Significance and Use 4.1 Reliability demonstration testing is a methodology for qualifying or validating a product’s performance capability. Demonstration methods are useful for components, devices, assemblies, materials, processes, and systems. Many industries require demonstration testing either for new product development and product introduction, in validating a change to an existing product or as part of an audit. Test plans generally try to answer the questions, “How long will a product last?” or “What is its reliability?”, under stated conditions at some specific time. When time is being used as a life variable, it must be cast in some kind of “time” units. Typical time units are hours (or minutes), cycles of usage, calendar time or some variation of these. In certain cases, “time” can be accelerated in order to reduce a plan’s completion time. In the automotive industry mileage may be used as the time variable. Certain means of accelerating tests involve the use of increased power, voltage, mechanical load, humidity, vibration, or temperature (often in the form of thermal cycling). 4.2 Two fundamental objectives in reliability test planning are: (a) demonstrating that a product meets a specific life requirement, and (b) demonstrating what a product can do – its life capability. In the first case, a requirement is specified; in the second case a series of test results are used to state a result at the present time – its current capability. Both cases share similar inputs and outputs. 4.3 Often a life distribution model is specified such as the Weibull, the exponential, the lognormal or the normal distribution. In addition, for the specific distribution assumed, a parameter is typically assumed (or a range of values for a parameter). For example, in the Weibull case, the shape parameter, β, is assumed; in the lognormal case the scale parameter, σ, is assumed and in the normal case the standard deviation, σ, is assumed. In other cases, a non-parametric analysis can be used. Non-parametric cases typically require a larger sample size than parametric cases. This standard will discuss conditions under which distributions and associated parameters can be assumed. 4.4 Generally, a life requirement is cast as a mission time and associated reliability, for example, to demonstrate a reliability of 99% at time t=1000 hours of usage. In another case the requirement might be cast as a Bp life requirement, such as the B5 life. For example, if B5 = 10 000 cycles are specified, this means to demonstrate a reliability of 95 % at t = 10 000 cycles. Other life requirements might be a mean life, a median life (B50) or a failure rate not to be exceeded at a specified time t. In other cases, the requirement might mean withstanding a load for some duration. Demonstration necessarily means to demonstrate with some statistical confidence. Thus, a confidence value is a standard input in any plan. Commonly used confidence values are 99 %, 95 %, 90 %, and 63.2 %. 4.4.1 When a requirement and a confidence value have been stated, a derived plan will determine a sample size, n, a test time, t, and a maximum number of failures, r, allowed by the plan. A test concludes and is successful if the n units tested result in not more than r failures by time t. In another scenario, the sample size, number of failures allowed and confidence value are first stated and the plan returns the test time requirement. 4.5 The “RC” nomenclature for specifying a test requirement is often used, where R stands for reliability and C for confidence. For example, to state a requirement of 2000 hours at R99C90 means that the requirement is to demonstrate 99 % reliability at 2000 hours with 90 % confidence. Alternatively, this also means to demonstrate a B1 life of 2000 hours with 90 % confidence. 4.6 This guide considers, the Weibull, lognormal and normal parametric cases as well as the basic non-parametric case for attribute reliability. The common exponential case is a Weibull distribution with assumed shape parameter β = 1, but is considered as a separate case, distinct from the Weibull.Scope 1.1 This standard covers fundamental concepts, applications and mathematical relationships associated with the planning of reliability demonstration tests as applied to components and materials testing. 1.2 The system of units for this guide is not specified. Quantities and examples are presented only as illustrations of a method or a calculation. Any examples used are not binding on any particular product or industry. 1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use. 1.4 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
