铁磁性管道腐蚀远场涡流检测性能的改进

远场涡流检测由于信号微弱、探头过长等缺点限制了其进一步应用.本文结合有限元仿真和实验方法,研究了改进远场涡流技术性能的几种方法,即: 采用电磁屏蔽、磁路进行传感器优化设计以缩短探头长度、增大信号强度;改进信号调理电路以提高细微缺陷检出能力;利用同态滤波、小波变换以及误差修正等数字信号处理技术确保检测结果的可靠性.实验结果表明,采用上述方法后系统性能得到很大提高,可以有效检测出管道外壁的细微缺陷.     

Transient Eddy Current NDE System Based on Fluxgate Sensor for the Detection of Defects in Multilayered Conducting Material

This paper describes a novel transient eddy current non destructive evaluation (NDE) system for the detection of defects in a multilayered conducting material by using fluxgate magnetometer as a sensor. In conventional eddy current NDE, the depth of defect detection is restricted due to the excitation frequency and its associated skin depth. Similarly, in conventional pulsed eddy current testing the time derivative of the secondary magnetic field, which decays much faster than the magnetic field itself, is measured by the induction coil. However, in this work we use fluxgate magnetometer which measures magnetic field directly and double “D” differential excitation coil in order to enhance the depth of investigation. In addition to this, the other instruments such as transmitter, transmitter controller and data acquisition system used for this work are the same one used for TEM based geophysical applications. The system has been used for the detection of an artificially engineered defect in an aluminum plate at a depth of 2 mm as well as 20 mm below the surface.     

Metallurgical Characterization of Bainitic Steel by Eddy Current Method

Journal of Failure Analysis and Prevention - Bainite is a decomposition product of austenite consisting of an aggregate of ferrite and carbides. In general, it forms at temperatures lower than...     

Numerical Analysis for Eddy Current Characteristics of Magnetically Saturated Ferromagnetic Tubes

Abstract

Magnetic saturation is applied to ferromagnetic tubes inspected by the encircling or inner coil because suppressing magnetic noise is important for the eddy current testing technique. Eddy current signal characteristics in magnetically saturated tubes are different from those in nonmagnetic tubes. In ferromagnetic tubes, defect signal phase angle is not useful for estimating defect depth because it does not depend on the defect depth. In this paper, numerical eddy current analysis has been done in order to explain the relationship between the defect depth and the phase angle in magnetically saturated tubes. This analysis is performed as follows: 1) The magnetic permeability near the defect is calculated as the non-linear magnetostatic problem. 2) The eddy current distribution is calculated as the linear magnetodynamic problem using the incremental permeability value calculated in step 1. The numerical analysis results reveal that the permeability around the defect remains inhomogeneous and it causes the unique eddy current characteristics. Based on these calculated results, a quantitative evaluation method of determining defect depth is proposed. After determining the defect shape by using signal characteristics obtained from the strongly magnetizing state, the defect depth can be estimated by using the signal amplitude.     

Finite-Element Calculations of Remote Field Eddy Current Interactions with Slit Defects

Abstract

Remote field eddy current (RFEC) excitation is a promising approach for detection of the very fine axial cracks typical of stress corrosion cracking (SCC) in pipelines. Interactions between adjacent cracks or slits can enhance responses in some cases. Detailed finite-element modeling was undertaken to establish the behavior and interactions of multiple slits such as those occurring in SCC. Three different field/slit configurations are considered, with anomalous source models used to aid interpretation of the results. The study noted that magnetic perturbations generated by ferromagnetic material tend to be vanishingly small, and that the interactions between multiple cracks give minimal enhancement, indicating that eddy current rather than magnetic field excitation is best for the detection of SCC. With eddy current excitation, field perturbations are generated by even very fine slits, and are larger in non-ferromagnetic material. For non-ferromagnetic pipes, the perturbations tend to merge as a circumferential separation between parallel axial cracks decreases, resulting in significant interaction and signal enhancement.     

A Novel Inclined Excitation Method for Crack Detection of Non-Ferromagnetic Materials Using Eddy Current Thermography

Strength of Materials - A novel inclined excitation method for eddy current thermography was proposed to eliminate the “fuzzy effect” together with the wavelet singularity-based image...     

Cinematography of Combustion in Diesel Engines

Methods used to film the injection and combustion processes in some forms of diesel combustion chamber are described. Rates of up to 15,000 frames/sec. were used with suitable lighting to show the distribution of injected fuel prior to self-ignition. Pressure, injector needle motion, crank angle and time were recorded in addition to the visual picture. Two synchronised cameras were used with semi-automatic control gear to get a correct sequence of operations. An improved accelerator was devised for the Fastax camera used for the visual pictures. The field of this camera was split into two halves with a supplementary lens in one path to cover two areas at different distances from the camera. Perspex (polymethyl methacrylate) was used for combustion chamber windows in spite of the high temperatures and pressures. Lighting, processing methods and choice of sensitive materials are also dealt with. In recording data on 70 mm. paper, an artifice was used to increase temporal resolution.     

Eddy Current Detection of Retained Austenite in Ni-Hard4 Cast Iron

In this article, the nondestructive eddy current (EC) method was used for microstructural evaluation of Ni-hard4 cast iron (NiHCI). Different destabilizing heat treatments were used to produce different microstructures and hardness. The microstructural analysis and hardness measurement were done for conventional characterization of the reference blocks. Then, the EC techniques were applied to evaluate the hardness and microstructural changes by the detection of EC parameters at different frequencies. Metallurgical investigations showed that increasing the destabilizing soaking time and temperature decreases the amount of retained austenite in NiHCIs. The nondestructive evaluation method (NDT) revealed that it can be good correlated between retained austenite amount and hardness with EC parameters. This shows that the EC technique can detect NiHCIs products according to the metallurgical characterization with good accuracy.     

321不锈钢疲劳早期损伤的涡流评估

采用涡流阵列系统对321不锈钢疲劳早期损伤进行了检测与评估。整个疲劳过程中涡流幅值的变化分为快速增长、稳定和加速增长三个阶段。第一阶段的涡流幅值与循环周次对数之间有近似线性关系。在裂纹萌生寿命内,疲劳损伤区的大小与循环周次无关。对不同循环周次下321不锈钢试样的微结构分析表明疲劳早期涡流幅值变化的主要来源是材料内部位错的增殖和运动,而不是马氏体相转变。     

柴油机工作不均匀性的振动检测方法

在某型号装备的柴油机上,采用断缸的方法模拟各缸工作不均匀试验,测量柴油机在不同工作状态下一个工作循环内的机体振动信号。利用局域波分解方法对柴油机振动信号进行时频分析,获得Hilbert时频谱。对柴油机在不同工作状态下振动信号的时频谱进行分析,对各缸段的振动信号频率变化进行统计分析,计算频率偏差,并分析其在不同工作状态下的变化趋势。分析结果表明,利用各缸段振动信号频率变化的偏差可以对柴油机工作不均匀性进行判别。     

Optimized Eddy Current Detection of Small Cracks in Steam Generator Tubing

A complete, computer based design methodology is described, aiming to develop an eddy current sensor with increased sensitivity to flaws, and reduced sensitivity to probe lift-off. The first part of the paper contains an analysis performed in order to establish detailed criteria for an effective design. Numerical investigations have been carried out and their results are discussed, regarding various problems of detectability and lift-off noise level. Based on these results, in the second part two probe arrangements are proposed, and it is shown how their performance parameters could be further improved.     

二冲程柴油机瞬时转速法诊断机理研究

为了探究瞬时转速法的诊断机理,以5S50ME型二冲程柴油机为研究对象,测得柴油机在正常状态和故障状态时不同工况下的瞬时转速信号,得到不同工况下瞬时转速的变化规律,在此基础上,通过分析各工况下的瞬时转速波形图和频谱图,提出谐波幅值比和波动率峰值两种能有效反应柴油机故障状态的特征参数。以5缸机为研究对象,若1次谐波分量和5次谐波分量的谐波幅值比大于某一阈值,则说明柴油机存在单缸故障,并且故障缸为波动率峰值最小的缸。     

The Influence of the Heating Rate and Thermal Energy on Crack Detection by Eddy Current Thermography

The capability of eddy current thermography in detecting cracks is investigated numerically and experimentally in relation to the crack orientation, the heating rate and the excitation period. The numerical investigation shows that, for cracks parallel to the heat flow the detection region increases with the increase of the heating rate, while for cracks perpendicular to the heat flow, it increases with the increase of the excitation period. The experimental results confirm that the detection of cracks parallel or perpendicular to the heat flow is improved by increasing the heating rate or the excitation period, respectively. The optimum time period for the detection of a crack depends on the crack orientation: For cracks parallel to the heat flow (i.e. perpendicular to the current flow), the best results are obtained at the beginning of the heating period. For cracks perpendicular to the heat flow, the optimum detection period is delayed with the distance of the crack from the heated area. If the crack is very close to the edge of the plate, both the detection period and the sharpness of the crack are reduced. The experimental results are compared to data obtained by identical experiments, where the use of a lower performance camera was combined with data processing techniques. The comparison indicates that a higher performance camera is more effective and may compensate for the improvements achieved in the detection of a crack through data processing techniques.     

Optimal Convergence of the Fixed-Point Method for Nonlinear Eddy Current Problems

A method is presented for the optimal choice of the fixed-point reluctivity in nonlinear eddy current problems solved for steady-state in the frequency and the time domain. The optimal selection is shown to ensure a minimal contraction number for the fixed-point method. The proposed technique is compared to earlier approaches using a 2-D example.      

Detection of the Subsurface Cracks in a Stainless Steel Plate Using Pulsed Eddy Current

The nondestructive method to detect subsurface defects is limited because conventional eddy current are concentrated near to the surfaces adjacent to the excitation coil. The PEC technique enables detection of cracks buried deeper under the surface with relatively small current density. In the present study, an attempt has been made to investigate detection of subsurface cracks using a specially designed double-D differential probe. The tested sample is a SS304 with a thickness of 5 mm; small EDM notches were machined in the test sample at different depths from the surface to simulate the sub surface cracks in a pipe. The designed PEC probe has two excitation coils and two detecting Hall-sensors. The difference between two sensors is the resultant PEC signal. The cracks under the surface were detected using peak amplitude of the detected pulse; in addition, for a clear understanding of the crack depth, the Fourier transform is applied. In time domain, the peak amplitude of the detected pulse is decreased, and in the frequency domain, the magnitude of the lower frequency component has been increased with an increase in the crack depth. The experimental results have indicated that the proposed differential probe has the potential to detect the sub surface cracks in a stainless steel structure.     

Enhancing Probability of Detection and Analysis of Bolt Hole Eddy Current

Fatigue cracks are prone to develop around fasteners found in multi-layer aluminum structures on ageing aircraft such as the CC-130 Hercules and CP-140 Aurora (P-3 Orion). Probability of Detection (POD) studies using eddy current techniques within the bolt holes contribute to risk assessments used in evaluating the serviceability of these aircraft. Improving POD by optimizing the inspection system can reduce the required frequency of inspections, since assurance of detection of smaller crack sizes extends the interval for which growth of cracks to a critical size may occur. In this work signal analysis and POD of laboratory grown fatigue cracks in the corners of bolt holes of 7075-T6 aluminum is examined. A number of parameters that enhance crack detection are identified, including the use of intimate contact probes versus steel sheath (non-contact), higher frequencies and the use of C-Scan display. Results demonstrate better detectability at 1600 kHz, than at the normally used 400 kHz, and enhanced recognition and assurance of identification of peak crack signal for data recorded and displayed in a C-Scan format. Results are compared with a previous POD study, which used current field techniques for detection.     

电涡流式振动位移传感器检定技术的研究

文章简述电涡流式振动位移传感器的工作原理,并根据《JJG644—2003振动位移传感器检定规程》,从理论和实践上对电涡流式振动位移传感器静态特性、动态特性的检定方法展开研究,以提高电涡流式振动位移传感器测量的准确性和量值的统一性。     

内燃机结构冲压件的低噪声设计研究

为了降低发动机结构冲压件的辐射噪声,运用形貌优化的方法,提高结构刚度来达到降噪的目的。以某发动机油底壳为研究对象,建立了薄壁结构的有限元实体模型。以某阶固有频率的最大化为设计目标,寻找最佳的加强肋分布,以及最小肋宽和起肋角,以提高结构刚度,减小其表面辐射噪声。同时,在形貌优化结果的基础上,结合冲压结构加工工艺性要求提出了改进方案。     

飞机结构腐蚀检测中的脉冲涡流无损检测技术

对脉冲涡流无损检测技术的工作原理进行了分析，建立了脉冲涡流的检测系统对加工的模拟飞机多层结构的试件进行了测试，提取其时域瞬态感应信号的峰值和过零时间作为特征量，对出现在飞机多层结构第二层中的腐蚀缺陷进行了定量检测，试验结果和理论分析相一致，证明了脉冲涡流检测方法的有效性，由于其具有快速和定量化的优点，因而在飞机结构的腐蚀检测中具有广阔的应用前景．