Reliability Concepts

Reliability Concepts

DOI: 10.4018/978-1-5225-4941-3.ch001
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The first chapter introduces basic concepts of Reliability and their relationships. Four probability functions—reliability function, cumulative distribution function, probability density function, and hazard rate function—that completely characterize the failure process are defined. Three failure rates—MTBF, MTTF, MTTR—that play important role in reliability engineering design process are explained here. The three patterns of failures, DFR, CFR, and IFR, are discussed with reference to the bathtub curve. Two probability models, Exponential and Weibull, are presented. Series and parallel systems and application areas of reliability are also presented.
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Products fail due to mistakes in design, variation in features such as dimensions, parameters, strength, etc., or due to the environment in which it would have to endure. The wear and degradation imposed by use and time also are to be considered. On production side, the effects of process variations on quality, yield, and cost should be minimum. On maintenance side, the product should be serviceable. Causes of failures include bad engineering design, faulty construction or manufacturing process, human error, poor maintenance, inadequate testing and inspection, improper use and lack of protection against excessive environmental stress.

Reliability in service can be improved if necessary precautions are taken during development and production. Greater care in design, more effort on training, use of better materials and processes along with more effective testing will enhance the Reliability of a product. Greater care and skill along with more effective inspection will give better quality to the product. However, there is an optimum level of effort that should be expended on quality and reliability, beyond which further effort would be counter-productive. Reduced failures of products will limit the cost both to the customer and manufacturer. Reliability and maintainability are not only an important part of the engineering design process but also necessary in life-cycle costing, cost benefit analysis, operational capability studies, repair and facility resourcing, inventory and spare part requirement determination, replacement decisions, and the establishment of preventive maintenance programs.

The history of formal reliability studies goes back to the days of the second world war when it was felt necessary to develop methods for estimating the success rate of complex weapons. After the war, these methods were applied to electronic devices and in space technology. Most of the applications concerned with non-repairable devices and systems where the first failure would terminate the useful life of the device or system. The primary reason for reliability and maintainability engineering is to improve the reliability and availability of the product or system being developed and thereby add to its value.

Essentially, reliability studies provide predictions. They predict the future behavior of a device or system, based on past information and experience. Since predictions cannot be made with certainty, they are inherently probabilistic. Obtaining reliability figure/index of complex systems requires collection of field data, life testing experiments etc., Reliability will be evaluated based on the function the product /component which is supposed to perform with respect to environment and time.

At a given point in time, a component or system is either functioning or it has failed. A working component or system will eventually fail. The failed state will continue forever, if the component or system is non-repairable. A repairable component or system will remain in the failed state for a period of time, while it is being repaired and then transcends back to the functioning state when the repair is completed. The change from a functioning to a failed state is failure while the change from a failure to a functioning state is referred to as repair. It is also assumed that repairs bring the component or system back to an “as good as new” condition. This cycle continues with the repair-to-failure and the failure-to-repair process; and then, repeats over and over for a repairable system.

Engineering products can fail in service due to many reasons (Misra, 2008) These include:

  • Variation of parameters and dimensions, leading to weakening, component mismatch, incorrect fits, vibration, etc.;

  • When applied stress exceeds the strength of a component (overstress) such as mechanical overstress leading to a fracture or bending of a beam or electrical overstress leading to local melting of an integrated circuit transistor or breakdown of the dielectric of a capacitor;

  • Wear-out due to time dependent mechanisms such as material fatigue, wear, corrosion, insulation deterioration, etc., which progressively reduce the strength of the component so that it can no longer withstand the stress applied.

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