3- Electromagnetic Testing Method Flashcards
Eddy current testing involves the use of ,,,,, and can be applied to any ,,,,
alternating magnetic fields
conductor
Leakage flux testing involves the use of a
,,,, and can be applied only to ,,,,
permanent magnet, DC or AC electromagnetic fields,
ferromagnetic materials
Ferromagnetic materials are the materials that are ,,,, They are able to create ,,,,,.
magnetic
permanent magnets
In eddy current testing, the alternating magnetic field sets up ,,,,, in the test part
circulating eddy currents
The magnetoelastic technique is used for characterizing and determining the amount of residual stress in magnetic materials by ,,,,, or ,,,,,
measuring magnetic
“Barkhausen noise.”
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
High-speed automatic testing is possible using multiple ,,,,
NDT methods
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
as the coil is brought near an electrically conductive material, the ,,,,,, penetrates the material and generates continuous, ,,,,,,
alternating magnetic field
circular eddy currents
As the penetration of the induced field
increases, the eddy currents become ,,,,,
weaker
The induced eddy currents produce an in a test coil,,,,,
opposing (secondary) magnetic field
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.
In effect, the impedance of the test coil is ,,,proportionally as eddy currents are increased in the test piece.
reduced
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
Changes in magnetic flux density may also be detected by ,,,,, devices,
Hall effect
the conductivity of a material has the greatest effect on coil ,,,,,
impedance
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.
Another important influence on coil impedance is the clearance or lift-off between the ,,,, and the,,,,
coil
conductive material surface
coil impedance remains unchanged regardless of any conductivity changes in the ,,,,
material
as the coil approaches the surface in the stepwise fashion, ,,,,,,eddy currents are induced in the material
stronger
When attempting to measure changes in conductivity, changes in spacing or lift-off are highly ,,,,,
undesirable
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
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
Coil impedance can be calculated for any known combination of ,,,,, and ,,,,,
conductivity
probe clearance
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
