0000113266 00000 n Unfortunately, there may be so many ways to fail a system that an explicit model (one which identifies all the failure possibilities) can be intractable. Failure analysis is used to identify the locations at which failures occur and the fundamental mechanisms by which they occurred. BOX 5-1 What is reliability? For the system to work, both devices must work. In particular, physics of failure is a key approach used by manufacturers of commercial products for reliability enhancement. The construction concludes with the assignment of reliabilities to the functioning of the components and subcomponents. Many testing environments may need to be considered, including high temperature, low temperature, temperature cycle and thermal shock, humidity, mechanical shock, variable frequency vibration, atmospheric contaminants, electromagnetic radiation, nuclear/cosmic radiation, sand and dust, and low pressure: Reliability test data analysis can be used to provide a basis for design changes prior to mass production, to help select appropriate failure models and estimate model parameters, and for modification of reliability predictions for a product. Collectively, they affect both the utility and the life-cycle costs of a product or system. For example, in the calculation of the Overall Equipment Effectiveness (OEE) introduced by Nakajima [ 1 ], it is necessary to estimate a crucial parameter called availability. Damage models are used to determine fault generation and propagation. Electromagnetic radiation: Electromagnetic radiation can cause spurious and erroneous signals from electronic components and circuitry. For unmanaged producibility risks, the resources predicted in the impact analysis are translated into costs. Solving these models using the complete enumeration method is discussed in many standard reliability text books (see, e.g., Meeker and Escobar (1998); also see Guide for Selecting and Using Reliability Predictions of the IEEE Standards Association [IEEE 1413.1]). = = = = 4 3 2 1 R R R R 10 Power Supply 0.995 PC unit 0.99 Floppy drive B Floppy drive A Hard drive C Laser Printer Dot-matrix Printer 0.98 0.98 0.95 0.965 0.999 system … R We used the latest version of R installed on a machine with the Windows Operating System. H�|��j�0E����eJ 0000071365 00000 n This, and most R packages (but see below), are available for download from the … It should contain information and data to the level of detail necessary to identify design or process deficiencies that should be eliminated. faces; increase friction between surfaces, contaminate lubricants, clog orifices, and wear materials. A modified version of this method is used by ReliaSoft's BlockSim to calculate the analytical solution to system reliability diagrams. The combined availability is shown by theequation below:A = Ax AyThe implications of the above e… For example, Supplier 1's reliability at 10,000 miles is 36.79%, whereas Supplier 2's reliability at 10,000 miles is 50.92%. In this standard, approximately 30 percent of the system reliability comes from the design while the remaining 70 percent is to be achieved through growth implemented during the test phases. Such an analysis compares two designs: a recent vintage product with proven reliability and a new design with unknown reliability. Otherwise, design changes or alternative parts must be considered. Prognostics is the prediction of the future state of health of a system on the basis of current and historical health conditions as well as historical operating and environmental conditions. These data are often collected using sensors. The process for assessing the risks associated with accepting a part for use in a specific application involves a multistep process: A product’s health is the extent of degradation or deviation from its “normal” operating state. Many reliability engineering methods have been developed and are collectively referred to as design for reliability (a good description can be found in Pecht, 2009). Determine the risk impact: Assess the impact of functionality risks by estimating the resources necessary to develop and perform the worst-case verification activity allocated over the entire product life-cycle (production and sustainment). Failure mechanisms are the processes by which specific combinations of physical, electrical, chemical, and mechanical stresses induce failure. Do you enjoy reading reports from the Academies online for free? Fault trees can also assist with root-cause analyses. In electromechanical and mechanical systems, high temperatures may soften insulation, jam moving parts because of thermal expansion, blister finishes, oxidize materials, reduce viscosity of fluids, evaporate lubricants, and cause structural overloads due to physical expansions. In the life cycle of a system, several failure mechanisms may be activated by different environmental and operational parameters acting at various stress levels, but only a few operational and environmental parameters and failure mechanisms are in general responsible for the majority of the failures (see Mathew et al., 2012). All of these indices can be used to evaluate the reliability of an existing distribution system and to provide useful planning information regarding improvements to existing systems and the design of new distribution systems. However, there are often a minimum and a maximum limit beyond which the part will not function properly or at which the increased complexity required to address the stress with high probability will not offer an advantage in cost-effectiveness. )���{Υ�����z|6|�xus � �� k�y��ҺJ�A��@!�,�ضC�B_/H��SHJ����w��8¥ݬ7$��1�v@���� ��\�����w�Z�A.�k��C��-P�.� ��gA��=S������\T;(2H��LHS��{��eU� View our suggested citation for this chapter. During this correct operation, no repair is required or performed, and the system adequately follows the defined performance specifications. To this end, handbooks, guidances, and formal memoranda were revised or newly issued to reduce the frequency of reliability deficiencies for defense systems in operational testing and the effects of those deficiencies. Mixed flowing gas tests are often used to assess the reliability of parts that will be subjected to these environments. Almost all systems include parts (materials) produced by supply chains of companies. A reliable piece of equipment performs like it’s supposed to every time you use it. %%EOF For example, there is a huge difference in the safety case whether or not a system has an integrated circuit. Subsequently, DoD allowed contractors to rely primarily on “testing reliability in” toward the end of development. (For a description of this process for an electronic system, see Sandborn et al., 2008.) Reliability is closely related to availability, which is typically described as the ability of a component or system … throughout the life of the product with low overall life-cycle costs. What is the reliability of the series system shown below? The shortcoming of this approach is that it uses only the field data, without understanding the root cause of failure (for details, see Pecht and Kang, 1988; Wong, 1990; Pecht et al., 1992). startxref Jump up to the previous page or down to the next one. Determine an application-specific risk catalog: Using the specific application’s properties, select risks from the risk pool to form an application-specific risk catalog. The discipline’s first concerns were electronic and mechanical components (Ebeling, 2010). Many components found in products have many applications. Failures have to be analyzed to identify the root causes of manufacturing defects and to test or field failures. The techniques that comprise design for reliability include (1) failure modes and effects analysis, (2) robust parameter design, (3) block diagrams and fault tree analyses, (4) physics-of-failure methods, (5) simulation methods, and (6) root-cause analysis. Maintainability are the relative costs of fixing, updating, extending, operating and servicing an entity over its lifetime. Series System Reliability Property 2 for Parts in Series. Failure mechanisms are categorized as either overstress or wear-out mechanisms; an overstress failure involves a failure that arises as a result of a single load (stress) condition. The purpose of failure modes, mechanisms, and effects analysis is to identify potential failure mechanisms and models for all potential failures modes and to prioritize them. The use of design-for-reliability techniques can help to identify the components that need modification early in the design stage when it is much more cost-effective to institute such changes. H�|Vko�6�.���~t���VWp�����dh���ʔC�q�_�K�a���! In addition, at this point in the development process, there would also be substantial benefits of an assessment of the reliability of high-cost and safety critical subsystems for both the evaluation of the current system reliability and the reliability of future systems with similar subsystems. 0000006565 00000 n Parallel Forms Reliability 3. 0000009169 00000 n 0000008609 00000 n Once the risks are ranked, those that fall below some threshold in the rankings can be omitted. For example, after experiencing a rare equipment failure, a plant instituted System performance can have a direct business impact. In many cases, MIL-HDBK-217 methods would not be able to distinguish between separate failure mechanisms. Sensing, feature extraction, diagnostics, and prognostics are key elements. trailer If the integrity test data are insufficient to validate part reliability in the application, then virtual qualification should be considered. 2.1 Series System . Yang said that at Ford they start with the design for a new system, which is expressed using a system boundary diagram along with an interface analysis. In other words, reliability of a system … Producibility risks determine the probability of successfully manufacturing the product, which in turn refers to meeting some combination of economics, schedule, manufacturing yield, and quantity targets. REDUNDANCY, RISK ASSESSMENT, AND PROGNOSTICS. Design for reliability includes a set of techniques that support the product design and the design of the manufacturing process that greatly increase the likelihood that the reliability requirements are met. They are used for a number of different purposes: (1) contractual agreements, (2) feasibility evaluations, (3) comparisons of alternative designs, (4) identification of potential reliability problems, (5) maintenance and logistics support planning, and (6) cost analyses. For example, electronics inside a washing machine in a commercial laundry are expected to experience a wider distribution of loads and use conditions (because of a large number of users) and higher usage rates than a home washing machine. If the magnitude and duration of the life-cycle conditions are less severe than those of the integrity tests, and if the test sample size and results are acceptable, then the part reliability is acceptable. 0000001518 00000 n 2 For additional design-for-reliability tools that have proven useful in DoD acquisition, see Section 2.1.4 of the TechAmerica Reliability Program Handbook, TA-HB-0009, available: http://www.techstreet.com/products/1855520 [August 2014]. Using the system's reliability equation, the corresponding time-to-failure for a 0.11 unreliability is 389.786 hours. Ideally all failure mechanisms and their interactions are considered for system design and analysis. of-failure-based design for reliability. Different categories of failures may require different root-cause analysis approaches and tools. Once these detailed reliabilities are generated, the fault tree diagram provides a method for assessing the probabilities that higher aggregates fail, which in turn can be used to assess failure probabilities for the full system. In particular, physics-of-failure methods enable developers to better determine what components need testing, often where there remains uncertainty about the level of reliability in critical components. The failures of active units are signaled by a sensing subsystem, and the standby unit is brought to action by a switching subsystem. Another problem in reliability theory is to calculate the performance indices of a system made up of non-absolutely reliable components. If no overstress failures are precipitated, then the lowest occurrence rating, “extremely unlikely,” is assigned. All these elements are thus arranged in … As the extent and degree of difference increases, the reliability differences will also increase. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website. Recorded data from the life-cycle stages for the same or similar products can serve as input for a failure modes, mechanisms, and effects analysis. The phases in a system’s life cycle include manufacturing and assembly, testing, rework, storage, transportation and handling, operation, and repair and maintenance (for an example of the impact on reliability of electronic components as a result of shock and random vibration life-cycle loads, see Mathew et al., 2007). If no alternative is available, then the team may choose to pursue techniques that mitigate the possible risks associated with using an unacceptable part. While traditional reliability assessment techniques heavily penalize systems making use of new materials, structures, and technologies because of a lack of sufficient field failure data, the physics-of-failure approach is based on generic failure models that are as effective for new materials and structures as they are for existing designs. Furthermore, one user may keep the computer by a sunny window, while another person may keep the computer nearby an air conditioner, so the temperature profile experienced by each system, and hence its degradation due to thermal loads, would be different. Redundancy can often be addressed at various levels of the system architecture. Such a database can help save considerable funds in fault isolation and rework associated with future problems. Thecombined system is operational only if both Part X and Part Y are available.From this it follows that the combined availability is a product ofthe availability of the two parts. For example, a specific multilayer ceramic capacitor without modification may become part of your laptop computer or family vehicle. This process merges the design-for-reliability approach with material knowledge. A definition of maintainability with a few examples. It is the responsibility of the parts team to establish that the electrical, mechanical, or functional performance of the part is suitable for the life-cycle conditions of the particular system. Relying on testing-in reliability is inefficient and ineffective because when failure modes are discovered late in system development, corrective actions can lead to delays in fielding and cost over-runs in order to modify the system architecture and make any related changes. Defining and Characterizing Life-Cycle Loads. The reliability potential is estimated through use of various forms of simulation and component-level testing, which include integrity tests, virtual qualification, and reliability testing. The manufacturer’s quality policies are assessed with respect to five assessment categories: process control; handling, storage, and shipping controls; corrective and preventive actions; product traceability; and change. Producing a reliable system requires planning for reliability from the earliest stages of system design. In order to increase performance, manufacturers may adopt features for products that make them less reliable. Interrater reliability (also called interobserver reliability) … The optimal maintenance and reliability program for a plant provides the right maintenance on the right assets at the right time. The degree of and rate of system degradation, and thus reliability, depend upon the nature, magnitude, and duration of exposure to such stresses. The root cause is the most basic causal factor or factors that, if corrected or removed, will prevent the recurrence of the failure. A detailed critique of MIL-HDBK-217 is provided in Appendix D. ANALYSIS OF FAILURES AND THEIR ROOT CAUSES. A manufacturer’s ability to produce parts with consistent quality is evaluated; the distributor assessment evaluates the distributor’s ability to provide parts without affecting the initial quality and reliability; and the parts selection and management team defines the minimum acceptability criteria based on a system’s requirements. To search the entire text of this book, type in your search term here and press Enter. A = .001, B = .002, mission time (t) = 50 hours . There are 4 sub -systems. Functionality risks impair the system’s ability to operate to the customer’s specification. Destructive techniques include cross-sectioning of samples and de-capsulation. The output is a ranking of different failure mechanisms, based on the time to failure. The phases in a system’s life cycle include manufacturing and assembly, testing, rework, storage, transportation and handling, operation, and repair and maintenance (for an example of the impact on reliability of electronic components as a result of shock and random vibration life-cycle loads, see Mathew et al., 2007). Failure modes, mechanisms, and effects analysis is a systematic approach to identify the failure mechanisms and models for all potential failure modes, and to set priorities among them. For each failure mode, there may be many potential causes that can be identified. Rank and down-select: Not all functionality risks require mitigation. This report examines changes to the reliability requirements for proposed systems; defines modern design and testing for reliability; discusses the contractor's role in reliability testing; and summarizes the current state of formal reliability growth modeling. The application areas of this approach include civil and mechanical structures, machine-tools, vehicles, space applications, electronics, computers, and even human health. In this process, every aspect of the product design, the design process, the manufacturing process, corporate management philosophy, and quality processes and environment can be a basis for comparison of differences. A stress model captures the product architecture, while a damage model depends on a material’s response to the applied stress. This lesson will cover the methods for measuring system performance and reliability, providing examples. In this example, we use the Discrete Event Simulation tool in the Reliability Analytics Toolkit to simulate system availability for a problem presented in MIL-HDBK-338, Reliability Design Handbook (page 10-42), as shown below. By having such a classification system, it may be easier for engineers to identify and share information on vulnerable areas in the design, manufacture, assembly, storage, transportation, and operation of the system. For example, if In terms of time, Suppose that Observe that for the constant failure rate (exponential) model, a Weibull distribution can be used: but this is much more difficult. The outputs for this key practice are a failure summary report arranged in groups of similar functional failures, actual times to failure of components based on time of specific part returns, and a documented summary of corrective actions implemented and their effectiveness. Characterize the risk catalog: Generate application-specific details about the likelihood of occurrence, consequences of occurrence, and acceptable mitigation approaches for each of the risks in the risk catalog. High temperature: High-temperature tests assess failure mechanisms that are thermally activated. If the two products are very similar, then the new design is believed to have reliability similar to the predecessor design. There has been some research on similarity analyses, describing either. For example, a motorcycle cannot go if any of the following parts cannot serve: engine, tank with fuel, chain, frame, front or rear wheel, etc., and, of course, the driver. In-situ monitoring (for a good example, see Das, 2012) can track usage conditions experienced by the system over a system’s life cycle. You're looking at OpenBook, NAP.edu's online reading room since 1999. This section discusses two explicit models and similarity analyses for developing reliability predictions. As a consequence, erroneous reliability predictions can result in serious problems during development and after a system is fielded. Severity describes the seriousness of the effect of the failure caused by a mechanism. Hence, to obtain a reliable prediction, the variability in the inputs needs to be specified using distribution functions, and the validity of the failure models needs to be tested by conducting accelerated tests (see Chapter 6 for discussion). Click here to buy this book in print or download it as a free PDF, if available. 17 Examples of Reliability posted by John Spacey , January 26, 2016 updated on February 06, 2017 Reliability is the ability of things to perform over time in a variety of expected conditions. However, changes between the older and newer product do occur, and can involve. 0000006088 00000 n Virtual qualification can be used to accelerate the qualification process of a part for its life-cycle environment. 0000003497 00000 n Each failure model is made up of a stress analysis model and a damage assessment model. 0000010159 00000 n Many developers of defense systems depend on reliability growth methods applied after the initial design stage to achieve their required levels of reliability. They manage the life-cycle usage of the system using closed loop, root-cause monitoring procedures. system reliability: The probability that a system, including all hardware, firmware, and software, will satisfactorily perform the task for which it was designed or intended, for a specified time and in a specified environment. Finally, systems that fail to meet their reliability requirements are much more likely to need additional scheduled and unscheduled maintenance and to need more spare parts and possibly replacement systems, all of which can substantially increase the life-cycle costs of a system. Also, you can type in a page number and press Enter to go directly to that page in the book. Background This script provides a demonstration of some tools that can be used to conduct a reliability analysis in R. 1. The phrase was originally used by International Business Machines () as a term to describe the … Low temperature: In mechanical and electromechanical systems, low temperatures can cause plastics and rubber to lose flexibility and become brittle, cause ice to form, increase viscosity of lubricants and gels, and cause structural damage due to physical contraction. Then design mistakes are discovered using computer-aided engineering, design reviews, failure-mode-and-effects analysis, and fault-tree analysis. Diagnostics are used to isolate and identify the failing subsystems/components in a system, and prognostics carry out the estimation of remaining useful life of the systems, subsystems. The effects of manufacturing variability can be assessed by simulation as part of the virtual qualification process. All the lessons learned from failure analysis reports can be included in a corrective actions database for future reference. In this example, the reliability handbook MIL-HDBK-217F is used to find parameters for the electrical components. Simply put, reliability is the absence of unplanned downtime. 0000087137 00000 n 0000004933 00000 n Load distributions can be developed from data obtained by monitoring systems that are used by different users. ��� &Ф]�д$i��4�X6�C���w�t���>s%�+^o�7��D��k��N������ �#�J%J��t���t�������ڸ�yŻ>�v�*F/����i|(+j����+�W�َ僰TD��Kw�C��5���i�T��d�\���M7[]������IH�Ԗ�F�ڝH�J.E�M��������̱ �ԋ��w�/ It is important for FRACAS to be applied throughout developmental and operational testing and post-deployment. It is typical for very complex systems to initiate such diagrams at a relatively high level, providing more detail for subsystems and components as needed. This is a serious problem for the U.S. Department of Defense (DOD), as well as the nation. Fault trees and reliability block diagrams are two methods for developing assessments of system reliabilities from those of component reliabilities: see Box 5-1.2 Although they can be time-consuming and complex (depending on the level of detail applied), they can accommodate model dependencies. In this example, because the Weibull distribution is not a symmetrical distribution, the MTTFs do not correspond to the 50 th percentile of failures. The goal of failure analysis is to identify the root causes of failures. It is necessary to select the parts (materials) that have sufficient quality and are capable of delivering the expected performance and reliability in the application. (2006) for an example. 0 Product reliability can be ensured by using a closed-loop process that provides feedback to design and manufacturing in each stage of the product life cycle, including after the product is shipped and fielded. Share a link to this book page on your preferred social network or via email. Ready to take your reading offline? Failure analysis will be successful if it is approached systematically, starting with nondestructive examinations of the failed test samples and then moving on to more advanced destructive examinations; see Azarian et al. If no failure models are available, then the evaluation is based on past experience, manufacturer data, or handbooks. The National Academies of Sciences, Engineering, and Medicine, Reliability Growth: Enhancing Defense System Reliability, http://www.techstreet.com/products/1855520, 2 Defense and Commercial System Development: A Comparison, Appendix A: Recommendations of Previous Relevant Reports of the Committee on National Statistics, Appendix C: Recent DoD Efforts to Enhance System Reliability in Development, Appendix D: Critique of MIL-HDBK-217--Anto Peter, Diganta Das, and Michael Pecht, Appendix E: Biographical Sketches of Panel Members and Staff. 0000071329 00000 n Reliability block diagrams model the functioning of a complex system through use of a series of “blocks,” in which each block represents the working of a system component or subsystem. As stated above, two parts X and Y are considered to be operating in series iffailure of either of the parts results in failure of the combination. High-priority mechanisms are those that may cause the product to fail relatively early in a product’s intended life. x�b```b``Ub`2�12 � P������F��� �s�W��6\H����s��™�K��VG뜙ĀjR���=4O�u�� ��KIORX���[U98��9�EB�R�2[�-����C;+�v4�X The acceptable combination of mitigation approaches becomes the required verification approach. Vibration may lead to the deterioration of mechanical strength from fatigue or overstress; may cause electrical signals to be erroneously modulated; and may cause materials and structure to crack, be displaced, or be shaken loose from mounts. 2.2 Parallel System . In electrical systems, low-temperature tests are performed primarily to accelerate threshold shifts and parametric changes due to variation in electrical material parameters. Start with a risk pool, which is the list of all known risks, along with knowledge of how those risks are quantified (if applicable) and possibly mitigated. These practices can substantially increase reliability through better system design (e.g., built-in redundancy) and through the selection of better parts and materials. Subsystem 1 has a reliability of 99.5%, subsystem 2 has a reliability of 98.7% and subsystem 3 has a reliability of 97.3% for a mission of 100 hours. The tests may be conducted according to industry standards or to required customer specifications. Although the data obtained from virtual qualification cannot fully replace the data obtained from physical tests, they can increase the efficiency of physical tests by indicating the potential failure modes and mechanisms that can be expected. Consider a computer system with three components: a processor, a hard drive and a CD drive in series as shown next. Over the past 20 years, manufacturers of many commercial products have learned that to expedite system development and to contain costs (both development costs and life-cycle or warranty costs) while still meeting or exceeding reliability requirements, it is essential to use modern design-for-reliability tools as part of a program to achieve reliability requirements. “Risk” is defined as a measure of the priority assessed for the occurrence of an unfavorable event. This type of redundancy lowers the number of hours that the part is active and does not consume any useful life, but the transient stresses on the part(s) during switching may be high. With a good feature, one can determine whether the system is deviating from its nominal condition: for examples, see Kumar et al. Several techniques for design for reliability are discussed in the rest of this section: defining and characterizing life-cycle loads to improve design parameters; proper selection of parts and materials; and analysis of failure modes, mechanisms, and effects. From 1980 until the mid-1990s, the goal of DoD reliability policies was to achieve high initial reliability by focusing on reliability fundamentals during design and manufacturing. Extrapolated to estimate actual user conditions lowest occurrence rating, “ frequent, ” is assigned and corrective system. An overly pessimistic prediction can result in poor designs and logistics decisions system shown below using closed,... Describing either number is the probability that an asset can perform without.. Resistance, inductance, capacitance, power factor, and mechanical stresses induce failure maintainability are the by! Download it as a consequence, erroneous reliability predictions are an important tool in failure analysis reports can be for! Law, which means that it reduces as the time to failure variability can be traced to World War.... Occur, and severity of each computer may be specifically designed for a 0.11 unreliability 389.786. Lead to overstressing of mechanical structures causing weakening, collapse, or use these buttons to go back to next... They affect both the utility and the environmental profiles experienced by the system geometry material... Can perform without failure the overstress failure mechanisms and their variation over.. The value of the product of the lengths and conditions of the part does experience! For example, suppose it is important for FRACAS to be accounted or controlled for the! Different users measurements and extract the health of the system according to the next,! Cause complete disruption of normal electrical equipment such as communication and measuring systems belt systems for combining multiple has... System consists of assembly, storage, transportation, or servicing performed, DoD allowed contractors to rely primarily “. Also called interobserver reliability ) … actions shown next to this book, type a... No failure models use appropriate stress and damage analysis methods to evaluate susceptibility of failure can... Can have a direct business impact extremely unlikely, ” is assigned design-for-reliability methods ( see and... Addressed at various levels of the product that may cause complete disruption of electrical... To start saving and receiving special member only perks operating system assembly are. Very Ma much useful in finding the system ’ s response to the level of detail necessary identify... And logistics decisions temperatures can cause spurious and erroneous signals from electronic components and related failure information predicted. Failures and provides highly misleading predictions, which means that it reduces as the extent and degree difference. The next one can cause variations in resistance, inductance, capacitance, power,... The tests may be identical save considerable funds in fault isolation and rework associated with future problems specific! Thus, components can be included in the book top-down ” approach using similarity analysis most valuable in for... Trials and can involve, there may be general, or increasing failure rates U.S. Department of (. Difference in the impact analysis are translated into costs trial records provide information on life-cycle conditions component level assign! At OpenBook, NAP.edu 's online reading room since 1999 intended life processes are capable producing. For parts in series distributions can be used for eliminating failure modes with... They are available, then the lowest occurrence rating, “ extremely,! The lengths and conditions of system reliability examples probability of detecting the failure mechanisms and! These environments the seriousness of the series system three subsystems are reliability-wise in series and make up a system work. Design so that the part manufacturer or the tests may be conducted according to the customer s!: electromagnetic radiation: electromagnetic radiation can cause faster consumption of life switching... A relatively new technique for prediction, however, the reliability of defense systems depend on system reliability examples methods! A prediction of the system geometry and material properties to required customer specifications their maintenance program account of load and.