Finite-element calculations of remote field eddy current interactions with slit defects

Remote field eddy current (RFEC) excitation is a promising approach for detection of the very fine axial cracks typical of stress corrosion cracking (SCC) in pipelines. Interactions between adjacent cracks or slits can enhance responses in some cases. Detailed finite-element modeling was undertaken to establish the behavior and interactions of multiple slits such as those occurring in SCC. Three different field/slit configurations are considered, with anomalous source models used to aid interpretation of the results. The study noted that magnetic perturbations generated by ferromagnetic material tend to be vanishingly small, and that the interactions between multiple cracks give minimal enhancement, indicating that eddy current rather than magnetic field excitation is best for the detection of SCC. With eddy current excitation, field perturbations are generated by even very fine slits, and are larger in non-ferromagnetic material. For nonferromagnetic pipes, the perturbations tend to merge as a circumferential separation between parallel axial cracks decreases, resulting in significant interaction and signal enhancement.     

Finite-Element Calculations of Remote Field Eddy Current Interactions with Slit Defects

Abstract

Remote field eddy current (RFEC) excitation is a promising approach for detection of the very fine axial cracks typical of stress corrosion cracking (SCC) in pipelines. Interactions between adjacent cracks or slits can enhance responses in some cases. Detailed finite-element modeling was undertaken to establish the behavior and interactions of multiple slits such as those occurring in SCC. Three different field/slit configurations are considered, with anomalous source models used to aid interpretation of the results. The study noted that magnetic perturbations generated by ferromagnetic material tend to be vanishingly small, and that the interactions between multiple cracks give minimal enhancement, indicating that eddy current rather than magnetic field excitation is best for the detection of SCC. With eddy current excitation, field perturbations are generated by even very fine slits, and are larger in non-ferromagnetic material. For non-ferromagnetic pipes, the perturbations tend to merge as a circumferential separation between parallel axial cracks decreases, resulting in significant interaction and signal enhancement.     

Finite-Element Analysis for Remote Field Eddy Current Responses from Near- and Farside Cracks

Results of three-dimensional finite-element analysis investigations of remote field eddy current (RFEC) signal responses from internal and external stress corrosion cracking (SCC) in steel pipes are presented. The fine shallow SCC cracks were simulated by double axial slits located on the near- and farside pipe surfaces under simulated RFEC excitation. Although it is normally a characteristic of RFEC testing that responses to interior or exterior defects are approximately equal, there is a considerable difference between the farside and nearside responses from very fine defects such as SCC cracks.     

Longitudinal AC interactions with axial slits in steel pipes

The Remote Field Eddy Current (RFEC) nondestructive inspection technique uses low frequency AC and through wall transmission to inspect pipes and tubes from the inside. In steel pipes, it has generally greater sensitivity to circumferential rather than axial slits because the perturbation of magnetic fields orthogonal to slits dominates. Circumferential AC magnetic fields, generated by passing AC axially along a steel pipe from an external supply, have therefore been tested in order to give greater sensitivity to axially aligned cracks characteristic of stress corrosion cracking in pipe-lines. Anomalous source missing magnetization defect models suggest that, as slit widths are reduced, the importance of magnetic interactions is reduced until eddy current interactions predominate. This suggests that, for very fine axial cracks, true RFEC geometry, which gives circumferential eddy currents, will give stronger signals than circumferential AC magnetic fields.Research supported by Natural Sciences and Engineering Research Council of Canada, Pipetronix Ltd., and Gas Research Institute.     

V7002罐封头开裂原因分析

国内现有最大克劳斯硫回收装置的V7002罐封头发生开裂,对该罐开裂原因进行了分析。该封头存在三种开裂：容器内硫化氢产生氢原子向钢中扩散并聚集形成氢鼓泡;不同层面上的相邻氢鼓泡裂纹在内压下相互连接,形成氢致开裂,和氢鼓泡同为内部开裂;在内压下鼓泡位置产生拉应力,在拉应力和湿硫化氢环境下发生硫化物应力腐蚀开裂,为外部开裂。介绍了检验方法和通过改善材料和防腐工艺避免开裂方法。     

Zusammenwirken von Spannungsrißkorrosion und Ermüdung bei der Schwingungsrißkorrosion eines hochfesten Stahles

Conjoint Action of Stress Corrosion Cracking and Fatigue on Corrosion Fatigue of a High Strength Steel The corrosion fatigue characteristics of a high strength, martensitic steel in 0.5 n NaCl solution is investigated with regard to the fatigue and stress corrosion cracking behaviour of the material. Test parameters are stress ratio and frequency, testing is carried out with fracture mechanics methods, the crack surfaces are examined fractographically. An analysis of the results reveals that corrosion fatigue in high strength steel is caused by fatigue or by stress corrosion cracking, depending on the kinetics of the two processes. Fatigue and stress corrosion cracking do not act cumulative or additive. Instead, the kinetically faster process causes crack advance. The crack growth characteristics are interpreted with respect to the fractographic appearance of the crack surfaces. Corrosion fatigue cracks propagate either intergranular relative to the prior austenite grain boundaries as stress corrosion cracks do or transgranular like fatigue cracks, depending on the crack growth rates of the two processes. Fatigue and stress corrosion cracking do not interact, at least in a measurable degree, because of the different crack path of the two fracture processes. Results can be assessed quantitatively with the “process competition model”.     

Techniques for depth-selective,low-frequency eddy current analysis for SQUID-based nondestructive testing

Conventional eddy current techniques are widely used for detection of surface-breaking cracks in metal structures. These techniques have limited success in the detection of deep, nonsurface-breaking flaws that require low frequency eddy currents, for which inductive pick-up probes have drastically reduced sensitivity. High resolution, Superconducting QUantum Interference Device (SQUID) magnetometers, which are very sensitive to do or low frequency magnetic fields, have been developed for detection of subsurface flaws. We have now extended SQUID NDE by utilizing a sheet inducer to produce an extended eddy current parallel to the surface in a conducting plate. The magnitude of the induced current density inside the plate reduces with the depth; however, the current component at a certain phase angle may increase with the depth. At a particular phase angle, the current density on the surface becomes zero, while the current inside the plate is large, so that the magnetic signal at that phase angle due to the surface structures can be minimized. With this method, we have detected simulated cracks in the sides of plugged holes in a thick plate, a hidden corrosion area in a specimen which consisted of two painted aluminum plates joined with sealant, as well as crack defects adjacent to fasteners in the second layer of lap joined aluminum plates. We present a theoretical model for simulation of the phase-related magnetic signal due to a flaw, which shows the relation between the phase angle and the depth of the flaw. The theoretical phase analysis is compared with the experimental results.     

Environmentally assisted cracking of aluminium alloys

A short review of the stress corrosion cracking (SCC) behaviour of aluminium alloys is given. Mechanisms of environmentally assisted cracking are outlined. For aluminium alloys, in which stress corrosion cracks propagate predominantly along grain boundaries, anodic dissolution and hydrogen embrittlement have been proposed. Transgranular stress corrosion cracking occurring at severe loading conditions has found particular interest concerning localised corrosion‐deformation interactions. Accelerated test methods for assessing the SCC behaviour are described, including the slow strain rate testing technique and the breaking load method. Results of recent studies on environmentally assisted cracking of aluminium alloys are summarised. Most of the work published in the last two decades has been on aluminium‐lithium based alloys and improved high strength Al‐Zn‐Mg‐Cu alloys in corrosion resistant retrogressed and re‐aged tempers.     

The failure of an armour faced conveyor connector

The combined use of modern metallurgical techniques for fracture examination, laboratory test data and fracture mechanics calculations allows metallurgical failures to be examined in a quantitative manner. Complex load histories and environments can result in more than one sub-critical cracking mechanism occurring in a component. Quantitative understanding of the rate determining cracking process is a necessary prerequisite to rectifying the problem. The following case study describes a connector from an armour faced conveyor which failed in service. The failure investigation involved fractography, stress analysis, material property evaluation and fracture mechanics calculations. Fractographic evidence indicated a stress corrosion failure mechanism. Calculations of critical crack sizes showed that stress corrosion cracking alone could not account for the fracture. It was concluded that the failure was due to a sequence of three cracking processes which preceded unstable ductile fracture. Firstly, frictional heating caused rubbing or quench cracks typically 0.5–1 mm deep. Secondly, corrosion fatigue cracks grew several millimetres allowing the third fracture process, stress corrosion cracking, (SCC) to initiate and grow. In the situation described here, this process was much faster than corrosion fatigue. The influence of defect size due to rubbing cracks and the influence of K_ISCC have been compared with the corrosion fatigue life of the component. An increase in K_ISCC and hence critical defect size for SCC has been shown to increase the corrosion fatigue life of the component by a large factor. A change in design would also alleviate the problem of SCC by reducing the static stress, which is the driving force for SCC.     

球罐应力腐蚀开裂研究与安全性分析

对球罐应力腐蚀开裂的原因和主要影响因素进行了分析.针对16MnR和SPV50Q球罐用钢,在分析湿硫化氢环境下应力腐蚀开裂形式的基础上,通过改进的WOL预裂纹试样的应力腐蚀开裂试验,对不同球罐用钢、不同硫化氢浓度、不同焊接状态条件下的应力腐蚀开裂进行了研究,并对设备的安全性能进行了分析,进而提出了防止应力腐蚀开裂的对策.     

流量计导压管断裂失效分析

某化工厂工艺管线上的316不锈钢材质的孔板流量计导压管断裂,导致介质泄漏发生火灾。为查明其失效原因,对断裂的仪表管进行成分、硬度、金相、断口形貌和腐蚀产物分析,确认仪表管发生断裂的原因是在安装应力、震动和环境中Cl元素的共同作用下,先发生了应力腐蚀形成裂纹源,裂纹达到门槛值后又以疲劳形式扩展,最终导致开裂。     

Eddy Current Signal Deconvolution Technique for the Improvement of Steam Generator Tubing Burst Pressure Predictions

Eddy current techniques are extremely sensitive to the presence of axial cracks in nuclear power plant steam generator tube walls, but they are equally sensitive to the presence of dents, fretting, support structures, corrosion products, and other artifacts. Eddy current signal interpretation is further complicated by cracking geometries more complex than a single axial crack. Although there has been limited success in classifying and sizing defects through artificial neural networks, the ability to predict tubing integrity has, so far, eluded modelers. In large part, this lack of success stems from an inability to distinguish crack signals from those arising from artifacts. We present here a new signal processing technique that deconvolves raw eddy current voltage signals into separate signal contributions from different sources, which allows signals associated with a dominant crack to be identified. The signal deconvolution technique, combined with artificial neural network modeling, significantly improves the prediction of tube burst pressure from bobbin-coil eddy current measurements of steam generator tubing.     

弹壳开裂失效分析

对某型H68黄铜弹壳进行检查时发现大量弹壳存在裂纹。通过对弹壳进行宏观和微观观察、能谱分析、金相检验和残余应力测定等,分析了弹壳开裂的原因。结果表明:弹壳的开裂性质为应力腐蚀开裂,其原因是由于收口位置存在一定的残余拉应力,且该位置的晶粒尺寸较大,在外界介质的协同作用下产生了应力腐蚀裂纹。     

Reconstruction of Cracks with Multiple Eddy Current Coils Using a Database Approach

This paper presents some recent studies dealing with flaw reconstruction using eddy current testing. The locations of parallel planar cracks in an Inconel 600 plate specimen are determined and the shapes of them are reconstructed at the same time using the signals from a multi-pancake coil probe. The reconstruction strategy, consisting of a data base used fast forward solver and a first order optimization algorithm, relies on the minimization of the nonlinear least square residual function. Validity of the inverse analysis method is investigated with respect to the influence of the arrangement of scanning paths. The efficiency of the computation is also discussed.     

Evaluation of Equibiaxial Compressive Stress Introduced into Austenitic Stainless Steel Using an Eddy Current Method

Equibiaxial compressive residual stress is introduced into steel after peening in order to improve both its resistance to stress corrosion cracking and its fatigue strength. Thus, a nondestructive and relatively quick method to evaluate the equibiaxial compressive residual stress in a surface layer modified by peening is required in order to evaluate the peening intensity needed to enhance the integrity of structural components. The purpose of the work reported here is to establish an eddy current method to evaluate equibiaxial compressive stress which can be applied to the residual stress introduced into various non-ferromagnetic materials after peening. To this end, hydraulic jacks were used to elastically deform specimens of the austenitic stainless steel, Japanese Industrial Standard (JIS) SUS316L, thereby introducing an equibiaxial compressive stress. In the case of SUS316L steel, stress-induced martensitic transformation is rare. The electromagnetic properties of these specimens were then measured. In addition, the eddy current signals from peened specimens were compared with these. The results demonstrate that it is possible to establish a method for evaluating the equibiaxial stress utilizing eddy current signals.     

飞机多层金属板隐藏缺陷远场涡流探头设计及试验研究

由于飞机多层金属板结构厚度大、复杂等特性,现有检测方法无法发现原位内部缺陷,对于多层板金属缺陷的检测一直都是航空无损检测的难题。远场涡流检测技术因打破趋肤效应的限制,涡流能量可穿透较厚的被测试件,对金属板结构中隐藏缺陷的检测具有潜在优势。该文针对飞机多层金属板隐藏裂纹的原位检测,建立多层金属板构件隐藏裂纹平面远场涡流检测有限元仿真模型,研究不同角度、不同深度裂纹检测幅值、相位的变化规律。为验证该仿真模型的正确性,开展远场涡流检测多层金属板的试验。试验结果表明:设计开发的远场涡流探头可检测埋深13 mm的裂纹缺陷,当裂纹倾斜角度为0°时,检测灵敏度最高,当裂纹倾斜角度为90°时,检测灵敏度最低,与仿真结果保持一致,且能够对缺陷进行精准定位,可为飞机多层金属板构件隐藏裂纹的定量检测提供依据。     

Use of magnetically-saturated regions in remote field eddy current tools

The remote field eddy current (RFEC) technique is of potential interest for inspecting pipelines for stress corrosion cracking. Magnetic saturation techniques would allow higher operating frequencies and scanning speeds to be used. The use of selectively-saturated regions (windows) near the exciter and detector offers additional advantages. Finite element, analytic, and experimental measurements are presented in this paper. They show that, while saturation techniques are helpful, the effects are less than initially estimated from simple skin depth approximations.     

Improving the properties of aluminium alloys by retrogression and reageing

Retrogression and reageing heat treatments offer the potential of improved tensile properties in combination with greatly increased resistance to stress corrosion cracking. The potential of this technique is reviewed with respect to the current application in the European and North American Aerospace Industry. To illustrate the performance increase associated with RRA treatments, the stress corrosion cracking performance of the established aerospace plate and forging alloy 7010 has been evaluated using an alternate immersion constant load tensile type test (ECSS‐Q‐70‐37A) Specimens were cut from a large aerospace rectilinear forging and tested in three different tempers, T652, T7452 and a retrogressed and reaged condition (RRA). In the T652 condition the material has been shown to be highly susceptible to intergranular corrosion and stress corrosion cracking. In the T7452 and RRA conditions 7010 showed much improved resistance to SCC but pitting corrosion resulted in failure of some specimens within the 30day requirement of the test standard.     

Failure Analysis of AISI-304 Stainless Steel Styrene Storage Tank

This paper presents the failure analysis of AISI-304 stainless steel tank that was fabricated by welding and used for the storage of styrene monomers. After about 13 years of satisfactory operation, significant cracking was observed adjacent to the weld joints and in base plate near tank foundation. Weld repair was by shielded gas arc welding using AISI 308 stainless steel filler wire. The failed base plate was replaced with the new AISI 304 base plate of same thickness. After a short period of time, seepage was observed along the weld bead. Upon nondestructive testing cracks were found in the heat-affected zone and in the base plate. The failure investigation was carried out on welded and base plate samples using spectroscopy, optical and scanning electron microscopy, fractography, SEM–EDS analysis, microhardness measurements, tensile and impact testing. The results revealed transgranular cracks in the HAZ and base plate, and the failure was attributed due to stress corrosion cracking. Cracks initiated as a result of combined action of stresses developed during welding and the presence of a chloride containing environment due to seawater. It was further observed that improper welding parameters were employed for weld repair which resulted in sensitization of the structure and postweld heat treatment to remove weld sensitization and minimize the residual stresses was not done.     

Micromechanics of dislocation channeling in intergranular stress corrosion crack nucleation

While driven by a combination of stress, a susceptible microstructure and an aggressive environment, the mechanistic origin of stress corrosion cracking remains poorly understood. The emergence of localized deformation as a key process in SCC has resulted in considerable experiment and simulation studies. The effectiveness of irradiation in localizing deformation into dislocation channels has provided a tool for studying the interaction between channels and grain boundaries. Experiment and simulation have shown that normal stress can be in excess of twice the applied stress and that cracking correlates well with the high normal stress. Shear stresses in the channel can add an additional component to the normal stress at the channel-boundary intersection. While the exact role of localized deformation in stress corrosion cracking is not yet full understood, it is known that the degree of localized deformation correlates well with SCC susceptibility. Further, both experiments and simulations indicate that cracks preferentially nucleate in grain boundaries that are perpendicular to the loading direction, are non-special high angle boundaries, are not oriented for easy deformation under the applied load, and are effective barriers to slip transmission. This paper will review recent progress in understanding the behavior of localized deformation and the impact on stress corrosion cracking.