The effect of error and regularization norms on strain and damage identification via electrical impedance tomography in piezoresistive nanocomposites

Electrical impedance tomography (EIT) has recently emerged as a promising tool for the structural health monitoring (SHM) and non-destructive evaluation (NDE) of piezoresistive nanofiller-modified composites. With few exceptions in SHM and NDE, the vast majority of studies have formulated the EIT problem such that it endeavors to minimize a voltage error vector in the least-squares sense while simultaneously using an additional least-squares term for regularization. Unfortunately, the use of least-squares minimization and regularization has important limitations especially in the context of SHM and NDE. For example, the least-squares error minimization is extremely sensitive to outlier data. Such outlier data is a very real possibility in SHM applications due to damaged electrodes. Additionally, utilizing a least-squares term for regularization renders EIT incapable of resolving discontinuous conductivity changes such as would be induced by fracture events. More sophisticated minimization and regularization techniques that obviate these limitations are well known to medical and mathematical practitioners of EIT; however, these methods have not been explored in SHM and NDE applications. This is an important oversight because the needs of SHM and NDE differ considerably from the needs of medical imaging. Therefore, we herein explore the use of different minimization and regularization norms on EIT for experimental strain identification, detection of artificially induced through-hole damage, and detection of impact damage.     

Probabilistic modeling and sizing of embedded flaws in ultrasonic non-destructive inspections for fatigue damage prognostics and structural integrity assessment

The paper presents a systematic method and procedure for probabilistic fatigue life prediction using non-destructive testing data under uncertainty. The procedure is developed using uncertainty quantification models for detection, sizing, fatigue model parameters and inputs. The probability of detection model is based on a classical log-linear model coupling the actual flaw size with the NDE reported size. Using probabilistic modeling and Bayes theorem, the distribution of the actual flaw size is derived for both NDE data without flaw indications and NDE data with flaw indications. Fatigue damage and structural integrity assessment are suggested based on the developed method and procedure. A turbine rotor example with realistic NDE inspection data is presented to demonstrate the overall methodology. Calculation and interpretation of the results based on risk recommendations for industrial applications are given. The influence of the NDE detection threshold to the assessment results, and error analysis of the assessment results are discussed in detail.     

Experimental characterization of granite damage using nonlinear ultrasonic techniques

In this paper, a nondestructive evaluation (NDE) technique based on the nonlinear second harmonic wave theory is developed and used to characterize damage of granite samples subjected to compressive loadings. The nonlinear parameter defined in the new NDE technique is measured and compared with two traditional parameters including ultrasonic pulse velocity and dynamic modulus. The nonlinear parameter is found to be much more sensitive to the damage development in granites than traditional parameters. It is shown that the increase of nonlinear parameter is close to an exponential trend with respect to the increased loading level, which also indicates a faster increase rate of the nonlinear parameter corresponding to the internal damage of granite samples. A practical damage index is thus defined based on the exponential increasing trend of the nonlinear parameter. The new damage index based on nonlinear parameter is found to have a positive correlation with the loading level. This observation suggests that the new damage index may become a valuable indicator of loading level (or correspondingly material degradation) of granites in the in situ NDE tests.     

Quantifying creep damage in a metallic alloy using acousto-ultrasonics

Due to elevated temperatures and excessive stresses, turbine components may experience creep behavior. As a result, it is desirable to monitor and assess the current condition of such components. This study used the acousto-ultrasonics (AU) method in an effort to monitor the state of the material at various percentages of expended (i.e., used-up creep life) creep life in the nickel base alloy, Udimet 520. A stepped specimen (i.e., varying cross-sectional area) was used, which allowed for a post mortem nondestructive evaluation (NDE) analysis of the various levels of expended life. The overall objectives in this study were two-fold: First, a user-friendly, graphical interface AU system was developed, and second, the new AU system was applied as a NDE tool to assess distributed damage resulting from creep. The experimental results demonstrated that the AU method was able to detect material changes as a function of expended creep life.     

碳纤维增强复合材料层间界面缺陷超声成像技术

针对复合材料层压结构层间界面缺陷的无损检测与评估,通过分析超声波在碳纤维增强树脂基复合材料中的传播规律,研究入射声波在复合材料中的反射特性,利用入射声波在复合材料层间界面产生的反射信息,通过超声（B,T）扫描成像方法揭示复合材料层间界面缺陷及其三维分布。试验结果表明,采用短波长脉冲超声成像检测技术,可以有效地再现碳纤维增强树脂基复合材料层压结构中层间界面缺陷三维分布特征以及层间界面、铺层方向、纤维束取向等信息,为复合材料提供了一种有效的层间界面结构表征和缺陷检测方法。     

Assortment of ultrasonic nondestructive evaluation signals using the centroid searching method with the LMS algorithm and SAFT

NDE using ultrasonic signals is a very useful technique for the assessment of solid materials, construction, food, and biomedicine. Among many NDE methods, the ultrasonic inspections may involve the extraction of an appropriate set of features or a neural network for the classification of the signals in the feature space. This paper presents an approach that uses a geometric method and the LMS (Least Mean Square) algorithm to determine the coordinates of the ultrasonic probe followed by the SAFT (Synthetic Aperture Focusing Technique) and centroid searching technique to estimate the location of the ultrasonic reflector. The proposed method is employed to classify ultrasound NDE signals from cracks and deposits within steam generator tubes in a nuclear power plant.     

Non-destructive evaluation of smart materials by using extrinsic Fabry–Perot interferometric and fiber Bragg grating sensors

The real-time non-destructive evaluation (NDE) technologies of engineering structures are very important to assess the performance of in-service structures. It is very difficult to carry out the on-line structural integrity monitoring by using classical NDE methods. Fiber optic sensors in smart structures provide a unique opportunity to real-time monitor the structural health status by using embedded sensors. This paper examines the possibility of extrinsic Fabry–Perot interferometric (EFPI) and fiber Bragg grating (FBG) sensors for NDE of composite and aluminum structures. The experiments of 3-point bending measurement of composite laminates and aluminum plates with and without damages by using the EFPI and FBG sensors are performed, respectively. The experimental results show that the flexural strain of damaged structures is much higher when compared with relevant undamaged structures under same bending load by using both EFPI and FBG sensors. Furthermore, an excellent accordance was found between the results of the 3-point bending tests by using surface-mounted EFPI and FBG sensors     

QNDE的核心科学与工程基础进展(英文)

对材料和结构缺陷的无损检测(NDT)已经历了50多年的进化演变。在美国,它已经从一个要求零缺陷的检测策略(NDT),向基于损伤可容度设计的检测和评估技术(NDE)过渡。这里是假设部件始终包含一个缺陷,只有那些比断裂力学确定的临界尺寸更大时,需要通过检查将其消除,以重新确定产品的服务周期。介绍了这些因素对于推动模式转变及转换至定量无损检测方面起到的至关重要的作用。一系列的重大研究项目被启动,用于更新无损检测以满足新的要求。重点介绍了在第一项目发展中的研究重点,以及用于定量缺陷定义的DARPA/AFML跨学科项目。它有三个目的:发展新的核心科学/人员基础,使检测技术满足新的要求,确立发展新的领域,即适当的工程设备的阶段,并继续开展定量无损检测(QNDE)系列会议。从这个和其他方案的进展已导致对所涉及的任何检查和技术的各项测量的基本模型的链接为基础的定量无损评价(QNDE)的科学核心。除了讨论这些模式和它们的联系,还将定义核心结构。利用这些模型,一种新的强大的工程工具集已经开发,包括UT,RT和EC技术的模拟程序。这些工具的应用将成为亮点,在包括结构健康监测和状态检修的工作中将令人注目。最后,讨论了QNDE未来机会、远景和方向。     

铝基牺牲阳极的溶解过程和负差异效应

用旋转环一盘电极、电子探针和形貌分析等手段研究了铝基牺牲阳极的溶解过程及其负差异效应。结果表明,铝基牺牲阳极的溶解是从第二相组织边界处引发,从而破坏表面膜而活化。阳极溶解过程中的自腐蚀(表现为析氢)是负差异效应的主要原因,它随外加电流而线性增大。铝基牺牲阳极的负差异效应系数为12.8%。     

基于声信号递归Hilbert变换的轴承故障诊断研究

轴承缺陷检测与损伤程度检测一直是旋转机械领域内非常重视的问题,虽然目前针对振动信号的研究已经取得相当好的结果,但是对于难以安装振动传感器的情况,诊断效果仍需改进。针对强背景噪声下故障轴承产生的声音,提出一种基于递归Hilbert变换和一维卷积神经网络的诊断方法来提取抽象特征并进行模式识别。卷积神经网络结构中引入了全局平均池化层来加速网络的运行。最后,通过数据集验证了所提方法的有效性,与其他常用分类方法进行对比,验证了该方法的优越性。结果表明：所提算法不仅能够准确识别轴承的损伤部位,而且能够准确区分部件的损伤程度。     

Image fusion of ultrasonic and thermographic inspection of carbon/epoxy patches bonded to an aluminum plate

Inspecting damaged structures repaired with composite patches is one of the most challenging issues of different industries, especially in aeronautical vehicles. Various inspection methods such as ultrasonic (UT) and thermographic methods are commonly applied to the patches and bondings. Although, both ultrasonic and thermographic inspections have their own capabilities, they are also associated with some limitations especially in the inspection of complicated structures such as multilayers. Different data fusion techniques can be used to overcome these limitations, by exploiting the advantages of each inspection technique to achieve the goal to have a more precise and reliable defects assessment. In this paper, different image fusion algorithms are used to fuse the UT C-scan and thermal images obtained from thermographic inspection of carbon/epoxy patches bonded to an aluminum plate. The inspected specimen contained several delaminations of various sizes and locations along with some disbond defects which were artificially embedded in five samples of composite patches. The resulting images of 28 fusion algorithms and the input images have been quantitatively compared using Average Differences (AD) to clarify the efficiency of the fusion algorithm. Comparing results revealed that contrast pyramid was selected as the best image fusion algorithms which can be used for NDE fusion of carbon/epoxy patches bonded to an aluminum plate.     

Ultrasonic NDE image compression by transform and subband coding

In this paper, two categories of transform coding and subband coding are comparatively studied for compressing ultrasonic NDE images. The data compression performance of the discrete cosine transform-, the Karhunen-Loève transform-, and the discrete wavelet transform-based codecs are examined by being applied to ultrasonic NDE images. The effects of these compression methods on the analysis of ultrasonic information in compressed ultrasonic images are examined. Moreover, evaluation of the main elements in image coding system is performed to explore the important factors, which are worth paying more attention when developing a special image compression technique for ultrasonic NDE images.     

钻孔过程中切削温度对CFRP亚表面的影响

研究了碳纤维增强聚合物(CFRP)/Ti叠层构件钻孔时切削温度对钻孔质量的影响,分析了有无钛合金支撑层制孔时CFRP的切削力、切削热、孔壁表面和亚表面质量,并提出了亚表面损伤评价方法。结果表明,钻孔时钛合金支撑层对CFRP切削力及表面质量的影响较小,但对切削温度和亚表面质量影响显著。刀具同时加工钛合金和CFRP时会产生大量切削热,导致CFRP孔出口处温度大幅升高,高温导致CFRP树脂基体的刚度和粘结性能下降,使得CFRP孔出口附近纤维层上出现了严重的亚表面损伤。同时,采用提出的亚表面损伤评价方法对亚表面损伤进行评价,发现靠近出口处的纤维层亚表面损伤最为严重。在远离出口平面的方向上,亚表面损伤程度逐渐降低。因此,CFRP/Ti叠层构件钻孔过程中切削温度显著影响CFRP孔的亚表面质量,且亚表面的损伤程度是评价CFRP加工质量的重要因素。     

电子束辐照对多壁碳纳米管的影响（英文）

在室温下采用透射电子显微镜中汇聚的电子束辐照多壁碳纳米管。结果表明,在能量为100 keV的电子束辐照下除了碳纳米管管壁有一些弯曲外没有其他结构被破坏;当电子能量增加到200 keV时,纳米管有明显的损伤,可以观察到纳米管的无定型化、纳米管外壁的凹坑和缺口。200 keV的电子束辐照还能形成碳洋葱和2根多壁纳米管的焊接。多壁碳纳米管的离位阀能为83~110 keV。能量超过阀能的电子束可以很轻易地损伤纳米管而低于阀能的电子束则很难损坏纳米管,其损伤机理为溅射和原子离位。     

超导量子干涉器无损检测的应用与研究进展

超导量子干涉器（SQUID）由于其高磁场灵敏度、带宽宽、高空间分辨力在无损检测（NDE）领域有着重要的应用前景。近年来,针对SQUID NDE的研究取得了较快的进展,这为其用于实际工程奠定了良好的基础。综述了SQUID NDE的研究成果,包括应用领域、空间分辨力、噪声处理以及检测系统等。指出该技术的特点、存在的问题和发展方向。     

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.     

Wall thickness evaluation of single-crystal hollow blades by eddy current sensor

Advanced high-pressure turbine blades of jet engines are hollow and monocrystalline. The external wall thickness of these blades has to be checked systematically and quickly after manufacturing in order to guarantee the blade strength. Thickness evaluation is made difficult by the presence of internal partitions and by the crystalline anisotropy of the superalloy used in blade manufacturing. In this paper, the authors present the advantages of the eddy current (EC) technique in comparison to other non-destructive evaluation (NDE) techniques for wall thickness evaluation. A dedicated EC sensor was developed and implemented. The thickness evaluation was carried out with a neural network inverse model, and the results show the high accuracy and reliability of the proposed method.     

Time reversal and microwave techniques for solving inverse problem in Non-Destructive Evaluation

Non-Destructive Evaluation (NDE) methods are used to inspect materials without damaging their usefulness. The key problem in NDE is the inverse problem which involves reconstructing materials’ physical profiles, like inner discontinuity etc., using information in the measured NDE signal. Inverse problem solutions in NDE can be classified as model-based and system-based approach. In model-based approach, an accurate forward model is used in an iterative framework. This approach provides a resultant materials' physical profile that minimizes the error between the measured signal and a simulated signal. However, this approach requires repeated calculations of a numerical model in each iteration, making it computationally demanding. This paper presents a model-based method that also provides a direct approach to inversion using principles of time reversal. Time reversal focusing is based on the fact that when a wave solution is reversed in time and back-propagated the wave comes to focus at the source. Using a computational model, this paper applies principles of time reversal to microwave NDE data to solve the inverse problem of defect detection in dielectric materials. A two-dimensional finite difference time domain (FDTD) model, for simulating the propagation of forward and time reversed wave fields, is developed. A dielectric sample with artificial defects, illuminated by a Gaussian modulated pulse, is used in the simulations. The microwave measurements are recorded, time reversed and propagated using the FDTD model to highlight the scatterer/defect. Maxima in the energy image indicate locations of defects. Simulation results demonstrate the feasibility of the technique to detect defects in dielectric materials. The FDTD model is validated using experimental data.     

Remanufacturing Shaping System Based on Pulsed Plasma Arc Welding

Based on the pulsed plasma arc welding(PPAW),a rapid remanufacturing shaping system,including a structured light scanning reverse engineering(RE)system,a robot system and a PPAW source system have been developed.The new rapid remanufacturing shaping system can rapidly and accurately repair the worn parts with different damaged degrees and thus extend their life cycle.In the present paper,the whole process of repairing worn parts is described.Firstly,the scanning model of a worn part is obtained by using RE system based on reconstructing approach.Compared with standard CAD model in the undamaged setting,the surface profile and the extent of damage area are obtained.Secondly,the weld repair path is designed by slicing the point cloud model of damage area.This path consists of multilayer parallel lines that produce parallel overlapped weld beads which cover the damaged area.By off-line programming techniques,the robot executable program as document format is transmitted to the robot controller for repairing worn parts.Finally,the weld repair experiments on worn steel plane plate and damaged column are performed to evaluate the remanufacturing system.     

A new theory for the negative difference effect in magnesium corrosion

An unusual feature known as the negative difference effect (NDE) can be observed in magnesium when recording corrosion current density–potential curves. More hydrogen is evolved at a more positive potential which does not occur in conventional metals. Several models have been proposed in the literature in order to explain the phenomenon of NDE. They succeed in explaining some effects, and fail to deal with others. A new model, which explains the NDE by two electron consuming processes, is presented in this paper. By potentiostatic investigations of magnesium in a chloride electrolyte, measurements of hydrogen evolution and chemical analysis of the electrolyte the new model was experimentally verified.