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t available or is extremely expensive to obtain by combining redundancy together with a high level of failure monitoring and the avoidance of common cause failures even a system with relatively poor single channel part reliability can be made highly reliable at a system level up to mission critical reliability no testing of reliability has to be required for this in conjunction with redundancy the use of dissimilar designs or manufacturing processes e g via different suppliers of similar parts for single independent channels can provide less sensitivity to quality issues e g early childhood failures at a single supplier allowing very high levels of reliability to be achieved at all moments of the development cycle from early life to long term redundancy can also be applied in systems engineering by double checking requirements data designs calculations software and tests to overcome systematic failures another effective way to deal with reliability issues is to perform analysis that predicts degradation enabling the prevention of unscheduled downtime events failures rcm reliability centered maintenance programs can be used for this physics of failure based approach edit for electronic assemblies there has been an increasing shift towards a different approach called physics of failure this technique relies on understanding the physical static and dynamic failure mechanisms it accounts for variation in load strength and stress that lead to failure with a high level of detail made possible with the use of modern finite element method fem software programs that can handle complex geometries and mechanisms such as creep stress relaxation fatigue and probabilistic design monte carlo methods doe the material or component can be re designed to reduce the probability of failure and to make it more robust against such variations another common design technique is component derating i e selecting components whose specifications significantly exceed the expected stress levels such as using heavier gauge electrical wire than might normally be specified for the expected electric current common tools and techniques edit many of the tasks techniques and analyses used in reliability engineering are specific to particular industries and applications but can commonly include physics of failure pof built in self test bit or bist testability analysis failure mode and effects analysis fmea reliability hazard analysis reliability block diagram analysis dynamic reliability block diagram analysis 22 fault tree analysis root cause analysis statistical engineering design of experiments e g on simulations fem models or with testing sneak circuit analysis accelerated testing reliability growth analysis re active reliability weibull analysis for testing or mainly re active reliability hypertabastic survival models thermal analysis by finite element analysis fea and or measurement thermal induced shock and vibration fatigue analysis by fea and or measurement electromagnetic analysis avoidance of single point of failure spof functional analysis and functional failure analysis e g function fmea fha or ffa predictive and preventive maintenance reliability centered maintenance rcm analysis testability analysis failure diagnostics analysis normally also incorporated in fmea human error analysis operational hazard analysis preventative planned maintenance optimization pmo manual screening integrated logistics support results from these methods are presented during reviews of part or system design and logistics reliability is just one requirement among many for a complex part or system engineering trade off studies are used to determine the optimum balance between reliability requirements and other constraints the importance of language edit reliability engineers whether using quantitative or qualitative methods to describe a failure or hazard rely on language to pinpoint the risks and enable issues to be solved the language used must help create an orderly description of the function item system and its complex surrounding as it relates to the failure of these functions items systems systems engineering is very much about finding the correct words to describe the problem and related risks so that they can be readily solved via engineering solutions jack ring said that a systems engineer s job is to language the project ring et al 2000 23 for part system failures reliability engineers should concentrate more on the why and how rather that predicting when understanding why a failure has occurred e g due to over stressed components or manufacturing issues is far more likely to lead to improvement in the designs and processes used 4 than quantifying when a failure is likely to occur e g via determining mtbf to do this first the reliability hazards relating to the part system need to be classified and ordered based on some form of qualitative and quantitative logic if possible to allow for more efficient assessment and eventual improvement this is partly done in pure language and proposition logic but also based on experience with similar items this can for example be seen in descriptions of events in fault tree analysis fmea analysis and hazard tracking logs in this sense language and proper grammar part of qualitative analysis plays an important role in reliability engineering just like it does in safety engineering or in general within systems engineering correct use of language can also be key to identifying or reducing the risks of human error which are often the root cause of many failures this can include proper instructions in maintenance manuals operation manuals emergency procedures and others to prevent systematic human errors that may result in system failures these should be written by trained or experienced technical authors using so called simplified english or simplified technical english where words and structure are specifically chosen and created so as to reduce ambiguity or risk of confusion e g an replace the old part could ambiguously refer to a swapping a worn out part with a non worn out part or replacing a part with one using a more recent and hopefully improved design reliability modeling edit reliability modeling is the process of predicting or understanding the reliability of a component or system prior to its implementation two types of analysis that are often used to model a complete system s availability behavior including effects from logistics issues like spare part provisioning transport and manpower are fault tree analysis and reliability block diagrams at a component level the same types of analyses can be used together with others the input for the models can come from many sources including testing prior operational experience field data as well as data handbooks from similar or related industries regardless of source all model input data must be used with great caution as predictions are only valid in cases where the same product was used in the same context as such predictions are often only used to help compare alternatives a reliability block diagram showing a 1oo3 1 out of 3 redundant designed subsystem for part level predictions two separate fields of investigation are common the physics of failure approach uses an understanding of physical failure mechanisms involved such as mechanical crack propagation or chemical corrosion degradation or failure the parts stress modelling approach is an empirical method for prediction based on counting the number and type of components of the system and the stress they undergo during operation reliability theory edit main articles failure rate and survival analysis reliability is defined as the probability that a device will perform its intended function during a specified period of time under stated conditions mathematically this may be expressed as r t p r t t t f x d x displaystyle r t pr t t int _ t infty f x dx where f x displaystyle f x is the failure probability density function and t displaystyle t is the length of the period of time which is assumed to start from time zero there are a few key elements of this definition reliability is predicated on intended function generally this is taken to mean operation without failure however even if no individual part of the system fails but the system as a whole does not do what was intended then it is still charged against the system reliability the system requirements specification is the criterion against which reliability is measured reliability applies to a specified period of time in practical terms this means that a system has a specified chance that it will operate without failure before time t displaystyle t reliability engineering ensures that components and materials will meet the requirements during the specified time note that units other than time may sometimes be used e g a mission operation cycles reliability is restricted to operation under stated or explicitly defined conditions this constraint is necessary because it is impossible to design a system for unlimited conditions a mars rover will have different specified conditions than a family car the operating environment must be addressed during design and testing that same rover may be required to operate in varying conditions requiring additional scrutiny two notable references on reliability theory and its mathematical and statistical foundations are barlow r e and proschan f 1982 and samaniego f j 2007 often knowledge of the statistical dependence between component failures is unknown or only partially constrained when it is desired to model such incomplete knowledge reliability theory allows for analyses that characterize ranges or bounds on system failure probabilities rather than unique distributions particularly for small multi component systems quantitative system reliability parameters theory edit quantitative requirements are specified using reliability parameters the most common reliability parameter is the mean time to failure mttf which can also be specified as the failure rate this is expressed as a frequency or conditional probability density function pdf or the number of failures during a given period these parameters may be useful for higher system levels and systems that are operated frequently i e vehicles machinery and electronic equipment reliability increases as the mttf increases the mttf is usually specified in hours but can also be used with other units of measurement such as miles or cycles using mttf values on lower system levels can be very misleading especially if they do not specify the associated failures modes and mechanisms the f in mttf 17 in other cases reliability is specified as the probability of mission success for example reliability of a scheduled aircraft flight can be specified as a dimensionless probability or a percentage as often used in system safety engineering a special case of mission success is the single shot device or system these are devices or systems that remain relatively dormant and only operate once examples include automobile airbags thermal batteries and missiles single shot reliability is specified as a probability of one time success or is subsumed into a related parameter single shot missile reliability may be specified as a requirement for the probability of a hit for such systems the probability of failure on demand pfd is the reliability measure this is actually an unavailability number the pfd is derived from failure rate a frequency of occurrence and mission time for non repairable systems for repairable systems it is obtained from failure rate mean time to repair mttr and test interval this measure may not be unique for a given system as this measure depends on the kind of demand in addition to system level requirements reliability requirements may be specified for critical subsystems in most cases reliability parameters are specified with appropriate statistical confidence intervals reliability testing edit the purpose of reliability testing or reliability verification is to discover potential problems with the design as early as possible and ultimately provide confidence that the system meets its reliability requirements the reliability of the product in all environments such as expected use transportation or storage during the specified lifespan should be considered 10 it is to expose the product to natural or artificial environmental conditions to undergo its action to evaluate the performance of the product under the environmental conditions of actual use transportation and storage and to analyze and study the degree of influence of environmental factors and their mechanism of action 24 through the use of various environmental test equipment to simulate the high temperature low temperature and high humidity and temperature changes in the climate environment to accelerate the reaction of the product in the use environment to verify whether it reaches the expected quality in r d design and manufacturing 25 reliability verification is also called reliability testing which refers to the use of modeling statistics and other methods to evaluate the reliability of the product based on the product s life span and expected performance 26 most product on the market requires reliability testing such as automotive integrated circuit heavy machinery used to mine nature resources aircraft auto software 27 28 reliability testing may be performed at several levels and there are different types of testing complex systems may be tested at component circuit board unit assembly subsystem and system levels 29 the test level nomenclature varies among applications for example performing environmental stress screening tests at lower levels such as piece parts or small assemblies catches problems before they cause failures at higher levels testing proceeds during each level of integration through full up system testing developmental testing and operational testing thereby reducing program risk however testing does not mitigate unreliability risk with each test both statistical type i and type ii errors could be made depending on sample size test time assumptions and the needed discrimination ratio there is risk of incorrectly rejecting a good design type i error and the risk of incorrectly accepting a bad design type ii error it is not always feasible to test all system requirements some systems are prohibitively expensive to test some failure modes may take years to observe some complex interactions result in a huge number of possible test cases and some tests require the use of limited test ranges or other resources in such cases different approaches to testing can be used such as highly accelerated life testing design of experiments and simulations the desired level of statistical confidence also plays a role in reliability testing statistical confidence is increased by increasing either the test time or the number of items tested reliability test plans are designed to achieve the specified reliability ...
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