Sequencing MIL-STD-810 tests methods can be a challenge when developing an Environmental Test and Evaluation Master Plan (ETEMP). Determining a representative test sequence is essential for generating representative cumulative environmental stressors that will provide an accurate evaluative process. The standard provides, in most cases, vague and general guidance in Part 1 and in each of the methods.
Test Program Parameters
Test programs can vary greatly dependent on the type of equipment under test, the size of the equipment, and the number of available units for testing. When multiple units are available multiple path testing can be performed. As an example, Low Pressure, High Temperature, Ballistic Shock, and Sand and Dust could be performed on a unit or set of units while a different series of tests are being conducted on separate units.
Consideration should always be given to perceived vulnerabilities in the equipment under test. Gaskets and seals are often at risk of degradation through thermal extremes, deformations due to shock, and material deterioration due to exposure to chemical agents. These concerns should be taken into consideration when immersion or water jet testing is to be conducted.
Sever testing parameters can also be an area of special concern for sequencing. In certain cases, performing Pyroshock testing early in the test program may be advantageous if a failure would result in significant redesign and retest.
Pyroshock, even for far field, can involve amplitudes approaching 1,000 g’s with frequencies up to 3 kHz. This can cause multiple types of failures that would not be seen in other types of testing. If cumulative effects are of concern then Pyroshock could be performed again at the end of a sequence.
MIL-STD-810 Sequencing Guidance
MIL-STD-810 offers guidance throughout parts 1 and 2 of the document on test sequencing. In most cases the guidance advises that the anticipated life cycle be used to access the cumulative effects but concedes, that in most cases, that there is not a single definable sequence. A number of factors including situation dependent usage and test program assets will contribute to the final sequence selection.
Hard and Fast Sequencing Rules
There are certain hard and fast rules in MIL-STD-810 sequencing that must be followed. Certain methods should be considered end of sequence methods because the Unit Under Test (UUT) will have been severely degraded or will be rendered unsuitable for further testing. These methods include Contamination by Fluids, Corrosive Atmosphere, Acidic Atmosphere, Ballistic Shock, and Sand and Dust.
A UUT should not be introduced into an environmental chamber after Contamination by Fluids or Acidic Atmosphere testing because it could contaminate the chamber. Sand and Dust testing can leave deposits of dust that would provide nutrients for Fungus testing. Sand and Dust will also degrade finishes and provide misleading results if followed by Corrosive Atmosphere. Guidance for these concerns is provided in each method.
Take Aways
Sequencing of MIL-STD-810 is an often overlooked element in developmental test and evaluation. Inadequate attention in developing a rationale can result in misleading results from laboratory testing. However, no matter how well a plan has been put together, test failures, requisite redesigns, lab scheduling, and test item availability can all require sequence changes.
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