Standard Guide for General Reliability
This guide covers fundamental concepts, applications, and mathematical relationships associated with reliability as used in industrial areas and as applied to simple components, processes, and systems or complex final products. This guide summarizes selected concepts, terminology, formulas, and methods associated with reliability and its application to products and processes.
4.1 The theory of reliability is used for estimating and demonstrating the probability of survival at specific times or for specific usage cycles for simple components, devices, assemblies, processes, and systems. As reliability is one key dimension of quality, it may be more generally used as a measure of quality over time or over a usage or demand sequence.
4.1.1 Many industries require performance metrics and requirements that are reliability-centered. Reliability assessments may be needed for the determination of maintenance requirements, for spare parts allocation, for life cycle cost analysis and for warranty purposes. This guide summarizes selected concepts, terminology, formulas, and methods associated with reliability and its application to products and processes. Many mathematical relationships and methods are found in the annexes. For general statistical terms not found in Section 3, Terminology E456 and ISO 3534-1 can be used for definitional purposes and ISO Guide 73 for general terminology regarding risk analysis.
4.2 The term “system” implies a configuration of interacting components, sub-assemblies, materials, and possibly processes all acting together to make the system work as a whole. Parts of the system may be linked in combinations of series and parallel configuration and redundancy used in some parts to improve reliability. Additional conditions of complex engineering may have to be considered.
4.3 Process reliability concerns the assessment of any type of well-defined process. This can include manufacturing processes, business processes, and dispatch/demand type processes. Assessment typically measures the extent to which the process can continually perform its intended function without “upset” as well as process robustness.
4.4 A number of reliability metrics are in use. For example, mean time to failure (MTTF) is a common measure of average life or average time to the first time a unit fails. For this reason it is said to apply to non-repairable systems. Other life percentiles (or quantiles) are in use such as for example a Bp life or that life at which there is p % expected failure. Thus, the B50 or median life is the life at which 50 % of items would be expected to fail as well as survive; The B0.1 life is the life at which would be expected a 1 in 1000 failure probability 0.1 % failure) and a 99.9 % reliability.
4.4.1 Failure rate and average failure rate are also common metrics in reliability. With failure rates, it is important to understand that a rate may be changing with time and this may be increasing, decreasing or some combination of these over the life of a product or service. The failure rate may also be constant.
4.5 Bench testing of a device is used to obtain early reliability assessment or to demonstrate a specific reliability requirement or a related metric. There are a number of key methodologies that are used for this purpose. Demonstration testing may be dependent on the assumption of a distribution of failure time or may be carried out using nonparametric methods.
4.6 When a system is repaired following failure and placed back into service, we refer to the object as a repairable system. A key metric for this is the mean time between failure (MTBF); and this is not to be confused with MTTF. When a system is repaired, it may not be the case that its expected remaining life is as good as a new one. There may be a reduction in expected life following a repair and this may continue with continuing repair cycles. The MTBF metric applies to all such sequences of repair and restoration cycles over a service life period. This includes the first time to failure, the 2nd time, the 3rd time, etc.
1.1 This guide covers fundamental concepts, applications, and mathematical relationships associated with reliability as used in industrial areas and as applied to simple components, processes, and systems or complex final products.
1.2 The system of units for this guide is not specified. Quantities in the guide are presented only as illustrations of the method or of 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.
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