TM 1-1500-204-23-7
d.
Field Measurement. The measurement of magnetic flux or field strength, either within a part or at the part's
surface, is extremely difficult. There have been several attempts at developing practical methods or devices. These
methods or devices have all been limited in success and contain serious limitations They do serve a purpose in
technique development if their limitations are understood. A procedure or technique will be developed for a particular
part using rule of thumb and past experience. The actual part will then be subjected to the proposed procedure and the
devices or method used to check the field strength at critical points.
e.
Sensitivity Level. Any factor that affects the formation of magnetic indications at a discontinuity affects the
sensitivity of that magnetic particle inspection. Two of the most important of these factors are the amperage of the
magnetizing current and the control of the magnetic particle Inspection media.
(1)
Amperage. The formation of magnetic particle indications at discontinuities depends upon the strength of
the leakage fields at the discontinuities. Since the leakage fields are a part of the field generated by the magnetizing
current, the greater the magnetizing current, the greater will be the strength of the leakage fields. Thus the sensitivity of
a magnetic particle inspection is directly related to the current amperage. Too low an amperage may produce leakage
fields too weak to form readily discernable indications. Too high an amperage creates a heavy background accumulation
of particles which may mask an indication. In circular magnetization, too high an amperage may burn current contact
points of a part. In actual practice, amperage requirements are not normally calculated. The more direct approach of
estimating the magnetizing current or determining it by experiment is sufficiently accurate for the purpose.
(2)
Inspection media. Sensitivity level is affected not only by the current amperage but also by the kind of
magnetic particle inspection media, its control and its applications.
4-9. Circular Magnetization. Circular magnetization Is used for the detection of radial discontinuities around edges of
holes or openings in parts. It is also used for the detection of longitudinal discontinuities which lie in the same direction
as the current flow either in part or in a part which a central conductor passes through.
a.
Technique. Two techniques are used to obtain circular magnetization in parts: by passage of electric current
through the parts themselves, called the direct contact method, or, by passage of the current through a central conductor
that passes through the part, called the central conductor method. Each is explained In the following paragraphs.
(1)
Direct contact method. Direct contact to parts is generally made by placing them between clamping heads.
Lead face plates or copper braid pads must be used to prevent arcing, overheating, and splatter. Wetting of the contact
plates with the suspension vehicle prior to current application helps prevent overheating.
(2)
Central conductor method. A part can be circularly magnetized by passing electrical current through a
conductor positioned coaxially in a hole or opening in a part. A magnetizing field does exist outside a central conductor
carrying current so the walls surrounding a central conductor become magnetized making possible the detection of
discontinuities which parallel the central conductor. Central conductors are any high conductive material, such as a
copper bar or cable, placed in the center of the part to be magnetized. The central conductor method shall be used if
longitudinal discontinuities on the inside of tubular or cylindrically shaped parts are to be detected.
b.
Amperage. The magnetizing force at any point on the outside surface of a part 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 part, 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 part at that point. It is this magnetic flux density which 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 which 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 which approximately parallel the
direction of current flow with all or part of the circular field generated by current crossing them.
4-4
