Some engineers specialize in the reliability of military, naval, medical, and financial equipment, where “up time” is very important and “downtime” (at least unscheduled downtime) is to be avoided. While some reliability engineers focus on software, others (and this article) focus on the reliability and durability of hardware, the kinds of stresses that can shorten the life of hardware, and appropriate tests to uncover weaknesses.
One group of those stresses and tests can be called climatics. This group includes natural environments, such as thermal, altitude (low pressure), pressure (immersion), humidity, sand-and-dust, salt fog, corrosion, etc.
Another group of stresses and tests can be called dynamics. It includes man-made environments, such as EMI (electromagnetic interference, also called EMC or electromagnetic compatibility and RFI or radio frequency interference), as well as vibration, mechanical shock, and noise. The latter are my fields and are given some emphasis in this article.
In-house environmental test labs
Many organizations own their own testing labs in which they have climatic chambers that can reproduce the natural environments listed earlier. Such a test may involve a low-temperature test on a military vehicle, a test that is far cheaper and faster than taking the vehicle to arctic terrain. On other days, such a chamber might be used for elevated temperature testing, high-humidity testing, and other climatic environmental testing.
Such an organization may well also have a lab dedicated to dynamic (vibration and/or mechanical shock) testing. This type of testing may consist of a vibration test that simulates road inputs to a land vehicle, freeing test personnel from weather-related limitations on test tracks and highways. Figure 1 shows, for example, a package being drop-tested to verify that the contents will survive shipping and severe handling.
Commercial testing laboratories
Other organizations instead depend upon “outside” environmental testing labs, such as those listed at http://www.equipment-reliability.com/links.html. Some of these labs are independent. Others belong to rather vast organizations of multiple labs in various countries. Not all labs offer all kinds of environmental testing.
Many of these labs (in-house as well as independent, both in the U.S. and abroad) have become accredited to offer certain kinds of tests. A2LA (www.a2la.org), the American Association for Laboratory Accreditation is one of these accreditation bodies. Assessors visit labs that offer testing and evaluate their abilities to meet the requirements of International Standards Association ISO 17025.
Stimulation vs. simulation
In most of the environmental tests mentioned thus far, labs attempt to simulate real-world environments, both climatic and dynamic. The general idea is this: if hardware passes the lab test, it will probably survive in service.
However, many organizations want greater assurance that hardware won’t fail in service and won’t surprise users by failing in some unexpected way. Never let it be said, “We didn’t test for that.”
Envrionmental stress screening or ESS
High-reliability hardware is increasingly screened. Visualize 100 electronic, just-completed assemblies. There is a chance that some of them contain defects or latent failures. Sure, they work fine on the test bench. But (as in Figure 2) once a blast of heated and then refrigerated air is passed through flexible ducts and through the assemblies, inducing heating and cooling, expanding and contracting, some failures will occur. Subjecting the assemblies to multi-axis random vibration from pneumatic vibrators on the bottom of the softly-spring platform shown will precipitate other failures. If ESS is made more severe, accelerating the screen, the effort may be called HASS or Highly Accelerated Stress Screening.
Highly accelerated life testing (HALT)
If the foregoing is part of development of new hardware, the acronym HALT is applied. It is not a well-chosen acronym, but it’s widely used.
Thirsting for further knowledge?
The Equipment Reliability Institute (ERI) (www.equipmentreliability.com,) offers message boards at which you are invited to leave questions.
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