SENSITIVITY LEVEL MAGNETIC PARTICLE TEST

Any factor that affects the formation of indications at a discontinuity affects the sensitivity of magnetic particle testing. Two of the most important of these factors are the amperage of the magnetizing current and the control of the magnetic particle testing media.

Amperage

The formation of magnetic particle indications at discontinuities depends on the strength of the leakage fields at the discontinuities. Since the leakage fields are a part of the field generated by the magnetizing current, greater magnetizing current will produce greater leakage field strength. Thus, the sensitivity of a magnetic particle test is directly related to the current amperage.
Too low amperage may produce leakage fields too weak to form readily discemable indications. Too high an amperage creates a heavy background accumulation of particles that may mask an indication. In circular magnetization, too high amperage may bum current contact points of a test object. Actual amperage requirements should be calculated. Due to complex configurations, a shim or penetrameter can be used for verification of calculations.

Testing Media

The sensitivity level is affected not only by the current amperage, but also by the kind of magnetic particle testing media, its control and its applications.
Wet method media, because of their smaller particle sizes, are the most sensitive for the detection of surface discontinuities. Dry powders are better for detecting deep subsurface discontinuities.

Fluorescent magnetic particle materials have a higher apparent sensitivity than do those used with ordinary light, such as the blacks and reds. The fluorescent indications are more easily and quickly seen in the darkened areas associated with ultraviolet radiation tests.
Testing of objects made of materials only moderately retentive requires careful control of the way the testing media is applied. Usually, maximum sensitivity is obtained by applying the media while a test object is being magnetized and ending it before the magnetizing field is removed. This is also true in the case of
automatic wet method testing in which the main bath stream is shut off shortly before the magnetizing current is ended, to avoid washing off indications already formed.
Particle concentration in the baths must be closely controlled if maximum sensitivity is to be obtained. Sensitivity is lowered if concentrations are too low because of the lack of sufficient magnetic particles to be readily discernible. If too high, fine indications may be masked by heavy background accumulations.
Contaminants, particularly in wet baths, can result in lowered sensitivity. For example, lubricating oils and greases cause a blue background fluorescence that reduces contrast, causing fluorescent particle indications to be less visible.
Sensitivity of dry powders depends on the type of powder selected, how carefully it is applied and its color. Most powders are made for general use, and have a wide mix of particle sizes to favor the detection of both fine surface and deep subsurface discontinuities.

A powder color is usually selected that will provide the best color contrast with the color of the surface on which it is being used. Care is needed in applying the powder. A light tossing and air blowing action is needed to allow the particles to migrate to and be held by the leakage fields at discontinuities. Excessive
application of powder can cause indications to be lost in background
accumulation.
The magnetizing force at any point on the outside surface of a test object through which electric current is flowing will vary with the current amperage. The greater the amperage, the greater will be this magnetizing force. Inside the test object, just under the point on the surface, the magnetic flux density will be the product of this magnetizing force and the magnetic permeability of the test object at
that point. It is this magnetic flux density that determines the leakage field strengths at discontinuities. Thus, current amperage is directly related to the strength of leakage fields at discontinuities, and it is these leakage fields that capture and hold magnetic particles. The more difficult the discontinuities are to detect, the weaker the leakage fields for a given amperage, and greater
amperage will be required to form discernible magnetic particle indications. The discontinuities referred to in this case are those that parallel the direction of current flow.

Direct Contact

A problem arises when deciding what amperage to use for a given test object, particularly when the test object has a complicated shape. A rule of thumb suggesting 1000 A per inch of diameter is useful when the test object is reasonably uniform and cylindrical in shape. Except for some special alloys and cast irons, the use of 1000 A per inch of diameter will usually ensure more than enough field strength to detect surface and near surface discontinuities. In
highly permeable material, lower amperage per inch of produce adequate field strength within the test object.

Central Conductor

Amperage requirements using a central conductor will depend on the test object’s size and the diameter of the opening through which the conductor is to be located. In the case of a centrally located conductor, amperage requirements may range from 100 A per inch of hole diameter to as much as 1000 A per inch, depending on test object material and the nature of the suspected discontinuities. Keep in mind that the magnetizing field strength around a central conductor decreases with distance away from the conductor, with the strongest flux field being present at the surface inside the central conductor’s hole.
Radial discontinuities at the ends of holes and openings can also be detected using the central conductor method, since some portion of the magnetic lines of force will intercept these discontinuities. The central bar conductor should have an outside diameter as close as possible to the inside diameter of the hole of the test object. Use Eq.

to determine the number of times the test object must be rotated in equal movements and remagnetized to ensure complete testing has been obtained.

Longitudinal magnetization is used for the detection of circumferential discontinuities that lie in a direction transverse to, orat about right angles to, a test object’s axis. Circumferential discontinuities around a cylinder, for example, are detected by magnetizing the cylinder longitudinally in a direction parallel with
its axis. A portion of the longitudinal field will cross the discontinuities, creating leakage fields that can capture and hold magnetic particles to form indications at the discontinuities.

where DP is the diameter of the test object (inner diameter for central conductor, outer diameter for coil); De is outer diameter for central conductor, inner diameter for coil; and Sis number of turns required for complete overlapping coverage.
A test object is said to have been longitudinally magnetized when the field in it is about parallel with a major axis. A test object magnetized in a coil, for example, will be longitudinally magnetized in a direction parallel to the coil axis. A characteristic of a test object that is magnetized longitudinally will be the appearance of opposite magnetic poles, north and south, at the extreme ends of the test object. The existence of the poles is a disadvantage when magnetizing and testing because much of the leakage flux from the pole ends is not parallel with the test object surface. This reduces the magnitude of flux that is parallel, thereby weakening the leakage fields at discontinuities on the end regions. The poles are an advantage in demagnetizing because they make it easy to detect
magnetized test objects and to confirm removal of the residual fields after demagnetizing procedures.
Longitudinal magnetization is used for the detection of circumferential discontinuities that lie in a direction transverse to, or at about right angles to, a test object’s axis. Circumferential discontinuities around a cylinder, for example, are detected by magnetizing the cylinder longitudinally in a direction parallel with its axis. A portion of the longitudinal field will cross the discontinuities, creating leakage fields that can capture and hold magnetic particles to form indications at the discontinuities.

Coil Shot

The usual way to longitudinally magnetize a test object is by placing the test object in a rigid coil on a stationary magnetic particle testing unit. The test object may be laid on the bottom inside of the coil where the field is strongest, or the test object may be supported in the coil by the contact heads of the unit. Special
supports are provided on some testing units for long, heavy test objects permitting rotation of objects for testing. Coils are usually mounted on rails, permitting movement along a long test object for multiple tests (multiple coil shots). Because the effective field extends only 15 to 23 cm (6 to 9 in.) on either side of a coil, multiple tests are needed on long test objects.

Cable Wrap

Cable wrapping a coil around large or heavy test objects is a common practice. Flexible, insulated copper cable is used. A cable wrapped coil is connected to a magnetic particle mobile or portable power pack or it can be connected to the contact heads of a stationary test unit. The type of power source to be used will depend on the kind of current and amperages needed to accomplish the
particular desired test, both magnetizing and demagnetizing.
Cable lengths used to connect cable wrapped coils must be kept as short as possible to minimize cable resistance losses and aid in obtaining higher current amperages. In the case of alternating current, and to some extent half wave direct current, in addition to cable resistance, there is the inductance of the coil circuit that further reduces current output.

Twisting or taping the coil cable leads together aids in reducing the losses of the coil circuit. Coil inductance is the ratio of the total flux (sources and
variations) and the current. The magnetic flux and current are directly proportional to the coil opening’s area for straight coils. The henry (H) is the unit of inductance of a coil.
Coil inductance increases directly with the coil opening area, and increases as the square of the turns in the coil. Keeping each of these factors as small as possible, particularly when using alternating current, ensures the maximum amperage obtainable from the power supply. To keep coil areas low, cable coils should be wrapped directly on a test object or on some insulating material only a little
larger than the test object. Multiple tests along a long test object, using a coil of only a few turns is preferable to using a coil of many turns over the length of the test object.

The latter is occasionally done in cases where performing multiple tests is not possible or when a power pack having the required output voltage and current
capacity is available. Any cables and cable leads used with and for cable wrapped coils must have good quality electrical connections. Poor connections result in overheating and reduced coil amperage.
A number of factors must be considered when determining current amperage for longitudinal magnetization of test objects.
1 . The coil diameter and the number of turns.
2. The length-to-diameter ratio of the test object.
3. The size, shape and composition of the test object.
4. The position of the test object within the coil.
5. The kind of discontinuities being sought and their ease of detection.
6. The magnetic coupling component, or the fill factor of the coil to the test object.
The magnetizing field strength H in the center of the magnetizing coil increases or decreases in direct proportion to the coil current and its number of turns. Also, the field strength will decrease if the coil radius is made larger, or will increase if the radius is made smaller. The field is theoretically zero in the coil center and
increases to a maximum at the inside edge of the conductor(s). Thus, a test object placed against the inside of a coil, for example lying in the bottom of the coil,
will experience greater magnetizing field strength than when it is centered in the coil.
While being magnetized in a coil, a magnetic test object has magnetic poles generated at its ends. Rules of thumb have been developed experimentally that include the effects of a test object’s magnetic permeability, which is assumed to be about 500 or greater, and the demagnetizing effects of the poles at the test object ends.
These rules of thumb use the length-to-diameter ratio of a test object that for many regularly shaped test objects is easily determined from test object dimensions or can be estimated in the case of irregularly shaped test objects.

SENSITIVITY LEVEL MAGNETIC PARTICLE TEST