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ult tolerance 11 2 basic reliability and mission reliability 11 3 detectability and common cause failures 12 reliability versus quality six sigma 13 reliability operational assessment 14 reliability organizations 15 education 16 see also 17 references 18 further reading toggle further reading subsection 18 1 us standards specifications and handbooks 18 2 uk standards 18 3 french standards 18 4 international standards 19 external links toggle the table of contents reliability engineering 23 languages العربية català کوردی čeština deutsch español euskara فارسی suomi français हिन्दी hrvatski magyar italiano 日本語 한국어 nederlands română sicilianu svenska türkçe 粵語 中文 edit links article talk english read edit view history tools tools move to sidebar hide actions read edit view history general what links here related changes upload file permanent link page information cite this page get shortened url switch to legacy parser print export download as pdf printable version in other projects wikimedia commons wikidata item appearance move to sidebar hide from wikipedia the free encyclopedia sub discipline of systems engineering that emphasizes dependability reliability engineering is a sub discipline of systems engineering that emphasizes the ability of equipment to function without failure reliability is defined as the probability that a product system or service will perform its intended function adequately for a specified period of time or will operate in a defined environment without failure 1 reliability is closely related to availability which is typically described as the ability of a component or system to function at a specified moment or interval of time the reliability function is theoretically defined as the probability of success in practice it is calculated using different techniques and its value ranges between 0 and 1 where 0 indicates no probability of success while 1 indicates definite success this probability is estimated from detailed physics of failure analysis previous data sets or through reliability testing and reliability modeling availability testability maintainability and maintenance are often defined as a part of reliability engineering in reliability programs reliability often plays a vital role in the cost effectiveness of overall systems reliability engineering deals with the prediction prevention and management of high levels of lifetime engineering uncertainty and risks of failure although stochastic parameters define and affect reliability reliability is not only achieved by mathematics and statistics 2 3 nearly all teaching and literature on the subject emphasize these aspects and ignore the reality that the ranges of uncertainty involved largely invalidate quantitative methods for prediction and measurement 4 for example it is easy to represent probability of failure as a symbol or value in an equation but it is almost impossible to predict its true magnitude in practice which is massively multivariate so having the equation for reliability does not begin to equal having an accurate predictive measurement of reliability reliability engineering relates closely to quality engineering safety engineering and system safety in that they use common methods for their analysis and may require input from each other it can be said that a system must be reliably safe reliability engineering focuses on the costs of failure caused by the system downtime cost of spares repair equipment personnel and cost of warranty claims 5 history edit the word reliability can be traced back to 1816 and is first attested to the poet samuel taylor coleridge 6 before world war ii the term was linked mostly to repeatability a test in any type of science was considered reliable if the same results would be obtained repeatedly in the 1920s product improvement through the use of statistical process control was promoted by dr walter a shewhart at bell labs 7 around the time that waloddi weibull was working on statistical models for fatigue the development of reliability engineering was here on a parallel path with quality the modern use of the word reliability was defined by the u s military in the 1940s characterizing a product that would operate when expected and for a specified period in world war ii many reliability issues stemmed from the inherent unreliability of electronic equipment available at the time and from fatigue in 1945 m a miner published a seminal paper titled cumulative damage in fatigue in an asme journal a main application for reliability engineering in the military was for the vacuum tube as used in radar systems and other electronics for which reliability proved to be very problematic and costly the ieee formed the reliability society in 1948 in 1950 the united states department of defense formed a group called the advisory group on the reliability of electronic equipment agree to investigate reliability methods for military equipment 8 this group recommended three main ways of working improve component reliability establish quality and reliability requirements for suppliers collect field data and find root causes of failures in the 1960s more emphasis was given to reliability testing on component and system levels the famous military standard mil std 781 was created at that time around this period also the much used predecessor to military handbook 217 was published by rca and was used for the prediction of failure rates of electronic components the emphasis on component reliability and empirical research e g mil std 217 alone slowly decreased more pragmatic approaches as used in the consumer industries were being used in the 1980s televisions were increasingly made up of solid state semiconductors automobiles rapidly increased their use of semiconductors with a variety of microcomputers under the hood and in the dash large air conditioning systems developed electronic controllers as did microwave ovens and a variety of other appliances communications systems began to adopt electronics to replace older mechanical switching systems bellcore issued the first consumer prediction methodology for telecommunications and sae developed a similar document sae870050 for automotive applications the nature of predictions evolved during the decade and it became apparent that die complexity wasn t the only factor that determined failure rates for integrated circuits ics kam wong published a paper questioning the bathtub curve 9 see also reliability centered maintenance during this decade the failure rate of many components dropped by a factor of 10 software became important to the reliability of systems by the 1990s the pace of ic development was picking up wider use of stand alone microcomputers was common and the pc market helped keep ic densities following moore s law and doubling about every 18 months reliability engineering was now changing as it moved towards understanding the physics of failure failure rates for components kept dropping but system level issues became more prominent systems thinking has become more and more important for software the cmm model capability maturity model was developed which gave a more qualitative approach to reliability iso 9000 added reliability measures to the design and development portion of certification the expansion of the world wide web created new challenges of security and trust the older problem of too little reliable information available had now been replaced by too much information of questionable value consumer reliability problems could now be discussed online in real time using data new technologies such as micro electromechanical systems mems handheld gps and hand held devices that combine cell phones and computers all represent challenges to maintaining reliability product development time continued to shorten through this decade and what had been done in three years was being done in 18 months this meant that reliability tools and tasks had to be more closely tied to the development process itself in many ways reliability has become part of everyday life and consumer expectations overview edit reliability is the probability of a product performing its intended function under specified operating conditions in a manner that meets or exceeds customer expectations 10 objective edit the objectives of reliability engineering in decreasing order of priority are 11 to apply engineering knowledge and specialist techniques to prevent or to reduce the likelihood or frequency of failures to identify and correct the causes of failures that do occur despite the efforts to prevent them to determine ways of coping with failures that do occur if their causes have not been corrected to apply methods for estimating the likely reliability of new designs and for analysing reliability data the reason for the priority emphasis is that it is by far the most effective way of working in terms of minimizing costs and generating reliable products the primary skills that are required therefore are the ability to understand and anticipate the possible causes of failures and knowledge of how to prevent them it is also necessary to know the methods that can be used for analyzing designs and data scope and techniques edit reliability engineering for complex systems requires a different more elaborate systems approach than for non complex systems reliability engineering may in that case involve system availability and mission readiness analysis and related reliability and maintenance requirement allocation functional system failure analysis and derived requirements specification inherent system design reliability analysis and derived requirements specification for both hardware and software design system diagnostics design fault tolerant systems e g by redundancy predictive and preventive maintenance e g reliability centered maintenance human factors human interaction human errors manufacturing and assembly induced failures effect on the detected 0 hour quality and reliability maintenance induced failures transport induced failures storage induced failures use load studies component stress analysis and derived requirements specification software systematic failures failure reliability testing and derived requirements field failure monitoring and corrective actions spare parts stocking availability control technical documentation caution and warning analysis data and information acquisition organisation creation of a general reliability development hazard log and fracas system chaos engineering effective reliability engineering requires understanding of the basics of failure mechanisms for which experience broad engineering skills and good knowledge from many different special fields of engineering are required 12 for example tribology stress mechanics fracture mechanics fatigue thermal engineering fluid mechanics shock loading engineering electrical engineering chemical engineering e g corrosion material science definitions edit reliability may be defined in the following ways the idea that an item is fit for a purpose the capacity of a designed produced or maintained item to perform as required the capacity of a population of designed produced or maintained items to perform as required the resistance to failure of an item the probability of an item to perform a required function under stated conditions the durability of an object basics of a reliability assessment edit many engineering techniques are used in reliability risk assessments such as reliability block diagrams hazard analysis failure mode and effects analysis fmea 13 fault tree analysis fta reliability centered maintenance probabilistic load and material stress and wear calculations probabilistic fatigue and creep analysis human error analysis manufacturing defect analysis reliability testing etc these analyses must be done properly and with much attention to detail to be effective because of the large number of reliability techniques their expense and the varying degrees of reliability required for different situations most projects develop a reliability program plan to specify the reliability tasks statement of work sow requirements that will be performed for that specific system consistent with the creation of safety cases for example per arp4761 the goal of reliability assessments is to provide a robust set of qualitative and quantitative evidence that the use of a component or system will not be associated with unacceptable risk the basic steps to take 14 are to thoroughly identify relevant unreliability hazards e g potential conditions events human errors failure modes interactions failure mechanisms and root causes by specific analysis or tests assess the associated system risk by specific analysis or testing propose mitigation e g requirements design changes detection logic maintenance and training by which the risks may be lowered and controlled at an acceptable level determine the best mitigation and get agreement on final acceptable risk levels possibly based on cost benefit analysis the risk here is the combination of probability and severity of the failure incident scenario occurring the severity can be looked at from a system safety or a system availability point of view reliability for safety can be thought of as a very different focus from reliability for system availability availability and safety can exist in dynamic tension as keeping a system too available can be unsafe forcing an engineering system into a safe state too quickly can force false alarms that impede the availability of the system in a de minimis definition the severity of failures includes the cost of spare parts man hours logistics damage secondary failures and downtime of machines which may cause production loss a more complete definition of failure also can mean injury dismemberment and death of people within the system witness mine accidents industrial accidents space shuttle failures and the same to innocent bystanders witness the citizenry of cities like bhopal love canal chernobyl or sendai and other victims of the 2011 tōhoku earthquake and tsunami in this case reliability engineering becomes system safety what is acceptable is determined by the managing authority or customers or the affected communities residual risk is the risk that is left over after all reliability activities have finished and includes the unidentified risk and is therefore not completely quantifiable the complexity of the technical systems such as improvements of design and materials planned inspections fool proof design and backup redundancy decreases risk and increases the cost the risk can be decreased to alara as low as reasonably achievable or alapa as low as practically achievable levels reliability and availability program plan edit implementing a reliability program is not simply a software purchase it is not just a checklist of items that must be completed that ensure one has reliable products and processes a reliability program is a complex learning and kn...
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