Is the sequence that environmental tests are applied important?

Yes for several reasons but it is very dependent upon the test strategy.

When the intention is to simulate the worst possible sequence of environments the equipment may experience over its life.

The sequence is devised to be a worst case. The tests are performed in an order which would cause the most damaging cumulative effect; some tests are repeated at various stages in the sequence where the cumulative effect is important. For example dry heat tests are performed first to “age” the equipment followed by damp heat which puts moisture into hairline cracks formed by the aging process, this is followed by low temperature which freezes moisture in the cracks causing the crack to open. Repeated damp and cold cycles open the crack further allowing the moisture to penetrate further until failure occurs e.g. moisture shorts out a circuit. There is some debate where mechanical tests should be included in this type of sequence. Since repeating mechanical tests may result in unrepresentative fatigue. Mechanical tests performed early could result in damage that climatic tests may exploit to cause failure e.g. vibration could cause cracks which could allow moisture to penetrate. But climatic tests could be performed first degrading materials that then fail when applied to vibration or shock.

When there is no requirement to consider the cumulative effects of the tests but there is a shortage of test samples.

The tests are considered separate and cumulative effects are not considered. But because there are limited samples each sample will perform many tests. In this case it is important that the preceding test does not influence the following test. For this reason tests like contamination and salt corrosion which leave residues on the sample tend to be performed last or on samples which are used for no other test.

When the intention is to simulate particular events that will occur in an equipments life

This approach is used where a particular sequence is known to exist. It addresses combinations of environments as well as sequence. Most commonly the sequences represent aircraft missions and include vibration temperature and altitude cycles applied in the order in which they would occur.

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