Metal Thickness

T.O. 33B-1-1 4-81 c. Excessive curvature of part surface. 4.7.4.5 Metal Thickness. If metal thickness is less than the effective penetration of the eddy currents, the measured conductivity will differ from the true value. Note that the effective penetration depth is approximately three times the standard depth of penetration. With meter equipment it is important to determine the operating frequency of the instrument. The operating frequency must not exceed the effective penetration depth of the material being tested. Impedance plane analysis equipment has a very wide range of operating frequencies, and the frequency can be adjusted to limit penetration to less than the effective depth. The standard depth can be determined by using the equation in section 4.2.3.3.1. Special sliderules are available for calculation of depth of penetration. Effective depth is approximately three times greater than the standard depth calculated by this equation. The material thickness must be greater than the effective depth or errors in conductivity measurement will occur. 4.7.4.6 Edge Effects. If the electromagnetic field of the probe is affected by the geometry of the edge of the part, an error will occur in the measurement of the conductivity. The probe should be located several probe diameters away form the nearest edge or transition boundary. 4.7.4.7 Curvature. Lift-off effects caused by the probe-to-curve surface fit-up will cause an error in the conductivity measurement. On curved surfaces, the smallest practical probe should be used to minimize lift-off effects. 4.7.5 Effects Of Variations In Test Conditions. 4.7.5.1 Frequency. Because frequency affects distribution of eddy currents within the test part, it affects the minimum thickness which can be measured without special adjustments. Higher frequencies permit measurement of thinner metals without compensation for thickness. Frequencies that provide less than 3 standard depths of penetration in the metal being tested are necessary for reasonably accurate conductivity measurement. However, the higher frequencies are more strongly affected by localized variations in conductivity or by conductive coatings and cladding on metals. Excessively high frequencies should not be used for conductivity measurements. 4.7.5.2 Probes For conductivity Measurements. With instruments designed for conductivity measurement, probes are carefully matched to the instruments and are usually obtained from the instrument manufacturer. Probes for conductivity measuring instruments are larger than those normally employed for defect detection. This design provides for averaging of conductivity over a relatively large area. Probes are designed with plastic or ceramic shoes to prevent damage to the coil. With continued use, wear on the face of the probe reduces the coil-to-surface distance, and calibration cannot be obtained. As wear occurs, the probe shoe must be changed and the instrument recalibrated. 4.7.5.3 Lift Off Effects On conductivity. Meter type conductivity measuring eddy current instruments often have a pre-set lift-off adjustment. The lift-off adjustment is usually set during calibration of the instruments. Applicable maintenance manuals describe the procedures that can be performed by trained NDI personnel. With probe wear and changes in instrument electrical components over a period of time, lift-off adjustment can change. Therefore, when conductivity measurements are to be performed on rough surfaces or through thin nonconductive coatings, lift-off adjustment should be checked prior to performing the measurements. After calibrating an instrument against the conductivity standards, lift-off adjustment should be checked against a specimen with conductivity representative of the test part. Lift-off greater than the amount of preset lift-off adjustment (if any) results in errors in conductivity reading. 4.7.5.4 Temperature Effects On Conductivity Measurements. Higher temperature increases the thermal activity, of the atoms in a metal lattice. The thermal activity causes the atoms to vibrate at a high amplitude about their position in the lattice. This thermal vibration of the atoms increases the chances of a collision with electrons in the material. This increases the resistance to electron flow, thereby