Reliability Growth Enhancing Defense System Reliability (2015) / Chapter Skim
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5 System Design for Reliability
5 System Design for Reliability
Pages 63-84
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From page 63... ...
This chapter describes techniques to improve system design to enhance system reliability. 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.
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From page 64... ...
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. The stress at each failure site is obtained as a function of both the loading conditions and the system geometry and material properties.
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From page 65... ...
In particular, physics of failure is a key approach used by manu acturers f of commercial products for reliability enhancement. 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.
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From page 66... ...
• They design to the quality level that can be controlled in manu facturing and assembly, considering the potential failure modes, failure sites, and failure mechanisms, obtained from the physics of-failure analysis, and the life-cycle profile. • They verify the reliability of the system under the expected life cycle conditions.
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From page 67... ...
There are three methods used to estimate system life-cycle loads relevant to defense systems: similarity analysis, field trial and service records, and in-situ monitoring: 1 This is one of the limitations of prediction that is diminishing over time, given that many systems are being outfitted with sensors and communications technology that provide comprehensive information about the factors that will affect reliability.
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From page 68... ...
• Field trial records provide estimates of the environmental profiles experienced by the system. The data are a function of the lengths and conditions of the trials and can be extrapolated to estimate actual user conditions.
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From page 69... ...
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. It supports physics
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From page 70... ...
The potential failure mechanisms are considered individually, and they are assessed with models that enable the design of the system for the intended application. Failure models use appropriate stress and damage analysis methods to evaluate susceptibility of failure.
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From page 71... ...
For the wear-out failure mechanisms, the ratings are assigned on the basis of benchmarking the individual time to failure for a given wear-out mechanism with overall time to failure, expected product life, past experience, and engineering judgment. 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.
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From page 72... ...
Virtual qualification can be used to optimize the product design in such a way that the minimum time to failure of any part of the product is greater than its desired life. 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.
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From page 73... ...
The information required for designing system-specific reliability tests includes the anticipated life-cycle conditions, the reliability goals for the system, and the failure modes and mechanisms identified during reliability analysis. The different types of reliability tests that can be conducted include tests for design marginality, determination of destruct limits, design verification testing before mass production, on-going reliability testing, and accelerated testing (for examples, see Keimasi et al., 2006; Mathew et al., 2007; Osterman 2011; Alam et al., 2012; and Menon et al., 2013)
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From page 74... ...
• Nuclear/cosmic radiation: Nuclear/cosmic radiation can cause heat ing and thermal aging; alter the chemical, physical, and electrical properties of materials; produce gasses and secondary radiation; oxidize and discolor surfaces; and damage electronic components and circuits. • Sand and dust: Sand and dust can scratch and abrade finished sur
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From page 75... ...
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. Test data can also be used to create guidelines for manufacturing tests including screens, and to create test requirements for materials, parts, and sub-assemblies obtained from suppliers.
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From page 76... ...
is defined, and should be implemented, as a closed-loop process for iden tifying and tracking root failure causes, and subsequently determining, implementing and verifying an effective corrective action to eliminate their reoccurrence. The FRACAS accumulates failure, analysis and corrective action information to assess progress in eliminating hardware, software and process-related failure modes and mechanisms.
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From page 77... ...
This section discusses two explicit models and similarity analyses for developing reliability predictions. Two Explicit Models 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)
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From page 78... ...
As is the case for reli ability block diagrams, fault trees are initially built at a relatively coarse level and then expanded as needed to provide greater detail. The construction concludes with the assignment of reliabilities to the functioning of the components and subcomponents.
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From page 79... ...
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)
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From page 80... ...
The application properties most likely to be used to create the risk catalog include functionality, life cycle environments (e.g., manufacturing, shipping and handling, storage, operation, and possibly end-of-life) , manufacturing char acteristics (e.g., schedule, quantity, location, and suppliers)
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From page 81... ...
4. Classify risks: Classify each risk in the risk catalog in one of two categories: functionality risks and producibility risks.
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From page 82... ...
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. 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)
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From page 83... ...
. The prognostics and health management process does not predict reliability but rather provides a reliability assessment based on in-situ monitoring of certain environmental or performance parameters.
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