MIL-STD-1537C
stock is reading "a" minus reading "b". Repeat for 3 specimens and take the average. CAUTIONS: (a) Prior to machining curved surfaces on flat stock, establish that the conductivity of the flat stock does not vary appreciably throughout the depths to be exposed. (b) Do not over heat the part during machining since this will appreciably change the conductivity.
5.1.4 Lift-off correction factors.
a. Establish lift-off correction factors for material or parts having a nonconductive coating exceeding 0.003 inch (0.76 mm) as follows:
b. Obtain plastic shim stock or paper of the same thickness as the nonconductive layer on the parts to be tested. Where paint thickness is not known, it will be necessary to remove or measure the paint thickness, for example, nonuniform coatings.
c. Place the shim or paper on a bare specimen of the same nominal conductivity (+5% IACS) as the parts to be tested.
d. Note the difference in readings of conductivity between tests run when the probe is placed directly on the surface of the bare specimen and when the probe is placed on the paper or plastic shim stock on top of the specimen. Use this difference as the lift-off correction factor for readings on the parts to be tested.
5.1.5 Operating precautions.
a. When necessary to rub or slide probe tip against a surface, do it gently to save wear on the probe surface.
b. Do not bang probe tip against surface being measured. c. A firm consistent probe pressure is desirable.
d. Clean the surface of the material to be measured, if required.
e. Keep the probe tip clean by wiping often with a clean lint-free cloth or a suitable commercial wiper.
5.2 Conductivity requirements.
The conductivity measurements for acceptance of heat-treated aluminum alloys shall be within the conductivity range for the specific alloy and type of heat treatment condition defined by the applicable contracting agency materials specifications.
6 NOTES
6.1 Intended use.
Hardness must be known for the conductivity test to be meaningful in measuring heat treatment. Figure 2 shows a typical variation of hardness and conductivity of an aluminum alloy with heat treat condition. Note that both hardness and conductivity values are required to define a given heat treat condition.
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