Magnetic Materials

T.O. 33B-1-1 4-79 4.7.2.2 Magnetic Materials. Use of general purpose instruments may be extended to the separation of magnetic materials where the product of permeability and conductivity of each of the alloys is clearly different. Conductivity meters will not measure the conductivity of magnetic materials. 4.7.2.3 Typical Application. Eddy current techniques are used to separate metal parts or raw materials of similar geometry which have lost alloy and/or temper identification and have become mixed in manufacture or storage. Such procedures can be applied at any stage in the processing, storage, or service of the material. 4.7.2.4 Control Of Heat Treatment. The relationship between electrical conductivity and heat treat condition has permitted the use of eddy current techniques for checking the adequacy of heat treatment in aluminum alloys. In this application, conductivity measurements by eddy current techniques are used to supplement a minimum amount of tensile testing and/or hardness testing. Eddy current conductivity measurements are particularly valuable for determining the uniformity of heat treatment of large and complex aluminum alloy structures when tensile specimens are not obtainable and part geometry limits accessibility for hardness testing. Adequacy of heat treatment of aluminum alloys is determined by conformance of the material to the pre-established conductivity ranges. This method of heat treat control has been applied extensively to aluminum alloys. Eddy current (electromagnetic) techniques are used also for evaluation of heat treatment of steels. Generally, more sophisticated instrumentation is employed for steels, but general purpose instruments can be used for many applications. Acceptance standards are usually employed for eddy current inspection of steel. Conductivity measurement is applied to a lesser degree for heat treat control of copper and magnesium alloys. Eddy current techniques can be employed for heat treat control in any alloy system where consistent but different conductivity ranges or permeability values occur with the various heat treating conditions. Conductivity measurement has not been established as a method of determining heat treat response in titanium alloys. Differences in conductivity between various heat treat conditions for most titanium alloys are insufficient to permit determination of temper. 4.7.2.5 Determination Of Heat And Fire Damage. A common application of conductivity measurement in field applications is the determination of heat and/or fire damage to aircraft structures. Because of the extensive use of aluminum alloys for aircraft structures and their sensitivity to mechanical property losses at relatively low temperatures, greatest experience and data have been generated for these materials. Heat and fire damage to other metals can be detected if temperatures become high enough to affect both conductivity and/or permeability and mechanical properties. For aluminum alloys, damage is detected as deviations in conductivity from the specified range for the alloy and temper being inspected. Heat and fire damage usually vary over a part because of nonuniform application of heat. Nonuniform heat application, in turn, results in variations in electrical conductivity. Unless the temperature and time of heat application is known, or testing is performed on a number of parts with the same history of heat application, quantitative values of mechanical properties cannot be established from the electrical conductivity values. 4.7.3 Test Equipment. 4.7.3.1 Direct Conductivity Measurement. To determine conductivity directly, eddy current instruments are available which provide a value of conductivity in % IACS. These instruments do not cover the entire conductivity range. The most common range of instrument coverage is approximately 8 to 110 % IACS. Although instruments with other conductivity ranges are available, they have more limited application and are not generally available at field bases. % IACS measuring instruments usually require only two standards of known conductivity for calibration. US Navy unit - .5-105% with variable frequency. 4.7.3.2 General Purpose Equipment. If direct conductivity measuring equipment is not available, general purpose eddy current equipment may be adapted for measuring conductivity. Use of general purpose equipment requires a larger number of standards to establish a calibration curve. The number of standards necessary for a conductivity measuring application is determined by the range of conductivity to be covered and the accuracv required. General purpose equipment can also be employed in a