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A review of three magnetic NDT technologies
Journal of Magnetism and Magnetic Materials 324 (2012) 382–388
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Review
A review of three magnetic NDT technologies Z.D. Wang n, Y. Gu, Y.S. Wang Department of Mechanics, School of Civil Engineering, Beijing Jiaotong University, Beijing 100044, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 May 2011 Received in revised form 26 August 2011 Available online 7 September 2011
Magnetic techniques are most important NDT technologies to characterize the mechanical features of ferromagnetic materials based on the physical principle of magnetic-stress coupling. A review is presented in this paper about the development of the magnetic NDT technologies. After a brief outline of the theoretical studies of the magnetic-stress coupling effect, the three popular magnetic NDT technologies are reviewed, which are magnetic flux leakage (MFL), magnetic Barkhausen noise (MBN) and recently developed metal magnetic memory (MMM). The first two are ascribed to the active magnetic method, and the last one is the passive method. Based on an extensive literature survey in this field, this paper focuses on the discussion of the physical mechanism and some important experimental results relevant to the three NDT technologies. The challenges for each technique in this field are also summarized. & 2011 Elsevier B.V. All rights reserved.
Keywords: Non-destructive technique Magnetic-stress coupling effect Magnetic flux leakage Barkhausen noise Metal magnetic memory
1. Introduction
2. Magnetic-stress coupling theory
A fundamental feature of ferromagnetic materials, which consist of numerous small magnetic domains in the microstructure, is the coupling between stress and magnetic field [1–6]; that is, the magnetization may result in variations in the dimension of ferromagnetic materials, namely magnetostriction [7,8], and on the other hand the stress may also change the magnetization of ferromagnetic materials, the so-called piezomagnetic effect [9,10]. These macro-phenomena are related to magnetic-moments rotation and domain-wall displacement in the micro-structure when ferromagnetic materials are subjected to an applied magnetic field or mechanical stress [11,12]. Compared with the magnetostriction, the piezomagnetic effect has received more attention in non-destructive test (NDT) field, because it is the physical basis to evaluate the stress status of ferromagnetic structures and components by magnetic measuring methods. As a result, a great number of non-destructive magnetic techniques have been developed over the last decades such as magnetic flux leakage (MFL), magnetic Barkhausen noise (MBN), magnetoacoustic emission (MAE), stress-induced magnetic anisotropy (SMA) and recently developed metal magnetic memory (MMM) [13,14]. In the remainder of this paper, the concept and theory of magnetic-stress coupling models in ferromagnetic materials will be presented in Section 2, the development of three typical NDT magnetic techniques are described in Section 3 and the conclusions are presented in Section 4.
It is well-known that magnetic properties of ferromagnetic materials are stress-dependent. For example, the coercivity Hc and permeability m of ferromagnetic materials may be changed in the order of 100% by the stress within the elastic limit [15]. Fig. 1 presents a schematic description of a cyclic stress affecting the magnetization of ferromagnetic materials. The total magnetization M includes the reversible component, Mre, and irreversible component, Mirr. In every loading cycle, an irreversible magnetization difference DMirr is obtained when the domains pass through pinning sites under the action of the mechanical stress [16]. From the microstructural point of view, a ferromagnetic sample is composed of numerous magnetic domains. Every domain is 10 8–10 12 m3 in volume and includes approximately 1012–1015 atoms. The magnetic moments distribute randomly in the initial state, and no magnetization appears in the macro-scale. When an external load is applied to a ferromagnetic sample, a tension tends to orient the domains in the direction of the applied load for a positive magnetorestrictive material and a compression orients the domains perpendicularly to the loading direction due to the piezomagnetic effect [17]. Fig. 2a displays a symmetric structure in which the applied field H¼0. Only in the case of Ha0 (see Fig. 2b), the symmetric structure is destroyed and the ferromagnetic sample is magnetized. Thus, the presence of an external field (e.g. an artificial field or Earth’s magnetic field) is a necessary condition for all magnetic NDT techniques. This may also be concluded from the following experiment: the spontaneous stray field signal is detected when plastically deforming a ferromagnetic sample under the action of the Earth’s magnetic field, but not obtained when the sample was plastically deformed in a magnetic-prohibited environment. Theoretical studies of the magnetic-stress coupling effect provide a physical understanding of various magnetic NDT technologies.
n
Corresponding author. E-mail address: [email protected] (Z.D. Wang).
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Z.D. Wang et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 382–388
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magneto-mechanical effect (see e.g. [19–21]), as follows: dl dB ¼ , ds H dH s
Fig. 1. Schematic diagram of the stress affecting the magnetization of a ferromagnetic s Þ remains in sample. The irreversible component of the residual magnetization ðDMirr every loading cycle.
Fig. 2. Magnetic field and stress affecting the movement of magnetic domains in the micro-structure. Tensile stress orients the domains in the loading direction and compressive stress causes the domains perpendicularly to the loading direction. The magnetic field destroys the systematic domain structure and lead to magnetization (a) without defect and (b) corrupt wall.
In fact, the conceptual description of the interaction between the stress and magnetism may date back to more than half a century ago. Brown [18] first presented a theoretical analysis of the magneto-mechanical effect in ferromagnetic materials by replacing the applied stress with an equivalent field in 1949. Later on, Cullity [19] focused on such phenomena in terms of Le Chatelier’s principle. Further, Sablik et al. [20] considered the variations in hysteresis of ferromagnetic materials under a constant stress. Additionally, the studies of the magneto-mechanical effect based on the concepts of the ‘‘effective field theory’’ and ‘‘law of approach’’ were performed by Jiles [21–24]. The classical magneto-elastic constitution for a ferromagnetic material is generally expressed as [25] (
e ¼ es þ eH ¼ Sr þ Ds H, B ¼ Bs þBH ¼ DH r þ lH,
ð1Þ
where S, l Ds and DH are tensors of the stiffness matrices, effective magnetic permeability, magnetic–elastic coefficient and elastic–magnetic coefficient, respectively; r and H are tensors of the stress and external magnetic field, respectively; the functions of e and B denote strain tensor and effective magnetic field tensor, respectively; es and eH denote the strain components caused by the stress and magnetic field, respectively; Bs and BH denote the magnetization components caused by the stress and magnetic field, respectively. For an isotropic ferromagnetic material, a number of representative macro-models have been presented in order to characterize the
ð2Þ
dM 1 dMan ¼ 2 sð1cÞðMan Mirr Þ þ , ds ds k
ð3Þ
3 Es ¼ ls cos2 y: 2
ð4Þ
In Eq. (2), (dl/dH)s is the changing rate of the magnetostriction with the applied field at a constant stress. (dB/ds)H is the change of the magnetic induction with the stress at a constant field. Eq. (2) quantifies the relationship between the magnetostrictive effect and magneto-elastic effect. Eq. (3) describes the dependence of the material magnetization on the stress as well as anhysterestic magnetization—Man (the ideal or lossless magnetization of a material) and Mirr (the irreversible component of magnetization); k and c are material constants. Eq. (4) gives the dependence of the stress energy (Es) on the stress (s), bulk magnetostriction (l) and the angle between the applied stress and field (y). Under consideration of the different physical mechanisms of the magnetization of ferromagnetic materials at the different stages, nonlinear magnetic–elastic models, involving the magneto-mechanical effect, have been proposed in [26–28], which however do not differentiate elastic and plastic deformation. It is well-known that elastic and plastic deformation represents different deformation modes in the microstructure. The former involves an increase or decrease in the atomic spacing while the latter may result in the generation and accumulation of various defects such as dislocation, twinning and shear band. Clearly, the different deformation modes have different impacts on the magnetic behavior of ferromagnetic materials. In order to characterize such a difference in quantity, Wang et al. [29] recently proposed a magnetic–elastic–plastic model by considering the different mechanisms of the elastic and plastic deformation: H m a , Magnetization strength : M ¼ Ms coth total 0 m0 a Htotal e p Effective field : Htotal ¼ HH þ Hs þ Hs , Magnetic-induced effective field : HH ¼ H þ aM, 3seq cos2 b Elastic-induced effective field : Hse ¼ m0 ðl11 þ l12 sÞ þ 2ðl21 þ l22 sÞM 2 M, Plastic-induced effective field :
Hsp ¼ k9ep 9, where k ¼
1 b/ep S , m0 2Ms
ð5Þ where Ms is the saturation magnetization; m0 the permeability of vacuum; a mean field parameter representing the interdomain coupling; seq the equivalent stress for a complex stress state; b the angle between the magnetization and equivalent-stress directions; l11, l12, l21 and l22 magnetostriction coefficients; ep plastic deformation; /epS average pinning energy of the site for 1801 wall; b material’s constant. This model leads to some results verified by experimental observations: elastic tensile stress in the field direction accelerates the magnetization for positive magneto-restrictive materials, but the compressive stress opposes the magnetization; different from the elastic deformation, both tensile and compressive plastic deformation may decrease the magnetization of ferromagnetic materials. However, similar to most proposed magnetic-stress coupling models, the actions of the applied field and stress in the magnetization are decoupled in Eq. (5). Clearly, the decoupling in the magnetic-stress coupling models results in a confusing conclusion; that is, a ferromagnetic sample may be magnetized by the stress even if no magnetic field exists, which is in conflict
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with the reported experimental observations. Therefore more convincible models should be proposed in order to have a thorough understanding of the magnetic-stress coupling effect of ferromagnetic materials.
3. Magnetic NDT technology Magnetic NDT technologies have been extensively adopted in engineering to ensure the operating safety of ferromagnetic structures and components [30]. In this section, the studies of three typical magnetic NDT technologies (MFL, MBN and MMM) will be summarized. Among them, MFL and MBN techniques may be ascribed to active magnetic test methods in which a strong magnetic field is applied. However, the MMM technique is a weak-field test method in which the Earth’s magnetic field instead of an artificial field is used as the stimulus. 3.1. Magnetic flux leakage Magnetic flux leakage (MFL) is one of the traditional electromagnetic NDT techniques, originated from magnetic particle technique. Hoke first discovered the MFL phenomenon in 1918. However, due to the lack of magnetization techniques in the early time, the first application of the MFL technique was performed by Watts in 1933 in assessing the quality of the welded joints. Since 1960s, this technique has been extensively used as an inspection technique in the petrochemical engineering and transportation, energy and metal industries [31–37]. One successful application of the MFL technique is the device, called ‘‘pipeline-pig’’, which is developed to detect the corrosion and metal loss in oil and gas in-service pipelines [38–40]. Fig. 3 shows a scheme of the structure and operating principle of the ‘‘pipelinepig’’. A strong permanent magnet in the ‘‘pipeline-pig’’ nearly saturates the pipe wall when it is propelled by the oil/gas pressure or driving equipments. No flux is leaked out if the pipe wall is perfect (Fig. 3a). However, the flux ‘‘leaks’’ out of the wall at the location of a metal loss defect (Fig. 3b). The ‘‘leakage flux’’ is detected by an array of circumferentially distributed sensor assembly. The MFL data are sampled and stored using an onboard data acquisition system, and subsequently analyzed offline by trained data analysts. As a classical NDT method, the principle of the MFL technique is relatively simple. That is, when a strong magnetic field is applied to a ferromagnetic material, any geometrical discontinuity in the test object will cause the field to leak out of the object into the air (see Fig. 4). The flux leakage can be monitored by a magnetic field sensor and used to estimate the dimensions of the defect. Although the MFL phenomenon is easily understood, the design and analysis of MFL systems involve complicated interactions between the excitation field, leakage flux and the defects in the material. There are several important aspects to be considered. Firstly, the level of
the excitation magnetic flux should be large and homogenous to allow the magnetic flux variation to occur at the location of a defect; secondly, the sensors should be located close to the position at which the changes in the magnetic field density, originating from the defect, are distinct from the background noise; additionally, developing an effective inversion method to identify the defective characters by the recorded MFL signals is difficult since the defect is irregular. A great number of efforts have been devoted to develop a simple analytical model to explain the formation of the MFL signal [41–49]. Those developed models may be classified into two types. One was ¨ originated from Forster model [41] by considering the change of the magnetic parameters (e.g. permeability and coercive field) in the local defect region, and the other was developed from Zatsepin– Scherbinin’s model [42] where magnetic dipoles were assumed in ¨ the surface of the defect. Clearly, Forster model described the MFL phenomenon in the macroscopic view but Zatsepin–Scherbinin’s model in the microscopic view. In order to consider the non-linear magnetic behavior of ferromagnetic materials, more complicated models have been proposed in [43–49]. The reported models can effectively describe the fundamental features and the topography of the MFL field for regular defects (e.g. rectangular slot), but not for irregular defects. In order to analyze the MFL signal generated by irregular defects, some integral equations in describing the defect-induced magnetic charges were given based on the linear approximation of ferromagnetic materials, which can be solved numerically using the iteration method [50–53]. Compared with the theoretical analysis, the magnetic finite element method (FEM) is a powerful tool for the investigation of the MFL signal due to its flexibility in the simulations of varied irregular geometrical defects. 2D magnetic FEM methods [54,55] provide sufficient information for sharp-shaped defect characterization, but do not accurately quantify the natural defects, e.g. stress corrosion cracks. Therefore, 3D magnetic FEM has received more attention in recent years. In this aspect, the study in [56] presented comparisons between 2D and 3D models. The studies in [57–60]
Fig. 4. Schematic representation of the flux leakage in the presence of a geometric discontinuity.
Fig. 3. Structure and principle of the ‘pipeline-pig’. (a) No magnetic flux leakage for perfect pipe wall; (b) magnetic flux leakage in defect position.
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investigated the characteristics of MFL signals due to corrosion defects. Additionally, the effect of the local dent-induced stresses on the MFL signal was studied using both magnetic FEM modeling and experimental tests in [61]. A 3D simulation, aimed at studying the influence of the defect geometry and lift-off value, was performed in [62]. Further, the study in [63] examined the effects of both dent geometry and localized residual stresses on the MFL signal using a 3D magnetic FEM method. It may be pointed out that the MFL technique is one of the most popular magnetic NDT techniques and extensively used in various engineering fields. In the MFL test, the key is how to inversely determine the defects of the investigated object using the recorded MFL signals. Various new algorithms (e.g. wavelets, neutral networks and genetic algorithm) were applied in [64,65]. However, there are two primary obstacles in the defect inversion. Firstly, the defect in the reality is commonly complicated and usually characterized by many parameters such as the width, thickness, location and edge condition of the defect, which in turn significantly impact on the measured MFL signals. Clearly, it is generally difficult to characterize every parameter based on the measured MFL signals. Secondly, it is still a challenge to deal with the elastic–plastic zone near the cracks, and idealized material properties are commonly assumed. This is in part due to the fact that the effects of the plastic deformation on the magnetic characteristics of ferromagnetic materials are not thoroughly understood. 3.2. Magnetic Barkhausen noise Different from the MFL technique developed for inspecting the macro-defects, the magnetic Barkhausen noise (MBN) technique is usually used to determine the magnetic easy axis [66], the residual and applied stresses [67] and the grain size [68] of ferromagnetic materials. In 1919 Barkhausen [69] found that when a ferromagnetic material was magnetized by an increasing field, the noise in the form of voltage pulses was generated in a coil placed near the material. Fig. 5a shows a schematic of the Barkhausen noise during the magnetization process. The test system is presented in Fig. 5b. From the microstructural point of view, the development of Barkhausen noise is due to the abrupt motion of 1801 domains across local pinning sites [70–72]. The parameter commonly used in the analysis of the detected MBN signal voltage V is the MBNenergy defined as X Z MBNenergy ¼ A ð6Þ V 2 dt: events
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Generally, MBNenergy may be displayed in a polar scale and described by [73] MBNenergy ¼ a cos2 ðyjÞ þ b,
ð7Þ
where y is the angle of the applied magnetic field with respect to the reference loading direction; a the angular dependence of the variation of the MBN signal; b the angular independent signal (isotropic background); j the direction of the magnetic easy axis with respect to the reference direction. The MBN signal is generated due to the irreversible movement of magnetic domain walls. The amplitude depends on residual and applied stresses. Thus, it is a useful NDT technique to detect the presence of the residual and applied stresses in ferromagnetic samples. In steels with a positive magnetostrictive constant, experiments [74–81] confirmed that the MBN amplitude was increased due to the elastic tensile stress in the direction of the magnetization but decreased due to the compressive stress. It is due to the different effects of tensile and compressive stresses on the movement of domain walls [82–84]: tensile stress increases the number of mobile 1801 domain walls, reorients domains or modifies the pinning barriers, but compressive stress makes the flux closure domain formation by decreasing the number of 1801 domain walls. Plastic deformation results in a significant increase in the dislocation density and changes the domain wall energy at some pinning sites. It should be noted that the effect of plastic deformation on the MBN signals is more complicated than that of elastic deformation [85–87]. Such complexity is especially reflected by some contradictory claims reported regarding the dependence of MBN signals on plastic deformation. For example, most studies [88–93] reported a continuous decrease of the Barkhausen noise as the plastic strain increases, while in the other studies [73,94–97] an increase at the low plastic strain followed by a decrease at the high plastic strain was observed. The above-mentioned results were obtained by the quasistatic experiments. Further work has also been performed on fatigue loads affecting the MBN signal. Kettunen and Ruuskanen [98] measured the MBN signals of two low-carbon structural steels under tension–compression fatigue cycles. It was found that the MBN signals increased in the beginning and then kept an approximate constant value without large oscillations until the final failure of the specimens. Karjalainen and Moilanen [99] further tested the MBN signals of the mild steel under the constant deflection bending fatigue. It was demonstrated that the overall signal amplitude decreased during the fatigue test. In some cases, there existed a sudden increase in the signal level
Fig. 5. Barkhausen noise and test system. The left figure shows the development of Barkhausen noise during the magnetization of specimen, and the right figure shows of MBN test system.
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prior to the failure. Furuya et al. [100] tested the MBN signal in the low cycle fatigue experiments in mild steels. They pointed out that the MBN amplitude decreased continuously during the cyclic loading and no particular changes in the amplitude were observed before the failure. Lindgren and Lepisto¨ [101] used a continuous monitoring method to test the MBN signal during the whole fatigue loading process. It was demonstrated that the maximum MBN signal increased with the increase of the stress amplitude but the minimum value was approximately constant. The changes of the MBN amplitude as a function of fatigue cycles were found to be different between the mild steel and high strength steel samples. Other interesting results have been reported by Refs. [102–106]. Clearly, the above reports are somewhat contradictory. This is because the MBN signals are very sensitive to the microstructure of ferromagnetic materials [107,108] and measuring systems [109,110]. In general, developing an efficient model to quantify the interaction between microstructure features and Barkhausen noise is still a challenge in the field. 3.3. Metal magnetic memory As above-mentioned, both MFL and MBN are active magnetic test techniques where a strong applied field is used to magnetize the tested object. Thus, the two techniques are usually timeconsuming and even impractical for some irregular structures. For the purpose of developing a more simple and effective NDT magnetic technique to meet the requirements in engineering, the passive magnetic technique has received a great interest in
Fig. 6. Schematic representation of SMFL distributions in the stress-concentration zone. Hp(x) reaches the maximum and the normal component Hp(y) is zero in maximum stress-concentration zone.
the recent years. In the late 1990s, a passive magnetic NDT technique, namely metal magnetic memory (MMM) technique, was first proposed by a Russian group [111,112]. The advantage of the MMM technique is that the Earth’s magnetic field instead of an artificial strong field plays as the role of the stimulus source. In addition, the MMM technique displays other attractive advantages, as follows:
it is effective in assessing the early damage and developed defects;
it has a detecting depth of up to a few millimeters in comparison with macrons in X-ray diffraction technique;
it is easy-operation and time-saving, and has the maximum measuring velocity of up to several meters per second. The physical mechanism of the MMM technique is described in [113–116]. Under the effect of the earth field and mechanical load, self-magnetic-flux-leakage (SMFL) signals are generated in the stress-concentration zones where the tangential SMFL component Hp(x) reaches the maximum and the normal component Hp(y) changes polarity and has a zero value (see Fig. 6). The magnetic state is still retained even if the load is removed. Therefore, the stress concentration zones can be detected by measuring the SMFL signals on the surface of the structure. In the view of micro-structures, it is due to the irreversible orientation of magnetic domains caused by plastic deformation in the maximum stress-concentration zone. The MMM technique has received extensive attention in engineering due to its advantages of easy-operation, time-saving and simple criteria. Various applications have been reported for diagnosing gas and oil pipelines, rails, turbine wheels, pressure vessel and others in [113]. Fig. 7 shows the testing results of normal component Hp(y) of three rings. The much lower amplitude of Hp(y) about the left ring means that it is under welloperation condition. However, the much higher amplitude of Hp(y) about the right one indicates that this one is seriously deteriorated. Moreover, the results also show the possible locations of the stress-concentration (SC) zone as labeled in the right picture of Fig. 7. The MMM technique is suitable for many engineering practices. However, as a comparatively new test method, it still has a large room to be improved, as shown in the following section. One of the critical points is that more accurate and quantitative criteria are required. Up to now this technique is only used as a qualitative test method to determine the possible dangerous positions without quantitative results (e.g. stress-concentration intensity and residual fatigue life). In order to understand the physical mechanism and provide more information about the
Fig. 7. Testing results of normal component Hp(y) of three rings. The results show that the left ring is well and the right is seriously damaged.
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relations between the SMFL signal and structure characters, further studies have been recently performed. Dong et al. [117] measured the SMFL signal for a series of tensile 18CrNiWA steels. The magnetic curve on the measured line shows a good linearity after loading. It rotates in the counterclockwise direction with the increase of the tensile stress during the elastic deformation, while gathers together in the plastic deformation stage. Wang et al. [118,119] performed numerical simulations of the SMFL signals using a revised magnetic-charge model where critical factors (e.g. the size, location and density of the local plastic zone and the lift-off values of the sensor) affecting the SMFL signal are presented. Yao et al. [120] experimentally confirmed that the SMFL signal and its gradient were significantly different during which Q235-steel specimens are deformed from elastic to plastic deformation under the tension, but no detectable change could be found during the whole compressive loading. They presented an explanation by considering the different movement modes of the domain structures subjected to tensile and compressive loads. Shi et al. [121] measured the SMFL signal and its gradient during which 18CrNi4A steel specimens were subjected to tension–tension fatigue loading where the effect of the local stress concentration factor on magnetic test results was especially considered. Leng et al. [122] tested the SMFL signal of 18CrNi4A steel specimens induced by cyclic bending stresses. The experimental results were qualitatively explained by the ‘‘effective field model’’ developed by Jiles [24]. Wilson et al. [123] introduced a novel three-axis magnetic sensor to confirm that the parallel component Bx is more related to the applied stress than the perpendicular component Bz. Roskosz and Gawrilenko [124] further presented experimental and numerical analyses of the SMFL distribution in loaded notched samples. The MMM technique, although has received considerable attention over the last decade, still needs further studies, based on the following reasons: (1) Lack of physical models in quantifying the relations between the SMFL signal and plastic deformation. As mentioned above, the MMM technique is developed for inspecting the initial damage or plastic deformation induced by the local stress concentration. Naturally, traditional magneto-elastic models [19–21] cannot describe such phenomenon. Although the newly developed magneto-plastic model [29] differentiated the actions of the elastic and plastic deformation on the magnetization of ferromagnetic materials, no direct relationship between the plastic strain and SMFL signal has been presented. (2) Lack of quantitative criteria in capturing the features of defects. There are only two qualitative criteria for which the tangential component Hp(x) reaches the maximum value, and the normal component Hp(y) changes positive–negative symbols and thus has a zero in the location of maximum stress. Due to the lack of quantitative defect criteria, the MMM technique can only be used as an assistant tool to diagnose the possible dangerous zone presently. More accurate results must be determined by other techniques. In fact, there must be some quantitative relations between SMFL signals and defect characters in MMM tests. In this aspect, systematic experiments and numerical simulations should be performed.
4. Conclusions The magnetic-stress coupling effect is one of the basic characters of ferromagnetic materials. Based on such an effect, various magnetic NDT techniques in assessing the stress status of ferromagnetic structures have been developed, some of which have been successfully used in engineering. However, understanding of the
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magnetic-stress coupling effect is incomplete due to the complexity of the magnetic-stress action and high-sensitivity to the material’s microstructure. Thus, the applications of the NDT techniques in industry still face considerable challenges. Further studies of the NDT techniques are desirable. In this paper, the current physical models for magnetic-stress coupling are introduced, and three representative magnetic NDT techniques (MFL, MNB and MMM) are reviewed. The fundamental features, advantages, disadvantages and future work for these NDT techniques are summarized based on a literature review of more than one hundred references.
Acknowledgments This work was funded by Natural Science Foundations of China (no. 11072027), and the Fundamental Research Funds for the Central Universities and Ministry of Education of the People’s Republic of China (NECT).
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Review
A review of three magnetic NDT technologies Z.D. Wang n, Y. Gu, Y.S. Wang Department of Mechanics, School of Civil Engineering, Beijing Jiaotong University, Beijing 100044, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 May 2011 Received in revised form 26 August 2011 Available online 7 September 2011
Magnetic techniques are most important NDT technologies to characterize the mechanical features of ferromagnetic materials based on the physical principle of magnetic-stress coupling. A review is presented in this paper about the development of the magnetic NDT technologies. After a brief outline of the theoretical studies of the magnetic-stress coupling effect, the three popular magnetic NDT technologies are reviewed, which are magnetic flux leakage (MFL), magnetic Barkhausen noise (MBN) and recently developed metal magnetic memory (MMM). The first two are ascribed to the active magnetic method, and the last one is the passive method. Based on an extensive literature survey in this field, this paper focuses on the discussion of the physical mechanism and some important experimental results relevant to the three NDT technologies. The challenges for each technique in this field are also summarized. & 2011 Elsevier B.V. All rights reserved.
Keywords: Non-destructive technique Magnetic-stress coupling effect Magnetic flux leakage Barkhausen noise Metal magnetic memory
1. Introduction
2. Magnetic-stress coupling theory
A fundamental feature of ferromagnetic materials, which consist of numerous small magnetic domains in the microstructure, is the coupling between stress and magnetic field [1–6]; that is, the magnetization may result in variations in the dimension of ferromagnetic materials, namely magnetostriction [7,8], and on the other hand the stress may also change the magnetization of ferromagnetic materials, the so-called piezomagnetic effect [9,10]. These macro-phenomena are related to magnetic-moments rotation and domain-wall displacement in the micro-structure when ferromagnetic materials are subjected to an applied magnetic field or mechanical stress [11,12]. Compared with the magnetostriction, the piezomagnetic effect has received more attention in non-destructive test (NDT) field, because it is the physical basis to evaluate the stress status of ferromagnetic structures and components by magnetic measuring methods. As a result, a great number of non-destructive magnetic techniques have been developed over the last decades such as magnetic flux leakage (MFL), magnetic Barkhausen noise (MBN), magnetoacoustic emission (MAE), stress-induced magnetic anisotropy (SMA) and recently developed metal magnetic memory (MMM) [13,14]. In the remainder of this paper, the concept and theory of magnetic-stress coupling models in ferromagnetic materials will be presented in Section 2, the development of three typical NDT magnetic techniques are described in Section 3 and the conclusions are presented in Section 4.
It is well-known that magnetic properties of ferromagnetic materials are stress-dependent. For example, the coercivity Hc and permeability m of ferromagnetic materials may be changed in the order of 100% by the stress within the elastic limit [15]. Fig. 1 presents a schematic description of a cyclic stress affecting the magnetization of ferromagnetic materials. The total magnetization M includes the reversible component, Mre, and irreversible component, Mirr. In every loading cycle, an irreversible magnetization difference DMirr is obtained when the domains pass through pinning sites under the action of the mechanical stress [16]. From the microstructural point of view, a ferromagnetic sample is composed of numerous magnetic domains. Every domain is 10 8–10 12 m3 in volume and includes approximately 1012–1015 atoms. The magnetic moments distribute randomly in the initial state, and no magnetization appears in the macro-scale. When an external load is applied to a ferromagnetic sample, a tension tends to orient the domains in the direction of the applied load for a positive magnetorestrictive material and a compression orients the domains perpendicularly to the loading direction due to the piezomagnetic effect [17]. Fig. 2a displays a symmetric structure in which the applied field H¼0. Only in the case of Ha0 (see Fig. 2b), the symmetric structure is destroyed and the ferromagnetic sample is magnetized. Thus, the presence of an external field (e.g. an artificial field or Earth’s magnetic field) is a necessary condition for all magnetic NDT techniques. This may also be concluded from the following experiment: the spontaneous stray field signal is detected when plastically deforming a ferromagnetic sample under the action of the Earth’s magnetic field, but not obtained when the sample was plastically deformed in a magnetic-prohibited environment. Theoretical studies of the magnetic-stress coupling effect provide a physical understanding of various magnetic NDT technologies.
n
Corresponding author. E-mail address: [email protected] (Z.D. Wang).
0304-8853/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2011.08.048
Z.D. Wang et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 382–388
383
magneto-mechanical effect (see e.g. [19–21]), as follows: dl dB ¼ , ds H dH s
Fig. 1. Schematic diagram of the stress affecting the magnetization of a ferromagnetic s Þ remains in sample. The irreversible component of the residual magnetization ðDMirr every loading cycle.
Fig. 2. Magnetic field and stress affecting the movement of magnetic domains in the micro-structure. Tensile stress orients the domains in the loading direction and compressive stress causes the domains perpendicularly to the loading direction. The magnetic field destroys the systematic domain structure and lead to magnetization (a) without defect and (b) corrupt wall.
In fact, the conceptual description of the interaction between the stress and magnetism may date back to more than half a century ago. Brown [18] first presented a theoretical analysis of the magneto-mechanical effect in ferromagnetic materials by replacing the applied stress with an equivalent field in 1949. Later on, Cullity [19] focused on such phenomena in terms of Le Chatelier’s principle. Further, Sablik et al. [20] considered the variations in hysteresis of ferromagnetic materials under a constant stress. Additionally, the studies of the magneto-mechanical effect based on the concepts of the ‘‘effective field theory’’ and ‘‘law of approach’’ were performed by Jiles [21–24]. The classical magneto-elastic constitution for a ferromagnetic material is generally expressed as [25] (
e ¼ es þ eH ¼ Sr þ Ds H, B ¼ Bs þBH ¼ DH r þ lH,
ð1Þ
where S, l Ds and DH are tensors of the stiffness matrices, effective magnetic permeability, magnetic–elastic coefficient and elastic–magnetic coefficient, respectively; r and H are tensors of the stress and external magnetic field, respectively; the functions of e and B denote strain tensor and effective magnetic field tensor, respectively; es and eH denote the strain components caused by the stress and magnetic field, respectively; Bs and BH denote the magnetization components caused by the stress and magnetic field, respectively. For an isotropic ferromagnetic material, a number of representative macro-models have been presented in order to characterize the
ð2Þ
dM 1 dMan ¼ 2 sð1cÞðMan Mirr Þ þ , ds ds k
ð3Þ
3 Es ¼ ls cos2 y: 2
ð4Þ
In Eq. (2), (dl/dH)s is the changing rate of the magnetostriction with the applied field at a constant stress. (dB/ds)H is the change of the magnetic induction with the stress at a constant field. Eq. (2) quantifies the relationship between the magnetostrictive effect and magneto-elastic effect. Eq. (3) describes the dependence of the material magnetization on the stress as well as anhysterestic magnetization—Man (the ideal or lossless magnetization of a material) and Mirr (the irreversible component of magnetization); k and c are material constants. Eq. (4) gives the dependence of the stress energy (Es) on the stress (s), bulk magnetostriction (l) and the angle between the applied stress and field (y). Under consideration of the different physical mechanisms of the magnetization of ferromagnetic materials at the different stages, nonlinear magnetic–elastic models, involving the magneto-mechanical effect, have been proposed in [26–28], which however do not differentiate elastic and plastic deformation. It is well-known that elastic and plastic deformation represents different deformation modes in the microstructure. The former involves an increase or decrease in the atomic spacing while the latter may result in the generation and accumulation of various defects such as dislocation, twinning and shear band. Clearly, the different deformation modes have different impacts on the magnetic behavior of ferromagnetic materials. In order to characterize such a difference in quantity, Wang et al. [29] recently proposed a magnetic–elastic–plastic model by considering the different mechanisms of the elastic and plastic deformation: H m a , Magnetization strength : M ¼ Ms coth total 0 m0 a Htotal e p Effective field : Htotal ¼ HH þ Hs þ Hs , Magnetic-induced effective field : HH ¼ H þ aM, 3seq cos2 b Elastic-induced effective field : Hse ¼ m0 ðl11 þ l12 sÞ þ 2ðl21 þ l22 sÞM 2 M, Plastic-induced effective field :
Hsp ¼ k9ep 9, where k ¼
1 b/ep S , m0 2Ms
ð5Þ where Ms is the saturation magnetization; m0 the permeability of vacuum; a mean field parameter representing the interdomain coupling; seq the equivalent stress for a complex stress state; b the angle between the magnetization and equivalent-stress directions; l11, l12, l21 and l22 magnetostriction coefficients; ep plastic deformation; /epS average pinning energy of the site for 1801 wall; b material’s constant. This model leads to some results verified by experimental observations: elastic tensile stress in the field direction accelerates the magnetization for positive magneto-restrictive materials, but the compressive stress opposes the magnetization; different from the elastic deformation, both tensile and compressive plastic deformation may decrease the magnetization of ferromagnetic materials. However, similar to most proposed magnetic-stress coupling models, the actions of the applied field and stress in the magnetization are decoupled in Eq. (5). Clearly, the decoupling in the magnetic-stress coupling models results in a confusing conclusion; that is, a ferromagnetic sample may be magnetized by the stress even if no magnetic field exists, which is in conflict
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with the reported experimental observations. Therefore more convincible models should be proposed in order to have a thorough understanding of the magnetic-stress coupling effect of ferromagnetic materials.
3. Magnetic NDT technology Magnetic NDT technologies have been extensively adopted in engineering to ensure the operating safety of ferromagnetic structures and components [30]. In this section, the studies of three typical magnetic NDT technologies (MFL, MBN and MMM) will be summarized. Among them, MFL and MBN techniques may be ascribed to active magnetic test methods in which a strong magnetic field is applied. However, the MMM technique is a weak-field test method in which the Earth’s magnetic field instead of an artificial field is used as the stimulus. 3.1. Magnetic flux leakage Magnetic flux leakage (MFL) is one of the traditional electromagnetic NDT techniques, originated from magnetic particle technique. Hoke first discovered the MFL phenomenon in 1918. However, due to the lack of magnetization techniques in the early time, the first application of the MFL technique was performed by Watts in 1933 in assessing the quality of the welded joints. Since 1960s, this technique has been extensively used as an inspection technique in the petrochemical engineering and transportation, energy and metal industries [31–37]. One successful application of the MFL technique is the device, called ‘‘pipeline-pig’’, which is developed to detect the corrosion and metal loss in oil and gas in-service pipelines [38–40]. Fig. 3 shows a scheme of the structure and operating principle of the ‘‘pipelinepig’’. A strong permanent magnet in the ‘‘pipeline-pig’’ nearly saturates the pipe wall when it is propelled by the oil/gas pressure or driving equipments. No flux is leaked out if the pipe wall is perfect (Fig. 3a). However, the flux ‘‘leaks’’ out of the wall at the location of a metal loss defect (Fig. 3b). The ‘‘leakage flux’’ is detected by an array of circumferentially distributed sensor assembly. The MFL data are sampled and stored using an onboard data acquisition system, and subsequently analyzed offline by trained data analysts. As a classical NDT method, the principle of the MFL technique is relatively simple. That is, when a strong magnetic field is applied to a ferromagnetic material, any geometrical discontinuity in the test object will cause the field to leak out of the object into the air (see Fig. 4). The flux leakage can be monitored by a magnetic field sensor and used to estimate the dimensions of the defect. Although the MFL phenomenon is easily understood, the design and analysis of MFL systems involve complicated interactions between the excitation field, leakage flux and the defects in the material. There are several important aspects to be considered. Firstly, the level of
the excitation magnetic flux should be large and homogenous to allow the magnetic flux variation to occur at the location of a defect; secondly, the sensors should be located close to the position at which the changes in the magnetic field density, originating from the defect, are distinct from the background noise; additionally, developing an effective inversion method to identify the defective characters by the recorded MFL signals is difficult since the defect is irregular. A great number of efforts have been devoted to develop a simple analytical model to explain the formation of the MFL signal [41–49]. Those developed models may be classified into two types. One was ¨ originated from Forster model [41] by considering the change of the magnetic parameters (e.g. permeability and coercive field) in the local defect region, and the other was developed from Zatsepin– Scherbinin’s model [42] where magnetic dipoles were assumed in ¨ the surface of the defect. Clearly, Forster model described the MFL phenomenon in the macroscopic view but Zatsepin–Scherbinin’s model in the microscopic view. In order to consider the non-linear magnetic behavior of ferromagnetic materials, more complicated models have been proposed in [43–49]. The reported models can effectively describe the fundamental features and the topography of the MFL field for regular defects (e.g. rectangular slot), but not for irregular defects. In order to analyze the MFL signal generated by irregular defects, some integral equations in describing the defect-induced magnetic charges were given based on the linear approximation of ferromagnetic materials, which can be solved numerically using the iteration method [50–53]. Compared with the theoretical analysis, the magnetic finite element method (FEM) is a powerful tool for the investigation of the MFL signal due to its flexibility in the simulations of varied irregular geometrical defects. 2D magnetic FEM methods [54,55] provide sufficient information for sharp-shaped defect characterization, but do not accurately quantify the natural defects, e.g. stress corrosion cracks. Therefore, 3D magnetic FEM has received more attention in recent years. In this aspect, the study in [56] presented comparisons between 2D and 3D models. The studies in [57–60]
Fig. 4. Schematic representation of the flux leakage in the presence of a geometric discontinuity.
Fig. 3. Structure and principle of the ‘pipeline-pig’. (a) No magnetic flux leakage for perfect pipe wall; (b) magnetic flux leakage in defect position.
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investigated the characteristics of MFL signals due to corrosion defects. Additionally, the effect of the local dent-induced stresses on the MFL signal was studied using both magnetic FEM modeling and experimental tests in [61]. A 3D simulation, aimed at studying the influence of the defect geometry and lift-off value, was performed in [62]. Further, the study in [63] examined the effects of both dent geometry and localized residual stresses on the MFL signal using a 3D magnetic FEM method. It may be pointed out that the MFL technique is one of the most popular magnetic NDT techniques and extensively used in various engineering fields. In the MFL test, the key is how to inversely determine the defects of the investigated object using the recorded MFL signals. Various new algorithms (e.g. wavelets, neutral networks and genetic algorithm) were applied in [64,65]. However, there are two primary obstacles in the defect inversion. Firstly, the defect in the reality is commonly complicated and usually characterized by many parameters such as the width, thickness, location and edge condition of the defect, which in turn significantly impact on the measured MFL signals. Clearly, it is generally difficult to characterize every parameter based on the measured MFL signals. Secondly, it is still a challenge to deal with the elastic–plastic zone near the cracks, and idealized material properties are commonly assumed. This is in part due to the fact that the effects of the plastic deformation on the magnetic characteristics of ferromagnetic materials are not thoroughly understood. 3.2. Magnetic Barkhausen noise Different from the MFL technique developed for inspecting the macro-defects, the magnetic Barkhausen noise (MBN) technique is usually used to determine the magnetic easy axis [66], the residual and applied stresses [67] and the grain size [68] of ferromagnetic materials. In 1919 Barkhausen [69] found that when a ferromagnetic material was magnetized by an increasing field, the noise in the form of voltage pulses was generated in a coil placed near the material. Fig. 5a shows a schematic of the Barkhausen noise during the magnetization process. The test system is presented in Fig. 5b. From the microstructural point of view, the development of Barkhausen noise is due to the abrupt motion of 1801 domains across local pinning sites [70–72]. The parameter commonly used in the analysis of the detected MBN signal voltage V is the MBNenergy defined as X Z MBNenergy ¼ A ð6Þ V 2 dt: events
385
Generally, MBNenergy may be displayed in a polar scale and described by [73] MBNenergy ¼ a cos2 ðyjÞ þ b,
ð7Þ
where y is the angle of the applied magnetic field with respect to the reference loading direction; a the angular dependence of the variation of the MBN signal; b the angular independent signal (isotropic background); j the direction of the magnetic easy axis with respect to the reference direction. The MBN signal is generated due to the irreversible movement of magnetic domain walls. The amplitude depends on residual and applied stresses. Thus, it is a useful NDT technique to detect the presence of the residual and applied stresses in ferromagnetic samples. In steels with a positive magnetostrictive constant, experiments [74–81] confirmed that the MBN amplitude was increased due to the elastic tensile stress in the direction of the magnetization but decreased due to the compressive stress. It is due to the different effects of tensile and compressive stresses on the movement of domain walls [82–84]: tensile stress increases the number of mobile 1801 domain walls, reorients domains or modifies the pinning barriers, but compressive stress makes the flux closure domain formation by decreasing the number of 1801 domain walls. Plastic deformation results in a significant increase in the dislocation density and changes the domain wall energy at some pinning sites. It should be noted that the effect of plastic deformation on the MBN signals is more complicated than that of elastic deformation [85–87]. Such complexity is especially reflected by some contradictory claims reported regarding the dependence of MBN signals on plastic deformation. For example, most studies [88–93] reported a continuous decrease of the Barkhausen noise as the plastic strain increases, while in the other studies [73,94–97] an increase at the low plastic strain followed by a decrease at the high plastic strain was observed. The above-mentioned results were obtained by the quasistatic experiments. Further work has also been performed on fatigue loads affecting the MBN signal. Kettunen and Ruuskanen [98] measured the MBN signals of two low-carbon structural steels under tension–compression fatigue cycles. It was found that the MBN signals increased in the beginning and then kept an approximate constant value without large oscillations until the final failure of the specimens. Karjalainen and Moilanen [99] further tested the MBN signals of the mild steel under the constant deflection bending fatigue. It was demonstrated that the overall signal amplitude decreased during the fatigue test. In some cases, there existed a sudden increase in the signal level
Fig. 5. Barkhausen noise and test system. The left figure shows the development of Barkhausen noise during the magnetization of specimen, and the right figure shows of MBN test system.
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prior to the failure. Furuya et al. [100] tested the MBN signal in the low cycle fatigue experiments in mild steels. They pointed out that the MBN amplitude decreased continuously during the cyclic loading and no particular changes in the amplitude were observed before the failure. Lindgren and Lepisto¨ [101] used a continuous monitoring method to test the MBN signal during the whole fatigue loading process. It was demonstrated that the maximum MBN signal increased with the increase of the stress amplitude but the minimum value was approximately constant. The changes of the MBN amplitude as a function of fatigue cycles were found to be different between the mild steel and high strength steel samples. Other interesting results have been reported by Refs. [102–106]. Clearly, the above reports are somewhat contradictory. This is because the MBN signals are very sensitive to the microstructure of ferromagnetic materials [107,108] and measuring systems [109,110]. In general, developing an efficient model to quantify the interaction between microstructure features and Barkhausen noise is still a challenge in the field. 3.3. Metal magnetic memory As above-mentioned, both MFL and MBN are active magnetic test techniques where a strong applied field is used to magnetize the tested object. Thus, the two techniques are usually timeconsuming and even impractical for some irregular structures. For the purpose of developing a more simple and effective NDT magnetic technique to meet the requirements in engineering, the passive magnetic technique has received a great interest in
Fig. 6. Schematic representation of SMFL distributions in the stress-concentration zone. Hp(x) reaches the maximum and the normal component Hp(y) is zero in maximum stress-concentration zone.
the recent years. In the late 1990s, a passive magnetic NDT technique, namely metal magnetic memory (MMM) technique, was first proposed by a Russian group [111,112]. The advantage of the MMM technique is that the Earth’s magnetic field instead of an artificial strong field plays as the role of the stimulus source. In addition, the MMM technique displays other attractive advantages, as follows:
it is effective in assessing the early damage and developed defects;
it has a detecting depth of up to a few millimeters in comparison with macrons in X-ray diffraction technique;
it is easy-operation and time-saving, and has the maximum measuring velocity of up to several meters per second. The physical mechanism of the MMM technique is described in [113–116]. Under the effect of the earth field and mechanical load, self-magnetic-flux-leakage (SMFL) signals are generated in the stress-concentration zones where the tangential SMFL component Hp(x) reaches the maximum and the normal component Hp(y) changes polarity and has a zero value (see Fig. 6). The magnetic state is still retained even if the load is removed. Therefore, the stress concentration zones can be detected by measuring the SMFL signals on the surface of the structure. In the view of micro-structures, it is due to the irreversible orientation of magnetic domains caused by plastic deformation in the maximum stress-concentration zone. The MMM technique has received extensive attention in engineering due to its advantages of easy-operation, time-saving and simple criteria. Various applications have been reported for diagnosing gas and oil pipelines, rails, turbine wheels, pressure vessel and others in [113]. Fig. 7 shows the testing results of normal component Hp(y) of three rings. The much lower amplitude of Hp(y) about the left ring means that it is under welloperation condition. However, the much higher amplitude of Hp(y) about the right one indicates that this one is seriously deteriorated. Moreover, the results also show the possible locations of the stress-concentration (SC) zone as labeled in the right picture of Fig. 7. The MMM technique is suitable for many engineering practices. However, as a comparatively new test method, it still has a large room to be improved, as shown in the following section. One of the critical points is that more accurate and quantitative criteria are required. Up to now this technique is only used as a qualitative test method to determine the possible dangerous positions without quantitative results (e.g. stress-concentration intensity and residual fatigue life). In order to understand the physical mechanism and provide more information about the
Fig. 7. Testing results of normal component Hp(y) of three rings. The results show that the left ring is well and the right is seriously damaged.
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relations between the SMFL signal and structure characters, further studies have been recently performed. Dong et al. [117] measured the SMFL signal for a series of tensile 18CrNiWA steels. The magnetic curve on the measured line shows a good linearity after loading. It rotates in the counterclockwise direction with the increase of the tensile stress during the elastic deformation, while gathers together in the plastic deformation stage. Wang et al. [118,119] performed numerical simulations of the SMFL signals using a revised magnetic-charge model where critical factors (e.g. the size, location and density of the local plastic zone and the lift-off values of the sensor) affecting the SMFL signal are presented. Yao et al. [120] experimentally confirmed that the SMFL signal and its gradient were significantly different during which Q235-steel specimens are deformed from elastic to plastic deformation under the tension, but no detectable change could be found during the whole compressive loading. They presented an explanation by considering the different movement modes of the domain structures subjected to tensile and compressive loads. Shi et al. [121] measured the SMFL signal and its gradient during which 18CrNi4A steel specimens were subjected to tension–tension fatigue loading where the effect of the local stress concentration factor on magnetic test results was especially considered. Leng et al. [122] tested the SMFL signal of 18CrNi4A steel specimens induced by cyclic bending stresses. The experimental results were qualitatively explained by the ‘‘effective field model’’ developed by Jiles [24]. Wilson et al. [123] introduced a novel three-axis magnetic sensor to confirm that the parallel component Bx is more related to the applied stress than the perpendicular component Bz. Roskosz and Gawrilenko [124] further presented experimental and numerical analyses of the SMFL distribution in loaded notched samples. The MMM technique, although has received considerable attention over the last decade, still needs further studies, based on the following reasons: (1) Lack of physical models in quantifying the relations between the SMFL signal and plastic deformation. As mentioned above, the MMM technique is developed for inspecting the initial damage or plastic deformation induced by the local stress concentration. Naturally, traditional magneto-elastic models [19–21] cannot describe such phenomenon. Although the newly developed magneto-plastic model [29] differentiated the actions of the elastic and plastic deformation on the magnetization of ferromagnetic materials, no direct relationship between the plastic strain and SMFL signal has been presented. (2) Lack of quantitative criteria in capturing the features of defects. There are only two qualitative criteria for which the tangential component Hp(x) reaches the maximum value, and the normal component Hp(y) changes positive–negative symbols and thus has a zero in the location of maximum stress. Due to the lack of quantitative defect criteria, the MMM technique can only be used as an assistant tool to diagnose the possible dangerous zone presently. More accurate results must be determined by other techniques. In fact, there must be some quantitative relations between SMFL signals and defect characters in MMM tests. In this aspect, systematic experiments and numerical simulations should be performed.
4. Conclusions The magnetic-stress coupling effect is one of the basic characters of ferromagnetic materials. Based on such an effect, various magnetic NDT techniques in assessing the stress status of ferromagnetic structures have been developed, some of which have been successfully used in engineering. However, understanding of the
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magnetic-stress coupling effect is incomplete due to the complexity of the magnetic-stress action and high-sensitivity to the material’s microstructure. Thus, the applications of the NDT techniques in industry still face considerable challenges. Further studies of the NDT techniques are desirable. In this paper, the current physical models for magnetic-stress coupling are introduced, and three representative magnetic NDT techniques (MFL, MNB and MMM) are reviewed. The fundamental features, advantages, disadvantages and future work for these NDT techniques are summarized based on a literature review of more than one hundred references.
Acknowledgments This work was funded by Natural Science Foundations of China (no. 11072027), and the Fundamental Research Funds for the Central Universities and Ministry of Education of the People’s Republic of China (NECT).
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