Failure modeling is a key to reliability engineering. Validated failure rate models are essential to the development of prediction techniques, allocation procedures, design and analysis methodologies, test and demonstration procedures, control procedures, etc. In other words, all of the elements needed as inputs for sound decisions to insure that an item can be designed and manufactured so that it will perform satisfactorily and economically over its useful life.

Inputs to failure rate models are operational field data, test data, engineering judgment, and physical failure information. These inputs are used by the reliability engineer to construct and validate statistical failure rate models (usually having one of the distributional forms described previously) and to estimate their parameters.

There are three basic ways in which the failure rate can change with time. The failure rate may be decreasing, increasing, or constant (DFR, IFR or CFR). We can tell much about the causes of failure and about the reliability of the item by appreciating the way the failure rate changes.

For example, a failure mode which does not occur for a finite time and then exhibits an increasing rate of occurrence (IFR) is characteristic of fatigue brought about in items where a proportion of the items is weaker than the application requires and after failure is either rejected or repaired in such a way that the correct strength is restored. “Burn-in” of electronic equipment to show up such defects as substandard components or poor connections is a good example of the way in which knowledge of a DFR is used to generate an improvement in reliability, by operating the items under failure provoking stress conditions for a time before delivery. As substandard items fail and are replaced or repaired, the failure rate decreases, i.e., reliability increases as time proceeds. A constant failure rate (CFR) is characteristic of failures which are caused by the application of loads in excess of the design strength at a constant average rate. For example, overstress failures due to accidental or transient circuit overload or maintenance induced failures of mechanical equipment typically occur randomly and at a generally constant rate. A CFR is also typical of complex systems subject to repair and overhaul, where different components exhibit different failure rate patterns, and parts of the system may have different ages. The combination of different failure rate patterns tends to a CFR as the system complexity increases.

References:

1. MIL-HDBK-338, Electronic Reliability Design Handbook, 15 Oct 84
2. Bazovsky, Igor, Reliability Theory and Practice
3. O’Connor, Patrick, D. T., Practical Reliability Engineering
4. Birolini, Alessandro, Reliability Engineering: Theory and Practice