MIL-STD-202G
METHOD 107G
THERMAL SHOCK
PURPOSE. This test is conducted for the purpose of determining the resistance of a part to exposures at extremes of high and low temperatures, and to the shock of alternate exposures to these extremes, such as would be experienced when equipment or parts are transferred to and from heated shelters in arctic areas. These conditions may also be encountered in equipment operated noncontinuously in low-temperature areas or during transportation. Although it is preferred that the specimen reach thermal stability during the exposure specified, in the interest of saving time, parts may be tested at the minimum exposure durations specified, which will not insure thermal stability but only an approach thereto. Permanent changes in operating characteristics and physical damage produced during thermal shock result principally from variations in dimensions and other physical properties. Effects of thermal shock include cracking and delamination of finishes, cracking and crazing of embedding and encapsulating compounds, opening of thermal seals and case seams, leakage of filling materials, rupturing, or cracking of hermetic seals and vacuum glass to metal seals, and changes in electrical characteristics due to mechanical displacement or rupture of conductors or of insulating materials.
APPARATUS. Suitable temperature controlled systems shall be used to meet the temperature requirements and test conditions specified in table 107-I or table 107-III. The liquid method is more severe and may damage some components that might not be degraded by the air method. It is not intended for use on nonhermetically sealed components.
2.1 Environmental chambers. A system of sufficient thermal capacity shall be used to change ambient chamber conditions to meet test requirements and to reach specified temperature conditions of steps 1 and 3 of table 107-I. The supply air temperature of the chambers shall reach the specified temperature within a recovery time of 5 minutes after the specimens have been transferred to the appropriate chamber.
2.2 Liquid baths. Suitable temperature controlled baths containing liquids (see table 107-IV) shall be chosen to maintain the specified test conditions (see table 107-III) within the indicated tolerances. A liquid media shall not be used without prior approval of the qualifying activity.
3. PROCEDURE.
3.1 Environmental chambers. Specimens shall be placed so that there is substantially no obstruction to the flow of air across and around the specimen. When special mounting is required, it shall be specified. The specimen shall be subjected to the specified test condition of table 107-I. The first five cycles shall be run continuously. After five cycles, the test may be interrupted after the completion of any full cycle, and the specimens allowed to return to room ambient temperature before testing is resumed. One cycle consists of steps 1 through 4 of the applicable test condition. Specimens shall not be subjected to forced circulating air while being transferred from one chamber to another. Whether single or multiple chambers are used, the effective total transfer time from the specified low temperature to the specified high temperature, or the reverse, shall not exceed 5 minutes. Direct heat conduction to the specimen should be minimized. In the case of multiple chambers, the transfer time shall be defined as the time between withdrawal from the low temperature chamber and introduction into the high temperature chamber or the reverse.
NOTE: In single compartment chambers, in which the temperature extremes of steps 1 and 3 are achieved without physical movement of the specimens, steps 2 and 4 are not applicable.
METHOD 107G 28 March 1984
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