184.108.40.206 Eddy Current Instrument Standardization. Eddy current inspection equipment and standards required by the
procedures in this manual are listed in Table 1-7. Reference blocks, instrument settings, and standardization instructions
for the eddy current instrument, are included in each eddy current procedure. Instrument settings, as they are given in
this manual, should be considered typical and present a test block display shown in Figure 1-7. Additional nulling will be
required to reestablish the position of the "flying spot' with the probe on the part/area to be inspected. (Use Teflon tape
listed in Table 1-8 on the probe to reduce wear. Instrument settings shall be made with Teflon tape on the probe, if
220.127.116.11 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 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.
1.4.12 Ultrasonic (UT) Method.
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
Ultrasonic inspection uses high frequency sound waves as a probing medium to provide information as to the state of
various materials. This method is effective for the inspection of most metals for surface and subsurface damage. The
method requires that at least one surface of the part be accessible for transducer contact in the vicinity of the area to be
examined. The inspection is accomplished by inducing the ultrasound into the part by coupling the transducer to the part
and picking up reflections of this sound from within the part. Any marked changes in acoustic properties: defect,
interface, or back surface will reflect sound back to the transducer. The detected ultrasonic reflections are electronically
displayed on a CRT and interpreted for indications of defects. Accessory wedges can be used to provide adequate
transducer mating to curved surfaces, change the angle, and/or the mode of propagation of the sound beam.
18.104.22.168 Safety Precautions During Ultrasonic Inspection. Follow safety precautions and instructions contained in this
manual and the Nondestructive Inspection Methods manual listed in Table 1-1.