A high percentage of defense systems fail to meet their reliability requirements. Virtual qualification can be used to accelerate the qualification process of a part for its life-cycle environment. Redundancy exists when one or more of the parts of a system can fail and the system can still function with the parts that remain operational. This pattern points to the need for better design practices and better system engineering (see also Trapnell, 1984; Ellner and Trapnell, 1990). In order to increase performance, manufacturers may adopt features for products that make them less reliable. Prognostics and health management consists of technologies and methods to assess the reliability of a system in its actual life-cycle conditions to determine the likelihood of failure and to mitigate system risk: for examples and further details, see Jaai and Pecht (2010) and Cheng et al. The following is an example of reliability analysis for a moderately complex system, which includes series and parallel connections, as well as m-out-of-n sub-systems. 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. Ideally, a virtual qualification process will identify quality suppliers and quality parts through use of physics-of-failure modeling and a risk assessment and mitigation program. 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. = = = = 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 parameters are added to the model, and the reliability for the complete system can be computed, … At the design stage, these reliabilities can either come from the reliabilities of similar components for related systems, from supplier data, or from expert judgment. Thus, components can be modeled to have decreasing, constant, or increasing failure rates. Failures categorized as system damage can be further categorized according to the failure mode and mechanism. It should contain information and data to the level of detail necessary to identify design or process deficiencies that should be eliminated. An overly pessimistic prediction can result in unnecessary additional design and test expenses to resolve the perceived low reliability. Decision Consistency Below we tried to explain all these with an example. 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. Reliability Growth evaluates these recent changes and, more generally, assesses how current DOD principles and practices could be modified to increase the likelihood that defense systems will satisfy their reliability requirements. The ratings of the part manufacturer or the user’s procurement ratings are generally used to determine these limiting values. 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. System reliability is the probability that an asset can perform without failure for a specific period of time and under normal operating conditions. startxref 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. 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. An alternative method is to use a “top-down” approach using similarity analysis. For example, consider an unreliability value of [math]F(t)=0.11\,\![/math]. Design for reliability is a collection of techniques that are used to modify the initial design of a system to improve its reliability. The process allows qualification to be incorporated into the design phase of product development, because it. These methods can also accommodate time-phased missions. A modified version of this method is used by ReliaSoft's BlockSim to calculate the analytical solution to system reliability diagrams. Failure tracking activities are used to collect test- and field-failed components and related failure information. Another problem in reliability theory is to calculate the performance indices of a system made up of non-absolutely reliable components. Failures do link hierarchically in terms of the system architecture, and so a failure mode may, in turn, cause failures in a higher level subsystem or may be the result of a failure of a lower level component, or both. An important tool in failure analysis is known as FRACAS or failure reporting, analysis and corrective action system. 0 ��J� ��EIm ��Ρ �DL 2��1�� f�9�` �HS �T�@Ǝ ;4��W�� ��� �anj� �.uT�"��@��]�wS�T� զ ��� }�������fj.��#�-�Ic����"6u�S�]�0 �;�] As the extent and degree of difference increases, the reliability differences will also increase. 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. In hot standby, the secondary part(s) forms an active parallel system. 0000006565 00000 n This, and most R packages (but see below), are available for download from the … 0000006088 00000 n With a good feature, one can determine whether the system is deviating from its nominal condition: for examples, see Kumar et al. 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. In life data analysis and accelerated life testing data analysis, as well as other testing activities, one of the primary objectives is to obtain a life distribution that describes the times-to-failure of a component, subassembly, assembly or system. rkov models are very Ma much useful in finding the System Reliability in various life situations. 0000002223 00000 n In particular, physics of failure is a key approach used by manufacturers of commercial products for reliability enhancement. x�bb�f`b``Ń3� ���ţ�1�c� 6A� Nonconstant failure rates can be handled by assessing the probability of failure at different times using the probability of failure for each component at each time, rather than using the component’s mean time between failure. %%EOF Producing a reliable system requires planning for reliability from the earliest stages of system design. (2006) for an example. 0000003497 00000 n And in each application, the basic technology may be identical. The basic elements of a fault tree diagram are events that correspond to improper functioning of components and subcomponents, and gates that represent and/or conditions. (This assumes that all unmanaged risks are producer risks.). The approach is based on the identification of potential failure modes, failure mechanisms, and failure sites for the system as a function of its life-cycle loading conditions. 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. 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. Failures have to be analyzed to identify the root causes of manufacturing defects and to test or field failures. 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. trailer Furthermore, reliability failures discovered after deployment can result in costly and strategic delays and the need for expensive redesign, which often limits the tactical situations in which the system can be used. The upper series of images relate to head pulleys used on conveyor belt systems. This approach is inaccurate for predicting actual field failures and provides highly misleading predictions, which can result in poor designs and logistics decisions. 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. 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. (2012). Feature extraction is used to analyze the measurements and extract the health indicators that characterize the system degradation trend. 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. Maintainability are the relative costs of fixing, updating, extending, operating and servicing an entity over its lifetime. (2012) and Sun et al. There are 4 sub -systems. They use failure data at the component level to assign rates or probabilities of failure. Sensing, feature extraction, diagnostics, and prognostics are key elements. 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). The stress at each failure site is obtained as a function of both the loading conditions and the system geometry and material properties. “Risk” is defined as a measure of the priority assessed for the occurrence of an unfavorable event. Do you enjoy reading reports from the Academies online for free? 0000004933 00000 n For unmanaged producibility risks, the resources predicted in the impact analysis are translated into costs. Example 1. Example: Calculating Reliability of a Series System Three subsystems are reliability-wise in series and make up a system. Defining and Characterizing Life-Cycle Loads. What is reliability? 501 0 obj<>stream This section discusses two explicit models and similarity analyses for developing reliability predictions. endstream endobj 531 0 obj<>/W[1 1 1]/Type/XRef/Index[59 440]>>stream allows design, manufacturing, and testing to be conducted promptly and cost-effectively. 0000008609 00000 n This process merges the design-for-reliability approach with material knowledge. Destructive techniques include cross-sectioning of samples and de-capsulation. 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. It appears to the panel that U.S. Department of Defense (DoD) contractors do not fully exploit these techniques. 1a) is such, which fails if any of its elements fails. The acceptable combination of mitigation approaches becomes the required verification approach. Wear-out mechanisms are analyzed using both stress and damage analysis to calculate the time required to induce failure as a result of a defined stress life-cycle profile. 0000001518 00000 n 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]). Field trial records provide estimates of the environmental profiles experienced by the system. In many cases, MIL-HDBK-217 methods would not be able to distinguish between separate failure mechanisms. Do you want to take a quick tour of the OpenBook's features? 0000071329 00000 n Sand and dust: Sand and dust can scratch and abrade finished sur-. As the “new” product is produced and used in the field, these data are used to update the prediction for future production of the same product (for details, see Pecht, 2009). Each failure model is made up of a stress analysis model and a damage assessment model. 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. Such an analysis compares two designs: a recent vintage product with proven reliability and a new design with unknown reliability. Virtual qualification uses computer-aided simulation to identify and rank the dominant failure mechanisms associated with a part under life-cycle loads, determine the acceleration factor for a given set of accelerated test parameters, and determine the expected time to failure for the identified failure mechanisms (for an example, see George et al., 2009). In both of these methods, a generic average failure rate (assuming average operating conditions) is assumed. What is the reliability of the series system shown below? H�|Vko�6�.���~t���VWp�����dh���ʔC�q�_�K�a���! 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. The degree of and rate of system degradation, and thus reliability, depend upon the nature, magnitude, and duration of exposure to such stresses. Once the risks are ranked, those that fall below some threshold in the rankings can be omitted. 0000001899 00000 n Classify risks: Classify each risk in the risk catalog in one of two categories: functionality risks and producibility risks. This lesson will cover the methods for measuring system performance and reliability, providing examples. 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. 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. ��X ��sx(DY"�\���M%�������(�F�(�`/mE:�ĥ{�z�|"��7�� ��(���8���W��8� Register for a free account to start saving and receiving special member only perks. If the part is not found to be acceptable after this assessment, then the assessment team must decide whether an acceptable alternative is available. R A = reliability of device A = probability that device A will work … A stress model captures the product architecture, while a damage model depends on a material’s response to the applied stress. Ideally, such data should be obtained and processed during actual application. However, changes between the older and newer product do occur, and can involve. Two common types of redundancy are active and standby. Once the components and external events are understood, a system model is developed. Failure analysis techniques include nondestructive and destructive techniques. ��`�u��n���8� �>��L �AB�i�zϨx����C����u\��POk�̭�h2��%p}�i����0��M��qv���� ?4��e����U�y�1 ������{��n��t�ӽ���֚��C؂��#$n�݆����@��l��P�|�d���|��0��ۂ[�#��k�B�7�4�jɅ���T�e�B�Z䯼�o�����O�ɱ��k&;]�7=eR�9�Y�)��$DO�FV��Maxw���d2�tf=c��9�J}"ǏΞ�wId\���z�L�`�ܕ�ZbY�~��ܕ_]Ľ�{�,P迓�����L\���efJ�/�KH�.B� ��r.tۄ\4ӈ�����h��.�E^,:��Mk����fh��k�O�tɄ�_^O�4���ӥ��T��5�Ņ�X���ݩ�i�7��j���Q�Kx�03AQ�JG�"`���� a�u�u�}. Simply put, reliability is the absence of unplanned downtime. (For a description of this process for an electronic system, see Sandborn et al., 2008.) 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