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design life
The design life of a component or product is the period of time during which the item is expected by its designers to work within its specified parameters; in other words, the life expectancy of the item. It is the length of time between placement into service of a single item and that items onset of wearout.
The design life of components and products differs from the items mean time between failure (MTBF), in that MTBF is a measure of the rate of occurrence of random failures in time where these failures are not due to a wear-out mechanism. For example, the MTBF of a device may be 100,000 hours and the design-life is 20,000 hours. In this example, across the population of products, one failure will occur, on average, every 100,000 population operating hours (100,000 units operating for 1 hour each = 100,000 population operating hours). None of these units will ever approach reaching 100,000 operating hours as it will fail due to wear-out and be replaced by a new unit. Aluminum electrolytic capacitors, fans, and batteries are classic examples of components that will fail due to wear-out well before they could achieve the operating time indicated by their individual MTBF.
Another use of the term design-life deals with consumer products. Many products employ design-life as one factor of their differentiation from competing products and components. A disposable camera is designed to withstand a short life, whilst an expensive single-lens reflex camera can be expected to have a design life measured in years or decades. (Clearly in this example there are other differentiators).
==Long design lives==
Some products designed for heavy or demanding use are so well-made that they are retained and used well beyond their design life. Some public transport vehicles come into this category, as do a number of artificial satellites and spacecraft.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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