SPRADI9 June 2024 AM623 , AM625
HTOL-based models are limited in two ways that do not apply for design-based reliability approach.
Typically, HTOL-based models apply a constant failure rate assumption. This is bottom of the bathtub curve as shown in Figure 8-1. Importantly, this paradigm does not credibly represent the useful lifetime of the product, as logically, lifetime coincides with the onset of the wear-out stage, characterized by increasing instantaneous failure rate.
Reliability estimates are limited by fixed HTOL sample size and test duration. Usually one or both of these factors lead to a result of no observed failures. This limits viability of the model to characterize lifetime because no failures means minimal information to asses changes in failure rate vs. time. Mathematically, the case of terminating the test at a fixed duration (usually due to practical reasons) regardless of number of failures is called Type I right censoring. The term right censoring is used because when the test is terminated, it is unknown when the survivors at that point can later fail. The failure rate (in reciprocal time dimension) for Type I right censoring is represented by the following equation as shown in Equation 1.
Where
To convert the Average Failure Rate (AFR) to CDF (Fail Fraction), use the following general identity:
In this case, the Average Failure Rate is the same as the Constant Failure Rate. However, more boradly, AFR is a non-constant function of time.