ability of a given article to maintain its capacity for work for a certain duration of time, with the necessary interruptions for technical maintenance and repairs.

The term of service of an article is determined by its design features, operating conditions, and sphere of use. For many unrepairable articles (for example, lamps, gears, and the units of household electrical and radio appliances), the term coincides with the failure of the article. In many instances, the end of service is defined as the period of an increased intensity of failures. Such a method determines the term of service for automatic control components which perform responsible functions. The use of the service life method is required because of the reduction in the effective operation of the articles and the increased intensity of failures in its components, as well as by the violation of safety requirements. The operating period of unrepairable articles is established from the results of special testing and is incorporated in the technical specifications for the articles. If information cannot be obtained ahead of time on a change in the intensity of failures, the term of service of the article is determined by direct examination during the operating process.

The term of service of repairable articles is determined by the ineffectiveness of their further operation because of aging and frequent failures or an increase in repair expenditures. In certain instances, for example, in transport, a violation of safety requirements can be the criterion for the term of service of repairable articles. The term of service can also be set by obsolescence.

The service-life indexes also differ in characterizing durability in accrued operating time and in calendar service time. The index characterizing the service life of the article in terms of accrued operating time is termed the reserve; the index characterizing the service life in terms of calendar time is the lifetime. A distinction is made between the reserve and lifetime up to the first major overhaul, between major over-hauls, and up to the discarding of the article.

The service life of buildings and installations is the limiting term of the buildings and installations during which they maintain the required operating qualities. A distinction is made between obsolescent and physical service life. Obsolescent service life (the time of obsolescence) is characterized by the life of the buildings and installations up to that moment when they cease to meet the changing operating conditions or the conditions of the production processes. The physical service life is determined by the duration of the wear of the basic bearing structures and elements (such as the frame, walls, and foundations) under the effect of loads and physicochemical factors. Here, certain structural elements and parts of the buildings and installations (the light wall partitions, roofing, coverings, floors, window casings, doors, and so forth) can have a shorter durability and be replaced in a major overhaul.

The gradual physical wear of the structures occurs unevenly during the overall lifetime of the building. Physical wear occurs more quickly (because of the deformations of the structures, the uneven settling of the ground, and so forth) during the initial period after construction and more slowly in the subsequent period of normal wear, which is much longer. At the end of the initial period, special post-settling repairs of individual structural elements may be required.

Service life is reduced through incorrect use of the buildings and structures, overloading of the structural elements, and harsh, destructive environmental influences (the effect of moisture, wind, frost, and so forth). The correct selection of the design factors with relation to the particular features of the climate and the operating conditions is of great significance in extending service life. The service life is improved by using building and insulating materials that possess high resistance in freezing and thawing, moisture resistance, and bioresistance and by protecting the structures against the penetration of destructive agents, especially moisture. The construction standards and rules in effect in the USSR have established the three degrees of service life for enclosing structures: the first degree has a lifetime of at least 100 years; the second, 50 years; and the third, 20 years.

the accrued operating time of a machine or system before the machine or system reaches a state in which further operation is impossible or undesirable because of reduced efficiency or increased danger to human beings. Service life is a random quantity because the length of time a device operates before reaching the limit state depends on many factors that cannot be measured, such as environmental conditions and the structure of the device itself.

Three variations of service life are distinguished: mean life, gamma-percentage service life, and specific service life. The mean service life is the mathematical expectation with respect to service life. The gamma-percentage service life is the accrued operating time during which the device will not, with a given probability (the gamma percentage), reach the limit state. The specific service life is the accrued operating time after which the device is either taken out of operation and written off or examined expressly to determine the device’s technical condition. The length of the specified service life is determined by the conditions under which the device can be safely operated.