This chapter describes techniques to improve system design to enhance system reliability. 0000008609 00000 n These factors include the type or technology of the part under consideration, the quantity and type of manufacturer’s data available for the part, the quality and reliability monitors employed by the part manufacturer, and the comprehensiveness of production screening at the assembly level. However, such methods can dramatically increase system reliability, and DoD system reliability would benefit considerably from the use of such methods. A failure mode is the manner in which a failure (at the component, subsystem, or system level) is observed to occur, or alternatively, as the specific way in which a failure is manifested, such as the breaking of a truck axle. View our suggested citation for this chapter. An emerging approach uses physics-of-failure and design-for-reliability methods (see, e.g., Pecht and Dasgupta, 1995). Assessment of the reliability potential of a system design is the determination of the reliability of a system consistent with good practice and conditional on a use profile. 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). These practices, collectively referred to as design for reliability, improve reliability through design in several ways: Reviewing in-house procedures (e.g., design, manufacturing process, storage and handling, quality control, maintenance) against corresponding standards can help identify factors that could cause failures. Producing a reliable system requires planning for reliability from the earliest stages of system design. For example, there is a huge difference in the safety case whether or not a system has an integrated circuit. That number is the product of the probability of detection, occurrence, and severity of each mechanism. ...or use these buttons to go back to the previous chapter or skip to the next one. Many components found in products have many applications. 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. 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. Background This script provides a demonstration of some tools that can be used to conduct a reliability analysis in R. 1. In the case of wear-out failures, damage is accumulated over a period until the item is no longer able to withstand the applied load. Decide whether the risk is acceptable: If the impact fits within the overall product’s risk threshold and budget, then the part selection can be made with the chosen verification activity (if any). Equipment misapplication can result from improper changes in the operating requirements of the machine. In this method, each limit-state function is linearized using Taylor expansion at the most likelihood point (MLP), then the multivariate saddle-point approximation is used for all linearized functions to compute system … We stress that the still-used handbook MIL-HDBK-217 (U.S. Department of Defense, 1991) does not provide adequate design guidance and information regarding microelectronic failure mechanisms. In some cases, it may cause complete disruption of normal electrical equipment such as communication and measuring systems. The main idea in this approach is that all the analysts agree to draw as much relevant information as possible from tests and field data. However, changes between the older and newer product do occur, and can involve. If no failure models are available, then the evaluation is based on past experience, manufacturer data, or handbooks. Some users may shut down the computer every time they log off; others may shut down only once at the end of the day; still others may keep their computers on all the time. The life of the hot standby part(s) is consumed at the same rate as active parts. In this Paper, we present the Markov Model for a system comprising 4-Elements to determine the Reliability R(t), Failure Probability F(t) and Mean Time to In a system with standby redundancy, ideally the parts will last longer than the parts in a system with active redundancy. They identify the potential failure modes, failure sites, and failure mechanisms. Determine the impact of unmanaged risk: Combine the likelihood of risk occurrence with the consequences of occurrence to predict the resources associated with risks that the product development team chooses not to manage proactively. For example, misapplication of a component could arise from its use outside the operating conditions specified by the vendor (e.g., current, voltage, or temperature). Life-cycle profiles include environmental conditions such as temperature, humidity, pressure, vibration or shock, chemical environments, radiation, contaminants, and loads due to operating conditions, such as current, voltage, and power. Variable frequency vibration: Some systems must be able to withstand deterioration due to vibration. 0000000016 00000 n 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. Thus, components can be modeled to have decreasing, constant, or increasing failure rates. 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. operation of a system. 0000009480 00000 n For wear-out mechanisms, failure susceptibility is evaluated by determining the time to failure under the given environmental and operating conditions. Reliability, maintainability, and availability (RAM) are three system attributes that are of great interest to systems engineers, logisticians, and users. 2.2 Parallel System . The construction concludes with the assignment of reliabilities to the functioning of the components and subcomponents. 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. The combined availability is shown by theequation below:A = Ax AyThe implications of the above e… Show this book's table of contents, where you can jump to any chapter by name. The study of component and process reliability is the basis of many efficiency evaluations in Operations Management discipline. An active redundant system is a standard “parallel” system, which only fails when all components have failed. Transient, The magnetic strip on … All these elements are thus arranged in … “Risk” is defined as a measure of the priority assessed for the occurrence of an unfavorable event. 0000001518 00000 n Determine the verification approach: For the risks that are ranked above the threshold determined in the previous activity, consider the mitigation approaches defined in the risk catalog. throughout the life of the product with low overall life-cycle costs. Defining and characterizing the life-cycle stresses can be difficult because systems can experience completely different application conditions, including location, the system utilization profile, and the duration of utilization and maintenance conditions. If no overstress failures are precipitated, then the lowest occurrence rating, “extremely unlikely,” is assigned. 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). These data are often collected using sensors. For example, suppose it is required to estimate the reliability of the system according to the results of stand tests on the components. Details on performing similarity analyses can be found in the 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 … The data to be collected to monitor a system’s health are used to determine the sensor type and location in a monitored system, as well as the methods of collecting and storing the measurements. For example, a specific multilayer ceramic capacitor without modification may become part of your laptop computer or family vehicle. In this example, the reliability handbook MIL-HDBK-217F is used to find parameters for the electrical components. A specific approach to design for reliability was described during the panel’s workshop by Guangbin Yang of Ford Motor Company. If the part is not found to be acceptable after this assessment, then the assessment team must decide whether an acceptable alternative is available. 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. 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. In many cases, MIL-HDBK-217 methods would not be able to distinguish between separate failure mechanisms. There has been some research on similarity analyses, describing either. A = .001, B = .002, mission time (t) = 50 hours . 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). 0000009169 00000 n General methodologies for risk assessment (both quantitative and qualitative) have been developed and are widely available. In cold standby, the secondary part(s) is completely shut down until needed. A standby system consists of an active unit or subsystem and one or more inactive units, which become active in the event of a failure of the functioning unit. Examples of reliability requir… 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. There are three conceptual types of standby redundancy: cold, warm, and hot. 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. This example appears in the System Analysis Reference book. 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. 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. 0000007912 00000 n Failure susceptibility is evaluated by assessing the time to failure or likelihood of a failure for a given geometry, material construction, or environmental and operational condition. 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. Reliability, availability and serviceability (RAS), also known as reliability, availability, and maintainability (RAM), is a computer hardware engineering term involving reliability engineering, high availability, and serviceability design. For example, after experiencing a rare equipment failure, a plant instituted Low pressure: Low pressure can cause overstress of structures such as containers and tanks that can explode or fracture; cause seals to leak; cause air bubbles in materials, which may explode; lead to internal heating due to lack of cooling medium; cause arcing breakdowns in insulations; lead to the formation of ozone; and make outgassing more likely. An alternative method is to use a “top-down” approach using similarity analysis. Reliability can be difficult to specify, since it is defined in qualitative terms. Howeve… 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. _PU�b�5�I|�$��2Ua�����#+3�xm�Ϊ���8��i �J�:@\.�f(V�ޔ� �p�`�Ri؂�'�.�=�����J*.`�hhZ�� r8����1����Q0�`hhhZT����($���dA�@�%@���\��l���,��ET��3N`|��͠ɪ�ϸ�)�Q����#;�C� L�K'�����`���4��PN�!���h&�~��dN0�Cv��hg�bmv�m@� �D�qg��n�|``d�a`8а�@� "� BOX 5-1 The tests may be conducted according to industry standards or to required customer specifications. There are probably a variety of reasons for this omission, including the additional cost and time of development needed. 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. 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