Failure modes components

Armstrong, M. 1997. Reliability-importance and dual failure-mode components. IEEE Trans, on Reliability 46(2) 212-221. 

The EMEA begins with the selection of a subsystem or component and then documents all potential failure modes. Their effect is traced up to the system level. A documented worksheet similar to Eigure 4 is used on which the following elements are recorded. 

Failure Mode. The failure mode identifies how the component/subsystem can fail to perform each required function. A function may have more than one failure mode. 

Failure Cause. The failure cause is the physical, chemical, electrical, thermal, or other design deficiency which caused the failure. The agent, physical process, or hardware deficiency causing the failure mode must be identified, ie, what caused the failure for each failure mode. There may be more than one cause. Failure Fffect. The failure effect is the local effect on the immediate component/subsystem and the global effect on system performance/operation. In commercial products, the effect on the customer, ie, the global effect, must be addressed. 

Flaw. A flaw can be defined as an imperfection in a material that does not affect its usefulness or serviceability. A component may have imperfections and still retain its usefulness. This fact is recognized by most material codes that permit, but limit, the size and extent of imperfections. This is particularly true of welds, which commonly contain harmless imperfections. It is not uncommon for failures to occur in the vicinity of flaws that have contributed nothing to the failure mode. 

Eailure Mode and Effects Analysis (EMEA) A failure identification methodology where the failure modes of a component sub-system are identified. An analysis of these failure modes on the safety of the entire system is performed. 

A hazard identification technique in which all known failure modes of components or features of a system are considered in turn, and undesired outcomes are noted... 

FMEA can be used to provide a quantitative measure of the risk for a design. Because it can be applied hierarchically from system through subassembly and component levels down to individual dimensions and characteristics, it follows the progress of the design into detail. FMEA also lists potential failure modes and rates their Severity (S), Occurrence (O) and Detectability ( )). It therefore provides a possible means for linking potential variability risks with consequent design acceptability and associated costs. Note that the ratings of Occurrence and Detectability are equated to probability levels. 

The link with FMEA brings into play the additional dimension of potential variability into the assessment of the failure modes and the effects on the customer. The Conformability Matrix also highlights those bought-in components and/or assemblies that have been analysed and found to have conformance problems and require further communication with the supplier. This will ultimately improve the supplier development process by highlighting problems up front. 

Once the variability risks, and q, have been calculated, the link with the particular failure mode(s) from an FMEA for each critical characteristic is made. However, determining this link, if not already evident, can be the most subjective part of the analysis and should ideally be a team-based activity. There may be many component characteristics and failure modes in a product and the matrix must be used to methodically work through this part of the analysis. Past failure data on similar products may be useful in this respect, highlighting those areas of the product that are most affected by variation. Variation in fit, performance or service life is of particular interest since controlling these kinds of variation is most closely allied with quality and reliability (Nelson, 1996). 

For example, the characteristic dimension A on the cover support leg was critical to the success of the automated assembly process, the potential failure mode being a major disruption to the production line. An FMEA Severity Rating (S) = 8 is allocated. See a Process FMEA Severity Ratings table as provided in Chrysler Corporation et al. (1995) for guidance on process orientated failures. The component cost, Pc = 5.93 and the number planned to be produced per annum, N = 50000. 

Following the eompletion of the variability risks table, a Conformability Matrix was produeed. This was used to relate the failure modes and their severity eoming out of the design FMEA to the results of the Component Manufaeturing Variability Risk Analysis. The portion of the matrix eoneerned with the moulded hub ean be found in Figure 2.34(d) and was eompleted using the Conformability Map. 

Figure 4.41 shows the Stress-Strength Interference (SSI) diagrams for the two assembly operation failure modes. The instantaneous stress on the relief section on first assembly is composed of two parts first the applied tensile stress,. v, due to the pre-load, F, and secondly, the torsional stress, t, due to the torque on assembly, M, and this is shown in Figure 4.41(a) (Edwards and McKee, 1991). This stress is at a maximum during the assembly operation. If the component survives this stress, it will not fail by stress rupture later in life. 

Before setting about the task of developing such a model, the product development process requires definition along with an indication of its key stages, this is so the appropriate tools and techniques can be applied (Booker et al., 1997). In the approach presented here in Figure 5.11, the product development phases are activities generally defined in the automotive industry (Clark and Fujimoto, 1991). QFD Phase 1 is used to understand and quantify the importance of customer needs and requirements, and to support the definition of product and process requirements. The FMEA process is used to explore any potential failure modes, their likely Occurrence, Severity and Detectability. DFA/DFM techniques are used to minimize part count, facilitate ease of assembly and project component manufacturing and assembly costs, and are primarily aimed at cost reduction. 

Potential Failure Mode. How could the component, product, process or system element fail to meet each aspect of the specification ... 

E S i c - ai it O Component/ assembly process description 9 e M a Failure Mode Description and FMEA Severity Rating (S) ... 

BS 5760 1991 Part 5 - Guide to Failure Modes, Effects and Criticality Analysis (FMEA and EM EC A). Reliability of. Systems, Equipment and Components. London BSI. 

We previously encountered failure modes and effects (FMEA) and failure modes effects and criticality analysis (FMECA) as qualitative methods for accident analysis. These tabular methods for reliability analysis may be made quantitative by associating failure rates with the parts in a systems model to estimate the system reliability. FMEA/FMECA may be applied in design or operational phases (ANSI/IEEE Std 352-1975, MIL-STD-1543 and MIL-STD-1629A). Typical headings in the F.Mld. A identify the system and component under analysis, failure modes, the ef fect i>f failure, an estimale of how critical apart is, the estimated probability of the failure, mitigaturs and IHissihiy die support systems. The style and contents of a FMEA are flexible and depend upon the. ilitcLiives of the analyst. 

A failure modes and effects analysis delineates components, their interaction.s ith each other, and the effects of their failures on their system. A key element of fault tree analysis is the identification of related fault events that can contribute to the top event. For a quantitative evaluation, the failure modes must be clearly defined and related to a numerical database. Component failure modes should be realistically and consistently postulated within the context of system operational requirements and environmental factors. 

The GIDEP Reliability-maintainability Data Bank (RMDB) has failure rates, failure modes, replacement rates, mean time between failure (MTBF) and mean time to repair (MTTR) on components, equipment, subsystems and systems. The RMDB includes field experience data, laboratory accelerated life test data, reliability and maintainability demonstration test results. The... 

Component Failure Mode Error Rale Upper Bound Number of Record-. 

The component fragility for a particular failure mode is expressed in terms of the ground-acceleration A. The fragility is therefore the frequency at which the random variable A is less than or equal to a critical value, a. The ground-acceleration capacity is, in turn, modeled a.s equatum 5.1-6, where A is the median ground-acceleration, random variable (with unit median) representing the ... 

Core damage and containment performance was assessed for accident sequences, component failure, human error, and containment failure modes relative to the design and operational characteristics of the various reactor and containment types. The IPEs were compared to standards for quality probabilistic risk assessment. Methods, data, boundary conditions, and assumptions are considered to understand the differences and similarities observed. 

The FMEA approach is a bottom-up approach, looking at component failures and establishing their effect on the system. An alternative approach is to use a top-down approach such as Fault Tree Analysis to postulate system failure modes and establish which processes, procedures, or activities are likely to cause such failures. 

Failure rates based on 14,000 lailures lor 300 component types. Mean values, upper and lower bounds are offered for different failure modes. 

The main objective of the In-Plant Reliability Data System (IPRDS) was to develop a comprehensive and component-specific data base for PRA and other component reliability-related statistical analysis. Data base personnel visited selected plants and copied all the plant maintenance wor)c requests. They also gathered plant equipment lists and plant drawings and in some cases interviewed plant personnel for Information on component populations and duty cycles. Subsequently, the maintenance records were screened to separate out the cases of corrective maintenance applying to particular components these were reviewed to determine such things as failure modes, severity, and, if possible, failure cause. The data from these reports were encoded into a computerized data base. 

Three reports have been issued containing IPRDS failure data. Information on pumps, valves, and major components in NPP electrical distribution systems has been encoded and analyzed. All three reports provide introductions to the IPRDS, explain failure data collections, discuss the type of failure data in the data base, and summarize the findings. They all contain comprehensive breakdowns of failure rates by failure modes with the results compared with WASH-1400 and the corresponding LER summaries. Statistical tables and plant-specific data are found in the appendixes. Because the data base was developed from only four nuclear power stations, caution should be used for other than generic application. 

The IEEE Std 500 document is based on a hierarchical structure of component types set down in the manual s table of contents. The preface for each subsection (defined by a component type) provides a tree diagram that clearly shows the way the component classes have been subdivided to determine "data cells". The failure modes for each component class are also hierarchically organized according to failure severity catastrophic, degraded, or incipient. Rates per hour and demand rates (per cycle) are both included, as well as upper and lower bounds. 

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