Term
|
Definition
– Cannot be magnetized
– Repelled by magnetism |
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Term
|
Definition
– Are affected by Magnetic Field
– Slightly attracted by magnetism |
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Term
|
Definition
– Strongly attracted to Magnetic Field
– High permeability
– Retentive in nature |
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Term
|
Definition
Lines of Force
never cross
They are most densely packed at the poles
of the magnet
They seek the path of least magnetic
resistance |
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Term
| Longitudinal Magnetization |
|
Definition
Longitudinal magnetization is said to exist
in an object when the flux lines traverse in a
direction essentially parallel to one of its
axes. |
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Term
|
Definition
Consequent poles exist when more than two
poles are present simultaneously in a
magnetized article.
[image] |
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Term
|
Definition
Distorted field is a combination of circular and longitudinal fields.
can produce a magnetic field at an angle perpendicular to the direction of a possible discontinuity |
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Term
|
Definition
If the bar is cut only partially through, two
opposite poles will appear and a leakage
field will exist in the area of the cut
[image] |
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|
Term
Electrically Induced Magnetic
Fields
How are they formed |
|
Definition
When an electric current passes through a
conductor, a magnetic field is formed in and
around the conductor |
|
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Term
|
Definition
Grasping the conductor in the right hand
with the thumb pointing in the direction of
current flow. The fingers will then point in
the direction of the lines of force |
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Term
|
Definition
|
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Term
|
Definition
| When a current-carrying conductor is formed into a loop, the lines of force circling the conductor form a magnetic field inside and outside the loop [image] |
|
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Term
|
Definition
Flux-per-unit area through an element which cuts the unit area at right angles to the direction of the flux.
Flux density is usually designated by the letter B,
its unit is the gauss. |
|
|
Term
|
Definition
The ease with which a magnetic flux is
established in a given material equal to B/H. |
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Term
|
Definition
• Reluctance is the opposition of a magnetic
material.
• Reluctance is analogous to the resistance in
an electric circuit. |
|
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Term
|
Definition
• Residual magnetism is the amount of
magnetism that a magnetic material retains
after the magnetizing force is removed. |
|
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Term
|
Definition
• Property to retain, to a greater or lesser
degree, a certain amount of residual
magnetism. |
|
|
Term
|
Definition
Reverse magnetizing force
necessary to remove the residual magnetism
so as to demagnetize an article. |
|
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Term
|
Definition
|
|
Term
| Hysteresis Loop Curve Characteristics |
|
Definition
Definite lag between the magnetization force and the flux throughout the cycle is called hysteresis.
• Wide hysteresis loop indicates that the material has high reluctance and is difficult to magnetize.
• Slender loop means the material has low retentivity and is easy to magnetize (low reluctance). |
|
|
Term
| Wide hysteresis loop indicates |
|
Definition
Low permeability
high reluctance
high retentivity
high coercive force
high residual magnetism |
|
|
Term
| Slender hysteresis loop means |
|
Definition
low retentivity
low reluctance |
|
|
Term
| Wide hysteresis loop Characteristics |
|
Definition
|
|
Term
| slender hysteresis loop Characteristics |
|
Definition
|
|
Term
| Circular Magnetization Induced into a article by: |
|
Definition
Induced into a article either by passing
current directly through the article (direct
magnetization) or by passing current
through a conductor inside the object to be
tested. |
|
|
Term
|
Definition
|
|
Term
|
Definition
Article to be magnetized is placed so that a current carrying conductor induces a magnetic field into the article.
“central conductor technique”. |
|
|
Term
| current and field in prods/ yoke pic |
|
Definition
|
|
Term
|
Definition
DC has
AC does not have
if
current is gradually reduced to zero |
|
|
Term
| Alternating Current characteristics |
|
Definition
| • Advantage of using ac is that the voltage can be stepped up or down. • Reversal of magnetic fields due to the alternating current, makes the magnetic particles more mobile facilitating their collection at leakage fields. |
|
|
Term
| Half-Wave Rectified Current characteristics |
|
Definition
The rectifier permits current to flow only
during the positive half cycles of the applied ac voltage
• Alternating current at any commercial frequency
may be used.
• Penetration is comparable to that of straight dc.
• The pulsating effect of the rectified wave is
helpful in adding mobility to the magnetic
particles.
• There is a definite advantage in locating deeplying discontinuities.
• It can be combined in the same equipment with ac. |
|
|
Term
Rectification of Alternating Current to
Half-Wave Direct Current pic |
|
Definition
|
|
Term
|
Definition
|
|
Term
| Demagnetization is usually done when |
|
Definition
• A strong residual field might interfere with
subsequent operations, such as welding or
machining.
• Article is moving part of an assembly and a
deposit of accumulated magnetized particles might
cause wear.
• Leakage fields might interfere with nearby
instruments.
• Residual fields might interfere with proper
cleaning of the article.
• The article is to be magnetized at a lower
magnetizing force in a different direction than the
original or previous test.
• Specified by procedural standards. |
|
|
Term
Demagnetization is usually not
required or necessary: |
|
Definition
• On articles of soft steel or iron where retentivity
is low.
• If, after the magnetic particles test, the article is to
be heat-treated.
• On large castings, weldments, or vessels where
residual fields will have no material amperage.
• If the article is to be magnetized again in another
direction at the same or higher amperage.
• If the article is likely to become remagnetized
during handling by being placed on a magnetic
chuck. |
|
|
Term
| White (Visible Light) requirements |
|
Definition
• Illumination level of at least 100 foot
candles (1000 lux) of visible light is
required at the surface. |
|
|
Term
|
Definition
• The minimum black light intensity
required by most codes is 1000 µW/cm2
• Typically from a distance of 15” to 18”.
• The reading from the meter is typically
recorded on a report, and is re-checked
periodically.
• Information about the meter and its
calibration record are also recorded. |
|
|
Term
| The best detection of defects occurs when the lines of force are ____________ to the longest dimension of the defect |
|
Definition
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|
Term
| A current carrying conductor induces a magnetic field that is ________ |
|
Definition
|
|
Term
| A material with a narrower hysteresis loop has ________ permeability |
|
Definition
|
|
Term
| A material with a narrower hysteresis loop has ______ coercivity |
|
Definition
|
|
Term
| A material with a wider hysteresis loop has ________ residual magnetism |
|
Definition
| Higher residual magnetism |
|
|
Term
| A material with a wider hysteresis loop has _____ reluctance |
|
Definition
|
|
Term
| A material is considered demagnetized when |
|
Definition
| Its residual magnetic field measures less than 3 gauss |
|
|
Term
| ________ has a very weak and negative susceptibility to magnetic field |
|
Definition
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|
Term
| The magnetic field circling each loop of wire in a coil combines with the fields from the other loops to produce a concentrated field where? |
|
Definition
| Down the center of the coil |
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|
Term
| Since all matter is composed of atoms |
|
Definition
| All materials are affected in some way by a magnetic field |
|
|
Term
| Ferromagnetic materials are made up of small polarized regions known as |
|
Definition
|
|
Term
| The central conductor technique is sometimes used because it: |
|
Definition
| Establishes a magnetic field that extends from the ID to the OD of a circular part |
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