MIL-DTL-24640C
The specimen shall next be placed into the oven such that a length of not less than the sum of 12 inches +20 times
the maximum specified overall cable diameter (see 3.1) remains straight and horizontal and such that the remainder
of the specimen is formed into the largest practicable horizontal coil (to permit free circulation of convection air
currents). The current source shall be connected between the two end conductors of the single series circuit within
the specimen. The specimen shall then be subjected to heat aging; the air temperature within the oven shall be
raised to 50±3 °C and the current source adjusted to produce a conductor temperature of 125±5 °C. These
temperatures shall be maintained for a continuous period of not less than 400 hours. The current source shall not be
shut off during heat aging, except for momentary shut off for taking resistance measurements when submethod a(2)
herein is used. Following heat aging, the specimen shall be allowed to cool throughout to room temperature. The
conductor temperature shall then be recorded. A recording which differs from room temperature by more than 5 °C
shall be considered to indicate failure of the conductor measurement means. If any such failure should occur, the
heat aging test shall be considered to be invalid and shall be repeated using a different specimen removed from the
same cable. (A possible advance indication of such a failure and the need to retest, may be the need to make
prominent adjustments of conductor current in order to maintain indicated conductor temperature during heat aging.)
Following heat aging, the specimen shall be subjected a second time to the insulation resistance and voltage
withstand tests as specified in 4.8.5 and 4.8.6, respectively. A sample, of length not less than 12 inches +20 times
the maximum specified overall cable diameter (see 3.1), shall the be removed from that portion of the specimen
which had been kept straight during the heat aging. This sample shall be subjected to the bending procedure as
specified in 4.7.2.5. A portion of the remainder of the specimen shall be subjected to the cable sealant removability
test as specified in 4.7.3.
b. Prior to heat aging, hot air oven method specimen 1 shall be subjected to the insulation resistance and
voltage withstand as specified in 4.8.5 and 4.8.6, respectively. The voltage withstand test shall use the voltages
specified (see 3.1). Both specimens shall then be placed within the oven, specimen number 1 shall be formed into
the largest practicable horizontal coil within the oven and specimen number 2 shall be positioned such that it is both
straight and horizontal. Both specimens shall be simultaneously subjected to heat aging. The air temperature within
the oven shall be raised to 125±3 °C. This temperature shall be maintained for a continuous period of not less than
400 hours. Following heat aging both specimens shall be allowed to cool to room temperature. Specimen 1 shall
then be subjected a second time to the insulation resistance and voltage withstand tests as specified in 4.8.5 and
4.8.6, respectively. A portion of specimen 1 shall next be subjected to the cable sealant removability test as
4.7.2.4 Observation. Any of the following shall constitute specimen failure:
a.
Falling away of any material from either end of any specimen during heat aging.
b.
Jacket sagging on any specimen.
c.
Specimen (specimen 1 if the hot air oven method is used) fails either of the insulation resistance tests (see
d.
Specimen (specimen 1 if the hot air oven method is used) fails either of the voltage withstand tests (see
Specimen (specimen 1 if the hot air oven method is used) fails the cable sealant removability tests (see
e.
4.7.2.5 Further procedure. The mandrel shall be secured to a fixed surface, one end of the specimen shall be
secured such that it is both in contact with the curved surface of the mandrel and perpendicular to the mandrel center
of the curvature axis. The opposite end of the specimen shall be bent circumferentially around the curved mandrel
surface through an arc of not less than 170 degrees. Bending shall be accomplished at the rate of approximately
20 degrees per second. The specimen shall then be secured to maintain its bent shape (as with a piece of twine,
stretched taut and tied between each end of the specimen) and shall be removed from the mandrel. Not less than two
thirds of the bent portion of the specimen shall then be dissected; each of its constituent components shall be
visually inspected for deterioration.
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