【作者】 张玉华；

【导师】 罗飞路；

【作者基本信息】 国防科学技术大学 ， 仪器科学与技术， 2010， 博士

【摘要】 结合某国防预研项目,以飞机机身结构和发动机特殊部件涡流检测的应用研究为背景,论文深入研究了常规/脉冲涡流检测技术在涡流场-电路耦合问题上的理论建模、提离等主要检测干扰源的抑制方法及特殊待测件的探头设计等方面的问题,其主要内容及创新点如下：以电磁场和电路理论为基础,导出了涡流线圈外接电压源和接入电桥电路两.种典型情况下三维涡流场-电路耦合问题的理论模型。针对复杂边界条件下三维涡流场-电路耦合问题,应用A-φ,A法描述涡流场定解问题必须满足的位函数方程和边界条件式,并结合外电路约束下涡流线圈所满足的电路方程,建立了基于线圈磁链的场-路直接耦合的数学模型。考虑采用数值法求解计算,用伽辽金加权余量法导出了涡流线圈接电压源和接入电桥电路两种典型情况下三维涡流场-路耦合问题的有限元方程组。给出了有限元仿真的基本步骤和实现方法,对其中一些关键性问题如涡流场开域问题的处理、离散网格的生成、网格密度的控制、节点自由度设置和边界条件设定等给出了解决方法。研究小曲率半径弧面涡流检测中探头提离和倾斜效应,提出了利用相位旋转和信号增强相结合抑制干扰的方法。建立三类检测表面-凹面、平面和凸面所对应的三维涡流场-路耦合分析的有限元模型,计算分析了表面形状、曲率半径变化对导体内涡流分布及线圈阻抗的影响,结果表明试件形状和曲率半径大小仅改变线圈阻抗变化的值,并不影响其变化规律。进一步着重研究了三类试件上方线圈提离变化所形成的阻抗轨迹特征及其对缺陷检测的影响,其结果表明提离轨迹可近似为一条直线,随检测频率增大,缺陷信号绕提离轨迹顺时针旋转,存在一个频率点使两者之间的夹角为90°,并从物理层面解释了上述变化的本质。研究了线圈倾斜时与导体之间电磁耦合作用的变化特征,对比分析了线圈倾斜度和提离增大时的阻抗轨迹,结果表明当线圈倾斜角小于某一特定角度时(约69°),倾斜轨迹也可近似为直线,且它几乎与提离轨迹重合,这一现象说明抑制提离干扰的同时,也可抑制极大部分倾斜干扰。基于上述结论,提出了用相位旋转和信号增强相结合抑制提离和倾斜干扰源的方法,并分析了当线圈接入检测桥路后,桥路连接方式及参数变化对上述阻抗信号特征及抑制方法产生的影响。通过实验数据验证了数值结果及理论分析的正确性,表明所提出的方法确实可行。针对盘孔裂纹检测问题,提出和设计了两种结构的涡流探头-单线圈绝对式探头(接入电桥电路)和三线圈差分式探头,并与一种双线圈正交式探头作了对比分析。根据盘孔几何结构和裂纹走向,选择探头插入式的检测方式,提出了两种不同结构形式的探头,并与另一种探头对比分析。针对三种探头对盘孔裂纹的检测,建立对应的三维涡流场-路耦合分析有限元模型,首先研究了探头中激励线圈的尺寸选取对检测灵敏度的改善,结果表明当激励线圈高度高于盘厚3.0mm时,探头具有最佳的检测灵敏度。然后根据三种探头自身的独特性,着重分析了其特定参数变化对检测灵敏度和裂纹信号的影响,结果表明绝对式探头内、外径增大均能提高检测灵敏度,但内径增大同时也会减小桥路输出信号的绝对变化量；差分式探头中激励线圈和检测线圈匝数比小于1有利于提高检测灵敏度和增强裂纹信号；减小正交式探头中检测线圈的长径比可增强裂纹信号,但这种探头的检测具有方向性,即它只对与检测线圈中轴线平行走向的裂纹最灵敏,对其它走向的裂纹,其检测灵敏度迅速下降,甚至为零。从检测特征及灵敏度上对比分析了三种探头的优、缺点。最后,用实际制作的涡流探头对某型飞机发动机篦齿盘标准件进行检测,其结果表明所提出的两种探头能够很好地实现盘孔周边微小裂纹的检测,其中差分式探头比绝对式探头具有更高的检测灵敏度,但同时它对两检测线圈的结构和位置对称性的要求严格；两种探头均可消除正交式探头的检测方向性问题。研究多层金属结构脉冲涡流检测中探头的提离效应,提出了利用“相位跳变点”从提离干扰中识别缺陷的方法。飞机机身多层胶(铆)接金属结构脉冲涡流检测中,探头提离变化极易对表面下缺陷信号造成干扰,使有效识别缺陷变得极为困难。为解决这一问题,建立对应的三维瞬态涡流场-路耦合分析的有限元模型,首先研究了仅有提离、仅有缺陷及提离和缺陷同时存在时三类检测信号Δξl、Δξc和Δξl+c的时域响应和时频域能量分布特征,结果表明随缺陷所处位置越深,Δξc的峰值越小,起始时间也越晚,整个信号的能量分布出现沿频率轴往下压缩,沿时间轴向右扩展的现象,从导体内瞬态感应场的渗透特性解释了上述变化的本质。提离信号Δξl的时域响应无延迟,且随探头提离增大其峰值增大；当提离变化较大时,信号能量分布沿频率轴轻微移动,主要表现在较高频部分,时间轴上则无明显变化。由此看出,Δξl和Δξc两者区别明显,但一旦缺陷信号中混杂进提离干扰,不论时域还是时频域,Δξl+c与Δξl,极其相似,无法判断是否存在缺陷。进一步地,根据信号波形的改变,着重分析了提离和缺陷所引起各频率成分的相位变化发现：提离增大导致幅值明显减小,但相位变化甚微,而缺陷则会使到达其所处位置的频率分量发生明显相变。基于这一特征,用相位信息从提离干扰中识别缺陷,并找到一种有效的分析工具-双复数小波变换,提取信号在时间-尺度平面上的相位,结果发现上述三类信号的相位曲线均存在一个非常明显的“相位跳变点”,在这一时间点小波系数的相位从“负”跳变到“正”。提离改变,其“相位跳变点”的位置几乎不变；缺陷信号则明显后移,且随缺陷位置越深,其“相位跳变点”越后；当提离和缺陷同时存在时,其跳变点介于上述两者之间。因此,提出了用“相位跳变点”从混杂了提离干扰的检测信号中识别缺陷的方法,通过实验验证了上述结论及理论分析的正确性,表明所提出的方法行之有效。更多还原

【Abstract】 Supported by National Defense Project to apply eddy current technique to inspect airframe structures and critical engine components, theoretic model of eddy current field-circuit coupled problem, method to suppress probe’s lift-off noise as a significant obstacle and the design of a novel probes for inspection of the specific specimen in Conventional/Pulsed eddy current testing are studied in this paper. The brief of these researches and the novel approaches are as follows:Based on electromagnetic field theory and electric circuit theory, mathematical model of three-dimensional eddy current field-circuit coupled problem are derived for two typical cases that coil is driven by voltage source and connected to bridge circuit. For three-dimensional eddy current-circuit coupled problem with complex boundary conditions, potential equation and boundary condition formulation of eddy current field problem are expressed by using A—φ, A method. And circuit equation of eddy current coil connected into outer circuit network is presented. Furthermore, mathematic model of field-circuit coupled directly is derived based on coil’s flux linkage. Numerical calculation is adopted here. Finite element formulation is derived by using Galerkin method as coil is driven by voltage source and connected to bridge circuit. Solution steps of finite element analysis are presented. Furthermore, some keys such as open boundary problem, mesh, element density, the degrees of freedom at a node and boundary condition are illustrated detailedly.A probe-coil’s lift-off and tilt effects for the curved specimen with small curvature radius in practical eddy current testing are analyzed. An approach using phase rotation and signal enhancement technique to eliminate lift-off and tilt noise is presented. For three typical detected surfaces-concave, plane and convexity, the relevant finite element models of three-dimensional eddy current field-circuit coupled problem have been built. The surface shape and curvature radius effects on eddy current distribution in the conductor and coil impedance have been investigated. The results show that the coil impedance magnitude is changed by the specimen’s shape and curvature radius, but its characteristic is not influenced. Furthermore, the characteristics of the impedance locus produced by coil’s lift-off variation and lift-off effect on defect detection for three specimens have been analyzed thoroughly. The results show that lift-off locus is always straight line. Defect signal is characteristic of rotating clockwise on lift-off locus as test frequency is increased. And the angle between defect signal and lift-off locus is 90 degree at a certain frequency. The physical essence of this characteristic is interpreted. Electromagnetic coupling between coil and specimen caused by coil’s tilt angle variation has been analyzed. The impedance locus produced by coil’s tilt has been compared with lift-off locus. The results show that the tilt locus is approximately straight line and close with lift-off locus as coil’s tilt angle is less than a certain angle (about 69°). The above-mentioned property shows that tilt noise is suppressed mostly with lift-off noise. On this merit, an approach using phase rotation and signal enhancement technique are presented to eliminate lift-off and tilt noise. Moreover, the mode of operation and element of the bridge effects on signal and the proposed method have been researched as coil is connected to bridge circuit. Numerical results and theoretical analysis are validated by experiments. It is showed that the proposed method to suppress the lift-off and tilt noise is feasible.For inspecting the crack breaking away from the aperture of a disk, two eddy current probes with different configuration-an absolute probe with a coil (connecting to bridge circuit) and a differential probe with three coaxial coils, are proposed and designed, and compared with another probe with two cross-axis coils. According to the geometry of disk aperture and the direction of a fatigue crack around a disk aperture, the plug-in detection mode is chosen. Moreover, two eddy current probes with different configuration are proposed and compared with another probe. For detecting crack around a disk aperture by three eddy current probes, three-dimension finite element models of eddy current field-circuit coupled problem have been built respectively. The excitation coil’s dimension effects on the test sensitivity have been analyzed. The results show that the probe has best sensitivity if excitation coil height is 3.0mm higher than disk thickness. Furthermore, according to the uniqueness of three probes, the specific parameters effects on the test sensitivity and crack signal have been investigated thoroughly. The results show that the larger an absolute probe’s inner and outer radius, the higher the probe sensitivity. But the absolute change of bridge circuit output voltage is increased inversely with coil inner radius. For a differential probe, it is advantageous to improve the sensitivity and enhance crack signal that the turn ratio of excitation coil to pickup coil is less than 1. For a cross-axis probe, defect signal can be enhanced by decreasing the ratio of pickup coil’s height to diameter. But the probe’s sensitivity has directionality. That is, its sensitivity to a crack parallel to pickup coil’s central axis is best. And its sensitivity to the cracks with another directions decreases rapidly to zero. The advantage and disadvantage over test performance and sensitivity of three probes are presented. Eddy current probes have been fabricated to inspect the standard castor gear specimen. The test results show that the absolute probe and differential probe can be applied to detect small crack breaking away from the disk aperture accurately. And the sensitivity of the differential probe is better than the absolute probe’s sensitivity, but in the differential probe, two pickup coils must be configured and located symmetrically about the excitation coil. Compared with the cross-axis probe, the proposed eddy current probes have no directionality in the test sensitivity. A probe’s lift-off effects for the multi-layered conductive structure inspected by pulsed eddy current technique are analyzed. An approach using "phase jump point" to distinguish a defect from lift-off noise is proposed. In order to suppress probe’s lift-off noise that masks signal produced by a defect beneath the surface of the first-layer conductor, the three-dimensional finite element model of transient eddy current-circuit coupled problem has been built. The time-domain reponse and signal energy distribution characteristic in the time-frequency plane of three test signals-Δξl caused by probe’s lift-off variation,Δξc produced by a defect, andΔξl+c caused by a defect together with lift-off, have been investigated. The results show that the deeper the crack location, the smaller the peak value ofΔξc, the longer the starting time ofΔξc and signal energy distribution in the time-frequency plane tends to compress downward in the frequency axis and extend rightward in the time axis. Based on penetration property of transient field into the conductor, the essence of signal features has been interpreted. The starting time of lift-off signalΔξl has no delay. And its peak value increases rapidly with probe’s lift-off. As probe’s lift-off is changed greatly, signal energy, especially the high frequency portion of the signal, tend to shift slightly in the frequency axis and remain unchanged in the time axis.Δξc is distinguished fromΔξl. ButΔξl+c are similar extremely toΔξl in time domain and the time-frequency plane. Consequently, a defect can not be identified visually from test signal with lift-off noise. Furhermore, according to the characteristic of signal shape, the phase of each frequency component in the signals caused by lift-off and defect has been analyzed. The results show that amplitude is decreased obviously and phase is unchanged almost by increasing probe’s lift-off. But phase of frequency components that penetrate into defect’s location is shifted significantly. On this merit, an approach using phase information to discriminate defect from lift-off noise is proposed. And an effective tool-dual-tree complex wavelet transform is adopted to extract signal phase in the time-scale plane. The results show that there is an important feature-"phase jump point (PJP)" in phase curves of three signals. The PJP is the time that wavelet coefficient’s phase of signal is from "negative" to "positive". The PJP ofΔξl caused by lift-off variation remains unchanged. The PJP ofΔξc produced by a defect appears later than the PJP ofΔξl. And the deeper the defect location is, the later the PJP ofΔξc is. The PJP ofΔξl+c caused by a defect together with lift-off is between the PJP ofΔξl and the PJP ofΔξc.On this merit, an approach using "phase jump point" to identify a defect from signal with lift-off noise is proposed. These conclusions and theoretical analysis are validated by experiments. It is showed that the proposed method is feasible.更多还原