Reliability Engineering

2Reliability Concepts

In Chapter 1, the reliability of a product was defined as “the ability of a product to perform as intended (i.e., without failure and within specified performance limits) for a specified time, in its life cycle conditions.” This chapter presents the fundamental definitions and measures needed for quantifying and communicating the reliability of a product. The focus in this chapter is on reliability and unreliability functions, the probability density function, hazard rate, conditional reliability function, percentiles of life, and time-to-failure metrics.

The purpose of, and the need for, a particular product determines the kind of reliability measures that are most meaningful and most useful. In general, a product may be required to perform various functions, each having a different reliability. In addition, at any given time (or number of cycles, or any other measure of the use of a product), the product may have a different probability of successfully performing the required function under the stated conditions.

2.1 Basic Reliability Concepts

For a constant sample size, n₀, of identical products that are tested or being monitored, if n_f products have failed and the remaining number of products, n_S, are still operating satisfactorily at any time, t, then

(2.1)

The factor t in Equation 2.1 can pertain to age, total time elapsed, operating time, number of ...