MIL-T-68410
Actual amount of compression in inches x 100
Percent compressibility = Original thickness (1)
Thickness after 5 minutes x 100 (2) Percent thickness of original after recovery = Original thickness
tr - tc
Formula to determine percent recovery: Percent recovery = 100 t0 - tc (3)
Where: t0 = original thickness
tc = compressed thickness tr = recovered thickness
The apparatus for tests shall consist of a small static load press, the platform and the presser foot of which shall be large enough to completely cover the sample under test. The moving head of the apparatus shall be equipped with suitable means for measuring thickness under the different compressions.
4.6.4 Low temperature resistance. Using a suitable testing machine, the load to compress a single layer specimen 25 percent shall be determined at 21° +3°C (70° +5°F). At least four equal weights shall be added at 1 minute +2 second intervals until at least 25 percent compression is obtained. The test shall be run in triplicate and results averaged. A single layer specimen shall be conditioned in cold chamber for 5 hours at
-40° +1°C (-40° +2°F). At the end of the conditioning period, the load which compressed the material 25
percent at room temperature shall be applied to the specimen in the cold chamber and the amount of compression recorded. At lease four equal weights shall also comprise the load and the weights added at 1 minute +2 second intervals. This test shall be conducted in triplicate and results averaged. Low temperatures
resistance in percent shall be calculated as follows:
Percent compression at -40° +1°C (-40° +2°F) under load produced 25 percent compression at
21° +3°C (70° +5°F) x 100
Low temperature resistance = 25 (4) (percent)
4.6.5 Adhesive strength. A test specimen 1 inch wide by 3 inches long shall be aged at 70° +1°C (158°
+2°F) for 7 days in a circulating hot air oven. The specimens shall be allowed to rest 24 hours after removal
from the oven before testing. A highly polished aluminum alloy panel conforming to QQ-A-250/4 (2024) shall be cleaned before the test by wiping with a cotton cloth soaked in cleaner conforming to MIL-C-38736
followed by wiping with a methyl alcohol soaked cloth. The panel shall be allowed to dry thoroughly before applying the test specimen. The Holland cloth or polyethylene shall be removed from the test specimen immediately before applying the tape or sheet to the polished panel. The tape and sheet shall be so applied that 2 inches are in contact with the panel allowing an overhang of 1 inch. The specimen shall be pressed on a
panel by rolling a 5 pound weight over the tape and sheet once in each direction. Care should be taken that the
1 inch overhang is not pressed to the supporting surface. After the specimen is applied to the panel, the panel shall be supported in a vertical position and a 3 pound weight suspended from the 1 inch overhang of the tape and sheet by means of a suitable clip, so that the weight is supported by the adhesive acting in shear. The
weight shall be applied gradually so that the tape and sheet are not subject to impact loading. The test shall be
run in triplicate.
4.6.6 Accelerated crazing. Crazing shall be tested against MIL-P-8184 acrylic, MIL-P-5425 acrylic, and MIL-P-83310 polycarbonate. Two adjoining specimens, each 1 by 7 inches, shall be cut from the sheet of plastic to be tested and shall be cleaned with aliphatic naphtha or any other suitable cleaner. The pieces shall be set up as cantilever beams as shown on figure 1, in a circulating air oven maintained at a temperature of
43°+1°C (110°+2°F) for 24 hours. The beams shall be loaded to produce an outer fiber tensile stress of 2000
psi at the support. The adhesive surface of 1- by 2-inch specimen, with the Holland cloth or polyethylene
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