1 Guide Rules
Test Z/AD is a cyclic temperature/humidity test which is designed to reveal defects in test specimens caused by "breathing" as distinct from the absorption of moisture.
This test differs from other cyclic damp heat tests in that it derives its increased severity from:
a) A greater number of temperature variations or "breathing" actions in a given time.
b) A greater cyclic temperature range.
c) A higher cyclic rate of change of temperature.
d) The inclusion of a number of sub-zero temperatures.
The accelerated breathing and the effect of the freezing of trapped water in cracks and fissures are the essential features of this test.
It is emphasized however that the freezing effect will occur only if the fissure dimensions are large enough to allow the penetration of a coherent mass of water, as is normally the case in fissures between seals and metal assemblies or between seals and wire terminations.
The degree of condensation will depend mainly upon the thermal time constant of the surface of the test specimens and may be negligible for very small specimens but copious for large specimens.
Similarly, the breathing effect will be more apparent on specimens which contain relatively large air-filled or gas-filled voids but again the severity of the test will depend to some extent on the thermal characteristics of the specimens.
Application of the test
For the reasons given above, it is recommended that this test procedure be limited to component type specimens when the construction of the specimens suggests a "breathing" type of damp heat test combined with icing and where the thermal characteristics are compatible with the rates of change of temperature of this test.
For solid type specimens, e.g. plastic encapsulated, where there may be small hairline cracks or porous material, the absorption or diffusion of water steam will predominate and a steady damp heat such as test C is preferred for investigating these effects.
For larger specimens such as equipment or when it is essential for components to ensure thermal stability during the various phases of the cycle, test Db should be employed, although due to the reduced number of cycles in a given period the degree of acceleration may not be so great. In this case, test Db shall normally form part of a sequence such as that defined in Chapter 7 of GB/T 2421-1999.
As in other damp heat tests, a polarizing voltage or electrical loading may be applied to the specimens. In the case of electrical loading, the loading should be such that the temperature rise of the specimens does not unduly affect the chamber conditions.
From the above, it is therefore evident that this test shall not be considered to be interchangeable with, or an alternative to, either steady state or other cyclic damp heat tests, but the choice of test procedure should be made with due regard for the physical and thermal characteristics of the test specimens and the types of failure mechanisms which are significant for each particular case.
Contents
1 Guide Rules
2 Object
3 General Description of the Test
4 Description of Test Apparatus
5 Severities
6 Testing Procedure
7 Information to be Given in the Relevant Specification
