3- Electromagnetic Testing Method Flashcards by Akmal Mughal

Eddy current testing involves the use of ,,,,, and can be applied to any ,,,,

alternating magnetic fields

conductor

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Leakage flux testing involves the use of a

,,,, and can be applied only to ,,,,

permanent magnet, DC or AC electromagnetic fields,

ferromagnetic materials

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Ferromagnetic materials are the materials that are ,,,, They are able to create ,,,,,.

magnetic

permanent magnets

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In eddy current testing, the alternating magnetic field sets up ,,,,, in the test part

circulating eddy currents

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The magnetoelastic technique is used for characterizing and determining the amount of residual stress in magnetic materials by ,,,,, or ,,,,,

measuring magnetic

“Barkhausen noise.”

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Eddy Current and magnetoelastic techniques an be combined with other methods, such as ,,,,, and ,,,,, to achieve multifunction high-speed testing

ultrasonic testing

laser dimensional measuring

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High-speed automatic testing is possible using multiple ,,,,

NDT methods

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When an alternating current is used to excite a coil, an ,,,, field is produced and magnetic lines of flux are concentrated at the ,,,, of the coil.

alternating magnetic

center

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as the coil is brought near an electrically conductive material, the ,,,,,, penetrates the material and generates continuous, ,,,,,,

alternating magnetic field

circular eddy currents

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As the penetration of the induced field

increases, the eddy currents become ,,,,,

weaker

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The induced eddy currents produce an in a test coil,,,,,

opposing (secondary) magnetic field

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The opposing magnetic field of, coming from the material, has a weakening effect on the ,,,,, and the test coil can sense this ,,,,

primary magnetic field

change.

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In effect, the impedance of the test coil is ,,,proportionally as eddy currents are increased in the test piece.

reduced

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A crack in the test material ,,, the eddy current flow ,,,,, the eddy current path, ,,,,, the secondary magnetic field, and increases the coil ,,,,,

obstructs
lengthens
reduces
impedance

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Changes in magnetic flux density may also be detected by ,,,,, devices,

Hall effect

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the conductivity of a material has the greatest effect on coil ,,,,,

impedance

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Coil impedance is dependent on the vector sum of the coil’s ,,,,,, and the test part’s resistance to the ,,,,,

inductive reactance

eddy current field.

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Another important influence on coil impedance is the clearance or lift-off between the ,,,, and the,,,,

coil

conductive material surface

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coil impedance remains unchanged regardless of any conductivity changes in the ,,,,

material

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as the coil approaches the surface in the stepwise fashion, ,,,,,,eddy currents are induced in the material

stronger

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When attempting to measure changes in conductivity, changes in spacing or lift-off are highly ,,,,,

undesirable

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In order to minimize variations in lift-off, eddy current coils may be recessed a,,,,, into the eddy current probe head, and the probe head may be ,,,, to maintain surface contact

short distance

spring loaded

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However, since the lift-off effect is ,,,, over a limited probe clearance range, eddy current probes can
be designed to measure ,,,,, coating thickness over uniformly conductive materials.

linear

nonconductive

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Coil impedance can be calculated for any known combination of ,,,,, and ,,,,,

conductivity

probe clearance

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The planar diagram shows that it is more difficult | to distinguish between ,,,, and ,,,,, with low conductivity materials.

defect indications | lift-off indications

,,,,,, coils are used more frequently than surface-mounted coils

Encircling

The degree of filling is the ratio of the ,,,, to the ,,,,,

test material cross-sectional area | coil cross-sectional area

For most applications, two coils are employed—the primary (field) coil generates the ,,,,, and the secondary (pickup) coil detects the ,,,,,,caused by the changes in conductivity and permeability

eddy currents | change in coil impedance

Eddy currents are generated in the material in accordance with ,,,,,

Maxwell’s law

according to Lenz’s law, these eddy currents must flow | in the ,,,,, to the current in the field coil

opposite direction

the magnitude of the eddy current depends on ,,,,,,of the test part

- frequency of the field current, - conductivity and permeability of the test material, - and geometry

When ferromagnetic materials are magnetized, magnetic lines of force (or flux) ,,,,, through the material and complete a ,,,, between the pole pieces.

flow | magnetic path

These magnetic lines of flux,,,, from zero at the center of the test piece and,,,, in density and strength toward the outer surface

increase | increase

By measuring the ,,,,,,of this leakage flux, we can | determine to some extent the severity of the defect

intensity

heavy buildup of magnetic particles is a ,,,,,pattern at the poles

three-dimensional

All of the fine magnetic particles near the magnets are drawn to the ,,,,, pieces

pole

The ideal permanent magnet should be easy to magnetize and hard to ,,,

demagnetize

If the material is not ,,,,,, some flux may also exit the far surface

too thick (<0.3in.)

For longitudinal flaw detection on round bars and tubes, a ,,,, yoke DC-MFL system is used.

rotational

for longitudinal and round bar/tube, The magnetic poles of the yoke are ,,,, apart with a series of magnetic sensors ,,,,,from the poles

180° degree | 90°

Transverse flaws are detected by passing the tube through a ring yoke that produces ,,,,,,,magnetization

longitudinal

The frequency of the alternating field is about ,,,, to ,,,

1 to 30kHz

Increases in intensity increase the depth of ,,,,,

saturation

with the differential coil (two-coil) system, the magnetic | lines of flux in the coils (shown by directional arrows) ,,,, each other

oppose

The differential method is also known as the self-,,,,, method because the adjacent sections of the material are compared to each other

comparison

The crack distorts the eddy currents, weakens the ,,,,, and increases the,,,,, and ,,,,,, of the pickup coil

primary magnetic field | impedance and voltage output

crack distorts the eddy current ,,,,,

path

Wide coils are used to measure ,,,, and narrow coils are used ,,,, detection

changes in conductivity | primarily for flaw

The majority of Flux Leakage sensors are ,,,,,or ,,,,,

inductive coil sensor | solid-state Hall effect sensors

For an induction coil to detect a magnetic field, the magnetic field must be,,,,,,, or ,,,,,

alternating or pulsating

A greater electromotive force (emf) will be induced into | the coil if the number of turns of wire is ,,,,,,,

increased

,,,,,,,probes are popular magnetic flux measuring devices

Hall effect

,,,,,,,,, is described as, strong magnetic fields skewed the equal potential lines in a conductor, thus producing a miniature voltage perpendicular to the direction of current flow

Hall effect

Hall effect principle: When a current-carrying conductor is placed in a magnetic field, a ,,,,, is produced, which is,,,,,,, to both the direction of current and the magnetic field.

Hall voltage | perpendicular

Since a 1 gauss (G) field produces a Hall voltage of about ,,,,,

30 mV

The more highly magnetized the ferromagnetic object, the higher its ,,,,,, will be for a given defect.

leakage flux field intensity

longitudinal defects in tubes, rods, and bars will be | more easily detected with ,,,, magnetic fields

circular

For typical Hall effect probes, operating speed is listed as ,,,,,maximum

100kHz

Test Frequency: | test frequency affects the ,,,,, of the coil

inductance

Test Frequency: | Lowering the test frequency ,,,, the depth of eddy current penetration

increases

Test Frequency: | Lower test frequencies are typically used with ,,,,, materials because of their,,,,,,

ferromagnetic | low permeability

Test Frequency: At a fixed frequency, eddy current penetration will be the ,,,,, in a metal with the ,,,,,,, International Annealed Copper Standard (% IACS) conductivity

greatest | lowest-percentage

Test Frequency: ,,,,,, is the frequency at which additional increases in frequency do not produce additional increases in eddy current losses.

The limit frequency

Test Frequency: | The limit frequency is also known as the ,,,,,,frequency of the material

“characteristic”

Test Frequency: | If the characteristic frequency is 100Hz, the test frequency that is required for an f/fg ratio of 10 is ,,,,,,

1.0kHz.

MFLT;

MAGNETIZATION FOR FLUX LEAKAGE TESTING

MFLT; | for MFLT testing to be effective, ferromagnetic parts must be magnetized to ,,,,,,,

saturation

MFLT: | Magnetic lines of flux exit the ,,,,, pole of a magnet and enter the ,,,,, pole

north | south

MFLT: | units of flux density will be given in ,,,, units or ,,,,

centimeter gram second (cgs) | gauss (G)

MFLT: | The characteristics of magnetic materials can be described by reference to ,,,,,or,,,,,,,

the materials’ magnetization or hysteresis curves

MFLT: | magnetization curve is a plot of,,,,,,,, on the vertical axis as a function of ,,,, on the horizontal axis

``` flux density (B) in guass magnetizing force (H) in oersteds ```

MFLT: | In a magnetizations curve, Non magnetized ferromagnetic material is represented by the origin, ,,,

point 0,0

MFLT: In a magnetization curve, As this material is gradually subjected to increasing magnetizing force, the magnetic flux density in the material increases from ,,.to ,,,,,

0 to Bmax

MFLT: In a magnetization curve, The force required to demagnetize the ferromagnetic part is known as the ,,,,,and the second quadrant of the magnetization curve is known as the ,,,,,

coercive force | coercive demagnetization curve.

MFLT: In a magnetization curve, The permeance or permeability (m) of a magnetic material depends on many factors, but for any set of conditions is defined as the ,,,,,

ratio of B/H

MFLT: | In a magnetization curve, As a general rule, hard materials have a ,,,, force and are not easily demagnetized

high coercive

Coupling: | In eddy current testing, the test piece is coupled to the test coil by the ,,,,,,

coil’s magnetic field

Coupling: | coupling efficiency is 100% when lift-off is ,,,,,

zero

Coupling: | Lift-off describes the change in electromagnetic coupling as a function of ,,,,,,

probe clearance

Coupling: | ,,,,,,,changes both the amplitude and phase of the eddy current signal

Lift-off

EMT

Electromagnetic Testing

EMT: | ,,,,,,, produced by DC saturation coils, is used when ferromagnetic materials are tested with encircling coils

Direct current (DC) saturation

EMT: When a coil’s magnetizing force is applied to ferromagnetic materials, the flux density in the material is much than the flux density generated by the test coil

greater

EMT: | the eddy current test coil detects the leakage flux emanating from ,,,,,, and,,,,,, defects.

small surface and near surface

EMT: Increases in alternating current field strength decrease the eddy current ,,,, in magnetic materials to some minimum value.

penetration

EMT: | When magnetic penetration is maximized, the part is magnetically ,,,,,,

saturated

MFLT:

MAGNETIC FLUX LEAKAGE TESTING

MFLT: | The magnetic flux leakage method is ,,,,,, and recommended by the ,,,,, for tubes with ,,,,, to,,,, wall thickness

dry, fast, online API small to medium

MFLT: | ,,,,,, is used over the entire cross section of pipes transversely for ,,,,,,,,and ,,,,,,,, flaws

DC field magnetization | internal and external

MFLT: ,,,,,, describes magnetic flux leakage as the most efficient method for integration of an automated, nondestructive test for detecting ,,,, flaws during the production of ,,,,,,, tubes.

ASTM E 570 rolling seamless, hot-rolled

GECA

General Eddy Current Applications

GECA: | General eddy current applications can be further subdivided into applications:

- Measurement of physical property differences ( such as; flaws & thickness) - Measurement of parameters relating more to conductivity (such as; hardness) - Application of permeability changes in ferromagnetic parts