In preceding chapters, evaluations of reliability were based on analysis of the interaction between loads on the system and the resistance of the system. A system would perform its intended function satisfactorily within a specified time period if its capacity exceeds the load. Instead of considering detailed interactions of resistance and load over time, in a time-to-failure (TTF) analysis, a system or its components can be treated as a black box or a lumped-parameter system, and their performances are observed over time. This reduces the reliability analysis to a one-dimensional problem involving time as the only variable describable by the TTF of a system or a component of the system. The time — to-failure is an important parameter in reliability analysis, representing the length of time during which a component or system under consideration remains operational. The TTF generally is affected by inherent, environmental, and operational factors. The inherent factors involve the strength of the materials, manufacturing process, and the quality control. The environmental factors include such things as temperature, humidity, air quality, and others. The operational factors include external load conditions, intensity and frequency of use, and technical capability of users. In a real-life setting, the elements of the factors affecting the TTF of a component are often subject to uncertainty. Therefore, the TTF is a random variable.
In some situations, other physical scale measures, such as distance or length, may be appropriate for system performance evaluation. For example, the reliability of an automobile could be evaluated over its traveling distance, or the pipe break probability owing to the internal pressure or external loads from gravity or soil could be evaluated based on the length of the pipe. Therefore, the notion of “time” should be regarded in a more general sense.
TTF analysis is particularly suitable for assessing the reliability of systems and/or components that are repairable. The primary objectives of the reliability analysis techniques described in the preceding chapters were the probability of
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the first failure of a system subject to external loads. In case the system fails, how and when the system is repaired or restored are of little importance. Hence such techniques are often used to evaluate the reliability of nonrepairable systems or the failure probability when systems are subject to extraordinary events. For a system that is repairable after its failure, the time period it would take to have it repaired back to the operational state, called the time-to-repair or restore (TTR), is uncertain.
Several factors affect the value of the TTR and include personal, conditional, and environmental factors (Knezevic, 1993). Personal factors are those represented by the skill, experience, training, physical ability, responsibility, and other similar characteristics of the personnel involved in the repair. The conditional factors include the operating environment and the extent of the failure. The environmental factors are humidity, temperature, lighting, noise, time of day, and similar factors affecting the maintenance crew during the repair. Again, owing to the inherently uncertain nature of the many elements, the TTR is a random variable.
For a repairable system or component, its service life can be extended indefinitely if repair work can restore the system like new. Intuitively, the probability of a repairable system available for service is greater than that of a nonrepairable system. Consider two identical systems: One is to be repaired after its failure, and the other is not to be repaired. The difference in probability that a system would be found in operating condition at a given instance would become wider as the age of the two systems increased. This chapter focuses on the characteristics of failure, repair, and availability of repairable systems by TTF analysis.
