Quantitative Fractography for Improved Probability of Detection (POD) Analysis of Bolt Hole Eddy Current

Fatigue cracks are prone to develop around the fasteners found in multilayer aluminum structures such as ageing aircraft. The determination of their probability of detection (POD) using eddy current techniques is critical for risk assessments used in evaluating serviceability. Normally, a large number of samples of both fatigue cracks and electric discharge machined notches are required for such a study. In this study, the use of quantitative fractography (QF) to efficiently gather POD data for bolt hole eddy current is demonstrated. The technique can be readily automated, significantly reducing man-hour requirements. The method is applied to mid-bore cracks, grown without starter notches, in order to simulate naturally occurring fatigue cracks. POD of the mid-bore cracks is examined using a probit analysis. An a _90/95 is the probability that 90% of the cracks of size a or larger will be detected 95% of the time. An a _90/95 of 0.34 mm was obtained for mid-bore crack depth. This value is consistent with skin depth considerations at the 1.6 MHz inspection frequency. An examination of the POD based on crack length found that it was related to that of depth according to the average length-to-depth aspect ratio of 2 to 1.     

Time-Frequency Analysis of Pulsed Eddy Current Signals

Detection and measurement of the material loss due to corrosion in aircraft fuselage lap splices are of great importance to life management of ageing aircraft. Pulsed eddy current (PEC) techniques have shown the potential to fully characterise these structures. However, variations in the probe lift-off, the interlayer gaps and material thinning produce similar PEC signals, making signal interpretation very difficult. This paper describes time-frequency analysis algorithms applied to pulsed-eddy current signals from lap-joint specimens containing simulated flaws. The effect of probe lift-off, interlayer gap, and material thinning on PEC signals is examined in three-dimensional space (amplitude-time-frequency). This study shows that the time-frequency analysis of PEC signals provides specific visual patterns that can be related to the interlayer gap, lift-off, and material loss.     

矩形脉冲涡流传感器参数变化对涡流分布影响的仿真分析

传统的圆柱形传感器在涡流检测中穿透深度较浅,同时检测灵敏度较低.本文采用矩形传感器进行了检测.在分析矩形涡流传感器榆测原理的基础上,采用大型电磁仿真软件ANsYS建立了脉冲涡流榆测模型,通过改变矩形传感器的尺寸参数和时间常数,研究了参数变化对铝板表面涡流分布以及铝板内涡流衰减的影响,仿真结果表明,铝板表丽均匀涡流区域的...     

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.     

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.     

轮辐螺栓孔挤孔模设计

对汽车轮辐上螺栓孔的挤压成形工艺进行了分析,并以微型车12×4.00B轮辐上的螺母座孔为例,介绍了挤压时预制孔径的确定办法、挤压力的计算及设备选择,是对有径向挤压的混合挤压的工艺计算的有益尝试.也对模具选材、结构及设计中应注意的主要问题作了详细介绍,对同类或相似零件的成形模具设计有较大的参考价值.     

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.     

Autocorrelation Central Hole in Clipped Photon-counting Detection Estimation from the Zero-count Probability Density

Abstract

In this paper we develop a procedure to avoid the autocorrelation central hole that appears in clipped photon-counting detection. This method can be understood as a higher order approximation than the Hofmann procedure. It can be applied in combination with any of the alternative methods to evaluate the autocorrelation function in clipped conditions proposed up until the present. We especially analyse that based on the zero-count probability measurement. Fairly good autocorrelation estimates are obtained for low intensity signals.     

基于最小二乘法的多频涡流检测信号参数分析

多频涡流检测技术是一种广泛应用的电磁无损检测技术.在检测过程中,微弱多频涡流信号的分析是检测系统的关键.本文提出了一种基于最小二乘法的涡流检测微弱信号参数分析方法.依据最小二乘法的求解原理,以双频涡流信号分析为例,建立了涡流信号参数分析数学模型,估计出各频率成分的时域特征参数,为涡流检测信号应用提供了依据,并对该算法进行了仿真和检测实验验证.实验结果表明:该方法效果良好,硬件成本低,具有较高的灵敏度,满足多频涡流检测信号参数分析的需要.     

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．     

Statistical Analysis of Eddy Current Data from Fastener Site Inspections

Recent work on reliably detecting and characterizing cracks in multi-layer airframe structures has used modeling and simulation to extract features from raw eddy current data, and to assist in the evaluation of probability of detection (POD). This paper focuses on the statistical analysis of the data from these studies. Hit/miss, linear, and physics-inspired methods are employed to evaluate POD. The Box-Cox transformation is used as a remedy for violations of the constant variance assumption. In addition, a bootstrapping method is introduced for confidence bound calculation on a 2nd order linear model. The objective of this work is to provide on insight how different models and assumptions impact POD evaluation.     

基于电磁差动涡流传感器和FPGA的金属板探伤研究

为了研究电磁涡流无损检测技术在金属板损伤探测上的应用,根据电涡流检测原理,设计了基于电磁差动传感器和FPGA的检测系统,实现对涡流传感器的正弦激励和阻抗分析.对不同长度、深度和材质的缺陷金属板进行了实验探测.通过对采集信号的相敏解调发现了阻抗变化的特征值与金属缺陷特征的对应关系.理论分析和实验测试表明该系统能较准确地检测出金属表面及内部的损伤.     

Nonlinear Eddy Current Analysis by BEM Utilizing Adaptive Equation Technique

Eddy currents induced in steel are analyzed by the boundary element method. As the periodic electromagnetic quantities in the steel are distorted due to the nonlinear magnetization property, they are expressed by a Fourier series, whose fundamental and harmonic waves are determined by solving the corresponding integral equations with the surface magnetic fields given as the boundary values. In this paper, we introduce a new combined integration scheme that enables efficient, volume-mesh-less treatment of the practical problems typically characterized by the geometrical singularities like edges and corners. Using this procedure, the internal and surface electromagnetic fields are obtained numerically one after another by applying an adaptive equation technique to obtain the boundary values.      

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.     

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.     

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.     

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...     

Pulsed Eddy Current Data Analysis for the Characterization of the Second-Layer Discontinuities

Acoustic emission (AE) was applied for detection of microcrack initiation in carbon fiber reinforced polymer composites subjected to shear stresses. Experimental materials were prepared from polyester bonded unidirectional (1D) non-crimp fabric and 2D plain-weave carbon fiber fabrics, using the resin transfer moulding technology. Control of epoxy resin/carbon textile proportions enabled variation of fiber volume content from small (34/35% for 2D/1D), through medium (51%) to high (68%). Rectangular samples (\(45 \times 4 \times 2\)  mm) were cut from 1D plates along [0] and across [90] fibers. Similar size samples from 2D plates were cut along warp/weft axes as well as in two orthogonal bias directions. Selected side surfaces were polished for microscopic (SEM) observations. Short-beam-strength tests were performed in 3-point bending (l/h\(=\)4), with two AE sensors attached for damage monitoring, which allowed to interrupt loading sequence before final failure. The acoustic emission historic index was the most effective AE parameter in damage initiation control. Microcracks developing on polished composite side-surfaces were observed under the SEM and direct microscopic evidence confirmed fiber debonding to be the principal mechanism of crack initiation in these materials and testing conditions before any further damage.     

Finite-Element Analysis for Remote Field Eddy Current Responses from Near- and Farside Cracks

Results of three-dimensional finite-element analysis investigations of remote field eddy current (RFEC) signal responses from internal and external stress corrosion cracking (SCC) in steel pipes are presented. The fine shallow SCC cracks were simulated by double axial slits located on the near- and farside pipe surfaces under simulated RFEC excitation. Although it is normally a characteristic of RFEC testing that responses to interior or exterior defects are approximately equal, there is a considerable difference between the farside and nearside responses from very fine defects such as SCC cracks.