An Alternative to the Exponential and Weibull Reliability Models

The two reliability models most widely used in engineering applications are the exponential and the Weibull. Both models are deficient because their failure rates do not conform to the bathtub curve nor specify the infant mortality period. In addition, Weibull applications require lifetime data which entails testing or field observations and is usually proprietary. The advantage of the exponential model is that it can be supported by both lifetime and MIL handbook data, the latter being publicly available. In this article, an alternative single parameter model—called lifetime—is formulated whose failure rate conforms to the shape of an asymmetric bathtub curve. It is shown that, unlike present multiparameter bathtub curve models, the alternative model can be supported by MIL handbook data. The reliability ramifications of the alternative model will be demonstrated through three applied examples: engine fan, microcomputer, and crystal oscillator. The data supportability claim is substantiated by constructing an estimation procedure demonstrated in the microcomputer and crystal oscillator examples. Calculations will show that both the exponential and Weibull models predict twice as many engine fan failures as the article model. For the microcomputer example, the exponential model gives three and one-half times as many faults.