PSI - Issue 19

Xavier Hermite et al. / Procedia Structural Integrity 19 (2019) 130–139 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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-  is the Weibull scale parameter, also called characteristic life, at which the probability of survival R(  ) = 36.8% when  = 0.

Fig. 4. Example of a Weibull analysis – Rank Regression

Identification of the most likely failure modes/mechanisms and their associated reliability, transformed to fit with the strength and mission profiles of the similar system under study, may reduce the tests number by reducing the number of failure mechanisms to test, and by demonstrating only the reliability (no characterization needed, a priori). Of course, if reliability is not acceptable for any of the failure mechanism or for the failure mechanisms competition, redesign should be performed, and characterization test could be needed.

3. Fatigue test specification for reliability demonstration

Since wear-out tests are the most expensive ones, in cost and duration, their ability to be reduced will be discussed in this section, but similar approach can be fitted to random failure tests (early failure cases should have been solved during HALT – HASS test program). Accelerating a wear-out mechanism can be performed by different means. Since some systems may have some time without working during their service, the first parameter to consider in such analysis is the working frequency. The test bench ensures the permanent loading of the component under testing, independently on its in-service passive states. For simple load cases and under specific conditions (for instance the sensitivity to the temperature increase induced by the test frequency), the exposure time can also be reduced by loading the component quicker (test frequency increase). Reliability demonstration implies the knowledge of the system mission profile, which means the usages and users of the system. The test will reveal the strength of the system relative to a failure mechanism. The reliability is then the probability that the mission profile exceeds the strength profile (stress – strength method). Mission profile is easier to estimate for systems, which have dedicated application under specific conditions. It is much more difficult to accurately estimate the mission profile of a consumer system, near impossible without experience feedback nor life situation measurement. For this last case, sensors choice and experimentation strategy combined with a physical approach of the failure mechanism can lead to a good overview of the researched mission profile. Different methods allow to specify the reliability demonstration test, each based on the use of an accelerated life model (High Accelerated Stress Test – HAST; for instance in fatigue, the commonly used model is the inverse power