He Cunfu

Circumferential thermoelastic waves in orthotropic cylindrical curved plates without energy dissipation

In this article, the propagation of guided thermoelastic waves in the circumferential direction of orthotropic cylindrical curved plates subjected to stress-free, isothermal boundary conditions is investigated in the context of the Green-Naghdi (GN) generalized thermoelastic theory (without energy dissipation). The coupled wave equations and heat conduction equation are solved by the Legendre orthogonal polynomial series expansion approach. The convergency of the method is discussed through a numerical example. The dispersion curves of thermal modes and elastic modes are illustrated simultaneously. Dispersion curves of the corresponding pure elastic cylindrical plate are also shown to analyze the influence of the thermoelasticity on elastic modes. The displacement, temperature and stress distributions are shown to discuss the differences between the elastic modes and thermal modes. A thermoelastic cylindrical plate with a different ratio of radius to thickness is considered to discuss the influence of the ratio on the characteristics of circumferential thermoelastic waves.

A new acoustic emission source location technique based on wavelet transform and mode analysis

For wave propagation in dispersive media, the arrival time of the acoustic emission signal to the sensor is dependent on the setting of the threshold voltage, which results in the inaccuracy of the acoustic emission location. Based on the wavelet transform and the theory of modal acoustic emission, a new method is proposed to improve the accuracy of acoustic emission source location. It is believed that the acoustic emission signal propagation in the structure has the characteristics of multi-mode and dispersion, and the acoustic emission source location should use the arrival time to sensors obtained from the output signals not only at the same mode but also at the same frequency. The wavelet transform is used to resolve the problem. By utilizing the time-frequency data of the wavelet, the frequency-dependent arrival time traveling is easily obtained; by numerical computation of the wave’s propagation in structure, the group velocity of the guided mode is also obtained, therefore the accuracy source location is realized. The acoustic emission source location experiments were conducted in a thin steel plate and results show that the technique is an effective tool for acoustic emission source location.

Theoretical and experimental studies of torsion deformation of a thin-walled tube with wound and pasted shape memory alloy wires

Restrained shape memory alloy (SMA) wires, as actuators, are wound and pasted onto the outside surface of a thin-walled circular tube at angle of α° with respect to the tube axis. Upon heating the wires, the martensitic residual strain will be restored, and the large recovery forces induced so that the tube will deform. This paper presents a mechanical model of a thin-walled tube, pasted with actuators (SMA wires), the analyses of the mixed deformation, of compression and torsion, of the tube and the relation between the angle of twist and the temperature in a temperature cycle. The theoretical predictions are validated with static experiments.

Determination of Material Properties of Functionally Graded Plate Using the Dispersion of Guided Waves and an Artificial Neural Network

Using guided wave dispersion characteristics, an inverse method based on artificial neural network (ANN) is presented to determine the material properties of functionally graded materials (FGM) plates. The group velocities of several lowest modes at several lower frequencies are used as the inputs of the ANN model; the outputs of the ANN are the distribution function of the volume fraction of the FGM plate. The Legendre polynomials method is used to calculate the dispersion curves for the FGM plate. Levenberg-Marquardt algorithm is used as numerical optimization to speed up the training process of the ANN model.

Detection of Surface Crack on the Substrate under Thermal Barrier Coatings Using Microwave Non-Destructive Evaluation

Abstract 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.

Megahertz-range magnetostrictive sensor for exciting longitudinal guided waves in steel rods/strands

A flexible printed meander coil (FPMC)-based magnetostrictive sensor (MsS) which has a central frequency around 1.25 MHz is presented in this paper. The FPMC-based MsS can be easily wrapped onto the twisted strand surface and disassembled from the strand. It can be also installed onto the central rod of the strand after being wrapped into multiple layers. That is, this FPMC-based MsS can fit for the tested sample of different diameters. With the proposed megahertz-range FPMC-based MsS, the rods/strands inspection system is potential to be applied for practical inspections, having higher axial inspection resolution and sensitivity to small defects.

Numerical Simulation of Multi-path Ultrasonic Flowmeter: Ultrasonic Path Error Analysis

The precision of ultrasonic flowmeter is mainly depending on ultrasonic path adaptability for velocity profile of pipe, especially for the special cases, such as ultrasonic flowmeter located near the elbow of pipe. In this paper, the flow fields of some typical pipes were numerically calculated with commercial CFD software, and the velocity profiles in different section of pipe were investigated. Based on the simulated velocity profile, flow rate in special section of the double bend pipe was measured using different ultrasonic path configuration methods, such as Gaussian-path, Jacobianpath and diametral-path. According to the error of the measured flow rate, it is easy to get the optimal layout of ultrasonic path for the particular velocity profile. It is proved that the numerical simulation of flow field is useful for the optimization of the ultrasonic paths of flowmeter.

Design of a piezoelectric transducer cylindrical phase modulator for simulating acoustic emission signals

To conveniently carry out the pipeline leak experiment in a laboratory, leak acoustic signals are simulated by using the converse piezoelectric effect of a piezoelectric transducer (PZT) cylindrical phase modulator. On the basis of the piezoelectric equations and electromechanical equivalence principle, the transfer function of a PZT cylindrical phase modulator is delivered. A PZT cylindrical phase modulator is designed, and the numerical simulation is conducted. Results prove that the PZT cylindrical phase modulator can effectively simulate leak acoustic emission signals when the frequency is lower than 25 KHz.

SH 0 导波在粘接结构中传播时的相位变化

对粘接结构进行超声导波无损检测与评估是一个有挑战性的前沿性课题.针对此问题，研究了SH 0 导波在界面为理想连接的三层板状粘接结构中传播时的相位变化情况.首先基于波传播的控制方程，建立了粘接结构中反射和透射SH 0 导波相对于入射SH 0 导波的相位差解析模型.然后利用数值模拟计算了铝/环氧树脂/铝粘接结构中反射和透射SH 0 导波的相位差曲线.最后分析了入射角度和频厚积的变化对反射和透射SH 0 导波相位差的影响.结果表明，对于具体的粘接结构，反射和透射SH 0 导波在其中传播时的相位差变化主要依赖于入射角度、频率等参数.在特定的频厚积下，当声波水平入射时，反射和入射SH 0 导波同相位.无论入射角度为多大，随着频厚积的增大，反射SH 0 导波的相位差曲线均会产生周期性谐振.对于透射SH 0 导波，当声波垂直入射时，其相位差曲线的改变无规律可循；但是随着入射角度逐渐增大，透射SH 0 导波的相位差曲线逐渐变规则.所得结果可为实验时研究板状粘接结构中SH 0 导波的传播特性以及提取SH 0 导波回波中的有用信息和定位提供一定的理论指导.

Characterization of Elastic and Plastic Behaviors in Steel Plate Based on Eddy Current Technique Using a Portable Impedance Analyzer

A portable impedance analyzer (PIA) was developed based on a TiePie-HS3 device to provide the comparable impedance measurement accuracy of the Agilent 4294a impedance analyzer in the frequency range of 0~250 kHz. Then the PIA was applied to monitor the tensile stress-induced variation of the eddy current sensor’s impedance in a medium-carbon steel sample. A model of equivalent magnetic field induced by the elastic stress and the number of pinning sites indicated that the inductance of the eddy current loop firstly increased with the increase in the tensile stress and then decreased at the yield point of the material. The experimental results testified that the variation of impedance amplitude, the variation of phase angle, and the shift of two featured frequencies demonstrated opposite variation trends before and after the yield point, as predicated by the model. A new parameter, which combined the impedance variation information of the selected two frequencies, was found to exhibit nearly monotonous dependency on the tensile stress in elastic and plastic stages. The new parameter together with the developed portable impedance analyzer provided the solution to identify the elastic and plastic behaviors in ferromagnetic materials in practical applications with an eddy current technique.