TM 1-1520-253-23
1.4.11.4 Sorting Metal Using Eddy Current. In addition to the more common usage for crack detection, eddy current
equipment may be used for metal sorting. Electrical conductivity and magnetic permeability are the material
characteristics evaluated during this type of inspection. The sorting technique cannot directly identify alloy or even the
type of metal. But, when there are limited possibilities, conductivity and/or permeability information may permit proper
classification (see Figure 1-8). Typically, the need for alloy sorting occurs when changes to parts are made to improve
performance. For example, a magnesium part that is experiencing severe corrosion is replaced by one made from
aluminum. Another example is the replacement of one aluminum part with another, also of aluminum, but made from an
alloy having improved strength or corrosion resistance. In both these examples, there may be a need to verify that
replacement has been made, and the electrical conductivity of the alloys involved may be sufficiently different to permit
verification by a sorting inspection. Another situation is the requirement to NDI a part to confirm a visual indication where
the material is not known and cannot be easily determined. Eddy current sorting will quickly determine if the part is
ferromagnetic and should be inspected using the magnetic particle method. Also, if the part is nonferromagnetic, which
test block (standard) most closely matches the conductivity of the part, and therefore, should be used to adjust the eddy
current equipment for crack inspection/verification.
NOTE
Ultrasonic inspection shall be performed in accordance with the general application and
techniques in TM 55-1500-335-23 (Nondestructive Inspection Methods manual) and the specific
requirements of this technical manual.
