Equipment For Magnetic Materials

T.O. 33B-1-1 4-80 go, no-go function to separate metals above and below a specified conductivity value. A standard representing the minimum acceptable or rejectable conductivity must be available. 4.7.3.3 Equipment For Magnetic Materials. Impedance plane analysis instruments can be used to measure the conductivity of ferromagnetic materials because the permeability and conductivity can be separated in phase. Meter type conductivity measurement equipment generally available at field level can not separate conductivity from permeability variations. However, the combination of conductivity and permeability, in many cases, can be related to variations in alloy, temper and strength. General purpose meter type instruments can then be used to separate or grade various levels of properties. The number of standards required depends on the number of categories of materials to be established. 4.7.4 Effects Of Variations In Material Properties. 4.7.4.1 Conductivity. Conductivity variations can occur in metals as a result of improper heat treatment or as a result of exposure to excessive temperatures during service and cold working. These are the conditions for which eddy current inspection is usually performed. Conductivity variations can stem from other sources. Separation of elements during solidification of metals can lead to either localized or uniform differences in conductivity. For instance, a variation in conductivity can exist with increasing depths beneath the part surface particularly in heavier sections which have not been worked extensively. Slight differences in heat treating time, temperature, or quenching rates imposed by limitations in heat treating facilities or changes in part configuration also lead to variations in conductivity of a part. Localized cold working of metals when not followed by heat treatment to relieve residual stress can reduce electrical conductivity. Many of the variations are considered normal to the processing of the parts and the conductivity lies within the acceptable range for the alloy specification and temper. Conductivities outside the specified range for a given alloy and temper should be considered unacceptable and further investigation should be performed using hardness testing techniques. 4.7.4.2 Clad Materials. Cladding will affect the measured conductivity of the base metal. The degree to which the cladding will affect the value obtained depends on the conductivity of the cladding, the thickness of the cladding, and the operating frequency. Present applications are usually limited to Alclad aluminum alloys in the range of 0.050 to 0.080 inch thick using conductivity meters with operating frequencies of 60 KHz. Special conductivity ranges are required for clad aluminum alloys. The thicknesses of cladding, which are usually based on a percentage range of the overall thicknesses, can vary slightly because of normal tolerances. At 60 KHz, conductivity readings from aluminum alloys less the 0.050 inch in thickness are affected by both cladding and part thickness. Eddy current testing of modern complex cladding systems is still in an experimental stage for the most part. 4.7.4.3 Magnetic permeability. Direct meter measurement of electrical conductivity is applicable to nonmagnetic materials with a relative magnetic permeability of one or nearly one. If magnetic permeability exceeds one, it will produce a bridge unbalance in the meter system which can not be separated from the conductivity measurement and erroneous readings will be obtained. For this reason, conductivity of steels, nickel, and other magnetic materials can not be determined with conventional eddy current conductivity meters. Some stainless steels (400 series) are essentially nonmagnetic in the annealed condition, but slight amounts of cold working or exposure to extremely low temperature can cause transformation to a magnetic structure. Impedance plane analysis equipment can readily separate magnetic permeability and conductivity, allowing an accurate measurement of conductivity of ferromagnetic materials. 4.7.4.4 Geometry. Any change in part configuration that affects distribution or penetration of eddy currents will result in erroneous electrical conductivity readings. The following sources of error are included in these categories: a. Proximity to part edges or adjoining structure. b. Metal thickness less than the effective depth of penetration in the metal.