飞机多层金属结构脉冲涡流检测提离效应的抑制

探头提离造成的信号畸变是脉冲涡流检测中主要干扰之一。本研究针对飞机多层铆接结构脉冲涡流检测中的提离效应进行抑制,采用一种基于提离数据库的峰值补偿方法对探头提离效应进行了一定程度的抑制。通过线性阵列探头获取提离补偿后的信号峰值,组成幅值矩阵,实现成像检测。对比探头提离的抑制效果,实验结果表明,该方法能够有效地运用于多层金属结构近表面及深层缺陷的提离检测。     

浅薄缺陷的脉冲漏磁信号特征量提取

为提高脉冲漏磁对浅薄缺陷的检测能力,提出了一种处理检测信号的二次差分方法,分离出在涡流效应的阻尼作用下产生的脉冲漏磁信号中的涡流分量,即二次差分信号。从二次差分信号中提取出了浅薄缺陷深度的新特征量——峰值时间Pt,并验证了二次差分方法的可行性。试验结果表明,和信号幅值大小相比,将Pt作为浅薄缺陷特征量,在检测浅薄缺陷时对缺陷深度的分辨率较高,同时能够克服检测探头提离所引起的缺陷特征量分辨率严重下降的问题。     

基于频谱分析的脉冲涡流检测提离消除技术

脉冲涡流检测中由于探头倾斜等原因容易产生提离效应,严重影响缺陷的定量精度。利用激励脉冲频率成分丰富的特点,提出了一种基于频谱分析的提离效应消除新方法。通过试验发现,经该方法处理后的有提离的原始检测信号在低频部分已基本重合,消除了提离效应对低频的影响,且可通过提取低频特征值对缺陷的深度进行定量。试验结果证明,该方法可以有效地消除提离效应,提高缺陷的定量检测精度。     

基于脉冲涡流差分信号提离交汇点的亚表面层材质劣化检测技术理论研究

多层金属结构广泛应用于石油、化工、航天和能源等工业领域的大型、复杂装备中。由于服役工况一般较为恶劣,该结构亚表面层材质性能会出现劣化,直接威胁到大型、复杂装备的安全运转。通过脉冲涡流检测理论仿真,发现了一种基于脉冲涡流差分信号的新型提离交汇点。通过对该交汇点的特性研究,提出了一种能够对多层结构亚表面层材质劣化进行检测和评估的方法。由于当前的脉冲涡流检测技术主要针对金属结构表面层材质劣化进行检测,所以该方法的提出拓展了脉冲涡流检测技术的应用领域。通过数值仿真,所提方法的正确性及实用性得到了验证。     

基于脉冲涡流的复合金属层材料鉴别

针对石油双层复合金属管的非破坏性原位鉴别问题,提出了一种基于脉冲涡流的复合金属层材料鉴别方法.该方法充分利用脉冲涡流频谱范围广、检测深度大的优势,采用脉冲涡流检测方法避免了包覆金属层的屏蔽效应,实现了对待测金属层的检测与识别.对不锈钢包覆的铜、铁、铝等3种模拟试样开展脉冲涡流检测试验,提取了信号主峰幅值、主峰面积、过零...     

发电机风扇叶片的柔性阵列涡流检测

以火电机组发电机风扇叶片为对象进行阵列涡流试验,通过研究检测频率、提离效应、扫查方向等变量对检测信号的影响,寻找合适的检测工艺,以替代传统的渗透检测。试验结果表明:火电机组发电机风扇叶片有效检测频率为300kHz^1 400kHz,与计算值吻合;风扇叶片上提离效应导致的信号降低可以通过增加增益补偿来解决;扫查方向对缺陷信号强度基本无影响。最后对发电机风扇叶片的阵列涡流现场检测应用提出了建议。     

奥氏体锅炉管氧化皮堆积厚度脉冲涡流检测

针对奥氏体锅炉管内氧化皮堆积带来的机组安全隐患问题,应用脉冲涡流技术对氧化皮堆积厚度进行定量检测。设计并制作了开合式传感器,该传感器具有结构简单、使用方便、通用性较强的优点。构建了脉冲涡流检测试验系统,并将其用于TP347H管样内氧化皮堆积厚度的检测,试验研究了传感器信号特征、提离影响及阵列接收效果。结果表明:检测信号峰值与氧化皮堆积厚度存在线性关系,可用于堆积厚度的准确定量;信号峰值随传感器提离增加而线性减小;以阵列接收信号差分峰值为特征量,可消除服役锅炉管电磁属性转变带来的影响。研究成果可为奥氏体锅炉管氧化皮堆积厚度检测提供应用参考价值。     

基于涡流效应的管道内壁缺陷检测与识别

在管道内检测中为了有效识别管道内壁缺陷,提出了基于涡流效应的检测方法,实现管道内壁缺陷的检测与识别。分析了缺陷大小及探头提离值对检测结果的影响,采用电桥式涡流检测方法对实验钢板进行检测的结果表明,系统最小能分辨孔径为2mm的缺陷;在涡流探头提离值为4mm时依然能有效检测到缺陷信号。     

涡流检测提离效应的自动补偿

提离(Lift-off)效应是便携式涡流探伤仪探头在工件表面进行扫查时,由探头和工件的间距变化所产生的信号,在工件表面凹凸不平、粗糙和操作不当时都会发生。提高效应是涡流探伤时的一种干扰信号,会对缺陷的判断、定量定性带来影响。为了抑制提离效应,当前国产仪器普遍采用人工调节方法,即调节电路中的可调元件,反复选择最佳状态,这种操作较为复杂,不易平衡,调节的精度较低。为此,我们设想采用微型计算机对提高效应进行自动补偿,以自动提高补偿来代替手动提离补偿,经过反复的试验和仪器研制,成功地实现了目动平衡的构想,操作简便、平衡速度快、调节精度较高,取得了令人满意的效果。     

脉冲涡流检测技术的进展

脉冲涡流检测技术主要用于检测亚表面及多层金属结构缺陷。脉冲激励与金属结构缺陷之间发生相互作用,在探头中引起的瞬态响应信号包含大量的缺陷信息,使之具有快速定量检测缺陷的潜力,但也增加了对响应信号解释的难度。评述了脉冲涡流瞬态响应的计算、提离噪声抑制等方面的国内外研究进展,分析了脉冲涡流检测技术的发展方向。     

Reduction of lift-off effects for pulsed eddy current NDT

The lift-off effect is commonly known to be one of the main obstacles for effective eddy current NDT testing as it can easily mask defect signals. Pulsed eddy current techniques, which are believed to be potentially rich of information, are also sensitive to the effect. An approach using normalisation and two reference signals to reduce the lift-off problem with pulsed eddy current techniques is proposed. Experimental testing on the proposed technique and results are presented in this report. Results show that significant reduction in the effect has been achieved mainly in metal loss and sub-surface slot inspection. The technique can also be applied for measurement of metal thickness beneath non-conductive coatings, microstructure, strain/stress measurement, where the output is sensitive to the lift-off effect.     

主成分分析在飞机多层结构层间腐蚀脉冲涡流检测中的应用

在飞机多层铆接结构层间腐蚀缺陷的脉冲涡流检测中,需要识别提离效应造成的干扰信号和缺陷信号,同时也需要判断缺陷深度。制作了模拟飞机多层铆接金属结构的试样,对不同深度和大小的腐蚀缺陷进行了检测。采用主成分分析(Principal Component Analysis,PCA)方法对实验数据进行处理,并提取前3个主成分进行分析。结果表明:应用PCA方法,可以将纯提离信号与带层间腐蚀缺陷的信号显著区别开来,可以将不带提离时的纯腐蚀信号的深度识别出;将PCA提取的主成分应用K-means算法进行聚类,可以将纯提离信号与纯腐蚀信号和腐蚀提离混合信号区别开来。而对于带提离的腐蚀,试验发现其PCA分布与不同深度的纯腐蚀出现混淆,因而不能准确识别这两种信号。     

Measurement of lift-off using the relative variation of magnetic flux in pulsed eddy current testing

Measurement of lift-off can be used to assess the thermal insulation thickness and it has the potential to reduce the lift-off effect in pulsed eddy current testing. In this paper, first, the relative variation of magnetic flux is proposed as a feature for the measurement of lift-off. And then, how to directly obtain the key parameters of the feature from the testing signals is provided. At last, the validity of the feature is verified by simulations and experiments, respectively. The results show that the feature is suitable when the lift-off is tens of millimeters and the plate is ferromagnetic.     

Transient and harmonic eddy currents: Lift-off point of intersection

It has been previously determined experimentally that the transient responses of pulsed eddy current excitation of a conductive semi-infinite plane pass through a common point, regardless of the lift-off value. The origin of this intersection point is investigated and related to the harmonic components of the Fourier series representation of the pulse. Discrete sinusoidal signals, when individually sent through the same inspection system, also exhibit this common point feature when only the lift-off is varied. Moreover, the transient responses can be reconstructed from the harmonic outputs by using truncated Fourier series.     

Signal characteristics of differential-pulsed eddy current sensors in the evaluation of plate thickness

Differential-pulsed eddy current (PEC) signals and their characteristics are investigated as tools for the evaluation of plate thickness by using the reflection-type PEC probe, which consists of an exciter coil and two sensor coils in a differential arrangement. Numerical and experimental signals are first compared to validate numerical simulation results, and good agreement between them is achieved. Investigation of differential PEC signals and their characteristics against thickness and conductivity variations shows that time-related features, such as the time-to-peak and the zero-crossing time, correspond well to plate thickness and conductivity. However, the feature quality of peak value turns out to be very poor. To improve this, the effects of coil characteristics on the input pulse, and consequently on differential PEC signals, are investigated. Results show that the feature quality of peak value may be improved by reducing the time constant, but doing so would worsen the quality of the other two features. Lift-off signals obtained by this probe show that the lift-off point of intersection also appears in the differential reflection-type PEC signals.     

Electrical conductivity measurement of ferromagnetic metallic materials using pulsed eddy current method

Pulsed eddy current testing (PECT) method for electrical conductivity measurement of ferromagnetic metallic materials is proposed. Based on time-domain analytical solutions to the PECT model of ferromagnetic plates, the conductivity and permeability are determined via an inverse problem established with the calculated and measured values of induced voltage. PECT method for conductivity measurement is verified by the four-point probe method on three carbon steel plates. In addition, the effects of the amplitude of pulsed excitation current and the lift-off of probe coils on measurement results are studied. PECT is an innovative, non-contacting method with good repeatability for electrical conductivity measurement.