Ultrasonic air-coupled testing of square-shape CFRP composite rods by means of guided waves

In aerospace industry, one of the most important parts of gliders and motor-gliders is a lightweight longeron reinforcement made of carbon fibre reinforced plastics (CFRP) rods, known as Graphlite SM315 composite. During manufacturing, the rods as constructional elements are glued together in epoxy-filled matrix in order to build the arbitrary spar profile. The defects present in single rods such as breakage of fibres, multiple delaminations due to the lack of bonding between fibres and reduction in density affect essentially the strength and the fail-safety of the overall construction.The aim of the present work is to investigate the effects (transmission, reflection, scattering and mode conversion) of guided waves propagation along a square-shape CFRP rod in the case of contactless excitation/reception and interaction with region of multiple delaminations applying the numerical model and performing experiments.The square-shape CFRP composite rods possessing internal artificial delamination type defects have been investigated by numerical modelling and experiments employing the developed air-coupled technique (pitch-catch set-up) for cases of conventional transmission and advanced back-scattering configurations. Numerical predictions of guided wave interaction with a multiple delamination type defect in a CFRP composite rod have been made and the interaction mechanism explained. It is possible to conclude, that the actual sizes of the internal defects have been clearly detected using reception of the back-scattered waves over the edges of the defective regions only.     

Autofocusing ultrasonic imagery for non-destructive testing and evaluation of specimens with complicated geometries

This paper introduces the autofocus problem for non-destructive testing and evaluation of specimens with complicated geometries using ultrasonic arrays. We present an algorithm for autofocusing imagery of point-like defects obtained from a flexible array with an unknown profile. The relative positions of the array elements are parameterised using a cubic spline function and the spline control points are estimated by iterative maximisation of an image contrast metric via simulated annealing. The resultant spline gives an estimate of the array profile and the profile of the surface that it has conformed to, allowing the generation of a well-focused image. The algorithm is demonstrated on experimental data obtained using a flexible array prototype.     

基于超声无损检测的扩散连接界面缺陷尺寸评估

针对扩散连接界面缺陷的无损检测问题,开展了人工缺陷试样的水浸超声检测试验研究,提出了扩散连接界面缺陷超声响应模型,开发了一种通过超声检测评估界面缺陷厚向尺寸的检测手段. 首先利用扩散连接的方法制备了带有人工缺陷扩散连接试样,采用30 MHz的入射波频率,对试样进行水浸超声无损检测. 基于超声无损检测原理提出了未焊合缺陷的超声响应模型,引入扩散连接界面劲度系数K作为桥梁建立起超声反射波反射率与界面缺陷尺寸之间的联系. 通过特定检测位置的超声反射波数据和实际微观缺陷尺寸数据拟合确定超声响应模型常数,进而实现基于超声无损检测的界面缺陷厚向尺寸评估. 结果表明,传统超声C扫描一般只是定性地反映缺陷存在与否,而厚向尺寸评估方法的提出在一定程度上为超声C扫描做了补充,有助于缺陷尺寸的定量检测,实现缺陷危害等级的评估.     

The effect of dispersion on long-range inspection using ultrasonic guided waves

The dispersion of ultrasonic guided waves causes wave-packets to spread out in space and time as they propagate through a structure. This limits the resolution that can be obtained in a long-range guided wave inspection system. A technique is presented for quickly predicting the rate of spreading of a dispersive wave-packet as it propagates. It is shown that the duration of a wave-packet increases linearly with propagation distance. It is also shown that the duration of a wave-packet after a given propagation distance can be minimised by optimising the input signal. A dimensionless parameter called minimum resolvable distance (MRD) is defined that enables a direct comparison to be made between the resolution attainable at different operating points. Some conclusions are made concerning the resolution of various operating points for the case of Lamb waves in an aluminium plate.     

LIBS for non-destructive testing of element distributions on surfaces

A laser-based technique for the quasi non-destructive evaluation of surfaces of building materials is presented. Laser-induced breakdown spectroscopy (LIBS) is an analytical tool to determine the chemical composition of laser evaporated material. After the material has been vaporized by the high energy laser pulse the elements emit their specific fluorescence. This radiation is used to identify the elements in the evaporated material by spectroscopic methods.The mass of the evaporated material is of the order of micrograms; this means LIBS is a quasi non-destructive method for building applications. The main advantages of LIBS are the direct measurement without any preparation and the possibility to measure on site and to obtain on-line results.LIBS is a well-known technique which has been successfully applied in the areas of material testing, medicine and industrial process control. This article reports on the progress of LIBS applications for building materials by the use of some examples: identification and analysis of the main components of concrete; determination of salt concentrations in building materials; and measuring the relative element composition to a depth of up to 10 mm with high geometrical resolution.     

Determination of the modulation frequency for thermographic non-destructive testing

We propose polynomial solutions of the inverse heat conduction problem to design thermographic non-destructive tests for detecting defects in composite and multi-layer materials. Inverse heat conduction in a multi-layer material slab with periodic temperature excitation is considered, and analytical quadrupole representation is used to derive a lumped parameters formulation. Predictions of the proposed polynomial representations are experimentally validated by detecting machined defects on thermally excited panels. For modulation frequencies outside the predicted detection range, defects appear on thermal images as blurry and unstructured; conversely, for modulation frequencies within the predicted range, defects are correctly represented on thermal images.     

Application of magnetic Barkhausen noise in non-destructive testing fields of residual stress

Magnetic Barkhausen noise （ MBN） is a phenomenon of electromagnetic energy due to the movement of magnetic domain walls inside ferromagnetic materials when they are locally magnetized by an alternating magnetic fields. According to Faraday＇s law of electromagnetic induction, the noise can be received by the coil attached to the surface of the material being magnetized and the noise carries the message of the characteristics of the material such as stresses, hardness, phase content, etc. Based on the characteristic of the noise, researching about the relationship between the residual stress in the welding assembly and the noise are carried out. Furthermore, data process is performed by RMS （Root Mean Square） equation and Power Spectrum analysis.     

Application of infrared thermography to the non-destructive testing of concrete and masonry bridges

Within recent years there has been an increase in the use of NDT methods to detect defects and anomalies in various civil engineering structures. Infrared thermography, which has been successfully used in the USA in civil engineering applications, is being increasingly applied in the UK as an NDT technique. For example, the technique is now included in the Building Regulations for the assessment of thermal insulation for all new non-domestic buildings from April 2002.One of the perceived limitations of infrared thermography is that in temperate climates it is too cold to use this technique since there is rarely the extreme solar exposure that has enabled the successful use of thermography to detect render debonding and concrete spalling utilising solar heating. However, with the advancements in modern technology it is now possible to detect smaller changes in temperature (down to 0.08 °C). This paper shows that even with the low ambient temperatures experienced in Europe it is possible to use infrared thermography to identify correctly known areas of delamination in a concrete bridge structure and also to investigate the internal structure of a masonry bridge.     

声波散射在焊接板结构超声导波合成孔径阵列成像中的作用

为揭示声波散射在焊接板结构超声导波合成孔径阵列成像中的作用机制,将散射回波矩阵和散射渡越时间矩阵用于分析描述超声水平剪切（shear horizontal,SH）导波声波散射,探讨了焊缝散射和边界散射在导波成像中的作用关系.结果表明,散射矩阵是揭示声波散射在超声导波阵列成像作用机制的有效工具,散射回波矩阵与散射渡越时间矩阵具有相同的散射矩阵因子,导波图像出现与散射矩阵因子成倍数关系的“条带”伪像,图像椭圆簇轨迹特征复杂化,图像背景噪声增强,图像信噪比减小.研究工作可为深入开展焊接板结构超声导波非直线阵列成像算法研究提供基础.     

Performance of non-destructive evaluation by diffracted SH ultrasonic waves in predicting degree of fatigue in cyclic bending of ferritic steel

The degree of fatigue in ferritic wrought steel during cyclic bending was determined by analysis of diffracted SH ultrasonic waves with the aid of multiple regression analysis. As the degree of fatigue increases, accompanied by residual stress, the incident waves curve to the interior region with positive stress field owing to the acoustoelastic effect. As a result, the propagation time of the launched waves lengthens, leading to a modulation of the received waveform. Multiple regression analysis for the waveform modulation produces a reliable estimation, with correlation coefficient of 0.948, for the degree of fatigue.     

一种用于焊接结构超声成像检测的SH导波换能器

板中水平剪切导波可用于焊接结构板材远距离成像检测,超声SH导波换能器的性能对较大尺度焊接结构板材超声导波成像检测至关重要.对板中SH导波进行了动力学分析,并结合半波长条件设计了合理的斜楔角度、压电晶片频率及尺寸等导波换能器的重要参数,通过加前背衬、取消后背衬、楔块前沿设置锯齿槽等措施,简化了换能器的工艺结构,减小了换能器内部回波干扰,提高了检测灵敏度.结果表明,设计研发的SH导波换能器激发的导波有较强的检测能力,可用于对较为复杂的T形一对焊焊接结构板材进行成像检测,能够有效表征焊接结构板材中与导波波长尺寸量级相当的缺陷,其性能指标达到了较大尺度焊接结构板材成像检测的要求.为开展较大尺度焊接结构超声导波成像检测奠定了基础.     

Spectral analysis of radar surface waves for non-destructive evaluation of cover concrete

This paper presents the use of spectral analysis of radar surface waves for the non-destructive evaluation of cover concrete. With only two commercial coupled antennas centred around 1.5 GHz, an original measurement mode is proposed for obtaining dynamic wide angle reflection refraction profiles. It consists in taking several profiles at the same place using different distances between the two antennas. After separation of the air wave and surface waves, the wrapped phase spectrum is obtained for a given distance between two receiver positions by means of a Fourier transform. For a given position, the speed is calculated for each frequency from the known wave travel distance. The repetition of the processing for other positions gives the speed variation along a profile. Moreover, the dispersion analysis can provide further information on the humidity of the material. Tests on a dry concrete surface and after wetting provide reliable results which allow the two moisture contents to be distinguished.     

小波分析在超声检测信号处理中的应用

小波分析技术以其多分辨率的突出特点成为目前超声信号时频表达的最佳分析方法,被广泛应用于超声信号的降噪处理。分析了小波技术用于超声降噪的理论基础和基本过程,基于国内外小波降噪的应用研究成果,从母小波和分解层数的选择以及小波系数非线性处理方法两个方面论述了小波分析在超声检测信号处理中的应用。     

Application of ultrasonic method in asphalt concrete testing for fatigue life estimation

This paper presents the possibilities of applying ultrasonic methods in asphalt concrete specimen testing in order to estimate fatigue life using ultrasonic characteristics of specimens. The procedures and results of testing cylinder shaped samples of both ultrasonic analysis and fatigue life testing are described. The ultrasonic method was first used in order to obtain the seismic characteristics of specimens. Then, the specimens were tested for fatigue lives using the repeated-loading indirect tensile test equipment (SDU-Asphalt Tester). To be able to model the fatigue lives, in addition to conventional fatigue model parameters new parameters from this ultrasonic analysis were taken into consideration. These additional parameters are acquired from the seismic measurements. Hence, only by examining the ultrasonic method, it will be possible to predict the fatigue lives of the specimens non-destructively.     

Ultrasonic and structural characterization of anisotropic austenitic stainless steel welds: Towards a higher reliability in ultrasonic non-destructive testing

The non-destructive testing of austenitic stainless steel welds of the primary coolant piping system is a significant problem for the nuclear industry. Ultrasonic techniques would be very helpful to detect, locate and size potential defects. Unfortunately, austenitic welds are coarse-grained, heterogeneous and anisotropic. This leads to aberration and scattering of the ultrasonic waves. In this paper, we present several experimental results of ultrasonic testing of two austenitic welds exhibiting high anisotropy. In order to explain the observed display of wave propagation phenomena such as beam deviation, we use finite element modeling. The modeling is associated with a complete characterization of the inspected welds. Two essential characteristics of the welds are determined: the average elastic constants of the weld and the grain orientations. The capability of the model is illustrated in different testing configurations. This work associating structural characterization and modeling shows that a better understanding of the phenomena of ultrasonic propagation should allow the interpretation and reliability of the industrial inspections of heterogeneous anisotropic welds to be improved.     

搅拌摩擦焊缝微细尺寸缺陷的无损检测技术

针对25 mm厚板2219铝合金搅拌摩擦焊缝中存在的微孔、疏松、“S”曲线弱连接、根部类裂纹等微细尺寸的缺陷,分别采用相控阵超声波扇形扫查、荧光粉渗透检测等技术进行无损检测,并与传统的X射线探伤结果进行对比,探讨不同的无损检测技术针对搅拌摩擦焊缝微细尺寸缺陷的工程适用性.经过对焊缝解剖,利用立体显微镜、光学显微镜、扫描...     

Scattering of guided shear waves in plates with discontinuities

This paper investigates the interaction of the SH0 mode with discontinuities in plate waveguides. The scattered fields are evaluated using a novel method that exploits the principle of reciprocity in elastodynamics. The results obtained compare to those provided by a finite element model. Very good agreement between the analytical and numerical models proves the effectiveness of the proposed approach, enabling us to clearly elucidate the role of the different size and shape parameters involved. The discontinuities considered are single and double sharp reductions of plate height of different amounts and lengths, where both symmetric and nonsymmetric one-sided notch cases are treated. Regimes related to low and high values of the product frequency and height of the plate are investigated, showing the dependence of reflection and transmission coefficients on length and height of the discontinuity, as well as the occurrence of mode conversion. The analytical approach proposed leads to a better understanding of the interaction of guided waves with discontinuities, which may stimulate the application of guided waves to defect sizing rather than to simple detection.     

Application of ultrasonic Rayleigh waves to thickness measurement of metallic coatings

The velocity of Rayleigh waves propagating in a layered medium depends on the frequency of the waves, the thickness of the coating and the properties of both the coating and the substrate materials. Rayleigh waves of various frequencies were generated using a broadband pulse and their velocities were measured as a function of the frequency and compared to the theoretical dispersion curve of the specimen. The thickness of the layer was deduced from comparison between these two curves. A special transducer was designed in order to achieve these measurements. Experiments were carried out on AISI 316L specimens coated with vacuum plasma sprayed (VPS) NiCoCrAlY of various thicknesses (190–330 μm) and various surface states. A good correlation was obtained between the thicknesses measured by means of ultrasonics and those obtained using photomicrographs of the cross-section of the specimens. The elastic characteristics of the coating and the substrate materials were measured using a transmission method for parallel-faced plates (disbonded samples of coating material). The velocity of the shear and compression waves propagating in various directions is obtained by changing the angle of incidence on the sample surface. A small anisotropy of the coating material was found. Finally, we demonstrated the efficiency of the use of surface waves for the detection of surface breaking cracks and disbonding at the interface between the coating and the substrate.     

Application of neuro-fuzzy techniques in oil pipeline ultrasonic nondestructive testing

This paper presents a novel approach to the problem of nondestructive pipeline testing using ultrasonic imaging. The identification of the flaw type and its dimensions are the most important problems in the pipeline inspection. Unlike typical methods, a decision based neural network is used for the detection of flaws. We train a generalized regression neural network to determine the dimensions of the corrosions and generate the whole image of both the internal and external walls of the oil pipeline. As an improvement to the detection algorithm, we introduce fuzzy decision-based neural network algorithms for the detection and classification of the corrosions. The simulation and experimental systems results show that these new methods outperform the existing methods.