空气耦合式超声检测在航空无损检测中的应用

针对复合材料的无损检测问题,探讨了一种基于穿透式异侧检测模式的空气耦合式超声波检测技术,阐述了该技术的基本原理。参照传统的液浸式超声波检测技术,以航空工业中重要的γ-TiAl基合金为研究对象,对检测过程中信号的衰减与损耗进行了详细的理论分析与数值计算。结果表明,超声波在介质交界面处的反射损耗和在空气中的传输衰减是目前空气耦合式超声检测技术应用遇到的主要难题,而研制新型超声波换能器是解决问题的关键。     

超声波在金属平行板夹层中的反射和透射

对超声波在金属平行板夹层面的反射和透射进行了分析和简化处理,从而导出了各层面回波之间的相互关系。     

Ultrasonic non-destructive on-line estimation of the tensile stiffness of a running paper web

A new method for measuring the elastic properties of paper on-line in a paper machine by using ultrasound has been developed. The unique feature of the method is that a continuous stochastic ultrasonic signal is used for measurements, which is generated by means of a dry friction between a running paper web and a rigidly fixed friction head. The signals from the web are picked up by an array of air-coupled ultrasonic receivers, which have no mechanical contact with the paper web. The ultrasound velocity in the web is obtained by means of the correlation processing. By using advanced digital signal processing, the tensile stiffness index (TSI) of the running web is determined. From the sensor we get the TSI values both in the machine direction and the cross-machine direction.The on-line ultrasonic sensors have been installed on two different paper machines for more than two years. This paper describes the background of the method, as well as some experiences and applications from two mill installations.     

Ultra-low noise front-end electronics for air-coupled ultrasonic non-destructive evaluation

Air-coupled ultrasonic inspection has been demonstrated to be a non-contact method of great interest in non-destructive evaluation (NDE) applications. The absence of direct contact or a liquid couplant provides this technique very attractive benefits in front of the well-known and well-developed liquid-coupled ultrasonic inspection systems. A wide range of defects can be detected by means of ultrasound coupled to and harvested from the specimen in absence of contact when using appropriate transducers designed for their operation in air.This paper presents an easy way to integrate air-coupled piezoelectric transducers in conventional ultrasonic NDE equipment. The design of a specific front-end electronics by using an ultra-low noise amplifier enables existing inspection systems to be used for the dry-coupled ultrasonic test of materials and structures. The amplifier provides the receiver with a signal-to-noise ratio large enough for good quality signal processing and imaging. System dynamic ranges of more than 100 dB are achieved.     

Lamb and SH waves generated and detected by air-coupled ultrasonic transducers in composite material plates

Electrostatic, air-coupled, ultrasonic transducers are used to generate and detect guided waves in anisotropic solid plates. Waves considered in this study are Lamb-type and SH-type, guided modes. If the plane of propagation coincides with a plane of symmetry of the material, then Lamb modes only are launched and detected by the transducers. If the plane of propagation does not coincide with a plane of symmetry of the material, then Lamb modes are still generated and detected, but guided, SH-like modes are, too. The variation of phase velocity with frequency is measured for several modes propagating in different directions along a glass–epoxy composite plate.A numerical model that takes into account the anisotropy of composite materials is developed to predict the dispersion curves (phase velocity, group velocity or wave-number versus frequency) and the displacement fields of plate waves, the plane of propagation being either a plane of symmetry or not.The experimental phase velocities are in good agreement with the predicted dispersion curves, thus showing that the forward problem concerning the propagation of plate waves in anisotropic, homogeneous, composite material plates is properly solved. The dispersion curves associated with the predicted displacement fields show that guided modes in composite plates have different behaviors depending on their direction of propagation.     

输油管道自校正超声波在线监测系统

超声波测厚是维护管道安全性的重要无损检测技术,其测量原理是利用超声波脉冲反射法在线监测石油管道的厚度和腐蚀情况.超声波脉冲反射法测厚的精度主要依赖于声速,而声速受环境温度的影响较大,故对超声波在固体中的传播速度和温度关系进行了大量的试验,在这些研究的基础上,提出了温度与声速的神经网络误差补偿模型和线性回归误差补偿模型....     

Noncontact fatigue crack visualization using nonlinear ultrasonic modulation

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.     

Application of the ultrasonic pulse-echo technique for quality control of the multi-layered plastic materials

The main aim of the ultrasonic pulse-echo technique application for quality control is to characterize the internal structure of the object under investigation. The advantage of such technique is a possibility to perform non-contact and one-side access measurements, and to investigate the internal structure of multi-layered materials as well. The presented novel application of the ultrasonic pulse-echo technique for characterization of the multi-layered plastic materials covers the complete attenuation measurement in the frequency domain and is based on ill-posed Tikhonov regularization task for each layer separately. The law of the frequency-dependent attenuation has been estimated from the inverse transfer function approximation in the frequency domain. Phase velocity dispersion curves have been estimated in two ways: from the experimental signal phase spectra and from the causal Kramers–Kronig relations. The developed method enables to predict waveforms of the reflected signals from the interfaces of the individual layers in on-line mode.According to this approach, the step-by-step iterative analysis has been performed for each layer using the information about the previous layers. During each step, the acoustic properties of an individual layer, such as density, absorption, ultrasound velocity and phase velocity dispersion, have been recovered using numerical optimization. Optimization has been performed comparing the real ultrasonic signal, reflected by multi-layered object, with the simulated response of the model. The comparison of the predicted waveforms with the experimental ones has shown a good correspondence.     

Comparison of normal and oblique incidence ultrasonic measurements for the detection of environmental degradation of adhesive joints

Normal and oblique incidence ultrasonic scans have been conducted on two and three layer adhesive joint specimens exposed to water at 50 °C for periods of up to 18 months. The joints consisted of aluminium alloy adherends, which were subjected to one of four different surface pre-treatments prior to being coated (for the two layer specimens) or bonded (for the three layer specimens) with an epoxy polymer. Overall, the oblique incidence technique detected the same types of defect as the normal incidence scans, except when the resolution was too poor to detect the smallest defects. No defect was detected using oblique incidence scans that could not be detected using normal incidence scans. In the two layer specimens, two main types of defects were detected: edge disbonds and micro defects. Micro defects were detected in regions remote from the edges and these small scale isolated defects took several forms. The detection of micro defects was a strong indicator that the interphase of the specimen had been attacked and weakened by the ingressing water molecules and, indeed, the number of such defects correlated to the loss of interfacial toughness. In the case of the three layer specimens, the extent of edge disbonding was much lower than in the corresponding two layer specimens. Also, no micro defects were detected ultrasonically in any of the three layer specimens. However, examination of some of the failure surfaces from the three layer specimens suggested that micro defects might have been present but that they were too small to be detected at the spatial resolution of the ultrasonic scans. Therefore, for the three layer joint specimens, there were no indicators from the ultrasonic scans, which could reveal whether the joint had suffered attack and weakening of the interphase regions by the ingressing moisture. There was no evidence in any of the tests of a gradual change in either the normal or oblique incidence reflection coefficient moving into an apparently well bonded region from either the edge disbonds or the micro defects.     

温度对钢板超声波检测结果的影响

在对某原油储罐钢板的超声检测过程中,发现部分底板弓形边缘板及坡口两侧有超出Ⅰ级验收标准的缺陷,有些甚至达到Ⅴ级。而在复检中,原检出缺陷级别有不同程度的降低。针对这一现象,讨论了钢板中声压反射率的变化规律,得出了同一缺陷的检出级别由于异质薄层的热胀冷缩,而昼夜发生变化的结论。     

基于空气耦合超声的激光焊缝质量评估

随着激光焊接技术在汽车及轨道交通上的广泛应用,对激光焊缝的无损检测技术要求越来越高. 针对3 mm以下的两层金属薄板激光焊缝为对象,探讨了空气耦合超声检测技术对激光焊缝检测的可能性. 利用数值解析和试验分析两种方法,在空气中对铝板激励兰姆(Lamb)波,通过Lamb波在激光焊缝试件中的传播模拟,分析了激光焊缝的宽度以及焊缝的良否对反射率及透射率的影响,解明了Lamb波在激光焊缝试件中的传播规律. 结果表明,可以利用Lamb波A0模式对激光焊缝的质量进行评估.     

Characterization of ultrasonic Rayleigh surface waves in asphaltic concrete

This research examines the application of ultrasonic Rayleigh surface waves to the non-destructive evaluation (NDE) of asphaltic concrete in the frequency range 40–100 kHz. Two ultrasonic parameters, the phase velocity and amplitude attenuation, are considered to be sensitive to local compositional and structural variations and are measured. A wedge technique is used to generate Rayleigh surface waves in an asphaltic concrete beam with uniformly distributed aggregate, and this technique is shown to be effective in launching Rayleigh waves in this highly viscoelastic (absorptive) and heterogeneous medium. Three different ultrasonic detection setups using contact and non-contact transducers as receivers are examined and their results are compared. The experimental results show that the wedge generation technique along with an air-coupled receiving transducer with a finite-size aperture is the most reliable for characterizing Rayleigh waves in asphaltic concrete. It is also experimentally demonstrated that the proposed setup may be used to detect aggregate segregation in asphaltic concrete.     

Assessment of porosity of mortar using ultrasonic Rayleigh waves

The relationship between Rayleigh wave velocity at ultrasonic frequencies and porosity is investigated in dry and fully saturated mortar. Porosity is varied by changing the water/cement ratio. Rayleigh wave (RW) pulse velocity is measured accurately using air-coupled ultrasonic transducers as transmitter and receiver. The experimental results are compared with two single-phase models. Due to relatively weak correlation between RW velocity and porosity, measured data and the models are compared using statistical hypothesis tests. Although the parameters of the model are not accurately predicted, these tests conclude on a reasonable agreement between measured data and the models. This result can be useful to develop simplified models of wave propagation in concrete, or to estimate porosity in concrete cover from ultrasonic measurements.     

Ultrasonic imaging of concrete members using an array system

The use of an ultrasonic array system is described, which can be used combined with 3D reconstruction calculations. In this way ultrasonic reflection and backscatter from the inside of concrete members can be imaged and interpreted. The application of the system is demonstrated for two examples: measuring the concrete cover of utility pipes in a tunnel and the examination of transversal ducts in a bridge plate.     

Low frequency bending piezoelectric actuator with integrated ultrasonic NDE functionality

A bending piezoelectric actuator has been manufactured with a 250 μm thick PZT plate embedded in five layers of unidirectional fiber Kevlar epoxy. The selected orientation of the fibers produces a gradient of residual stresses in the composite that enhances bending actuation. By segmenting the electrodes, it is possible to launch an ultrasonic pulse in the actuator so that structural health monitoring of the actuator itself can be performed using pitch catch ultrasonic NDE during the low frequency actuation. Qualitative experimental results of fatigue tests and damage detection are presented.     

Low frequency bending piezoelectric actuator with integrated ultrasonic NDE functionality

A bending piezoelectric actuator has been manufactured with a 250 μm thick PZT plate embedded in five layers of unidirectional fiber Kevlar epoxy. The selected orientation of the fibers produces a gradient of residual stresses in the composite that enhances bending actuation. By segmenting the electrodes, it is possible to launch an ultrasonic pulse in the actuator so that structural health monitoring of the actuator itself can be performed using pitch catch ultrasonic NDE during the low frequency actuation. Qualitative experimental results of fatigue tests and damage detection are presented.     

CFRP层板冲击损伤空耦超声Lamb波特征成像 ——基于时间反转损伤指数

为实现飞机碳纤维复合材料（Carbon Fiber Reinforced Plastics，CFRP）层板在役检测，采用同侧空气耦合超声兰姆波特征成像检测的方法对其缺陷进行检测。将非接触空气耦合超声传感器置于CFRP层板同侧，激发A0模态兰姆波，对其冲击损伤进行D扫描检测。引入时间反转损伤指数表征复合材料层板的冲击损伤。结合概率损伤算法，以该指数作为损伤重构成像的特征值，将不同扫描路径上的特征值数据进行融合，得到CFRP层板冲击损伤缺陷的兰姆波图像。结果表明，基于时间反转的兰姆波图像不仅能够直观地呈现损伤缺陷的位置和形状，而且能够通过避免基准信号选取和减少扫描步进次数显著提高检测效率。     

Application of Golay codes to improve the dynamic range in ultrasonic Lamb waves air-coupled systems

Air-coupled ultrasonic systems are usually limited by poor signal-to-noise ratios. The use of pulse compression techniques based on Golay codes can help to improve the ultrasound signal-to-noise ratio. This type of codes allows virtually side lobe free operation and causes the cancellation of self-induced noise. This paper proposes the use of Golay sequences to code Lamb waves excited in thin materials with a NDT system based on air-coupled piezoelectric arrays. Furthermore, a system for the generation of Lamb waves modulated by 16-bits Golay codes is also proposed. Successful results are presented in copper plates. An improvement in signal-to-noise ratio up to 21 dB is achieved when compared with conventional pulse transmission.     

Molecular beam epitaxial growth of zinc-blende FeN(1 1 1) on wurtzite GaN(0 0 0 1)

We report a study of the growth of iron nitride on gallium nitride using molecular beam epitaxy with Fe e-beam evaporation and rf N-plasma growth. Thin iron nitride layers of thickness about 16 nm were grown and monitored in situ using reflection high energy electron diffraction. The samples following growth were analyzed ex situ using a variety of techniques including X-ray diffraction, Rutherford Backscattering, and atomic force microscopy. By monitoring the structure, morphology, and lattice constant evolution of the iron nitride film, the crystal phase and orientation with respect to the GaN substrate are deduced; and from RBS data, the stoichiometry is obtained. The growth is discussed in terms of a 2-D to 3-D growth mode transition, and a critical thickness is estimated.     

Simulation investigation of through-feed centerless grinding process performed on a surface grinder

This paper proposes a simulation method for investigating the through-feed centerless grinding process performed on a surface grinder, where a compact centerless grinding unit, composed of a guide plate, an ultrasonic elliptic-vibration shoe, a blade, and their respective holders, is installed onto the worktable of a surface grinder, and the through-feed centerless grinding operation is performed as the workpiece located on the guide plate is fed into the space between the grinding wheel and ultrasonic shoe. The geometrical arrangement of the grinding apparatus including the contact lines on the grinding wheel, ultrasonic shoe, and blade are analyzed firstly for building a 3-D simulation model. Then, the workpiece forming process and the effects of major process parameters such as the workpiece eccentric angle, the stock removal, the ultrasonic shoe tilt angle and the applied voltage amplitude on the machining accuracy (i.e. workpiece cylindricity and roundness) are clarified by simulation and experiments. The obtained results indicate that higher machining accuracy can be achieved under the conditions of larger workpiece stock removal, smaller ultrasonic shoe tilt angle and higher applied voltage amplitude, while the workpiece eccentric angle is at 6°.