2.1 Selecting This Method
After examining requirements documents and applying the tailoring process in Part One of this Standard to determine where thermal shocks are foreseen in the life cycle of the materiel, use the following to confirm the need for this method and to place it in sequence with other methods.
2.1.1 Effects of Thermal Shock Environments
Effects of thermal shocks are usually more severe near the outer portions of materiel. The further from the surface (depending, of course, on the properties of the material involved), the slower and less significant are the thermal changes. Transit cases, packaging, etc., will lessen the effects of thermal shock on the enclosed materiel even more. Sudden temperature changes may either temporarily or permanently affect operation of materiel. The following are examples of problems that could result from thermal shock exposure that may relate to the materiel being tested. Consider the following typical problems to help determine if this method is appropriate for the materiel being tested. This list is not intended to be all-inclusive.
- a. Physical.
- (1) Shattering of glass vials and optical materiel.
- (2) Binding or slackening of moving parts.
- (3) Cracking of solid pellets or grains in explosives.
- (4) Differential contraction or expansion rates or induced strain rates of dissimilar materials.
- (5) Deformation or fracture of components.
- (6) Cracking of surface coatings.
- (7) Leaking of sealed compartments.
- (8) Failure of insulation protection.
- b. Chemical.
- (1) Separation of constituents.
- (2) Failure of chemical agent protection.
- c. Electrical.
- (1) Changes in electrical and electronic components.
- (2) Electronic or mechanical failures due to rapid water or frost formation.
2.1.2 Sequence Among Other Methods
- a. General. Use the anticipated life cycle sequence of events as a general sequence guide (see Part One, paragraph 5.5).
- b. Unique to this method. Use test item response characteristics and performance determination information obtained from the high and low temperature tests to better define the test conditions to be used for this procedure.
2.2 Selecting Procedure Variations
This method includes one test procedure with four variations – essentially in the length of the test and the shock itself. It employs constant temperature at each of the extreme shock conditions because, in many instances, the thermal shock itself so outweighs the other thermal effects that the test may be performed using two constant temperatures. This is particularly the case when more severe shocks are desired, such as for evaluation of safety or initial design, and when extreme values will be used. The four variations are:
- a. Procedure I-A - One-way Shock(s) from Constant Extreme Temperature.
- b. Procedure I-B - Single Cycle Shock from Constant Extreme Temperature.
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- c. Procedure I-C - Multi-Cycle Shocks from Constant Extreme Temperature.
- d. Procedure I-D - Shocks To or From Controlled Ambient Temperature.
2.2.1 Procedure Selection Considerations
When selecting this procedure, consider:
- a. The expected exposure temperatures in service.
- b. The materiel's logistic or deployment configuration.
2.2.2 Procedure Variations
The four procedure variations all involve temperature conditioning and performance testing. They differ on the number of shocks that, based on the LCEP, can vary from one shock (1/2 cycle) to six or more shocks (three or more cycles. Paragraph 2.3 includes five possible options, but only use 2.3c and d for cyclic situations.
2.3 Determine Test Levels and Conditions
Having selected this method (based on the test item's requirements documents and the tailoring process), complete the tailoring process by identifying appropriate parameter levels and applicable test conditions and techniques for the procedure. Base these selections on the requirements documents, the Life Cycle Environmental Profile (LCEP), requirements documents (see Part One, Figure 1-1), and information provided with this method. Consider tailoring known service extreme temperatures if the intent of the test is to reproduce induced strain rates found in service. Use values other than those suggested if realistic. This method addresses several exposure situations: aircraft flight exposure, air delivery - desert, and ground transfer – ambient to either cold regions or desert. Based on the anticipated deployment, determine which test variation is applicable. Rather than focusing solely on shocks from low to high temperatures or vice-versa, exposure temperatures could reflect shocks from controlled ambient temperature conditions to high or low temperatures. Base the exposure range on the expected service conditions, but extend the test levels as necessary to detect design flaws. Stabilize the whole test item temperature or, if known, the point of interest prior to transfer. However, if the LCEP indicates a duration less than that required to achieve stabilization, the duration from the LCEP should be used. The critical point of interest may be near the surface of the item. In such cases, a considerably shorter duration may apply rather than complete stabilization of the item. Justify any duration less than complete stabilization. Consider the following when selecting test levels.
- a. Aircraft flight exposure. This is appropriate if the materiel is to be exposed to desert or tropical ground heat and possible direct solar heating and, immediately afterwards, exposed to the extreme low temperatures associated with high altitude (see paragraph 1.2.1b). If not expended, the test item could subsequently be exposed to a potential thermal shock when the platform aircraft returns to a hot ambient environment. In addition, if not expended, the item could also be subjected to multiple thermal shocks.
- b. Air delivery - desert. This is appropriate for materiel that is delivered over desert terrain from unheated, high altitude aircraft, but use the ambient air temperature (no solar loading).
- c. Ground transfer - ambient to or from either cold regions or desert. This is intended to test materiel for the effects of movement to and from ambient conditions and cold regions or desert environments.
- d. Engineering design. This is used to detect issues related to marginal design.
2.3.1 Climatic Conditions
Identify the appropriate climatic conditions for the geographic areas in which the materiel will be operated and stored. Actual response temperatures achieved when materiel is exposed to the climatic conditions of the various ground climatic categories could be obtained from the test results of high and low temperature exposure (Methods 501.7, 502.7, and 505.6) for either the operational, or storage configuration. The latter assumption must take into account the induced effects of solar radiation during storage and transit in various climates.
2.3.2 Exposure Conditions
Select the test temperatures from field data or from the requirements documents, if available. If not available, determine the test temperatures from the anticipated deployment application or world areas in which the materiel will be deployed, or from the most extreme non-operating temperature requirements. Recommend using a range of temperatures that reflects that anticipated in-service, rather than some arbitrary extreme range.
MIL-STD-810H
METHOD 503.7
- a. Deployment application (aircraft flight exposure). The thermal stresses and rates that materiel will experience during exposure to the air flight operational environment are dependent on the ambient conditions, flight conditions, and performance of the onboard environmental control systems. The temperature and humidity at various altitudes can be found in MIL-HDBK-310 (paragraph 6.1, reference a).
- b. Air delivery/air drop. The test conditions for this exposure are based upon the probable conditions in the cargo compartment of the aircraft (or other transport location), and on the ground at the point of impact. Use a lower temperature extreme that assumes an unheated, unpressurized aircraft cargo compartment with the aircraft at an altitude of 8 kilometers (26,200 ft). This is the limiting altitude for cargo aircraft because of oxygen pressure requirements when the aircraft cargo compartment is unpressurized immediately before air drop operations. The temperature at this altitude can be found in MIL-HDBK-310. Determine the high temperature surface extremes from the appropriate tables in Method 501.7.
NOTE: Materiel packaging will normally mitigate thermal shocks to the packaged item. The air delivery/air drop scenario of packaged items may not involve significant thermal shock to the contents. However, the packaging may experience adverse effects due to the thermal shock.
- c. Ground transfer – ambient to or from cold regions or desert. In some regions of the world, materiel could experience thermal shocks during movement to and from environmentally conditioned buildings (enclosures) to extreme exterior ambient temperature conditions. Base selection of the outside ambient conditions upon the climatic categories or areas listed in the appropriate tables in Methods 501.7 or 502.7.
- (1) Cold regions. Typically, conditions for cold regions enclosures are indoor air at 18 °C to 24 °C (65 °F to 75 °F), with an accompanying RH of 30 to 50% (paragraph 6.1, reference d). These conditions roughly correspond to normal heating practices in cold regions.
- (2) Desert. For transfer from a desert environment to an air conditioned enclosure, determine if solar heating of the materiel will occur prior to the transfer.
- d. Engineering design. Use test conditions that reflect the extreme anticipated storage conditions.
2.3.3 Test Duration (number of shocks)
- a. Procedure I-A One-way shock(s) from constant extreme temperature. For materiel that is likely to be exposed only rarely to thermal shock in one direction, perform at least one shock for each appropriate condition, i.e., low to high temperature, or vice-versa (Figure 503.7-1 and paragraph 4.4.2.1a).
- b. Procedure I-B Single cycle shock from constant extreme temperature. For materiel that is likely to be exposed to only one thermal shock cycle (one in each direction), perform one shock for each appropriate condition, i.e., low-to-high temperature, and one in the opposite direction (Figure 503.7-2 and paragraph 4.4.2.1b).
- c. Procedure I-C Multi-cycle shocks from constant extreme temperature. There is little available data to substantiate a specific number of shocks when more frequent exposure is expected. In lieu of better information, apply a minimum of three shocks at each condition, i.e., three transfers from cold to hot, three transfers from hot to cold, and a stabilization period after each transfer. The number of shocks depends primarily on the anticipated service events (paragraph 503.7-3 and paragraph 4.4.2.1c). The objective of this test is to determine the effect of rapid temperature changes to the materiel. Therefore, expose the test item to the temperature extremes for a duration equal to either the actual operation, or to that required to achieve temperature stabilization within the limitations shown in paragraphs 1.2.1, 1.3, 2.1.1, and 2.3.5.
- d. Procedure I-D Shocks to or from controlled ambient temperature. This procedure essentially follows the durations of Procedures I-A to I-C, except all shocks are to and/or from controlled ambient conditions (Figure 503.7-4 and paragraph 4.4.2.1d).
2.3.4 Test Item Configuration
The configuration of the test item strongly affects test results. Therefore, use the anticipated configuration of the item during storage, shipment, or use. For small test items (e.g., radios), the test configuration should be representative of the in-Service condition, and provide a similar mounting platform thermal mass. As a minimum, consider the following configurations:
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- a. In a shipping/storage container or transit case, and installation of a thermally conditioned item into a container conditioned at another temperature.
- b. Protected or unprotected.
- c. Deployed (realistically or with restraints).
- d. Modified with kits for special applications.
- e. Packaged for airdrop.
- f. The installed environment and the effect upon the test item thermal response.
2.3.5 Temperature Stabilization
Stabilize the test item temperature (prior to transfer, and within the limitations shown in paragraphs 1.2.1, 1.3, and 2.1.1) or, if known, the point of interest prior to transfer, for as long as necessary to ensure a uniform temperature throughout at least the outer portions of the test item. However, if the LCEP indicates a duration less than that required to achieve stabilization, the duration from the LCEP should be used. The critical point of interest may be near the surface of the item. In such cases, a considerably shorter duration may apply rather than complete stabilization of the item. Justify any duration less than complete stabilization.
2.3.6 Relative Humidity
For most test programs, the relative humidity (RH) is not controlled. During the thermal shock test it may, however, have a significant effect on some materiel, e.g., cellulosic materials that are typically porous, into which moisture can migrate and then expand upon freezing. Do not attempt to control relative humidity unless specifically required.
2.3.7 Transfer Time
Ensure the transfer time reflects the time associated with the actual thermal shock in the life cycle profile. Make the transfer as rapidly as possible, but if the transfer takes more than one minute, justify the extra time.
2.4 Special Considerations
The test conditions as presented in this method are intended to be in general agreement with other extremes described in this document. The primary purpose in establishing these levels is to provide realistic conditions for the traverse between the two temperature extremes. Therefore, before transfer, stabilize the test item (within the limitations shown in paragraphs 1.2.1, 1.3, 2.1.1, and 2.3.5) at the most realistic temperature that would be encountered during the specific operation, or possibly the most extreme test item stabilization temperature if appropriate. Consider tailoring known service extreme temperatures if the intent of the test is to reproduce induced strain or heat transfer rates found in service.