depth in two types of medium carbon steels. Firstly, high-frequency induction quenching was applied in rod samples to increase the volume fraction of hard martensite to the soft ferrite/pearlite (or sorbite) in the sample surface. In order to determine the effective and total case depth, a complementary error function was employed to fit the measured hardness-depth profiles of induction-hardened samples. The cluster of minor hysteresis loops together with the tangential magnetic field (TMF) were recorded from all the samples and the comparative study was conducted among three kinds of magnetic parameters, which were sensitive to the variation of case depth. Compared to the parameters extracted from an individual minor loop and the distortion factor of the TMF, the magnitude of three-order harmonic of TMF was more suitable to indicate the variation in case depth. Two new minor-loop coefficients were introduced by combining two magnetic parameters with cumulative statistics of the cluster of minor-loops. The experimental results showed that the two coefficients monotonically linearly varied with the case depth within the carefully selected magnetisation region. [ABSTRACT FROM PUBLISHER]

A double-turn coil omnidirectional shear-horizontal wave magnetostrictive patch transducer (DC-OSH-MPT) array for imaging detection in composite plates is presented. DC-OSH-MPT parameters are optimized in order to make the transducer more suitable for imaging detection. Besides, the characteristic of the lowest shear-horizontal mode (SH0), which can be generated and received in all directions by DC-OSH-MPT bonded on composite plate, is beneficial for imaging. Due to anisotropic property of composite plates, the group velocity of SH0 mode in different azimuthal directions was experimentally measured in damage imaging. The location of simulated damage, which was a bonded circular cylinder, was estimated by DC-OSH-MPT array and ellipse imaging algorithm. The imaging results have demonstrated that DC-OSH-MPT can be considered as a spatially distributed array suitable for imaging detection in composite plates. [ABSTRACT FROM AUTHOR]

Intergranular corrosion (IGC) is one of the major localized corrosion problems faced by austenitic stainless steel components in ultra-supercritical power plants, which occurs along the grain boundaries of material and results in loss of engineering properties. Ultrasonic technique was applied to the early detection of IGC in austenitic stainless steel super304H. Acoustic attenuation and nonlinear parameter were measured at multiple excitation frequencies, and an equivalent attenuation factor and an equivalent nonlinear factor were defined to evaluate the IGC damage. Results indicated that both factors increased monotonously as the degree of IGC developed, and the equivalent nonlinear factor is more sensitive to IGC at early stage. Thus they can be utilized for early detection of IGC in austenitic stainless steel. [ABSTRACT FROM AUTHOR]

A multi-frequency instantaneous wavenumber damage imaging method based on ultrasonic guided waves induced by laser point-by-point excitation is proposed in this paper. The experiment involved using the Nd:YAG pulsed laser is used as the excitation source to excite ultrasonic guided waves point-by-point in the detection area, and the piezoelectric ceramic transducer is used to receive ultrasonic guided wavefield at a fixed position. The wavefield information at different frequencies is extracted by a three-dimensional Fourier transform. The spatial phase distribution of the extracted guided wavefield with different frequencies is obtained by the Hilbert transform and phase unwrapping method. Then, the wavenumber of each detection point and the dispersion relations of the test piece with different thicknesses are calculated. Based on the calculated dispersion relation, the mapping relationship between the wavenumber and the thickness of the test piece is obtained. Furthermore, based on the obtained wavenumber information and wavenumber thickness mapping relationship, the thickness reduction depth of the defect can be calculated. Finally, the quantitative detection of defects is achieved through data fusion. The proposed method is verified through quantitative detection of the rectangular groove defects and the flat bottom defects with variable thickness and complex contour in the aluminium plate. At the same time, the detection results of the proposed method are compared with those of the single-frequency instantaneous wavenumber imaging method, single-frequency local wavenumber imaging method, and multi-frequency local wavenumber imaging method. In the detection results, the method proposed in this paper presents a better defect detection effect. [ABSTRACT FROM AUTHOR]

AbstractThe mechanical properties of the graphite electrode will dynamically change during the charge-discharge cycle, which will affect the propagation characteristics of guided waves in lithium-ion batteries. Meanwhile, lithium-ion batteries experience fluctuations in operating temperature during cycling, which will also influence the propagation of guided waves. This research aims to investigate the guided wave propagation problem of porous electrode material with electrolyte versus temperature. The problem is under the framework of Green-Naghdi theory and Biot theory, and a numerical approach is presented to solve the thermoelastic wave propagation characteristics in such a multi-layered porous electrode. A frequency domain simulation for a multi-layered porous anode will be established. The simulation results confirm the feasibility and accuracy of the proposed approach. The porosity of the anode material shows obvious dynamic variation during cycling, which illustrates a strong correlation with the elastic modulus of the solid skeleton. Then, the effects of porosity and temperature variations on the propagation characteristics of thermoelastic guided waves in multi-layered porous graphite electrodes will be analyzed in detail. The phase velocity of fundamental modes appears regularly shift in the low frequency range. This may provide theoretical support for the acoustic nondestructive characterization of the operating states of the lithium-ion batteries. [ABSTRACT FROM AUTHOR]

15CrMo steel has good mechanical properties and is widely used in high-temperature pressure components. After long term service, it is easy to cause material spheroidisation. The metallographic and mechanical property tests are used to evaluate the spheroidisation of specimens. Ultrasonic backscattering signal is sensitive to microstructure and contains more microstructure information, which can be used for the evaluation of spheroidisation. However, the ultrasonic backscattering signal is nonlinear, and feature extraction is difficult. In this study, ultrasonic testing is used to scan different spheroidisation specimens, and the ultrasonic backscattering signal is extracted as the input to the deep learning model, which is used to extract features from the backscattering signal. The models are evaluated using classification and regression evaluation metrics. The results show that the proposed CNN-LSTM model has good identification performance for the classification of spheroidisation and the prediction of mechanical properties. The classification accuracy, recall, precision, and F1-score are all 1. Additionally, the maximum predicted RMSE and MAE values are only 2.33 MPa and 1.70 MPa, and the minimum R2 is only 0.97. The worst prediction is the tensile strength, with an average value of 442.2 MPa and a maximum value of 481.1 MPa. [ABSTRACT FROM AUTHOR]

AbstractProcess-induced initial gradient stress is inevitable during the fabrication of fiber reinforced polymer (FRP) composites. In this research, governing wave equations with variable coefficients are established for FRP plates under initial gradient stress, based on the acoustoelastic theories. The Legendre polynomial (LP) method is utilized to derive the solving algorithm. The dispersion curves are obtained without layer slicing, which provides a more realistic analysis model for gradient stress. The accuracy and convergence of the proposed method are discussed through numerical examples. The influences of initial gradient stress on dispersion slowness curves of Lamb and SH waves are analyzed, considering the parabolic initial gradient stress distribution, in which it’s common in the manufacturing process of FRP composites. The effect of initial gradient stress distribution on the phase velocity is investigated in detail. The influence of the anisotropic effect of FRP on guided wave propagation under initial gradient stress is discussed. The numerical analysis shows that the behavior of guided waves is influenced not only by the initial gradient stress but also dominated by propagation direction in anisotropic FRP composite plates. These results can provide a useful reference for the testing based on guided waves for FRP composite, especially when the initial gradient stress is encountered. [ABSTRACT FROM AUTHOR]

In this research, simulations and experiments were conducted for the bonding quality evaluation of an aluminum bonding structure, based on the SH guided wave analysis. In order to simulate the propagation of SH waves in the bonded structure, a 3D finite element model was established based on interfacial spring stiffness modeling. The frequency-wavenumber diagram was obtained by doing Fourier transform along the spatial axis in the time frequency domain simulation results. At the same time, the influence of the bonding interface weakening on the propagation characteristics of the guided wave was analyzed. The simulated dispersion curves of SH-guided waves in different bonding states were in good agreement with the theoretically results. In addition, a periodic permanent magnet electromagnetic acoustic transducer (PPM-EMAT) 3D finite element model was established. An optimized transducer was fabricated to excite pure SH0 mode. Then, a series of aluminum lap bonding structures with different bonding states were prepared. The effect of bonding interfaces on the propagation characteristics of SH waves was investigated. The interfacial spring stiffness can be determined when the STFT of the SH waves matches the theoretical dispersion curves. Thus, this method demonstrated an ultrasonic way to distinguish the weakening of the bonding structures. [ABSTRACT FROM AUTHOR]

Thermal barrier coatings (TBCs) have been widely used in gas turbine engines in order to protect the substrate metal alloy against high temperature and to enhance turbine efficiency. Thermal effects in the work process will produce a certain stress to induce the metal substrate surface defects such as cracks, which affect thermal conductivity of TBCs. Therefore, it is necessary to detect the cracks in order to ensure the quality of TBCs. In this paper, an approach for microwave non-destructive evaluation of surface crack on the substrate under thermal barrier coatings will be presented. The relationship between reflection coefficient and defects of TBCs were studied, based on the propagation feature of microwave isonified through multi-layer. A network analyzer operated in the frequency range of 26.5 GHz to 40 GHz was used to measure the reflection coefficient. Scanning images were obtained by using phase difference and amplitude at the optimum detection frequency. The results indicate that the amplitude, and the phase difference of the reflection coefficient, can represent the defects under TBCs using microwave non-destruction evaluation when the stand-off distance is suitable. The experimental setup that validates the microwave non-destructive inspection of TBCs is presented, and the feasibility to evaluate the cracks on the substrate surface using microwave non-destructive evaluation is verified in this paper. [ABSTRACT FROM AUTHOR]