M1L-STD-1537C
(3) 1f the thickness correction factor as determined above in (2) is greater than 5 %1ACS, stacking may lead to erroneous results. For correct results a higher eddy current instrument frequency shall be used.
e. Example #1: Suppose a lot of bare 0.040 inch (1.02 mm) 7075-T6 material is to be tested with a 60 kHz instrument and the acceptance range for this material is 30.0 to 34.5
%1ACS (see figure 1), at 60 kHz and 32 %1ACS, the minimum thickness is 0.050 inch (1.27 mm). Since EDP = 0.050 x 0.6 = 0.030 inch (0.76 mm), the 0.040 inch (1.02 mm) thick material shall be tested when stacked (see 4.4.3).
f. Example #2: Suppose three samples taken from the lot and tested while stacked, give readings of 34.0, 34.5 and 34.%1ACS. All readings are in the acceptable 30.0 - 34.5 %1ACS range. The average change in conductivity between the "unstacked" and "stacked" readings is
+2 %1ACS. This is within the acceptable +2.5 %1ACS range (see 4.4.3.1). The new acceptance range for this lot of 0.040-inch (1.02-mm) thick 7075-T6 material, unstacked, is then equal to
30.0 +2 to 34.5 +2 %1ACS or 32.0 to 36.5 %1ACS. The remaining specimens with "Unstacked" conductivity outside the 32.0 to 36.5 %1ACS range are rejected.
4.4.4 Clad Specimens.
Electrical conductivity measurement of clad specimens shall be permitted only when correlation has been established between the conductivity readings and the acceptable range of heat-treat response for the clad specimens. Correlation shall take into account the possible range of cladding allowed by the applicable material specification and limitations of the instrument operating frequency and specimen thickness. All procedures for conductivity measurements of clad aluminum alloys must be supported by documented evidence of correlation with heat treat response and hardness measurements over the allowable range of cladding thickness and shall be subject to review by the procuring agency.
4.4.5 Alclad specimens.
Alclad materials shall be inspected at 50 to 75 kHz. When testing alclad parts thicker than 0.091 inch (2.3 mm) the cladding shall be partially removed on at least one part per inspection lot by spot facing. 1f the value obtained in the spot-faced area is acceptable, a second reading shall be taken on the alclad surface of the same part. The difference in conductivity between the two readings shall be used as a cladding correction factor when inspecting the remaining parts in
the lot. 1n case of doubtful or marginal values, a second part from the same lot shall be
spot-faced, and the average difference shall be used for the cladding correction factor. The above procedure shall be performed on each lot of material or parts.
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