Magnetic detection of prestressing steel fractures in prestressed concrete

The paper deals with the remanent magnetism method (RM-method) to identify unsafe prestressed structures concerning sudden collapses of structural components due to fractures of the prestressing steel. It describes the non-destructive testing method that allows to localize fractures of single prestressing steel wires even in post-tensioned tendons. The method uses the magnetic field of externally magnetized tendons, measured at the concrete surface. The parameters that determine the characteristic fracture signal have been studied and are reported. The method has been successfully applied on full size units.     

Spannungsrißkorrosion an Spannstählen

Stress corrosion cracking of prestressing steels During the investigation of a post-tensioned bridge structure incipient cracks of the prestressing steels of the transverse prestressed members were observed. Defects related to non-injected ducts or the presence of corrosion inducing substances could not be detected. The prestressing steel used is a quenched and tempered steel, strength class St 140/160, which was produced in the former GDR. The cause for the cracks is the susceptibility of this type of steel to hydrogen-induced stress corrosion cracking as could be shown in laboratory tests. Under unfavourable conditions cracks can be initiated before grouting. Additional magnetic particle tests at selected areas of the longitudinal prestressed members did not indicate any signs for incipient cracks.     

Temporärer Korrosionsschutz von Spannstählen in unverpressten Hüllrohren

Temporary corrosion protection of prestressing steels in non‐injected ducts Prestressing steels may be subjected to corrosive conditions during the manufacturing process at building sites. Due to this a risk of hydrogen induced stress corrosion cracking of the steels may arise. Tests under practical conditions in a prestressed concrete beam were carried out where non‐injected ducts were treated with preheated scavenging air to prove this method being able to protect the prestressing steels against corrosion. The results yielded sufficient corrosion protection by this measure and therefore it may be an interesting alternative in comparison to corrosion protection by film forming agents which contain inhibitors.     

几种电磁无损检测方法的工作特征

从工作原理入手,对电磁无损检测中的涡流（单、多频和脉冲涡流）、交流漏磁、交变磁场等方法,剖析了它们在激励磁场的成分、频段、作用区域及检测磁场的特性方面的联系和区别。对比了各方法的用途和特点,以期对无损检测应用实践提供借鉴。     

Failure mechanisms of high strength steels in bicarbonate solutions under anodic polarization

High strength steels used in prestressing concrete structures are not exempt from the effects induced by corrosion on the normal concrete reinforcement. Carbonation of surrounding concrete or mortar is not unlikely for prestressing tendons and strands. Moreover, these steels undergo to brittle fracture as a consequence of stress corrosion cracking phenomena. To evaluate if concrete carbonation can promote this kind of failure, constant load tests in bicarbonate aqueous solutions under anodic polarization were carried out on high strength steel wires. Microscopic examination pointed out that the wires exhibited a brittle fracture mode, while its natural feature is ductile, as indicated by air testing. Failure mechanism was evaluated by a fracture mechanic approach. Cracks initiation was attributed to an anodic dissolution mechanism, while its propagation, interpreted by means of the surface mobility theory, was related to interaction between hydrogen atoms and magnetite at a crack tip.     

Influence of steel wrapping on magneto-inductive testing of the main cables of suspension bridges

The number of structurally deficient or functionally obsolete bridges is likely to increase in coming years due to continued bridge aging and deterioration and a decrease in funding needed to maintain and repair these structures. In the case of suspension bridges, problems arise due to corrosion and wire breaks within the main cables of the bridge. The magnetic flux leakage (MFL) method has been known for many years and is used for inspection work in different fields, such as the non-destructive evaluation (NDE) of pipelines, arial tramways or stay cable bridges. A special characteristic of the main cables of suspension bridges is the presence of steel wrapping, which is used to compact the cable and hold it in shape. If a NDE by means of magnetic methods is performed on such a cable, disturbances due to the wrapping can be expected in the measured signal. Furthermore, the very large diameters of these cables (up to 500 mm) poses an unsolved challenge in the application of magnetic non-destructive testing methods.The finite element method (FEM) and other simulation techniques are widely used in any field of engineering. The possibility to simulate different scenarios and the ability to quantify physical properties at any position where real measurements cannot be performed are among the main advantages of such methods. In this work, a FE model is presented to investigate the influence of the steel wrapping on MFL data. After the characterization of the disturbance, a postprocessing algorithm to eliminate the disturbances is discussed. Further, boundary conditions for a successful application of the proposed method in a real application are formulated.     

Capacity of the remanent magnetism method to detect fractures of steel in tendons embedded in prestressed concrete

Sudden collapses of structural components due to fractures of the prestressing steel have occurred. The paper describes the successful application of a nondestructive testing method to identify defective structures. The remanent magnetism method makes it possible to localize fractures of single prestressing steel bars in postensioned tendons. The tendons need not be accessible at any point, not even at the jacking ends. The ferromagnetic properties of the prestressing steel are used in this process. Characteristic magnetic leakage fields are caused by fractures. The magnetic field, measured at the concrete surface along the projection of the tendon profile, allows conclusions concerning existing fractures. Also the extent of damage (number of broken steel bars) can be evaluated.     

Assessment of test methods for evaluation stress corrosion cracking susceptibility of prestressing steels

Prestressing steels can be exposed in prestressed concrete structures in construction stage (ungrouted duct) to environmental conditions causing formation and growth of hydrogen induced cracks with brittle fracture of the steel. The risk of this hydrogen induced stress corrosion cracking can be minimized by appropriate treatment, but there is a requirement for construction to approve only prestressing steel having no enhanced susceptibility to stress corrosion cracking. The paper describes the historical development of the different test methods proposed in the past and gives an overview on the general requirements for a suitable test method to assess the hydrogen‐induced stress corrosion susceptibility of prestressing steels. Since 1982, based on investigations on construction sites and transfer of the results into laboratory tests, a test method is available enabling realistic estimation of suitable application possibilities of prestressing steels. This so called DIBt‐test is approved to distinguish between prestressing steels susceptible to hydrogen induced stress corrosion cracking and those suitable and therefore approvable steels at practical environmental conditions. Furthermore the test enables estimation of corrosion risk for newly developed prestressing steels with higher strength.     

基于场量测量和频率扫描技术的交变漏磁无损检测系统

交变漏磁检测方法由于兼具涡流检测和传统漏磁检测的特点，对表面缺陷具有很高的识别率。在对交变漏磁检测理论进行分析的基础上，提出了基于场量测量和频率扫描技术的快速识别和深度定量检测模型，并通过试验证明了该模型能实现表面缺陷的快速检测和缺陷深度的定量检测。     

Pulsed electromagnetic methods for defect detection and characterisation

The magnetic flux leakage (MFL) method has very good defect detection and location capabilities, but defect sizing capabilities, especially for sub-surface defect characterisation, are limited. The pulsed magnetic flux leakage (PMFL) technique has recently been introduced and shown to have great potential for automated defect sizing for surface-breaking defects using time-frequency signal processing techniques, but sizing of sub-surface defects has proved problematic. In this paper, pulsed magnetic reluctance (PMR), a new electromagnetic (EM) non-destructive evaluation (NDE) technique, is introduced and incorporated into a dual PMFL/PMR probe for the characterisation of surface and sub-surface defects in ferromagnetic materials. Experimental results from a comparison study of the two techniques using variety of defects analysed using time-frequency analysis show that the techniques offer complementary information, with PMFL providing defect location data and data for the characterisation of surface defects and PMR offering sub-surface defect characterisation capabilities. The work concludes that integration of these inspection techniques in the new pulsed EM probe can provide enhanced defect characterisation capabilities for flux leakage-based inspection systems using relatively simple time-frequency signal processing techniques.     

The effect of stresses approaching and exceeding the yield point on the magnetic properties of high strength pearlitic steels

The results of a study of the effects of stresses that approach and exceed the yield point on the magnetic properties of a sample of 50D pearlitic steel are reported. Unlike previous work which has only examined residual stress behaviour, measurements were made in-situ while sample remained under stress. Hysteresis loops, permeability curves and magnetostriction loops are presented and a variety of magnetic parameters analysed. The implications of this analysis for the application of non-destructive evaluation techniques such as magnetic flux leakage, magnetic Barkhausen noise and magnetoacoustic emission are discussed. Key changes in magnetic behaviour occur well before yield and this raises the possibility of developing magnetic NDE methods of predicting when a sample is approaching the yield point.     

基于三维有限元的平行钢丝拉索断丝漏磁场仿真

随着桥梁检测技术的发展,漏磁检测技术开始应用于在役拉索检测。由于拉索自身结构复杂,拉索漏磁检测具有大直径和大提离的特点,导致背景磁场较强。利用有限元方法建立三维漏磁检测模型,通过将存在断丝缺陷的磁场信号与无缺陷的磁场信号求差,获得了真实漏磁场信号;研究了断丝断口宽度、埋藏深度等参数对检测信号的影响,给出了漏磁场随上述参数的变化曲线,从而为拉索漏磁检测信号的解释提供依据,为断丝量化分析奠定了基础。     

常压储罐检验检测技术

综述了常压储罐在使用过程中可能出现的腐蚀和缺陷形态,并根据失效的可能性,给出了有效的检验检测方法,包括宏观检验、厚度检测、表面无损检测、埋藏缺陷检测、底板漏磁检测、声发射检测、真空试漏、附属设施及基础沉降检测等。分别介绍了各检测检验方法的特点以及如何在常压储罐检验中进行实施。     

在用油管剩余壁厚检测方法

提出采用主磁通与漏磁通相结合的方法检测在用油管剩余壁厚的方法。分析了检测原理，介绍了检测传感器的设计。检测时，沿油管周向磁化被测管段，油管螺旋前进，检测探头固定，主磁通用于测量油管的大面积剩余壁厚，漏磁通用于测量小面积壁厚变化，两种不同信号可用软件进行滤波区分，最终达到精确检测剩余壁厚的目的。试验表明了该方法可用于油管剩余壁厚分类。     

Corrosion‐technical properties of high‐strength stainless steels for the application in prestressed concrete structures

Experience with prestressed concrete over about half a century has indicated that the corrosion resistance of conventional prestressing steel does not always satisfy, especially the prestressing steels are susceptible to chloride attack (de‐icing salts) and hydrogen (hydrogen‐induced stress corrosion cracking). On the other hand corrosion agents, such as chloride, condensation water, can penetrate in the concrete and arrive at the surface of steels. Hence, corrosion damage of prestressing steels can happen and, in the extreme cases, the prestressed concrete structure collapsed resulting from the failure of the tendon. In this paper, consideration is made to use high‐strength stainless steels as prestressing tendon with bond in concrete. The high‐strength stainless steels of qualities 1.4301 (X5CrNi18‐10), 1.4401 (X5CrNiMo17‐12‐2), 1.4436 (X3CrNiMo17‐13‐3) and 1.4439 (X3CrNiMoN17‐13‐5) with sequence of increasing austenite stability were investigated. For application in prestressing tendon with bond in concrete the cold‐drawn high‐strength stainless steel of quality 1.4401 is an optimal proposition regarding its satisfactory resistance against pitting corrosion and stress corrosion cracking (SCC) in structure‐related corrosive conditions. The lower alloyed steel 1.4301 has an insufficient resistance against the chloride‐induced corrosion because of the lack of molybdenum and the content of deformation martensite due to the strong cold‐drawing of its unstable austenitic structure.     

Magnetization effect on hydrogen absorption in high-strength steels and its implications

Magnetization effect of hydrogen absorption in high-strength steels has been investigated and the implications of the results for magnetic flux leakage assessment used for determination of defects in line pipe steels are discussed. A strong and continuous remanent magnetic field in steels at a level commonly residual from magnetic flux leakage assessment practices can result in an increase of up to 1.62 times in the absorbed hydrogen concentration.     

Research on magnetic testing method of stress distribution

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A new optical technique for detection of defects in ferromagnetic materials and components

In this paper, we present the details of a new optical technique to detect the defects present in a ferromagnetic material or component, using a ferrofluid emulsion. This new flux leakage probe consists of monodispersed ferrofluid confined between two thin transparent glass slides or in a cuvette and a white light source for illumination. By employing ferrofluid droplets of suitable size and surfactant concentration, one can qualitatively identify the region where the defect is located in the test specimen by visually observing a color change in the ferrofluid cell, in the vicinity of a crack or defect in the test specimen. The origin of this color change in the back scattering direction is due to Bragg scattering from the droplet chains, formed by the leaked magnetic flux in the presence of a defect. We discuss in detail the procedure, the merits and potential uses of this new technique for non-destructive testing and evaluation (NDT and E) applications.     

基于小波包-Haar小波变换的漏磁检测信号降噪数据压缩方法

漏磁检测是对输油管道、油罐等设备缺陷进行无损检测的一种重要方法,但在检测过程中产生的信号数据量庞大,同时会产生多种干扰噪声,出现信号失真、漂移和信号被湮没等问题,对信号进行降噪滤波是缺陷漏磁检测的关键环节。探讨了信号降噪压缩方法,研究了基于小波包-Haar小波算法的漏磁检测信号降噪压缩算法,通过该算法,在对信号数据进行降噪压缩处理同时,保留了高频部分信号特征,从而在降噪后数据解压缩过程中避免了信号失真畸变的现象。     

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.