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Text of the page (random words):
e data collected from these life tests are used to predict laser life expectancy under the intended operating characteristics 30 reliability test requirements edit there are many criteria to test depends on the product or process that are testing on and mainly there are five components that are most common 31 32 product life span intended function operating condition probability of performance user exceptions 33 the product life span can be split into four different for analysis useful life is the estimated economic life of the product which is defined as the time can be used before the cost of repair do not justify the continue use to the product warranty life is the product should perform the function within the specified time period design life is where during the design of the product designer take into consideration on the life time of competitive product and customer desire and ensure that the product do not result in customer dissatisfaction 34 35 reliability test requirements can follow from any analysis for which the first estimate of failure probability failure mode or effect needs to be justified evidence can be generated with some level of confidence by testing with software based systems the probability is a mix of software and hardware based failures testing reliability requirements is problematic for several reasons a single test is in most cases insufficient to generate enough statistical data multiple tests or long duration tests are usually very expensive some tests are simply impractical and environmental conditions can be hard to predict over a systems life cycle reliability engineering is used to design a realistic and affordable test program that provides empirical evidence that the system meets its reliability requirements statistical confidence levels are used to address some of these concerns a certain parameter is expressed along with a corresponding confidence level for example an mtbf of 1000 hours at 90 confidence level from this specification the reliability engineer can for example design a test with explicit criteria for the number of hours and number of failures until the requirement is met or failed different sorts of tests are possible the combination of required reliability level and required confidence level greatly affects the development cost and the risk to both the customer and producer care is needed to select the best combination of requirements e g cost effectiveness reliability testing may be performed at various levels such as component subsystem and system also many factors must be addressed during testing and operation such as extreme temperature and humidity shock vibration or other environmental factors like loss of signal cooling or power or other catastrophes such as fire floods excessive heat physical or security violations or other myriad forms of damage or degradation for systems that must last many years accelerated life tests may be needed testing method edit a systematic approach to reliability testing is to first determine reliability goal then do tests that are linked to performance and determine the reliability of the product 36 a reliability verification test in modern industries should clearly determine how they relate to the product s overall reliability performance and how individual tests impact the warranty cost and customer satisfaction 37 accelerated testing edit main article accelerated life testing the purpose of accelerated life testing alt test is to induce field failure in the laboratory at a much faster rate by providing a harsher but nonetheless representative environment in such a test the product is expected to fail in the lab just as it would have failed in the field but in much less time the main objective of an accelerated test is either of the following to discover failure modes to predict the normal field life from the high stress lab life an accelerated testing program can be broken down into the following steps define objective and scope of the test collect required information about the product identify the stress es determine level of stress es conduct the accelerated test and analyze the collected data common ways to determine a life stress relationship are arrhenius model eyring model inverse power law model temperature humidity model temperature non thermal model software reliability edit further information software quality and site reliability engineering software reliability is a special aspect of reliability engineering it focuses on foundations and techniques to make software more reliable i e resilient to faults system reliability by definition includes all parts of the system including hardware software supporting infrastructure including critical external interfaces operators and procedures traditionally reliability engineering focuses on critical hardware parts of the system since the widespread use of digital integrated circuit technology software has become an increasingly critical part of most electronics and hence nearly all present day systems therefore software reliability has gained prominence within the field of system reliability there are significant differences however in how software and hardware behave most hardware unreliability is the result of a component or material failure that results in the system not performing its intended function repairing or replacing the hardware component restores the system to its original operating state however software does not fail in the same sense that hardware fails instead software unreliability is the result of unanticipated results of software operations even relatively small software programs can have astronomically large combinations of inputs and states that are infeasible to exhaustively test restoring software to its original state only works until the same combination of inputs and states results in the same unintended result software reliability engineering must take this into account despite this difference in the source of failure between software and hardware several software reliability models based on statistics have been proposed to quantify what we experience with software the longer software is run the higher the probability that it will eventually be used in an untested manner and exhibit a latent defect that results in a failure shooman 1987 musa 2005 denney 2005 as with hardware software reliability depends on good requirements design and implementation software reliability engineering relies heavily on a disciplined software engineering process to anticipate and design against unintended consequences there is more overlap between software quality engineering and software reliability engineering than between hardware quality and reliability a good software development plan is a key aspect of the software reliability program the software development plan describes the design and coding standards peer reviews unit tests configuration management software metrics and software models to be used during software development in modern distributed and microservices based systems reliability considerations increasingly extend beyond component level correctness to include the behavior of applications at the business logic level while traditional software reliability engineering and site reliability engineering sre focus on system level indicators such as availability and latency some sources and industry practitioners refer to this application focused perspective as application reliability engineering are this perspective emphasizes end to end user transactions and the correctness of business workflows including metrics such as transaction success rates data consistency and workflow completion across multiple interacting services it addresses scenarios in which individual system components may appear operational while failures arise from service interactions or logic errors potentially leading to degraded user experience or incorrect outcomes practices associated with application level reliability include end to end observability synthetic transaction testing and validation of business critical processes complementing traditional infrastructure focused reliability methods a common reliability metric is the number of software faults per line of code floc usually expressed as faults per thousand lines of code this metric along with software execution time is key to most software reliability models and estimates the theory is that the software reliability increases as the number of faults or fault density decreases establishing a direct connection between fault density and mean time between failure is difficult however because of the way software faults are distributed in the code their severity and the probability of the combination of inputs necessary to encounter the fault nevertheless fault density serves as a useful indicator for the reliability engineer other software metrics such as complexity are also used this metric remains controversial since changes in software development and verification practices can have dramatic impact on overall defect rates software testing is an important aspect of software reliability even the best software development process results in some software faults that are nearly undetectable until tested software is tested at several levels starting with individual units through integration and full up system testing all phases of testing software faults are discovered corrected and re tested reliability estimates are updated based on the fault density and other metrics at a system level mean time between failure data can be collected and used to estimate reliability unlike hardware performing exactly the same test on exactly the same software configuration does not provide increased statistical confidence instead software reliability uses different metrics such as code coverage the software engineering institute s capability maturity model is a common means of assessing the overall software development process for reliability and quality purposes structural reliability edit main article structural reliability structural reliability or the reliability of structures is the application of reliability theory to the behavior of structures it is used in both the design and maintenance of different types of structures including concrete and steel structures 38 39 in structural reliability studies both loads and resistances are modeled as probabilistic variables using this approach the probability of failure of a structure is calculated comparison to safety engineering edit main article safety engineering reliability for safety and reliability for availability are often closely related lost availability of an engineering system can cost money if a subway system is unavailable the subway operator will lose money for each hour the system is down the subway operator will lose more money if safety is compromised the definition of reliability is tied to a probability of not encountering a failure a failure can cause loss of safety loss of availability or both it is undesirable to lose safety or availability in a critical system reliability engineering is concerned with overall minimisation of failures that could lead to financial losses for the responsible entity whereas safety engineering focuses on minimising a specific set of failure types that in general could lead to loss of life injury or damage to equipment reliability hazards could transform into incidents leading to a loss of revenue for the company or the customer for example due to direct and indirect costs associated with loss of production due to system unavailability unexpected high or low demands for spares repair costs man hours re designs or interruptions to normal production 40 safety engineering is often highly specific relating only to certain tightly regulated industries applications or areas it primarily focuses on system safety hazards that could lead to severe accidents including loss of life destruction of equipment or environmental damage as such the related system functional reliability requirements are often extremely high although it deals with unwanted failures in the same sense as reliability engineering it however has less of a focus on direct costs and is not concerned with post failure repair actions another difference is the level of impact of failures on society leading to a tendency for strict control by governments or regulatory bodies e g nuclear aerospace defense rail and oil industries 40 fault tolerance edit main article fault tolerance safety can be increased using a 2oo2 cross checked redundant system availability can be increased by using 1oo2 1 out of 2 redundancy at a part or system level if both redundant elements disagree the more permissive element will maximize availability a 1oo2 system should never be relied on for safety fault tolerant systems often rely on additional redundancy e g 2oo3 voting logic where multiple redundant elements must agree on a potentially unsafe action before it is performed this increases both availability and safety at a system level this is common practice in aerospace systems that need continued availability and do not have a fail safe mode for example aircraft may use triple modular redundancy for flight computers and control surfaces including occasionally different modes of operation e g electrical mechanical hydraulic as these need to always be operational due to the fact that there are no safe default positions for control surfaces such as rudders or ailerons when the aircraft is flying basic reliability and mission reliability edit the above example of a 2oo3 fault tolerant system increases both mission reliability as well as safety however the basic reliability of the system will in this case still be lower than a non redundant 1oo1 or 2oo2 system basic reliability engineering covers all failures including those that might not result in system failure but do result in additional cost due to maintenance repair actions logistics spare parts etc for example replacement or repair of 1 faulty channel in a 2oo3 voting system the system is still operating although with one failed channel it has actually become a 2oo2 system is contributing to basic unreliability but not mission unreliability as an example the failure of the tail light of an aircraft will not prevent the plane from flying and so is not considered a mission failure but it does need to be remedied with a related cost and so does contribute to the basic unreliability levels detectability and common cause failures edit when using fault tolerant redundant systems or systems that are equipped with protection functions detectability of failures and avoidance of common cause failures becomes paramount for safe functioning and or mission reliability reliability versus quality six sigma edit main article six sigma quality often focuses on manufacturing defects during the warranty phase reliability looks at the failure intensity over the whole life of...
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