M1L-STD-1537C
4.4.3 Sample thickness.
Test articles for which electrical conductivity readings are made without a thickness correction factor shall meet or exceed the minimum thickness (see figure 1) for the %1ACS range and frequency being used, except as shown (see 4.4.3.1) below. Note: The Effective Depth of Penetration (EDP) determined (see figure 1) are 2.66 times the Standard Depth of Penetration determined by equations (see 3.7.1 and 3.7.2).
4.4.3.1 Thin specimens.
Nonclad aluminum of the same heat treat condition but with a thickness as much as 40 percent below the minimum shown (see figure 1) or calculated EDP shall be tested as follows:
a. For each lot of material, three specimens identified as a, b, and c, shall be tested using stacked material specimens. Measurements of conductivity shall be made with the specimens stacked in the three arrangements shown below.
ARRANGEMENT
(1) |
(2) |
(3) |
|
Top specimen |
a |
b |
c |
Middle specimen |
b |
c |
a |
Bottom specimen |
c |
a |
b |
b. Unstacked specimens shall be placed on a nonconductive test surface at least 1/4 inch (6.35 mm) from any electrically conductive material and a measurement of the conductivity of each specimen made.
c. The average stacked and unstacked conductivity can be calculated. (Average "Stacked") = 1/3 (Stacked + Stacked + Stacked).
(1) (2) (3)
(Average "Unstacked") = 1/3 (Unstacked + Unstacked + Unstacked) (a) (b) (c)
d. The difference (Average Unstacked) - (Average Stacked) shall be the thickness correction factor to be added to the acceptance limits for the given material and shall be considered the acceptance criterion for measuring the balance of the lot in the unstacked condition in accordance with (b).
(1) 1f a conductivity measurement determined when specimens are stacked is outside the applicable acceptance range, the specimen in the top position in the measurement shall be rejected. A remeasured new specimen from the lot shall replace the rejected specimen and the stack shall be remeasured. 1n all measurements with stacked readings outside the acceptance range, the top specimen in the stack shall be rejected and a new specimen chosen as above.
(2) Any lot of parts or material thicker than 0.6 EDP (see figure 1) and with a thickness correction factor between 0 and 2.5 %1ACS may be tested without stacking. Parts or material with a thickness correction factor greater than 2.5 %1ACS shall be stacked before testing.
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