Non Destructive Method Theory - Basic Principles - https://www.tinker.af.mil/Portals/106/Documents/Technical%20Orders/AFD-101516-33B-1-1.pdf AF338-1-1-EC-CP4Sc0-Indice ROCarneval

NONDESTRUCTIVE TESTING HANDBOOK - Electromagnetic Testing
Manual de Ensaio Não Destrutivo - Ensaio Eletromagnético

  1. Parte 1. Introdução a Sondas de Correntes Parasitas
    1. Operação Básica das Sondas de Correntes Parasitas
    2. Formato das Sondas de Bobinas
      1. Configuração
      2. Técnica Senrora
      3. Geometria
    3. Fatores que Afetam as Sondas de Correntes Parasitas
      1. Curva de Liftoff
      2. Fator de Enchimento ("Fillfactor")
      3. Profundidade de Penetração
  2. Parte 2. Projeto das Sondas de Correntes Parasitas
    1. Experiências no Projeto de Sondas de Correntes Parasitas
    2. Projeto Analítico de Sondas de Correntes Parasitas
      1. Cálculo da Resistência da Sonda
      2. Cálculo da Reatância da Sonda
      3. Sondas com Múltiplas Bobinas
      4. Projeto Analítico de Sondas Complexas
    3. Projeto Numérico
      1. Sondas de Enrolamento Único versis Sondas de Enrolamento Duplo
      2. Parâmetro Liftoff
      3. Parâmetro Profundidade do Furo
      4. Efeitos da Redução de Ruído
      5. Verificação Experimental do Projeto da Sonda por Modêlo Numérico
    4. Sondas com Núcleo de Ferro
      1. Sondas com Núcleo na Forma de Copo
      2. Dados de Projeção do Campo
      3. Características do Liftoff
    5. Sondas Isoladas
      1. Características do Liftoff
    6. Sondas do Diâmetro Interno
    7. Sondas Envolventes
  3. Parte 3. Detectores de Efeito Hall
    1. Princípios dos Detectores de Efeito Hall
      1. Mudanças de Operação dos Detectores de Efeito Hall
      2. Ações dos Campos Magnéticos na Mudança de Operação dos Semicondutores
      3. Elemento Hall
      4. Circuito Elétrico do Elemento Hall
      5. Características Operacionais dos Detectores de Efeito Hall
      6. Resposta Direcional dos Detectores de Efeito Hall
    2. Configurações dos Detectores de Efeito Hall
      1. "Arrays" (matriz de senores)  Multidirecional
      2. "Arrays" (matriz de senores) Linear
      3. "Arrays" (matriz de senores) em Ponte
      4. Sistemas Diferenciais
  4. Parte 4. Sondas para Fuga de Campo Magnético
    1. Descrição do Método
    2. Sondas do Tipo Bobina
    3. Outras Formas de Indicação do Campo Magnético de Fuga
      1. Magnetodiodo
      2. Fita Magnética de Gravação
      3. Partículas Magnéticas
  5. Parte 5. ImageNs de Correntes Parasitas com Sensores Magnetoóticos
    1. Princípios da Imagem Magnetoótica
    2. Geração de Película de Corrente Parasita
    3. Combinação da Imagem Magnetoótica e dos Princípios de Correntes Parasitas
      1. Imagem Magnetoótica da Película de Corrente Parasita
    4. Indicações Magnetoótica
    5. Indicações Lineares versus Rotativas
    6. Interpretação da Imagem MGanetoótica
    7. Padrões de Referência e Imagens Magnetoóticas Selecionadas
    8. Sumário


1 INTRODUÇÃO A SONDAS DE CORRENTES PARASITAS


1.1 OPERAÇÃO BÁSICA DAS SONDAS DE CORRENTES PARASITAS

Nondestructive testing involves the application of a suitable form of energy to a test object and measuring the manner in which the energy interacts with the material. An electromagnetic measurement is usually made with a probe, or transducer, that converts the energy into an electrical signal. The probe output is fed to an appropriate instrument, which calculates the measurement variable of interest. The measured variable or signal is then either manually interpreted or analyzed by using signal processing algorithms to determine the state of the test object. The probes typically used in electromagnetic nondestructive testing are described below.

In nondestructive testing, the words probe and transducer are synonymous. Electromagnetic testing probes come in several forms, types and sizes. The most common types are coils, hall effect detectors and magnetic particles.

The electromagnetic field that is measured in nondestructive testing is usually three-dimensional and varies as a function of space. In most cases, the field varies as a function of time. Full characterization of the state of the test object usually makes it necessary to obtain as much information about the field as possible. Because the field varies as a function of time and space, the test object is usually scanned and measurements are taken at multiple points along the surface of the test object. In the case of magnetic particle testing, the particles are sprayed over the test area. Another approach is to use an array of sensors to cover the area of interest. An alternate technique is to use magnetooptic imaging devices.

The electromagnetic field that is measured in nondestructive testing is usually three-dimensional and varies as a function of space. In most cases, the field varies as a function of time. Full characterization of the state of the test object usually makes it necessary to obtain as much information about the field as possible. Because the field varies as a function of time and space, the test object is usually scanned and measurements are taken at multiple points along the surface of the test object. In the case of magnetic particle testing, the particles are sprayed over the test area. Another approach is to use an array of sensors to cover the area of interest. An alternate technique is to use magnetooptic imaging devices.

The field, being three-dimensional, is characterized by three independent components. An appropriate coordinate system (cartesian, cylindrical or spherical) is usually chosen and the field values are referenced accordingly. It is customary to use a point in the test object as the origin of the coordinate system and align one of the coordinates along the surface of the test object. Most probes are directional, sensitive to fields along a specific direction. Thus, a flat or pancake eddy current coil is sensitive to fields that are perpendicular (normal) to the plane of the coil. Similarly, a hall element detector output is proportional to the plane of the hall element. Both devices are insensitive to components of the field in the other directions. It is possible to use two or more of these devices to measure components in the field in other directions. The orientation of the probe with respect to the test object is critical.

Consider the situation shown in Fig. 1, wherea steel billet is being tested with the magnetic flux leakage technique. The test object is magnetized by passing a current through the billet. Figure 1b showsa cross section of the billet with the flux contours. The leakage flux has only two components if the billet is very long and the rectangular discontinuity runs along the entire length of the test object (Fig. 1c). If a hall element, whose plane is parallel to the surface of the billet, is passed over the surface, the output of the detector will appear as shown in Fig. 1d. If a hall element whose plane is perpendicular to the surface scans the billet in the direction indicated in the figure, then the signal shown in Fig. le is observed. The signals depicted in Figs. 1d and 1e are called the tangential component and the normal component, respectively. In this specific case, the component along the axis of the billet is zero. If the flux density components along the axial, normal and tangential components are denoted by Bx, By and By, respectively, then the magnitude of the total flux density is:

F01aF01bF01cF01dF01e

Ficure 1. Magnetic field in long steel billet: (a) diagram showing rectangular slot on surface that carries current; (b) magnetic flux contours within billet; (c) magnetic flux contours around slot; (d) tangential component of leakage field as region above slot is scanned; (e) normal component of leakage field.

1.2 FORMATO DAS SONDAS DE BOBINAS

Ficure 1. Magnetic field in long steel billet: (a) diagram showing rectangular slot on surface that carries current; (b) magnetic flux contours within billet; (c) magnetic flux contours around slot; (d) tangential component of leakage field as region above slot is scanned; (e) normal component of leakage field.

1.2.1 CONFIGURAÇÃO

Design flexibility lets probes be configured in different ways. Three of the most common eddy current configurations are (1) absolute probes, (2) differential probes and (3) absolute and differential array probes.

Absolute eddy current probes consist of a single coil. In this type of probe, the impedance or the induced voltage in the coil is measured directly (the absolute value rather than changes in impedance or induced voltage is considered). In general, absolute eddy current probes are the simplest and perhaps for this reason are widely used.

Differential eddy current probes consist of a pair of coils connected in opposition so that the net measured impedance or induced voltage is cancelled out when both coils experience identical conditions. The coils sense changes in the test material, so differential eddy current probes are used to react to changes in test materials while canceling out noise and other unwanted signals that affect both coils. Their sensitivity to discontinuities in materials is higher than that of absolute probes. Their sensitivity to liftoff variations and probe wobble is reduced because those effects tend to affect both coils equally.

Array probes consist of coils arranged in a circular, rectangular or some other form of an array.

1.2.2 TÉCNICA SENSORA

A second kind of probe classification is based on the technique used for sensing changes in probe characteristics: either (1) the impedance technique or (2) the transmit-receive technique. Because impedance changes in the coil cause changes in the coil voltage (for a constant current source) or in the coil current (for a constant voltage source), it is possible to monitor the driving coil to sense any material parameters that result in impedance changes. The transmit-receive technique uses a separate driving coil (or coils) and pickup coil (or coils). In this case, the voltage induced across the pickup coils is measured.

1.2.3 GEOMETRIA

A third way to classify probes is according to geometry. Common probe designs include (1) inside diameter probes, (2) encircling coils (outside diameter probes), (3) surface probes such as pancake units and (4) special designs such as plus point probes. The pancake probe has a coil whose axis is normal to the surface of the test material and whose length is not larger than the radius. The plus point probe consists of two coils that lie at a right angle to each other.

Inside diameter probes consist of circular coils inserted in tubes or circular holes. Encircling coils are similar in structure to inside diameter probes except for the fact that the test material is passed inside the coils. They are primarily used to test the outside surface of round materials such as tubes and rods. Surface coils are some of the most widely used eddy current probes. In most cases, they consist of flat coils and are used to test flat surfaces or surfaces with relatively large curvatures relative to their size. Surface probes may be curved to fit contours of the test object.

All of these probes may be used in any of the configurations described above. Thus, for example, an inside surface probe may be absolute or differential and either the impedance or the induced voltage may be measured.

1.3 FATORES QUE AFETAM AS SONDAS DE CORRENTES PARASITAS

1.3.1 CURVA DE LIFTOFF

An eddy current probe has an initial impedance (quiescent impedance) that depends on the design of the probe itself. This is an intrinsic characteristic of any eddy current probe and is sometimes called infinite liftoff impedance. As the probe is moved closer to the test object, the real and imaginary parts of the impedance begin to change until the probe touches the material surface. This is called the zero liftoff impedance. The impedance curve described by the probe as it moves between these two points is the liftoff curve and is a very important factor to consider in eddy current testing.

Because of the nature of the eddy current probes, the curve is not linear (the change in the field is larger close to the coils). In many cases, especially with small diameter probes for which the field decays rapidly, the range in which measurements may be taken is very small and the effect of liftoff can be pronounced. In other cases, such as with large diameter probes or with forked probes, the effect may be considerably smaller.

Liftoff, because it is troublesome in many cases, is often considered an effect to be minimized. Liftoff effects may be reduced by techniques such as surface riding probes? or compensated for by making multifrequency measurements.* At the same time, some important eddy current tests depend on the liftoff effect. Measurements of nonconductive coating thicknesses over conducting surfaces and testing for surface evenness are two such tests.

1.3.2 FATOR DE ENCHMENTO ("FILLFACTOR")

For encircling coils, the coupling factor, analogous to the liftoff effect, is referred to as fill factor. Fill factor is a measure of how well the tested article fills the coil. The largest signal is obtained with the material completely filling the coil — fill factor is almost equal to 1.0. Although it is usually desirable to maximize fill factor, some tests rely on fill factor variations. Fill factor is determined by the intersection of the impedance curve with the vertical or imaginary axis of the impedance plane.

1.3.3 PROFUNDIDADE DE PENETRAÇÂO

When the eddy current probe is placed on the test object, the eddy currents induced in the test object are not uniformly distributed throughout the material. The eddy current density is high at the surface and decays exponentially with depth in the material; the phenomenon that accounts for this density difference is called the skin effect. A measure of the depth to which eddy currents penetrate the material is called the depth of penetration, or skin depth. The standard depth ofpenetration can be defined as:

Eq02

where f is frequency (hertz), 5 is the standard depth of penetration (meter), Ho is the magnetic permeability of free space, 1, is the relative magnetic permeability and o is the conductivity of the material.

The standard depth of penetration is a convenient figure at which, under precisely controlled conditions, the eddy current density has decayed to 1-e! (37 percent) of its surface value. It is an important figure for practical purposes because, at about five standard depths of penetration (under precisely defined conditions), the eddy current density is less than 0.7 percent of the surface value.

As Eq. 2 shows, the standard depth of penetration depends on conductivity, permeability and frequency but is relatively small for most metals, about 0.2 mm (0.008 in.) for copper at 100 kHz. The skin effect has two important effects on the design of eddy current probes: (1) the probes are more useful for surface testing and (2) lower frequencies may be necessary for subsurface testing. The standard depth of penetration can be increased in the case of ferromagnetic test objects by magnetically saturating them, thereby reducing their relative magnetic permeability mu r,


2. PROJETO DAS SONDAS DE CORRENTES PARASITAS

2.1 EXPERIÊNCIAS NO PROJETO DE SONDAS DE CORRENTES PARASITAS

2.2 PROJETO ANALÍTICO DE SONDAS DE CORRENTES PARASITAS
2.2.1 CÁLCULO DA RESISTÊNCIA DA SONDA
2.2.2 CÁLCULO DA REATÂNCIA DA SONDA
2.2.3 SONDAS COM MÚLTIPLAS BOBINAS
2.2.4 PROJETO ANALÍTICO DE SONDAS COMPLEXAS
2.3 PROJETO NUMÉRICO
2.3.1 SONDAS DE ENROLAMENTO ÙNICO VERSUS SONDAS DE ENROLAMENTO DUPLO
2.3.2 PARÂMETRO LIFTOFF
2.3.3 PARÂMETRO PROFUNDIDADE DO FURO
2.3.4 EFEITOS DA REDUÇÃO DE RUÍDO
2.3.5 VERICICAÇÃO EXPERIMENTAL DO PROJETO DA SONDA POR MODÊLO NUMÉRICO
2.4 SONDAS COM NÚCLEO DE FERRO
2.4.1 SONDAS COM NÚCLEO NA FORMA DE COPO
2.4.2 DADOS DE PROJEÇÃO DO CAMPO
2.4.3 CARACTERÍSTICAS DO LIFTOFF
2.5 SONDAS ISOLADAS
2.5.1 CARACTERÍSTICAS DO LIFTOFF
2.6 SONDAS DO DIÂMETRO INTERNO
2.7 SONDAS ENVOLVENTES


3. DETECTORES DE EFEITO HALL
3.1 PRINCÍPIOS DOS DETECTORES DE EFEITO HALL
3.1.1 MUDANÇAS DE OPERAÇÃO DOS DETECTORES DE EFEITO HALL
3.1.2 AÇÕES DOS CAMPOS MAGNÉTICOS NA MUDANÇA DE OPERAÇÃO DOS SEMICONDUTORES
3.1.3 ELEMENTO HALL
3.1.4 CIRCUITO ELÉTRICO DO ELEMENTO HALL
3.1.5 CARACTERÍSTICAS OPERACIONAIS DOS DETECTORES DE EFEITO HALL
3.1.6 RESPOSTA DIRECIONAL DOS DETECTORES DE EFEITO HALL
3.2 CONFIGURAÇÕES DOS DETECTORES DE EFEITO HALL
3.2.1 "ARRAYS" (MATRIZ DE SENSORES) MULTIDIRECIONAL
3.2.2 "ARRAYS" (MATRIZ DE SENSORES) LINEAR
3.2.3 "ARRAYS" (MATRIZ DE SENSORES) EM PONTE
3.2.4 SISTEMAS DIRECIONAIS


4. SONDAS PARA FUGA DE CAMPO MAGNÉTICO
4.1 DESCRIÇÃO DO MÉTODO
4.2 SONDAS DO TIPO BOBINA
4.3 OUTRAS FORMAS DE INDICAÇÃO DO CAMPO MAGNÉTICO DE FUGA
4.3.1 MAGNETODIODO
4.3.2 FITA MAGNÉTICA DE GRAVAÇÃO
4.3.3 PARTÍCULAS MAGNÉTICAS
5. IMAGENS DE CORRENTES PARASITAS COM SENSORES MAGNETOÓTICOS
5.1 PRINCÍPIOS DA IMAGEM MAGNETOÓTICA
5.2 GRAÇÃO DE PELÍCULA DE CORRENTE PARASITA
5.3 COMBINAÇÃO DA IMAGEM MAGNETOÓTICA E DOS PRINCÍPIOS DE CORRENTES PARASITAS
5.3.1 IMAGEM MAGNETOÓTICA DA PELÍCULA DE CORRENTES PARASITAS
5.4 INDICAÇÕES MAGNETOÓTICA
5.5 INDICAÇÕES LINEARES VERSUS ROTATIVAS
5.6 INTERPRETAÇÃO DA IMAGEM MAGNETOÓTICA
5.7 PADRÕES DE REFERÊNCIA E IMAGEM MAGNETOÓTICAS SELECIONADAS
5.8 SUMÁRIO


Autores:
  • Satish Upda, Michigan State University, East Lansing, Michigan
  • Nathan Ida, University of Akron, Akron, Ohio (Parts 1 and 2)
  • Gerald L. Fitzpatricj, PRI Research and Development Corporation, Kirkland, Washington (Part 5)
  • Tatsuo Hiroshima, Marktec Corporation, Taiei, Japan (Part 4)
  • Michael L. Mester, Lower Burrell, Pennsylvania (Part 4)
  • William C.L. Shih, PRI Research and Development Corporation, Torrance, California (Part 5)
  • Roderic K. Stanley, NDE Information Consultants, Houston, Texas (Part 3 and 4)
  • Lalita Upda, Michigan State University, East Lansing, Michigan (Part 1)



Referências
MUDAR
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  12. Burrows, C.W. United States Patent 1686 679, Apparatus for Testing Magnetizable Objects (October 1928).
  13. Zuschlag, T. “Magnetic Analysis Inspection in the Steel Industry.” Symposium on Magnetic Testing, 1948 [Detroit, Michigan, June 1948]. Special Technical Publication 85. West Conshohocken, PA: ASTM International (1949): p 113-122.
  14. Black, W.A. “Eddy Current Testing of Steel Tubing, 1929-60.” Materials Evaluation. Vol. 43, No. 12. Columbus, OH: American Society for Nondestructive Testing (November 1985): p 1490, 1492-1493, 1495-1498,
  15. Forster, F. Sections 36-42. Nondestructive Testing Handbook, first edition: Vol. 2. Columbus, OH: American Society for Nondestructive Testing (1959).
  16. Hochschild, R. “Eddy Current Testing by Impedance Analysis.” Nondestructive Testing. Vol. 12, No. 3. Columbus, OH: American Society for Nondestructive Testing (May-June 1954): p 35-44.
  17. Kraus, J.D. The Big Ear. Powell, OH: 17. Cygnus-Quasar Books (1976).

Bibliografia

Electromagnetic Induction Techniques
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Rail Testing
  • Clarke, J.G. and C.R. Spitzer. “Electronic Locator for Salvaging Trolley Rails.” Electronics. Vol. 17. New York, NY: McGraw-Hill January 1944): p 129.
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  • Keevil, W.R. “History and Development of Rail Flaw Detector Cars.” Materials Evaluation. Vol. 49, No. 1. Columbus, OH: American Society for Nondestructive Testing (January 1991): p 71-76.
  • “Rail Testing Cars, 1928-49.” Materials Evaluation. Vol. 50, No. 2. Columbus, OH: American Society for Nondestructive Testing (February 1992): p 307-310
  • Wickre, J.M. “ Fishing for Fissures: Sources for the History of Rail Testing Cars, 1927-60.” Materials Evaluation. Vol. 43, No. 4. Columbus, OH: American Society for Nondestructive Testing (March 1985): p 372-379.

Wire Rope
  • Cavanagh, PE. “Some Changes in Physical Properties of Steels and Wire Rope during Fatigue Failure.” Transactions. Montreal, Quebec, Canada: Canadian Institute of Mining and Metallurgy (july 1947): p 401-411.
  • Cavanagh, PE. and RS. Segsworth, “Nondestructive Inspection of Mine Hoist Cable.” Transactions. Vol. 38. Materials Park, OH: ASM International (1947): p 517-550.
  • Gee, J. “Testing and Inspection of Wire Ropes.” Mine and Quarry Engineering. Vol. 14. London, United Kingdom: Electrical Press, Limited (December 1948): p 375.
  • Weischedel, H.R. “Electromagnetic Wire Rope Inspection in Germany, 1925-40.” Materials Evaluation. Vol. 46, No. 6. Columbus, OH: American Society for Nondestructive Testing (May 1988): p 734-736


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