V. Reliability Testing
B. Testing during development
Describe the purpose, advantages, and limitations of each of the following types of tests, and use common models to develop test plans, evaluate risks, and interpret test results.
1. Accelerated life tests (e.g., single-stress, multiple-stress, sequential stress, step-stress) (Evaluate)
Build, conduct, and interpret accelerated testing takes more knowledge than briefly covered here.
Additional References
Select the Right Accelerated Life Test Approach (recorded webinar)
Accelerated life testing first steps (article)
Life Testing Starting Point (article)
Quick Quiz
1-132. Accelerated cycling is performed on a sample of devices for six months under normal operating conditions. What can be gained by a cycling program?
(A) It can reduce premature failures in use.
(B) It can reduce the constant failure rate probability.
(C) It can ensure acceptable customer quality.
(D) It can discover all failure mechanisms.
(A) It can reduce premature failures in use.
Thermal cycling is a commons stress products experience and if there are early life failures susceptible to thermal cycling stress, they will fall out in the testing. This permits the team to conduct detailed failure analysis and improve the design or assembly process as needed to minimize the specific failures.
Testing at normal operating conditions for 6 months may be the entire expected lifetime of a product, yet more often (and we’re not given any indication of the expected lifetime) product are expected to last long then 6 months. The product will likely experience thermal cycling in normal use and if it experiences one such cycle per day, it is possible to test more cycles per day in thermal chambers, providing an acceleration factor.
Without more details there is little to use to form a clear picture of the benefits of the testing. (C) implies this test will ensure customer quality, which is unlikely accomplished by one test using a single stress factor. (D) has the word “all” which is a key as no single test is able to reveal all failure mechanisms.
(B) is possibly an answer, yet it is addressing constant failure rates and accelerated cycling generally addresses design/assembly type mistakes or longer term wear out or fatigue failure mechanisms which are generally not considered contributors to a constant failure rate.
1-137. An expensive mechanical part needs to be evaluated to determine its adherence to design requirements. Why would accelerated life testing be conducted on such a part?
(A) because test to provide adequate reliability performance information under normal operating conditions would take too long
(B) because the error in the population resulting from part-to-part variation is too large
(C) because the probability density function of the product follows either a lognormal or a Weibull distribution
(D) because the current sample size and the life distribution indicate that wear-out is likely
(A) because test to provide adequate reliability performance information under normal operating conditions would take too long
Accelerated life testing is done to cheat time. To shorten the time it takes to learn about the relationship between stress and time to failure. In some cases, a failure mechanism occurs with the same fundamental pattern at a higher applied stress. If we have or can determine a relationship between stress and time to failure we can effectively shorten the time to failure in a meaningful manner.
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