electromagnetic testing reading three.pdf

Electromagnetic TestingReading-3 Fact CardsMy ASNT Level III Pre-Exam Preparatory Self Study Notes 2nd April 2015

Charlie Chong/ Fion Zhang

Aerospace Applications



Drone SDTI

Wing Loong Drone



My Mickey Drone

Fion Zhang at Shanghai2nd April 2015

http://meilishouxihu.blog.163.com/



Reading 03-01The TWI 31 – Eddy Current Methods for NDT


Magnetic field is the magnetic effect of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude (or strength); as such it is a vector field. The term is used for two distinct but closely related fields denoted by the symbols B and H, where H is measured in units of amperes per meter (symbol: A·m−1 or A/m) in the SI. B is measured in Teslas (symbol: T) and Newton per meter per ampere (symbol: N·m−1·A−1 or N/(m·A)) in the SI. B is most commonly defined in terms of the Lorentz force it exerts on moving electric charges.

Magnetic fields can be produced by moving electric charges and the intrinsic magnetic moments of elementary particles associated with a fundamental quantum property, their spin. In special relativity, electric and magnetic fields are two interrelated aspects of a single object, called the electromagnetic tensor; the split of this tensor into electric and magnetic fields depends on the relative velocity of the observer and charge. In quantum physics, the electromagnetic field is quantized and electromagnetic interactions result from the exchange of photons.

http://en.wikipedia.org/wiki/Magnetic_field


In everyday life, magnetic fields are most often encountered as a force created by permanent magnets, which pull on ferromagnetic materials such as iron, cobalt, or nickel, and attract or repel other magnets. Magnetic fields are widely used throughout modern technology, particularly in electrical engineering and electromechanics. The Earth produces its own magnetic field, which is important in navigation, and it shields the Earth's atmosphere from solar wind. Rotating magnetic fields are used in both electric motors and generators. Magnetic forces give information about the charge carriers in a material through the Hall effect. The interaction of magnetic fields in electric devices such as transformers is studied in the discipline of magnetic circuits.

http://en.wikipedia.org/wiki/Magnetic_field


φMagnetic FluxMagnetic flux φ is the product of the average magnetic field times the perpendicular area that it penetrates. It is a quantity of convenience in the statement of Faraday's Law and in the discussion of objects like transformers and solenoids. In the case of an electric generator where the magnetic field penetrates a rotating coil, the area used in defining the flux is the projection of the coil area onto the plane perpendicular to the magnetic field

B = magnetic flux densityφ in weber (Wb) (in derived units: volt-seconds)

http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magcon.html#c1


Magnetic Flux Illustrations

The contribution to magnetic flux for a given area is equal to the area times the component of magnetic field perpendicular to the area. For a closed surface, the sum of magnetic flux is always equal to zero (Gauss' law for magnetism). No matter how small the volume, the magnetic sources are always dipole sources (like miniature bar magnets), so that there are as many magnetic field lines coming in (to the south pole) as out (from the north pole).



Magnetic Field Strength HThe magnetic fields generated by currents and calculated from Ampere's Law or the Biot-Savart Law are characterized by the magnetic field B measured in Tesla(symbol:T) and newtons per meter per ampere [symbol: N·m−1·A−1 or N/(m·A)] . But when the generated fields pass through magnetic materials which themselves contribute internal magnetic fields, ambiguities can arise about what part of the field comes from the external currents and what comes from the material itself. It has been common practice to define another magnetic field quantity, usually called the "magnetic field strength" designated by H A·m−1 or A/m. It can be defined by the relationship:

H = B0/μ0 = B/μ0 - Mand has the value of unambiguously designating the driving magnetic influence from external currents in a material, independent of the material's magnetic response. The relationship for B can be written in the equivalent form:

B = μ0(H + M)H and M will have the same units, amperes/meter. To further distinguish B from H, B is sometimes called the magnetic flux density or the magnetic induction. The quantity M in these relationships is called the magnetization of the material.



Inductance of a CoilCoil's reaction to increasing current. For a fixed area and changing current, Faraday's law becomes

Since the magnetic field of a solenoid is

then for a long coil the emf is approximated by

From the definition of inductance

we obtain


InductorsInductance is typified by the behavior of a coil of wire in resisting any change of electric current through the coil. Arising from Faraday's law, the inductance L may be defined in terms of the emf generated to oppose a given change in current:


Inductor AC ResponseYou know that the voltage across an inductor leads the current because the Lenz' law behavior resists the buildup of the current, and it takes a finite time for an imposed voltage to force the buildup of current to its maximum.


Inductive ReactanceThe frequency dependent impedance of an inductor is called inductive reactance.

Impedance = Angular frequency x Inductance


Capacitor AC ResponseYou know that the voltage across a capacitor lags the current because the current must flow to build up the charge, and the voltage is proportional to that charge which is built up on the capacitor plates.


Capacitive ReactanceThe frequency dependent impedance of a capacitor is called capacitive reactance.


ResistanceThe electrical resistance of a circuit component or device is defined as the ratio of the voltage applied to the electric current which flows through it:

If the resistance is constant over a considerable range of voltage, then Ohm's law, I = V/R, can be used to predict the behavior of the material. Although the definition above involves DC current and voltage, the same definition holds for the AC application of resistors.

Whether or not a material obeys Ohm's law, its resistance can be described in terms of its bulk resistivity. The resistivity, and thus the resistance, is temperature dependent. Over sizable ranges of temperature, this temperature dependence can be predicted from a temperature coefficient of resistance.


Resistivity and ConductivityThe electrical resistance of a wire would be expected to be greater for a longer wire, less for a wire of larger cross sectional area, and would be expected to depend upon the material out of which the wire is made. Experimentally, the dependence upon these properties is a straightforward one for a wide range of conditions, and the resistance of a wire can be expressed as

The factor in the resistance which takes into account the nature of the material is the resistivity . Although it is temperature dependent, it can be used at a given temperature to calculate the resistance of a wire of given geometry.

It should be noted that it is being presumed that the current is uniform across the cross-section of the wire, which is true only for Direct Current. For Alternating Current there is the phenomenon of "skin effect" in which the current density is maximum at the maximum radius of the wire and drops for smaller radii within the wire. At radio frequencies, this becomes a major factor in design because the outer part of a wire or cable carries most of the current.

The inverse of resistivity is called conductivity. There are contexts where the use of conductivity is more convenient.

Electrical conductivity = σ = 1/ ρ


Electrical conductivity or specific conductance is the reciprocal of electrical resistivity, and measures a material's ability to conduct an electric current. It is commonly represented by the Greek letter σ (sigma), but κ(kappa) (especially in electrical engineering) or γ (gamma) are also occasionally used. Its SI unit is Siemens per meter (S/m) and CGSE unit is reciprocal second (s−1).


IACSThe International Annealed Copper Standard (IACS) establishes a standard for the conductivity of commercially pure annealed copper. The standard was established in 1913 by the International Electrotechnical Commission.

The Commission established that, at 20°C,commercially pure, annealed copper has a resistivity of 1.7241x10-8 ohm-meter or 5.8001x107 Siemens/meter when expressed in terms of conductivity. For convenience, conductivity is frequently expressed in terms of percent IACS. A conductivity of 5.8001x107 S/m (58 x 106 S/m) may be expressed as 100% IACS at 20°C. All other conductivity values are related back to this standardvalue of conductivity for annealed copper.

Therefore, iron with a conductivity value of 1.04 x 107 S/m, has a conductivity of approximately 18% of that of annealed copper and this is reported as 18% IACS. An interesting side note is that commercially pure copper products now often have IACS conductivity values greater than 100% IACS because processing techniques have improved since the adoption of the standard in 1913 and more impurities can now be removed from the metal. Wire of high purity has been produced having a conductivity of slightly over 103% IACS, which is very near the value expected for copper without any impurity.


Leading and lagging in AC currentThere are three options in a circuit for current. It can be leading, lagging, or in phase with voltage. These can all be seen when one maps current and voltage of alternating (AC) circuits against time. The only time that the voltage and current are in phase together is when the load is resistive. If at some point in the phase shift the current leads the voltage by more than 90 degrees, it can then be stated that the current lags that voltage by 180 degrees minus the phase shift. Ninety degrees phase shift is the determining point if the current is either leading or lagging the voltage.

Each of the main components of a circuit (resistor, capacitor, and inductor) can be seen as an impedance. All of them produce resistance in either fractional or exponential ways. Here are their complex number forms:

Resistor, R = RCapacitor, Zc = 1/jω CInductor, Zl = jωLω =2 π f


Lagging Voltage against current- Capacitive

CurrentVoltage

XC= 1/ ωC


Leading Voltage against current - Inductive

CurrentVoltage

XL= ωL


Capacitive Reactance

http://www.electronics-tutorials.ws/filter/filter_1.html


Inductive Reactance

http://www.electronics-tutorials.ws/inductor/ac-inductors.html


R2 XL2 Z2

Resistance Inductive Reactance Impedance


Defect DetectionDefect Detection –– Perturbation of Eddy currentPerturbation of Eddy current

Magnetic FieldFrom Test Coil

Magnetic Field From

Eddy Currents

Eddy Currents

Crack


Impedance Phasor Diagrams


and


and

10

100


and


Conductivity vs Impedance

constant frequency

0

0.2

0.4

0.6

0.8

1

0 0.1 0.2 0.3 0.4 0.5Normalized Resistance

Nor

mal

ized

Rea

ctan

ce

StainlessSteel, 304

CopperAluminum, 7075-T6

Titanium, 6Al-4V

Magnesium, A280

Lead

Copper 70%,Nickel 30%

Inconel

Nickel


Apparent Eddy Current Conductivity

• high accuracy ( 0.1 %)

• controlled penetration depth

specimen

eddy currents

probe coil

magnetic field

0

0.2

0.4

0.6

0.8

1.0

0.10 0.2 0.3 0.4 0.5

lift-offcurves

conductivity

curve(frequency)

Normalized Resistance

Nor

mal

ized

Rea

ctan

ce

= 0

= s

1

23

4


Nor

mal

ized

Rea

ctan

ce


Lift-Off Curvature

inductive(low frequency)

capacitive(high frequency)

“Horizontal” Component

“Ver

tical

”C

ompo

nent

lift-off

.

conductivity

σ2

σ1

σ

ℓ = s ℓ = 0

“Horizontal” Component

“Ver

tical

”C

ompo

nent

.

conductivity

lift-off

σ2

σ1

σ

ℓ = s ℓ = 0

Magnetic Susceptibility

0

0.2

0.4

0.6

0.8

1.0

0.10 0.2 0.3 0.4 0.5

lift-off

frequency(conductivity)

Normalized ResistanceN

orm

aliz

ed R

eact

ance

permeability


Nor

mal

ized

Rea

ctan

ce

0

1

2

3

4

0 0.2 0.4 0.6 0.8 1 1.2

2

3

1

µr = 4permeability

moderately high susceptibility low susceptibility

paramagnetic materials with small ferromagnetic phase content

increasing magnetic susceptibility decreases the apparent eddy current conductivity (AECC)

frequency(conductivity)

Thickness versus Normalized Impedance

thickness loss due to corrosion, erosion, etc.

probe coil

scanning

0

0.2

0.4

0.6

0.8

1

0 0.1 0.2 0.3 0.4 0.5 0.6

thickplate


Nor

mal

ized

Rea

ctan

ce

thinplate

lift-off

thinning

-0.2

0

0.2

0.4

0.6

0.8

1

0 1 2 3Depth [mm]

Re

{ F

}

f = 0.05 MHzf = 0.2 MHzf = 1 MHz

aluminum (σ = 46 %IACS)

/ /( ) x i xF x e e

Defect Detections


0

0.2

0.4

0.6

0.8

1

0 0.1 0.2 0.3 0.4 0.5

conductivity(frequency)

crackdepth

flawlessmaterial

ω1

lift-offN

orm

aliz

ed R

eact

ance

ω2

apparent eddy current conductivity (AECC) decreasesapparent eddy current lift-off (AECL) increases

Conductivity, Magnetic Susceptibility, AECL & AECC ResponsesKey:apparent eddy current lift-off (AECL)apparent eddy current conductivity (AECC)


Conductivity VS Alloying & Temper

IACS = International Annealed Copper Standard

σIACS = 5.8107 Ω-1m-1 at 20 °C

ρIACS = 1.724110-8 Ωm

20

30

40

50

60

Con

duct

ivity

[% IA

CS]

T3 T4

T6

T0

2014

T4

T6T0

6061

T6

T73T76

T0

70752024

T3 T4

T6

T72T8

T0

Various Aluminum Alloys


Inductive Lift-Off Effect

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

0.1 1 10 100Frequency [MHz]

Rel

ativ

e ΔA

ECC

[%] .

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0


Rel

ativ

e ΔA

ECC

[%] .

63.5 μm50.8 μm38.1 μm25.4 μm19.1 μm12.7 μm6.4 μm0.0 μm

-100

1020304050607080


AEC

L [μ

m]

.

-100

1020304050607080


AEC

L [μ

m]

. .

63.5 μm50.8 μm38.1 μm25.4 μm19.1 μm12.7 μm6.4 μm0.0 μm

4 mm diameter 8 mm diameter

1.5 %IACS 1.5 %IACS


Instrument Calibration

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0


AEC

C C

hang

e [%

] .

12A Nortec 8A Nortec 4A Nortec 12A Agilent 8A Agilent 4A Agilent 12A UniWest 8A UniWest 4A UniWest 12A Stanford 8A Stanford 4A Stanford

Nortec 2000S, Agilent 4294A, Stanford Research SR844, and UniWest US-450

conductivity spectra comparison on IN718 specimens of different peening intensities.

Magnetic Susceptibility versus Cold Work

10-4

10-3

10-2

10-1

100

101

0 10 20 30 40 50 60Cold Work [%]

Mag

netic

Sus

cept

ibili

ty

SS304L

IN276

IN718

SS305

SS304SS302

IN625

cold work (plastic deformation at room temperature) causesmartensitic (ferromagnetic) phase transformation

in austenitic stainless steels

Thickness Correction

1.0

1.1

1.2

1.3

1.4

0.1 1 10Frequency [MHz]

Con

duct

ivity

[%IA

CS]

1.0 mm1.5 mm2.0 mm2.5 mm3.0 mm3.5 mm4.0 mm5.0 mm

6.0 mm

thickness

Vic-3D simulation, Inconel plates (σ = 1.33 %IACS)

ao = 4.5 mm, ai = 2.25 mm, h = 2.25 mm

Non-conducting Coating

non-conductingcoating

probe coil, ao

t

d

ℓ

conducting substrate

ao > t, d > δ, AECL = ℓ + t

-100

1020304050607080


AEC

L [μ

m]

-100

1020304050607080


AEC

L [μ

m]

63.5 μm

50.8 μm

38.1 μm

25.4 μm

19.1 μm

12.7 μm

6.4 μm

0 μm

ao = 4 mm, simulatedlift-off:

ao = 4 mm, experimental

Conducting Coatingconducting

coating

probe coil, ao

t

d

ℓ

conducting substrate (µs,σs)

approximate: large transducer, weak perturbation

equivalent depth:

e1AECC( )

2 s sf

f

21( ) AECC

4 s sz

z

se 2

analytical: Fourier decomposition (Dodd and Deeds)

numerical: finite element, finite difference, volume integral, etc. (Vic-3D, Opera 3D, etc.)

zJe

z = δe

Crack Contrast and Resolution

probe coil

crack

0

0.2

0.4

0.6

0.8

1

0 1 2 3 4 5Flaw Length [mm]

Nor

mal

ized

AEC

C

semi-circular crack

-10% threshold

detectionthreshold

ao = 1 mm, ai = 0.75 mm, h = 1.5 mm

austenitic stainless steel, σ = 2.5 %IACS, μr = 1

Vic-3D simulation

f = 5 MHz, δ 0.19 mm


and


Characteristic Parameters

Pc= ωμσr -2


Characteristic Parameter dimensionless. It allows test coil operating point to be specified in terms of a single quantity rather than four independent variables.

Characteristic or Limit Frequency fg hertz, Characteristic Frequency Ratio f/fg dimensionless. It allows the test coil

operating point to be specified in terms of a single quantity rather than four independent variables.


The Characteristic Parameter (Pc) is a means of determining the operating point on the impedance curve. The formula for Pc depends on the units used, but the following is recommended:

PC = 4.6 × 10-3 f μr σr 2 (5.3a)

or

f = PC /(4.6 × 10-3 μr σr 2) (5.3b)

Where:Pc = characteristic parameter,f = frequency (kHz),μr = relative permeability,σ = conductivity (% IACS),r = mean coil radius (mm)

Note that the unit of frequency used in this formula, as in the other ‘applied’ eddy current formulae (for f90 and f/fg) is kHz, unlike the basic theoretical formulae for depth of penetration and inductive reactance.


Calculations of Pc are used in conjunction with FIG. 5.20, which relates the value of Pc to the operating point on the impedance curve for surface probes.

FIG. 5.20. Impedance diagram showing conductivity curves at a number of different values of lift-off to coil radius ratio (LO/r) (the solid curves) and a number of lift-off curves (dashed) Values of Pc are indicated for the zero lift-off conductivity curve. These values also apply to the corresponding locations on the other conductivity curves.


Example:Calculate the optimum frequency for maximum sensitivity in sorting commercially pure titanium (conductivity approximately 2.5% IACS) from Ti-6AI-4Valloy (conductivity 1% IACS), using a coil with outer diameter 10 mm wound on a 6 mm diameter plastic core.

Calculation:For maximum sensitivity to conductivity, and minimum response to lift-off, the operating point should be at or somewhat below the knee of the impedance curve. That is, Pc should be between 10 and approximately 200. The Pc value will be different for the two alloys, but, since Pc is proportional to conductivity, the two values will differ by a factor of 2.5/1 = I (the ratio of the conductivities). Suitable values would be 30 for titanium and 75 for stainless steel, though other similar values within the range 10 to 200 could be chosen.


That is:

f = Pc /(4.6 × 10-3 μrσ r2)Pc = 30 (for titanium),r = (10 + 6)/2 = 8 mm, so r = 4 mm,μr = 1σ = 1% IACS

Inserting these values into equation gives:f = 30/ (4.6 × 10-3 × 1 × 1 X 42)= 407.6 kHz or, rounding to a convenient figure:= approximately 400 kHz


When eddy current conductivity meters are used, this approach is normally not possible, because they operate at one or more fixed frequencies. Normally, these instruments are designed to give an operating point in the region of the knee of the curve at the lowest frequency, normally 60 kHz, and higher frequencies are used only if the material is too thin for testing at the lowest frequency.


Goal of Pc - For maximum sensitivity to conductivity, and minimum response to lift-off, the operating point should be at or somewhat below the knee of the impedance curve. The Pc = ωμσr -2 is chosen such that the operating point is below the knee where frequency of the coil could be selected.


Characteristic Frequency


Reading 03-02J.Younas– Eddy Current Methods for NDT


Fill Factor

Usually, 70-90% "fill-factor" is targeted for reliable inspection


fill factor: For encircling coil electromagnetic testing, the ratio of the cross sectional area of the test object to the effective cross sectional core area of the primary encircling coil (outside diameter of coil form, not inside diameter that is adjacent to the object).

For internal probe electromagnetic testing, the ratio of the effective cross sectional area of the primary internal probe coil to the cross sectional area of the tube interior.

NDT Handbook Vol5. Chapter 19


fill factor: For encircling coil electromagnetic testing, the ratio of the cross sectional

area of the test object to the effective cross sectional core area of the primary encircling coil (outside diameter of coil form, not inside diameter that is adjacent to the object). r2/Re

2

For internal probe electromagnetic testing, the ratio of the effective cross sectional area of the primary internal probe coil to the cross sectional area of the tube interior. r2/Ri

2

Re

r

Ri

Re

NDT Handbook Vol5. Chapter 19


Standard Depth

Eddy current density is greatest at surface Reduces exponentially with depth At standard δ = 1/e (37%) of surface value, δ = (ωμσ) -½


Effects of Frequency on δ


Effects of conductivity σ on δ


Effects of permeability μ on δ


Standard Depth δD

epth

Eddy Current Density

Dep

th

Eddy Current DensityLow FrequencyLow ConductivityLow Permeability

High FrequencyHigh ConductivityHigh Permeability

Standard Depthof

Penetration(Skin Depth)

1/e or 37 % of surface density


Type of probes

Surface Probe

External Probe

Internal Probe


Less Sensitive to drift due to temperature changes

Sensitive to drift due to temperature changes

Detect only ends of long flawsShow total length of long flaws

Difficult to interpretEasy to interpret

Not Sensitive gradual changes in properties

Sensitive to both sudden and gradual changes in properties.

Pair of coilsSingle Coil

Less Sensitive to probe wobbleSensitive to probe wobble

Changes in impedance or induced voltage is measured

Absolute value of impedance and induced voltage is measured

Differential ProbesAbsolute Probes


Reading 03-03UKHSE– Evaluation of the effectiveness of non-destructive testing screening methods

http://www.hse.gov.uk/research/rrpdf/rr659.pdf


Electromagnetic / Electrical TechniquesSLOFECSaturated Low Frequency Eddy current (SLOFEC) senses changes in permeability of steel plates for detection of corrosion. It uses strong magnets and eddy current coils. This combination generates magnetic field line and eddy current field line distribution to detect flux density changes within the material.

Testing field line changes within the material gives more sensitive results than conventional MFL techniques. The use of the eddy current technique allows variation of settings and analysis of signal amplitude & signal phase. Good for local defect detection such as detecting pitting corrosion and localisedmicrobiological corrosion (which is mainly present as localised corrosion but can rarely appear as general corrosion). Even small isolated pits are detected.



SLOFEC can detect & distinguish both internal and outer surface breaking defects. SLOFEC is suitable for inspection of ferritic and non-ferritic materials. Suitable for inspection of pipes, tank floors, vessel Wall thickness up to 30 mm, coatings up to 10 mm for tankfloors, vessels and drums. Wall thickness up to 25 mm, coatings up to 7 mm for pipes and pipelines (1”+ diameter) Sensitive for corrosion detection but not used for absolute wall thickness determination “Gradual” defects (greater than 300- 00mm in length) are not as easily detectable. Findings should be complemented by conventional ultrasonic inspection.



SLOPEC



Pulsed Eddy Current – PECPEC is a screening tool for inspecting remaining wall thickness under coatings and insulations. Not considered rapid. A coil is placed over the insulated pipe / vessel and a current pulse is sent through the coil. When the current is interrupted eddy currents are generated in the material, which decay in time. Measuring the rate of decay of the eddy currents determines the wall thickness. High wall thickness results in a slower decay. The PEC wall thickness is an average over its ‘footprint’, i.e. the area where eddy currents flow. The size of the footprint area depends on the distance between probe and metal surface. The footprint is approximately a circle with a diameter depending on the distance between probe and steel surface.

The PEC wall thickness readings are an average value over this footprint area. As a result, PEC can only detect general wall loss.

Localised corrosion such as pitting is not detected by PEC.



A rough rule of thumb is smallest detectable defect diameter is 50% of the lift-off, i.e. in 50mm of insulation the smallest detectable defect diameter is around 25mm. In principle PEC cannot differentiate between internal and external defects. PEC can be deployed on- tream for detection of erosion corrosion, flow accelerated corrosion and corrosion under insulation in carbon steel or low alloy ferromagnetic metals with wall thickness between 2-35 mm. PEC Can measure through any kind of non magnetic insulation upto 200 mm thick (e.g. rockwool, foamglas, concrete, marine growth, dirt, cladding and scaling) It can be applied in wet or underwater conditions and no surface preparation necessary.



Pulsed Eddy Current – IncotestIncotest is a pulsed eddy current system. Its principle of operation, strengths and weakness are similar to pulsed eddy current system PEC described in above. It is a screening tool for inspecting remaining wall thickness under coatings and insulations. Incotest can detect erosion corrosion, flow accelerated corrosion and corrosion under insulation in carbon steel or low alloy metals with wall thickness between 6-65 mm through insulation thickness up to 200mm Incotest can measure through any kind of non ferritic insulation e.g. rockwool, foamglas, concrete, marine growth, dirt, cladding, aluminium, stainless steel or galvanised (up to 1 mm) sheeting. No surface preparation is necessary The accuracy is roughly 5% and repeatability is 2%.



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MFLMFL technique uses magnets for inducing a magnetic field in the component being tested. In defect-free material, this field remains trapped within the material. Changes in material properties or geometry (i.e. defects) force the magnetic field to leak out of the material, where it is detected by the Hall- ffectMFL sensors.

MFL can detect corrosion, particularly pitting, in materials up to 20 mm thick at speeds upto 0.5m/ sec Inspection can be carried out through non-conducting coating (upto 6 mm thick) at sensitivity upto 20% underfloorcorrosion (as per supplier). Also suitable for ferritic vessels, pipes, boiler tubes, heat exchanger tubes.

MFL cannot differentiate between top side and bottom side corrosion.

Erosion defects may not be detected as the change between full wall thickness and the thinned area is gradual. MFL is a qualitative technique and requires use of ultrasound for estimation of wall loss and proofup.



MFL PipescanA MFL pipescan is a specially designed tool for inspection of pipes. A MFL scanning head is moved across the external surface of the pipe and will detect internal and external corrosion pitting. The system includes a separate electronics module with an operator adjustable alarm threshold control. The defect alarm consists of an audible alarm and L.E.D.s. Used as a screening tool, Pipescan is capable of detecting corrosion pitting originating from the internal and/or external surface of the pipe. The system has no sizing capabilities. Pipescan scanning heads cannot pass bends / elbows and severely distorted areas of pipe. Pipescan can inspect wall thickness up to 19 mm with coatings up to 6 mm.



RFT

Remote Field Remote FieldNear Field

exciter coilferromagnetic pipe sensing coil

ln(Hz)

z

low frequency operation (10-100 Hz)

Exponentially decaying eddy currents propagating mainly on the outer surface

cause a diffuse magnetic field that leaks both on the outside and the inside of the pipe.

0

1

rf

/0 zz zH H e

Without transition

RFT

http://www.mdpi.com/1424-8220/14/12/24098/htm


MicrowaveMicrowave signals readily penetrate inside non-conducting materials and may therefore be employed for defect detection within these materials.

Microwaves are radiated from the transducer to the specimen being tested. A detectable signal is returned at each interface where the dielectric constant changes (e.g. - where there are delaminations, cracks, holes, impurities or other defects).

The transducer may be moved relative to the specimen at any desired speed and the scanning speed need not be uniform. Once the data is collected, the software allows the image to be manipulated to enhance features. Also, since it is in digital form, the scan results can be stored and retrieved later to provide information on how a part or a defect has changed over time. This allows determination of the growth rate of a defect, which is critical to determining ultimate service life. Microwaves can be used for detection of defects such as delaminations, disbands and impact damage in dielectric materials such as fibre reinforced polymeric (FRP) materials. Full volumetric coverage can be obtained. Applicable to non-conductive materials only. Moisture or liquid may affect results.



Microwave



Others Non-Electromagnetic TechniquesThermographyThermography is a remote condition monitoring technique which identifies thesurface temperatures by the measurement of the intensity of infra-red radiation emitted from a surface. The higher the temperature of the object, higher will be the emitted infrared energy. This is the energy detected by infrared cameras. Resolution up to 0.1°C can be detected depending on camera and extra options utilised. A low resolution Flir camera will measure hotspots from 10mm at 5 metres (and hotspots from the size of 6mm for the higher resolution option). Cameras with 0.02°C thermal sensitivity are used for dynamic images. Results can be distorted if hot pipes and shiny cladding are present close to inspection area - something that is likely to happen in reality.



For CUI inspections a 30°C temperature gradient across the insulation (between pipe and environment) is desired to ensure detection. The presence of water will be measured in terms of temperature variations (e.g. variations of emitted infra red radiation) of the insulation weather proofing along the pipe, arising from a loss in thermal efficiency of the insulation system.



Neutron BackscatteringNeutron backscattering uses the neutron slowing down property of hydrogen for detection of moisture. A radioactive source (Am 241/Be or Cf 252) emits high energy neutrons into the insulation. The neutrons are slowed down or “moderated” by collisions with light elements, in particular hydrogen. Theythen diffuse back to a thermal neutron detector where the slow neutrons are counted.

Moisture in, for example, insulation increases the density of hydrogen nuclei so the number of slow neutrons detected will rise. Detects moisture under thermal insulation on pipe work or vessels. Detection method also works on metal clad insulation. Effective moisture detection capability even where insulation is several centimeters thick. Hand operated instrument giving on-line readout of results within a few seconds for each location.



Detector is sensitive to presence of hydrogen so can be used to detect presence of oil and other liquids with a high hydrogen content. Detector is very sensitive to water very close and almost completely insensitive to the presence of water farther away. This means that any water within the pipe or vessel will not effect the results. This claim is made by FORCE’s Moisture Probe only. Not suited for outdoors use during rainfall. Not suited for use on foam insulation and plastic cladding or any insulation with high hydrogen content. Radiological hazards associated with use of neutron source.



Neutron Backscattering



Reading 03-04Fact Sheets


Skin Effect / Standard Depth of Penetration (SDP)Eddy current density in a material is not uniform in the thickness (depth) direction. It is greatest on the material surface and decreases monotonously with depth (skin effect) and the eddy currents lag in phase with depth, allowing employ phase discrimination method to locate size and differentiate defects and disturbing variables. "Standard depth of penetration" (SDP) equation given above can be used to explain the capability of eddy current testing. For a uniform, isotropic and very thick material, SDP is the depth at which the eddy current density is 37% of its surface value. From the SDP equation, one can easily interpret that depth of penetration (delta) decreases with increasing frequency, conductivity, permeability (see flux line contours below). Thus, in order to detect very shallow defects (cracks, flaws) in a material and also to measure thickness of thin sheets, very high frequencies are to be used (see flux line contours below). Similarly, in order to detect sub-urface buried defects and to test highly conductive/ magnetic/ thick materials, low frequencies are to be employed.


Eddy Current - Skin Effect


Probes/Sensors for ECTAppropriate selection of probe coil is important in eddy current testing, as even an efficient eddy current testing instrument can not achieve much if it doesn’t get the right (desired) information from the coils. The most popular coil designs are:

Surface probes or pancake probes (with the probe axis normal to the surface), are chosen for testing plates and bolt-holes either as a single sensing element or an array - in both absolute and differential (split-D) modes.

Encircling probes for inspection of rods, bars and tubes with outside access and

Bobbin probes for pre-and in-service inspection of heat exchanger, steam generator, condenser tubes & others with inside access.

Phased array receivers also possible for enhanced detection and sizing.


Probes

Absolute Differential Reflection


Probes Considerations

I1

V2

11

V1

I2

2212 21,

V Z I

*wireZ i L R

sensitivity

thermal stability

eddy current

ferrite-core coil

high coupling

high coupling

eddy current

air-core coil

high coupling

low coupling

eddy current

flat air-core coilhigh coupling

flexible, low self-capacitance, reproducible, interchangeable, economic, etc.

I

V

1 11 12 1

2 12 22 2

V Z Z IV Z Z I

*12 12Z i L

topology


Probes Configurations

voltmeter

testpiece

oscillator

excitationcoil

sensing coil

~

voltmeter

testpiece

oscillator

coil

Zo

~

Hall or GMR detector

voltmeter

testpiece

oscillator

excitationcoil

~~

differential coils

coaxial rotatedparallel


These three types of probes can be operated in absolute or differential (left, last). They can also operate in send-receive mode (reflection mode) (separate coils for sending and receiving [again absolute or differential]). The EC probes consisting of a single sensing coil for excitation and reception are called absolute probes. Such probes are good for detection of cracks (long as well as short) as well as gradual variations. However, absolute probes are sensitive also to lift-off, probe tilt, temperature changes etc. Differential probes have two sensing coils wound in opposite direction and investigating two different regions of the material. They are good for high sensitive detection of small defects and they are reasonably immune to changes in temperature and probe wobble.


Eddy Current Testing SignalsAn eddy current signal is the trajectory of coil impedance formed upon scanning the coil over a material surface. Eddy current (impedance change) signal / data are analyzed in time-domain (strip-chart) and also in impedance plane (CRT or computer screen). Typical time-domain and impedance plane signals for a plate tested using a surface probe (absolute mode) are given on the right hand side.

CRT Screen or Impedance Plane Display


Electromagnetic Coupling (Lift-off / Fill-factor)Coupling of magnetic field to the material surface is important in ECT. For surface probes, it is called "liftoff“ which is the distance between the probe coil and the material surface. In general, uniform and very small lift-off is preferred for achieving better detection sensitivity to defects. Similarly, the electromagnetic coupling in the case of tubes/bars/rods is referred to as "fill-factor". It is the ratio of square of coil diameter to square of tube diameter, in the case of encircling coils and is expressed as percentage (dimensionless). Usually, 70-90% "fill-factor" is targeted for reliable inspection.


Eddy Current Testing ProcedureUsual EC test procedure involves first calibration. Artificial defects such as saw cuts, flat bottom holes, and electro-discharge machining (EDM) notches are produced in a material with similar chemical composition and geometry as that of the actual component. Well-characterized natural defects such as service induced fatigue cracks and stress corrosion cracks are preferred, if available. The test frequency, instrument gain and other instrument functions are optimized so that all specified artificial defects are detected, e.g. by thresholding of appropriate EC signal parameters such as signal peak-to- eakamplitude and phase angle. With optimized instrument settings, actual testing is carried out and any indication that is greater than the threshold level is recorded defective.


For quantification (characterization) master calibration graphs, e.g. between eddy current signal parameters and defect sizes are generated. In the case of heat exchanger tube ECT, calibration graph is between depth of ASME calibration defects (20%, 40%, 60%, 80% and 100% wall loss flat-bottom holes) and the signal phase angle. In order to detect and characterize defects under support plates multi-frequency EC testing which involves mixing of signals from different frequencies is followed and separate calibration graph is generated for quantification of wall loss.


FIGURE-Magnetic flux line contours of an eddy current probe in air, in an Inconel tube and in the tube surrounded by a carbon steel support plate. Freedom-loving flux lines are constrained by the tube wall and the support plate. This constraint (manifested as distortion / perturbation of eddy currents and associated impedance change) is what is measured to advantage in eddy current testing!


Advantages of Eddy Current Testing Eddy current test can nearly tested all metallic materials High inspection

speeds possible (~ 5 m/s) Eddy current test can readily detect very shallow and tight surface fatigue

cracks and stress corrosion cracks (~ 5 microns width and 50 microns depth)

High temperature and on-line testing is possible, even in shop floors Non-contact / remote / inaccessible testing is possible (Couplant is not required unlike in ultrasonic)

Recording and analysis of inspection data is possible (Computer based instruments / systems available with data acquisition, storage, analysis and database management)


Limitations of Eddy Current testingLike any other NDT technique ECT too has certain limitations, which are overcome to a large extent by the recent advances in the technique. A few key limitations are:

Only electrically conducting (metallic) materials can be tested Maximum inspectable thickness is ~ 6 mm (12 mm possible by tuning

frequency, probes, instrumentation etc.) Inspection of ferromagnetic materials is difficult using conventional eddy

current tests (Saturation ECT and Remote field ECT are possible for tubes) Use of calibration standards necessary Operator skill is necessary for meaningful testing and evaluation


Recent Trends / Advances in ECT Pulsed EC testing for sub-surface defect detection, Remote field EC testing for ferromagnetic tubes, Eddy current imaging to produce images or pictures of defects and to

automate inspection, Signal and image processing methods to extract more useful information

of defects for enhanced detection and characterization of defects, Low-frequency eddy current testing, Numerical modeling (finite element, boundary element / volume integral,

hybrid etc.) for simulation of inspection technique / situation, prediction of ECT signals for inversion & optimization of probes / test parameters,

Design of Phased-array and special focused probes, Realization of expert systems and data-base systems.


Figure: Eddy Current Image of a Stainless Steel Weld


Figure: This is an eddy current image of a small defect (hole) in a stainless steel plate. As compared to the time domain and impedance plane signals shown earlier, it is possible to have a visual feel of the defect and can have an idea about the spatial extent/shape/size of the defect.


Eddy Current Images of Small Fatigue Cracks

Al2024, 0.025-mil crack Ti-6Al-4V, 0.026-mil-crack

0.5” 0.5”, 2 MHz, 0.060”-diameter coil

probe coil

crack


Crystallographic Texture J E

1 1 1

2 2 2

3 3 3

0 00 00 0

J EJ EJ E

generally anisotropic hexagonal (transversely isotropic)

1 1 1

2 2 2

3 2 3

0 00 00 0

J EJ EJ E

cubic (isotropic)

1 1 1

2 1 2

3 1 3

0 00 00 0

J EJ EJ E

σ1 conductivity normal to the basal plane

σ2 conductivity in the basal plane

θ polar angle from the normal of the basal plane

σm minimum conductivity in the surface plane

σM maximum conductivity in the surface plane

σa average conductivity in the surface plane2 2a 1 2( ) 絒 sin (1 cos )]

2 2n 1 2( ) cos sin

M 2

1 2

2 2m 1 2( ) sin cos

x1

x3

x2basal plane

θ

surface plane

σnσm

σM


Electric “Birefringence” Due to Texture

1.00

1.01

1.02

1.03

1.04

1.05

0 30 60 90 120 150 180Azimuthal Angle [deg]

Con

duct

ivity

[%IA

CS]

highly textured Ti-6Al-4V plate equiaxed GTD-111

1.30

1.32

1.34

1.36

1.38

1.40

0 30 60 90 120 150 180Azimuthal Angle [deg]

Con

duct

ivity

[%IA

CS]

500 kHz, racetrack coil


Grain Noise in Ti-6Al-4V

as-received billet material solution treated and annealed heat-treated, coarse

heat-treated, very coarse heat-treated, large colonies equiaxed beta annealed

1” 1”, 2 MHz, 0.060”-diameter coil


Eddy Current versus Acoustic Microscopy

5 MHz eddy current 40 MHz acoustic

1” 1”, coarse grained Ti-6Al-4V sample


In-homogeneity

AECC Images of Waspaloy and IN100 Specimens

homogeneous IN100

2.2” 1.1”, 6 MHz

conductivity range 1.33-1.34 %IACS

±0.4 % relative variation

inhomogeneous Waspaloy

4.2” 2.1”, 6 MHz

conductivity range 1.38-1.47 %IACS

±3 % relative variation


Conductivity Material Noise

1.30

1.32

1.34

1.36

1.38

1.40

1.42

1.44

1.46

1.48

1.50

0.1 1 10Frequency [MHz]

AEC

C [%

IAC

S]

Spot 1 (1.441 %IACS)



Spot 4 (1.382% IACS)

as-forged Waspaloy

no (average) frequency dependence


Magnetic Susceptibility Material Noise1” 1”, stainless steel 304

f = 0.1 MHz, ΔAECC 6.4 %

f = 5 MHz, ΔAECC 0.8 %

intact

f = 0.1 MHz, ΔAECC 8.6 %

f = 5 MHz, ΔAECC 1.2 %

0.51×0.26×0.03 mm3 edm notch


Standards in Eddy Current TestingReference standards are used for adjusting the eddy current instrument’s sensitivity detection of cracks, conductivity, permeability and material thickness etc. and also for sizing. Some commonly used standards in eddy current testing are:

1. ASME, Section V, Article 8, Appendix 1 and 2), Electromagnetic (eddy current) testing of heat exchanger tubes

2. BS 3889 (part 2A): 1986 (1991) Automatic eddy current testing ofwrought steel tubes

3. BS 3889 (part 213): 1966 (1987) Eddy current testing of non-ferrous tubes

4. ASTM B 244 Method for measurement of thickness of anodic coatings of aluminum and other nonconductive coatings on nonmagnetic basematerials with eddy current instruments

5. ASTM B 659 Recommended practice for measurement of thickness ofmetallic coatings on nonmetallic substrates


6. ASTM E 215 Standardizing equipment for electromagnetic testing of seamless aluminum alloy tube

7. ASTM E 243 Electromagnetic (eddy current) testing of seamless copper and copper alloy tubes

8. ASTM E 309 Eddy current examination of steel tubular products using magnetic saturation

9. ASTM E 376 Measuring coating thickness by magnetic field or eddycurrent (electromagnetic) test methods

10. ASTM E 426 Electromagnetic (eddy current) testing of seamless and welded tubular products austenitic stainless steel and similar alloys

11. ASTM E 566 Electromagnetic (eddy current) sorting of ferrous metals 12. ASTM E 571 Electromagnetic (eddy current) examination of nickel and

nickel alloy tubular products 13. ASTM E 690 In-situ electromagnetic (eddy current) examination of non-

magnetic heat-exchanger tubes 14. ASTM E 703 Electromagnetic (eddy current) sorting of nonferrous

metals


Specific Applications of Eddy Current Testing1. Quality assurance and in-service inspection of austenitic stainless steel

tubes, plates and welds. 2. In-service inspection of heat exchangers, steam generators and

condensers for- Detection and sizing of defects in tubes (single frequency)- Detection and sizing of defects near support plates (multi-frequency)

3. Detection and sizing of defects in multi-layer aircraft structures (multi-frequency & pulsed eddy current tests)

4. Quality assurance and in-service inspection of ferromagnetic tubes. 5. Detection and characterization of intergranular corrosion (IGC) in stainless

steel 316L/304L. 6. Detection of weld centre line in austenitic stainless steel welds using eddy

current C-scan imaging. 7. Measurement of thickness of plates as well as thickness of coatings using

eddy currents. 8. Sorting of materials based on electrical conductivity and magnetic

permeability. 9. On-line eddy current detection of defects in materials. 10. High temperature and non-contact testing of materials.


Multiplexing

TimeSi

gnal

single-frequency time-multiplexed multiple-frequency

frequency-multiplexed multiple-frequencypulsed

Time

Sign

al

Time

Sign

alTime

Sign

al

excited signal (current) detected signal (voltage)

2D


Nonlinear Harmonic Analysissingle frequency, linear response

nonlinear harmonic analysis

Time

Sign

al

Time

Sign

al

H

B

ferromagnetic phase(ferrite, martensite, etc.)

Charlie Chong/ Fion Zhanghttps://www.yumpu.com/en/browse/user/charliechong

electromagnetic testing reading three.pdf

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