LFM激励信号在超声TOFD检测中的应用

对于大厚度焊接试件的超声衍射时差法检测(time of flight diffraction, TOFD),常规超声激励难以同时满足信噪比、检测距离以及检测分辨率的要求.文中将线性调频脉冲压缩技术应用在超声TOFD检测中,线性调频(linear frequency modulated, LFM)激励可综合改善检测信噪比(signal to noise ratio, SNR)与分辨率.首先对试验用10 MHz超声换能器进行了LFM信号参数测试,选择了合适的时宽和带宽.对埋藏3 mm高的横槽缺陷的钢板分别进行了超声LFM激励的TOFD检测及常规超声TOFD检测,对比发现超声LFM激励的TOFD检测精度高达0.01μs,可准确区分缺陷上下端衍射波.在较低的激励电压和系统增益下,实现了较高的检测信噪比及分辨率.     

Fourier-transform vibrothermography with frequency sweep excitation utilizing local defect resonances

The defect activation and detection efficiency of low-energy vibrothermography can be further improved by periodically sweeping the ultrasonic excitation frequency and Fourier-transforming the temperature data at the modulation frequency. This is due to the resulting periodic heat production. While typical local defect resonance investigations make use of known resonance frequencies, this technique instead employs a wideband ultrasonic excitation signal. It can therefore be used to detect defects quickly without knowing the LDR frequency in advance, while still keeping the efficient defect activation effects provided by LDR. Otherwise undetectably low thermal defect signals are enhanced by means of Fourier filtering and the defects’ resonance frequencies are characterized by their phase values.     

复杂结构焊缝的非线性超声检测方法

振动声调制技术是一种新出现非线性超声无损检测方法. 文中简要介绍了此技术的检测原理,并对其在复杂结构焊缝质量检测中的应用进行了初步研究. 制作了小径管座角焊缝试样,分别在单频激励和扫频激励条件下进行检测. 结果表明,缺陷试样的调制强度均高于参考试样,且在特定高频频率范围内,二者差别显著,更有利于焊缝质量判别. 而采用扫频技术可快速得到合适的高频频率范围,提高检测效率. 与超声C扫描检测结果相比,采用振动声调制技术对小径管座角焊缝进行检测是可行的,但缺陷的定位定量分析等问题还需要进一步研究.     

All-optical light modulation in anthraquinone dye doped waveguides based on an improved prism coupling method

In this paper, an improved all-optical light modulation scheme has been proposed with respect to photo-induced changes of the refractive index of polymer planar waveguides by the m-line spectroscopy technique. An improved prism coupling method was used to realize it. In this prism coupling configuration, the strong pump (writing) beam excited by an Ar⁺ laser at 514 nm is illuminated on film sample by passing through the upper base of prism, and induces local refractive index changes by heating. A probe (reading) beam, which is coupled into prism coupling system and forms waveguide modes in film, is modulated for its changes of reflectivity due to changes of refractive index of waveguide film. A shift of sharp guided wave resonance is observed and 20 ms of response time is obtained.     

多层铆接结构铆钉孔周裂纹的脉冲涡流检测

飞机多层金属铆接结构中铆钉周边裂纹的检测是无损检测领域中的一个难点和热点。基于脉冲涡流检测技术,设计检测探头,并对探头的激励线圈匝数、检测频率、接收传感器距铆钉距离等参数进行优化,研制一种使探头能够围绕铆钉进行旋转检测的装置。检测探头参数和传感器与铆钉之间的距离参数的变化对检测灵敏度的影响较大。旋转装置检测时:激励匝数为180匝、检测频率为100 Hz、探头与铆钉距离4 mm时,对于长度为1、2 mm的铆钉孔周边裂纹检测效果较好;对于长度大于2 mm的铆钉孔周边裂纹,探头距离铆钉10 mm的时候检测灵敏度较高。利用旋转装置检测和纯手动检测的结果对比表明,旋转检测装置能够很好地抑制探头与铆钉之间的距离变化带来的对信号的干扰,减少伪缺陷并提高检测效率。     

Pulsed remote field technique used for nondestructive inspection of ferromagnetic tube

Remote field eddy current (RFEC) technique is an effective method for measurement of ferromagnetic tube. However, traditional RFEC is unable to differentiate the internal and external defect and the probe has a long length. Pulsed eddy current techniques excite the induction coil with a pulsed waveform and have the richness of frequency harmonics. The wideband excitation is thought to be a potential in providing more information about the flaw. In this paper, pulsed RFEC technique is used to inspect ferromagnetic tube. The finite element analysis and experiment method is used to give a thorough analysis of the influence effect with the variations of the system parameters. Results show that this technique effectively combines the advantages of RFEC and pulsed excitation, which not only acquires more inspection information, including measurements of inner diameter of tubes, internal and external defects, but also reduces the length of probe and power consumption. The agreement between simulation and experiment shows that the present method is correct.     

Dual frequency fusion for defect signal enhancement in EC-GMR inspecton of riveted multilayer structures

Reliable detection of defects under fasteners at rivet sites is one of the most critical problems in inspection of multilayer structures. Rotating eddy current (EC) excitation with giant magnetoresistive (GMR) sensor array field pick up (EC-GMR) has proved to be a promising approach in this application. In this paper, fusion of dual frequency is applied to EC-GMR scan data acquired with a novel differential array probe, to suppress the strong fastener response and hence improve the detectability of defects. Performance of the data fusion algorithm is evaluated on a two-layer aluminum sample with steel fasteners and notches introduced in the bottom layer using the electric discharge machine (EDM). Experimental results demonstrate that defects of different sizes can be successfully detected by evaluating the residual field after fusion. Defect orientation angle has minimal impact on the sensitivity of the probe owing to rotating eddy current excitation.     

The application of acoustic emission for detecting incipient cavitation and the best efficiency point of a 60 kW centrifugal pump: case study

Pumps play a significant role in industrial plants and need continuous monitoring to minimise loss of production. To date, there is limited published information on the application of acoustic emission (AE) to incipient pump cavitation. This paper presents a case study where AE has been applied for detecting incipient cavitation and determining the best efficiency point (BEP) of a 60 kW centrifugal pump. Results presented are based on net positive suction head (NPSH) and performance tests. In conclusion, the AE technique was shown to offer early detection of incipient cavitation, furthermore, the technique has demonstrated the ability to determine the BEP of a pump.     

Feature extraction and selection for defect classification of pulsed eddy current NDT

Pulsed eddy current (PEC) is a new emerging nondestructive testing (NDT) technique using a broadband pulse excitation with rich frequency information and has wide application potentials. This technique mainly uses feature points and response signal shapes for defect detection and characterization, including peak point, frequency analysis, and statistical methods such as principal component analysis (PCA). This paper introduces the application of Hilbert transform to extract a new descending feature point and use the point as a cutoff point of sampling data for detection and feature estimation. The response signal is then divided by the conventional rising, peak, and the new descending points. Some shape features of the rising part and descending part are extracted. The characters of shape features are also discussed and compared. Various feature selection and integrations are proposed for defect classification. Experimental studies, including blind tests, show the validation of the new features and combination of selected features in defect classification. The robustness of the features and further work are also discussed.     

谱分析型多频涡流检测的激励信号设计与优化

基于虚拟仪器技术,设计了一种用于谱分析的多频涡流检测系统,重点对其激励信号的设计和优化问题进行了研究。激励信号通过软件方式进行合成,包括同步加法合成激励和调制频率合成激励。在同步加法合成激励方式中,对各频率分量的初始相位进行优化,降低峰值因数。在调制频率合成激励方式中,选择Tukey窗函数,可有效抑制检测信号频谱中的纹波。     

Metal defects sizing and detection under thick coating using microwave NDT

An experimental study to evaluate shapes and sizes of defect under thick coating by microwaves NDT is demonstrated. Specially fabricated thick fire protect coated steel panels with embedded defects are inspected using an X-band (8.2–12.4 GHz) open-ended rectangular waveguide. The fundamental idea behind using this probe is presented along with several experimental results to validate this method for defect detection under coating. The reflected signal related to the phase and magnitude of the reflection coefficient at the waveguide aperture is used to create images of these coated samples under test. These images indicate the ability of microwaves for identifying and sizing defects under thick coating layer. Linear sweep technique is used here to obtain multiple frequency spectrum variances. Principle Component Analysis (PCA) algorithms have been employed to enhance the resolution of our proposed method. A series of performance comparison with PCA algorithms are also provided to extract the defect features from thick coating layer influence. To evaluate the proposed technique, steel with known defect and five coated steel plates with unknown defect under different coating thickness are measured. Results indicate that the defect detection capability has been enhanced with the suitable use of signal processing methods.     

Determination of the modulation frequency for thermographic non-destructive testing

We propose polynomial solutions of the inverse heat conduction problem to design thermographic non-destructive tests for detecting defects in composite and multi-layer materials. Inverse heat conduction in a multi-layer material slab with periodic temperature excitation is considered, and analytical quadrupole representation is used to derive a lumped parameters formulation. Predictions of the proposed polynomial representations are experimentally validated by detecting machined defects on thermally excited panels. For modulation frequencies outside the predicted detection range, defects appear on thermal images as blurry and unstructured; conversely, for modulation frequencies within the predicted range, defects are correctly represented on thermal images.     

提高螺旋焊管超声波检测准确率的方法探讨

针对压力管道元件（螺旋焊管）制造单位存在超声波检测时缺陷检出率较低,导致生产的螺旋焊管质量不满足制造标准要求但却出厂的问题,通过对探头K值的研究,确定进行缺陷准确定位的探头选取原则以及对发现的缺陷进行定位的方法,为产品检验及监检人员审查螺旋焊管超声波检测报告的准确性和真实性提供了依据,减少了不合格产品出厂的几率。     

Improvement of ultrasonic testing of concrete by combining signal conditioning methods, scanning laser vibrometer and space averaging techniques

This paper deals with the combination of several techniques to improve the ultrasonic pulse echo testing of concrete elements when a laser doppler interferometer is used as the ultrasonic receiver. This techniques involves specially designed ultrasonic probes, a pulse compression technique, random speckle modulation and space time signal processing methods. The pulse echo technique is carried out by sending frequency modulated chirp signals and performing a cross-correlation between the received and the transmitted signals. In combination with the application of recently available ultrasonic concrete probes as transmitter, this leads to an improvement in the signal-to-noise ratio. A laser doppler interferometer, equipped with a random speckle modulator, is used as detector of the ultrasound. Finally, the data sets are processed with various methods, involving time signals of several space points. Examples are the space averaging and the synthetic aperture focusing technique (SAFT). The advantage of the suggested technique is demonstrated by practical measurements on a test specimen. The improvement in results as compared to standard laser interferometric measurements will increase the feasibility of laser interferometric detection for non-destructive testing in civil engineering.     

铸铝合金构件R角涡流检测试验研究

针对飞机机体结构R角区域检测难问题,设计研发了铸铝合金R角涡流检测探头,结合R角实际检测工况,提出采用硅钢作为涡流探头的屏蔽层,可有效屏蔽金属对检测信号的影响;进行了检测工艺参数优化试验,确定了最佳的检测频率,以实现对飞机机体结构R角区域裂纹的快速可靠检测.试验结果表明:在激励频率为400kHz时,检测信号幅值达到最大...     

New methods of signal excitation and processing in magneto-inductive testing of structures and alloys

This paper describes some results of applying noise excitation to eddy current testing. Pseudo random binary sequences (PRBS) were used as a noise equivalent. The advantage of this technique compared with conventional methods is mainly the simplicity of the equipment used. It is possible to produce a device using noise excitation on the basis of a microcalculator. Also, this technique gives a direct comparison between the reference object and the object under test as in the well known CS (controlled signals) method. The performance of the PRBS method is compared with a multifrequency eddy current method by solving the problem of separating various grades of steel. Both methods show nearly the same results.     

激励参数对脉冲涡流缺陷检测的仿真分析

脉冲涡流检测技术是一种新兴的电磁无损检测技术,激励参数与脉冲涡流检测灵敏度、涡流渗透深度密切相关,选择合适的激励参数十分必要。采用COMSOL有限元仿真软件建立了脉冲涡流圆柱型探头的有限元模型,分析了不同幅值、不同占空比以及幅值与时间的乘积相同的情况下,对金属部件表面下缺陷检测的影响。通过有限元仿真分析,提出增大激励幅值可以提高检测的灵敏度,增大占空比可以提高涡流的渗透深度的结论。该结论为工程实践金属内部缺陷检测提供了一定的参考价值。     

Decay mechanisms of photoexcitations in conducting polymers probed by picosecond photoinduced absorption

The dynamics of picosecond photoinduced absorption (PA) in a host of conducting polymers with varying degrees of ground-state degeneracy are compared for low-density excitation ( 10¹⁸ carriers/cm³). The results are consistent with dispersive recombination associated with the 1-D diffusion of the photoexcited species on chain segments distributed in length in the polymers polythiophene, trans-polyacetylene, and in polydiacetylene at high pressure. We find that, within the 5 ps time resolution of the experiment, the transient electronic response is approximately independent of probe wavelength in the range 1.2 to 2.1 eV for all polymers except polydiethynylsilane. In that material recombination occurs within 1.4 ps after photoexcitation at all energies except those between 1.8 and 2.0 eV, coincident with a narrow band in the PA spectrum that is linked to thermal modulation of the absorption edge.     

基于频谱分析的涡流探头工作频率确定方法及检测系统研究

涡流检测对某一被测量的检测灵敏度很大程度上依赖于试验频率。文章提出了一种基于频谱分析的涡流探头最佳试验频率获取方法,设计了数字化涡流阻抗检测系统。在该系统中,激励信号具备扫频输出和单频输出功能,探头检测信号拾取后数字化分解为实部和虚部。借助该系统,可方便、快捷、实时地获取每一探头的阻抗频谱,根据实际需要分析获得最佳试验频率。     

Defect identification in GRID-LOCK joints

Damage detection in aircraft structures is conducted using a variety of nondestructive evaluation (NDE) techniques, including visual inspection, ultrasonic inspection, eddy current inspection, radiography and optical methods. These techniques are well established and have distinct advantages and limitations. Optical NDE methods show significant promise for variety of aerospace structural components, including inspection of bonded metallic GRID-LOCK^® structures. In this paper, a full-field surface slope measurement technique (shearography) is utilized to confirm the effectiveness of optical NDE. Two bond defects (one disbond and one weak bond) are incorporated into a GRID-LOCK^® test structure and internal pressurization results in qualitative indications of damage on the shearograms. Because accurate characterization of structural defects is critical for flight safety, a quantitative nondestructive evaluation (QNDE) method using a scanning optical probe is also explored. This QNDE method involves use of radial basis function networks (RBFNs) trained and validated using finite element analysis nodal displacements.