微波无损检测热障涂层下金属表面裂缝的参数优化

基于微波在介电材料中的传播理论及其对金属界面特性的敏感性, 利用CST-微波工作室(computer simulation technology-microwave studio)对微波检测热障涂层下金属的裂缝进行了仿真计算, 研究了热障涂层的厚度、 裂缝方向对检测结果的影响, 仿真计算了热障涂层厚度为400 μm、 裂缝长边方向平行于矩形波导探头长边的不同宽度的裂缝。结果表明: 热障涂层厚度不同时, 微波检测金属表面裂缝的敏感工作频率不同; 裂缝方向与波导口长边的夹角为50°～55°时检测敏感度低。裂缝宽度小于8 μm时用本研究中的频率范围无法检测, 裂缝宽度在10～30 μm时检测效果不明显, 裂缝宽度在30 μm～1 mm范围内, 裂缝越宽微波的反射系数相位差越大。因此, 在合适的工作频率下能利用微波无损检测技术对热障涂层下金属表面的裂缝进行无损检测。     

Microwave detection of finite surface cracks in metals using rectangular waveguides

An electromagnetic model for finite surface crack detection is developed and its results are compared with those of the existing long crack model and some experimental results to verify its integrity. An open-ended waveguide sensor is used to scan a metal surface with a crack in it. The crack is then modelled as a small waveguide being fed by a larger one. The presence of the crack causes higher order modes to be generated, which in turn influence the properties of the standing wave set up inside the waveguide. Strategic probing of the standing wave properties is used to indicate the presence of a crack (detection). The distribution of the tangential electric field on a metal surface is investigated. This is done to check whether the boundary conditions have been properly forced. The behavior of crack characteristic signals as a function of varying crack length and location within the sensor aperture is also studied. The possibility of using the phase of the reflection coefficient for crack detection is investigated by a numerical simulation. A finite fatigue crack is finally detected at 38 GHz to demonstrate the practical feasibility and potential of this technique.     

Detection and Characterization of Buried Macroscopic Cracks Inside Dielectric Materials by Microwave Techniques and Artificial Neural Networks

The detection and characterization of macroscopic cracks inside dielectric materials is an important practical issue. Thus, there is a need to establish evaluation techniques, which can be used to characterize buried cracks; indeed, the knowledge of the geometrical configuration of a hidden crack is a key factor for fatigue crack engineering. Therefore, a microwave method for nondestructive characterization of macroscopic cracks inside dielectric materials is presented in this paper. This nondestructive and noncontact technique is based on the determination of the near-field reflection coefficient of an open-ended rectangular waveguide. The measurements are achieved by means of a microwave six-port-based system that operates at 35 GHz. We show that relatively small defects are detectable and demonstrate that the association of signal processing tools to this characterization method enables the retrieval of the crack profile in an acceptable manner. The reconstruction of a 1-D buried crack profile is performed by means of a multiple-multilayer-perceptron (MLP) approach. Several cases are investigated to demonstrate the capabilities of the method.      

Analysis technique for interaction of high-frequency rhombicinducer field with cracks in metals

In the nondestructive evaluation (NDE) involving the ac field measurement (ACFM), a current carrying structure is required to induce the eddy current in the work-piece and a probe to sample the field. Due to its flat profile, slender shape, and other advantages, the rhombic wire loop is a suitable inducer for developing linear flexible arrays for the ACFM inspection of large surfaces of ferrous and nonferrous metals. This paper introduces an analysis technique for the evaluation of the interaction of the field of the rhombic inducer carrying a high-frequency current, with long surface cracks of uniform depth in flat metal plates. The technique is accurate and very efficient computationally. It uses the two-dimensional Fourier transform together with a special boundary condition at the metal surface. The boundary condition takes into account the thin-skin nature of the eddy current in the metal as well as the flux leakage at the crack mouth. The analysis technique benefits from the use of scalar potential functions and can be extended to simply or multiply connected wire loops. Also, it is applicable to high-frequency (thin-skin) eddy current problems. Using the analysis technique, the tangential field below a rhombic inducer along its long diagonal when the inducer is located above the surface of aluminum and steel is given in the presence and absence of a crack. This field was found to have a nonuniform phase distribution. Near a crack, the phase change is significant, even for shallow cracks. The role of the nonuniform phase in the detection sensitivity is addressed. Also, simulated ACFM crack responses using an inducer with a linear probe attached along the long diagonal are presented and discussed. To support the validity of the analysis technique, experimental results obtained for some of the simulations are also reported. In addition to its application in predicting crack responses, the technique can be used for model-based inversion of crack signals     

Scattering of an induced surface electromagnetic field by fatiguecracks in ferromagnetic metals

In crack detection and sizing by the alternating current field measurement technique, U-shaped wires or coils excited by a high-frequency AC current source can be used to induce the surface field in the workpiece. The authors present a modeling technique for the interaction of a fatigue crack in a ferromagnetic metal with the surface field resulting from an inducer with two U-shaped wires. This work is an extension of a previous modeling technique to have developed for infinitely long (one-dimensional) cracks. In the present technique, the boundary of the fatigue crack is approximated by a circular arc, leading to a formulation for an efficient computation of the field-flaw interaction. Various numerical and experimental results supporting the modeling and illustrating the behavior of the magnetic field and electric potential at the metal surface around circular-arc cracks are presented     

圆柱类金属构件表面裂纹的激光超声识别方法研究

为了利用激光超声技术有效地识别圆柱表面裂纹,提出利用圆柱表面波信号增强和小波包-奇异值分解(WPT-SVD)方法识别圆柱类金属构件表面裂纹的位置和深度。建立了圆柱的激光超声显式有限元模型,分析了圆柱表面裂纹对表面波的模式转化作用。利用圆柱表面裂纹在激发源位置附近时激光超声扫描信号增强的现象,识别圆柱表面裂纹的位置。在已识别圆柱表面裂纹位置的基础上,通过分析圆柱表面裂纹检测信号的时频特点,利用WPT-SVD提取圆柱表面信号的时频特征,定义参数k r表征裂纹深度的变化,识别圆柱表面裂纹深度。搭建了激光超声圆柱表面裂纹检测实验系统,开展了实验研究,实验结果表明所提出的圆柱表面信号增强和WPT-SVD方法可以识别出圆柱表面裂纹的位置和深度。     

The measurement and analysis of semi-elliptical surface fatigue crack growth

Fatigue tests in which the development of crack shape was studied with the a.c. field measurement technique have been conducted on semi-elliptical cracks growing in flat plate specimens under tension and bending stresses. The a.c. field measurement technique, which was used to measure the cracks, exploits the fact that high frequency alternating current tends to flow in a thin skin along the metal surface and the crack depths can be interpreted from the changes in the voltage distribution produced by the presence of a crack. In order to interpret the measured voltages so as to provide accurate estimates of the crack length and depth, a theoretical analysis of the a.c. field around a semi-elliptical crack is required. This analysis was used to interpret the measured voltages in terms of the changes in the crack shape. Finally the interpreted data on the changes in crack shape were then used to test the accuracy of various stress intensity factor solutions.     

Numerical and Experimental Study of Crack Depth Measurement in Concrete Using Diffuse Ultrasound

This paper presents a combined numerical and experimental study on the diffuse ultrasonic measurement technique for determining the depth of surface breaking cracks in concrete. A finite element (FE) model for the dissipative diffusion in a two-dimensional domain with a surface breaking crack is developed using a commercial FE package; for this purpose, the dissipation term is eliminated by a simple change of variables. Three concrete blocks with a crack depth between 25.4 mm to 101.6 mm are prepared. Diffuse ultrasonic measurements are performed on uncracked and cracked concrete blocks, from which the diffuse energy evolution curves are obtained. The basic material parameters of the hardened concrete, i.e. the diffusivity and dissipation, are retrieved, which are needed for the numerical simulations. The crack depths are then determined by comparing the experimental and numerical arrival times of the average diffuse ultrasonic energy. Various geometrical configurations that arise in real-world concrete structures are simulated including an inclined crack, a partially closed crack, two parallel cracks, and a crack with an underlying reinforcement bar. The objective is to investigate the possible limitations of the diffuse ultrasonic measurement technique when implemented in real concrete structures. Finally, it is shown that the time of flight (TOF) of the average diffuse ultrasonic energy constitutes the theoretical basis of the present diffuse ultrasonic measurement of macroscopic cracks and therefore the present diffuse ultrasonic method forms another kind of TOF technique.     

ESEM observations of SCC initiation for 4340 high strength steel in distilled water

The stress corrosion cracking (SCC) initiation process for 4340 high strength steel in distilled water at room temperature was studied using a new kind of instrument: an environmental scanning electron microscope (ESEM). It was found that the applied stress accelerated oxide film formation which has an important influence on the subsequent SCC initiation. SCC was observed to initiate in the following circumstances: (1) cracking of a thick oxide film leading to SCC initiation along metal grain boundaries, (2) the initiation of pits initiating SCC in the metal and (3) SCC initiating from the edge of the specimen.All these three SCC initiation circumstances are consistent with the following model which couples SCC initiation with cracking of a surface protective oxide. There is a dynamic interaction between oxide formation, the applied stress, oxide cracking, pitting and the initiation of SCC. An aspect of the dynamic interaction is cracks forming in a protective surface oxide because of the applied stress, exposing to the water bare metal at the oxide crack tip, and oxidation of the bare metal causing crack healing. Oxide crack healing would be competing with the initiation of intergranular SCC if an oxide crack meets the metal surface at a grain boundary. If the intergranular SCC penetration is sufficiently fast along the metal grain boundary, then the crack yaws open preventing healing of the oxide crack. If intergranular SCC penetration is not sufficiently fast, then the oxidation process could produce sufficient oxide to fill both the stress corrosion crack and the oxide crack; in this case there would be initiation of SCC but only limited propagation of SCC. Stress-induced cracks in very thin oxide can induce pits which initiate SCC, and under some conditions such stress induced cracks in a thin oxide can directly initiate SCC.     

AN APPROACH TO MEASURE THE SHAPES OF THREE-DIMENSIONAL SURFACE CRACKS DURING FATIGUE CRACK GROWTH

This study presents a technique for measuring shapes of three dimensional surface cracks continuously during fatigue crack growth. The technique uses a laser interferometric system to measure crack compliance and a photomicroscopic system to measure surface crack length. Using a compliance expression for surface cracks, valid for the range of crack aspect ratios (a/c) of 0.2 to 2.0, aspect ratio calculations employing compliance and surface crack length measurements are demonstrated for cracks growing from EDM notches of different geometries (shallow or deep). The experimentally determined aspect ratio variations during cyclic crack growth are shown to be in good agreement with the expected variations in aspect ratio, predicted using the stress intensity factor equations for surface cracks. The effects of deviations in the compliance measurement location from the center of the surface crack due to assymetric crack growth are also accounted for through corrections of compliance measurements for crack-asymmetry. The fatigue crack growth rates of surface cracks, after incorporating the variations in aspect ratio in the calculations, agreed with the large-crack growth data for all crack geometries.     

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.     

Characterisation of crack-tip fields in biaxial fatigue based on high-magnification image correlation and electro-spray technique

This work presents a novel methodology for characterising fatigue cracks under biaxial conditions on a low carbon steel. It allows both short crack and early propagation stages to be studied in tubular specimens. Short crack growth is studied with a long-distance microscope acquiring images of the bare metal surface. Results showed oscillations in crack growth rate due to microstructure. Early propagation stage is studied with high magnification Digital Image Correlation (DIC) technique for measuring displacement and strain crack-tip fields. By applying micro-speckle pattern on the metal surface it is possible to achieve high magnification for DIC technique. Ultra-fine black and white speckles were created by electro-spray technique. The validity of this novel technique is demonstrated by direct comparison with extensometer measurements, under combined tension–compression and torsion conditions. It was also possible to estimate satisfactorily the mixed-mode stress intensity factor.     

Microwave Detection of Stress-Induced Fatigue Cracks in Steel and Potential for Crack Opening Determination

Fatigue crack detection in metals is an important practical issue in many industries. In this paper the results of detecting fatigue cracks, using the dominant mode approach, employing flange-mounted, open-ended, rectangular waveguides at several microwave frequencies are presented. The goal of this investigation has been to demonstrate the capability of this approach for detecting stress-induced cracks under various static loads. In addition, a correlation between the features of the measured crack characteristic signals and crack opening has been sought. The results show that at all of the investigated frequencies, cracks from being nearly closed to having openings of up to 0.0508 mm are detected effectively. Furthermore, it is found that the interaction of the flange edge with a crack results in features that can be used to enhance crack detection robustness significantly (i.e., increased probability of detection). Several features associated with these measured crack characteristic signals are shown to correlate linearly with crack opening. Such simple correlations may then be used to estimate a crack opening closely after it has been detected using this approach. A complete discussion of the results is also provided in this paper.     

Stress intensity factor solutions for a cracked bolt loaded by a nut

This paper presents the calculation of stress intensity factor (K) solutions for surface cracks in the thread ground of bolts subjected to axial loading directly applied by the nut. The stress-strain computations have been done by means of the finite element method with quarter-point singular isoparametric elements along the crack front. The stress intensity factor is calculated through the stiffness derivative method, by using a virtual crack extension technique to compute the energy release rate. Two modifications are made to improve the accuracy of the results: the displacement not only of the main node, but also of the quarter-point nodes located in the normal plane and the adjacent nodes in the crack line, avoiding both the change of the singularity and the crack curving. The results show that direct loading on the thread flank by a nut increases the stress intensity factor. This effect decreases with the crack length. For the deepest circular cracks, however, nut loading relaxes the K-value, mainly at the crack surface.     

Fatigue growth analysis of interacting and coalescing surface defects

Fatigue crack-growth is simulated for multiple surface cracks in a plate subjected to different combinations of tension and bending loads. The numerical technique employed is based on the step-by-step integration of a Paris' type of fatigue crack-growth law at a set of points of crack front, which enables the crack shape change during propagation to be traced. The stress intensity factors along the crack front are estimated by the three-dimensional finite element method. The technique has also accommodated an automatic procedure for the regeneration of finite element models as the crack grows, so that the prediction of complex crack shape change undergoing particularly during the crack coalescence is made possible and easy. The fatigue growth behaviours at three typical growth stages, i.e. pre-coalescence, coalescence and post-coalescence, are analysed for two multiple crack configurations. The predicted results are also compared with those obtained by the ASME XI code and a ‘no interaction and immediate transition (NIIT)’ simplified method. It is shown that each individual crack before they touch propagates almost independently and the interaction between them is generally limited despite being dependent on actual crack configurations. The analyses of fatigue lives show that both the ASME XI code and the NIIT method give more conservative results than the present simulation technique; however, the NIIT method is relatively less conservative than the ASME XI code which is excessively conservative for the crack configuration examined. This revised version was published online in July 2006 with corrections to the Cover Date.     

Interaction and evolution of short fatigue cracks

Distinguishing the different contributions to fatigue damage of short cracks having different sizes and locations on the specimen surface, three new concepts, referred to as effective short fatigue cracks (ESFCs), dominant effective short fatigue cracks (DESFC), and density of ESFCs, respectively, are introduced to facilitate an understanding of the mechanism of interaction and evolution of short cracks. These concepts are interrelated and in conjunction produce an 'effective short fatigue crack criterion'. Replica observations of 19 smooth axial specimens of 1Cr19Ni9Ti stainless steel weld metal during low-cycle fatigue tests reveal that the short cracks contribute to the fatigue damage of specimens due to the formation of a critical density of ESFCs. The density reflects the local microstructural growth conditions ahead of the DESFC tips. The DESFC behaviour is a result of interactive short cracks, and this behaviour is deemed suitable to describe the collective behaviour of short cracks. In the microstructural short-crack stage, the DESFC are located in the weakest zone. Due to an irregular microstructural barrier effect, the crack density is higher in this zone and increases with fatigue cycling to reach a maximum value at the transition point into the physical short-crack stage. Then, due to the effects of accelerating coalescence and the DESFC size shielding the formation of new cracks, the density decreases rapidly and tends gradually to a saturation value. This is why the short-crack growth rate is high initially and tends gradually to that of long-crack behaviour. The difference and change in local microstructural growth conditions ahead of DESFC tips are the intrinsic cause of the statistical behaviour of short cracks and the scatter of fatigue lives.     

Microwave Detection of Stress-Induced Fatigue Cracks in Steel and Potential for Crack Opening Determination

Abstract

Fatigue crack detection in metals is an important practical issue in many industries. In this paper the results of detecting fatigue cracks, using the dominant mode approach, employing flange-mounted, open-ended, rectangular waveguides at several microwave frequencies are presented. The goal of this investigation has been to demonstrate the capability of this approach for detecting stress-induced cracks under various static loads. In addition, a correlation between the features of the measured crack characteristic signals and crack opening has been sought. The results show that at all of the investigated frequencies, cracks from being nearly closed to having openings of up to 0.0508 mm are detected effectively. Furthermore, it is found that the interaction of the flange edge with a crack results in features that can be used to enhance crack detection robustness significantly (i.e., increased probability of detection). Several features associated with these measured crack characteristic signals are shown to correlate linearly with crack opening. Such simple correlations may then be used to estimate a crack opening closely after it has been detected using this approach. A complete discussion of the results is also provided in this paper.     

Developments in the analysis of interacting cracks

This paper presents an overview of the finite element alternating technique for the analysis of interacting cracks. To illustrate the ease and accuracy of this method the technique is used to analyse several problems associated with both widespread fatigue and multi-site damage, a problem which is attracting worldwide attention. Whilst this paper presents an overview of the technique for both two- and three-dimensional problems attention is focused on three-dimensional problems. In particular, the interaction effects between two fully embedded elliptical flaws and between two semi-elliptical surface flaws, and the effects of crack proximity and crack aspect ratio on the stress intensity factors are presented. For semi-elliptical surface flaws these results indicate that as the cracks approach each other the position of the point on the crack front with the highest stress intensity factor shifts. This subsequently suggests that surface cracks will tend to grow preferentially towards each other. The same trend is evidenced for fully embedded cracks. However, in this case there is no shift in the position of the maximum stress intensity factor. A discussion of the results in terms of stress intensity magnification factors is also presented.     

Finite-element modelling of crack propagation in elastic–plastic media: Part II Horizontal subsurface cracks

Horizontal subsurface cracks in an elastic–plastic material are analysed using finite-element techniques. The sliding surface is modelled as a rigid cylinder. The effect of such parameters as the friction between the cylinder and the material being indented, the elastic and plastic modulus of the material and the depth of crack location on the J-integral values at the left and right tips of a horizontal subsurface crack is considered. The prospective crack propagation direction is taken as the direction along which the J integral assumes a maximum as the indenter slides along the material surface. The left and right tip cracks were found likely to propagate at about 10° to the horizontal. This propagation direction was found to depend strongly on the location of the crack. Both crack tips are expected to propagate closer to the vertical direction as the depth of crack location is reduced. Also, horizontal cracks closer to the surface are found to have higher J integral values. While friction between the slider and the specimen did not affect the crack propagation direction, the crack-tip plasticity reduced the propagation direction, with respect to the horizontal.     

The cracking and fracture of mortar

The load-induced cracking of a small compact tension specimen of mortar was observed using a special loading device mounted in the specimen chamber of a scanning electron microscope. The crack was initiated at a load of 43.6 N and immediately extended about 12 mm, whereupon it became stable. Micromorphological aspects of the crack pattern are described in considerable detail. In general, it was observed that the crack geometry is complicated, with the crack path preferentially progressing at the interface between sand grains and cement paste, but not limited to these regions. A considerable amount of energy must be dissipated in creating the tortuous crack surface and in multiple or branch cracking. This technique permits resolution of much finer cracks than can be detected with ordinary optical methods.