Defining the Failure Mode

The Failure Mode and Effect Analysis (FMEA) is based on the systematic analysis of failure modes for each element of a system, by defining the failure mode and the consequences of this failure on the integrity of that system. It was first used in the 1960s in the field of aeronautics for the analysis of the safety of aircraft [15]. It is required by regulations in the USA and France for aircraft safety. It allows assessing the effects of each failure mode of a system s components and identifying the failure modes that may have a critical impact on the operability safety and maintenance of the system. It proceeds in four steps ... 

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. 

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 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. 

Based on the above level classification of risk probability and consequence, the risk level of each component is defined in accordance with the failure modes and consequent influence (Table 3). After determination of risk level, a preliminary judgment can be conducted. For low-risk area, generally it is unnecessary to adopt any measures and the devices can be used continuously with proper extension of inspection time intervals With respect to the middle risk area, the devices can be used within the specified inspection period, yet more importance should be attached to inspection or monitoring with moderate arrangement of maintenance plans, wherein the determination of inspection period subject to the fact that the risk level remains middle level in the next inspection As for the midhigh risk area, the reasons for the risks should be analyzed, and the inspection time intervals should be shortened, while the main positions causing midhigh risk should be restricted, or... 

The complexity of the devices used within the subsystem. A device will be less likely to be subject to systematic faults if the failure modes are well defined, the behaviour under fault conditions can be determined and there is sufficient failure data from field experience ... 

According to EC, 2009, a safety-related system is regarded as low complexity if the failure modes of each individual component are well defined, and the behaviour of the system under fault conditions can be completely determined. This paper focuses on SIS to which this definition does not apply. 

The second analysis is the FMEA, this is the bottom-up analysis. First, the failure modes of all the transmitter blocks are defined. Each of these modes is related to one or more effects on the transmitter functionality. Once the modes and effects are listed, the last step is the definition of the solution to mitigate the effect of the possible failures. These conclusions can result in changes in the transceiver design or changes in other components of the system, as DPU. Table 1 shows the summary of the results of the process where the most significant cases are presented. 

Consider the Failure Modes. Define the possible failure modes that would result in the realization of the potentials of hazards. What circumstances can arise that would result in the occurrence of an undesirable event What controls are in place that mitigate against the occurrence of such an event or exposure ... 

Occurrence classification/severity classification—from the discussions in Chapter I, it is seen that occurrence is a ranking number associated with the likelihood that the failure mode and/or associated causes of failure will occur. DEMEA/DFMECA looks for occurrence during the design phase of the product, whereas in PFMEA/PFMECA, the same is applicable during the production process. FMEA identifies failure modes in terms of probability of occurrence. These are actually relative values rather than absolute values, because they are measured in a defined scale. The probability of occurrence of each failure is grouped into discrete levels ... 

In the quantitative approach, alpha and beta values representing failure mode ratio and failure effect probability, respectively, are necessary. Alpha represents the probability, expressed as a decimal fraction, that the given part or item will fail in the identified mode. Beta represents the conditional probability that the failure effect will result in the identified criticality classification, given that the failure mode occurs. In the quantitative approach the value of each failure mode criticality number is defined as ... 

The failure modes of all constituent components are well defined and... 

The failure mode of at least one constituent component is not well defined or... 

For each of the defined proof test intervals, it is necessary to define which failure modes of the components will be tested, and which will not be tested [17]. 

Guideline 9 provides recommendation on the use of SCL and SCP for various geometries and states that for most axis5mimetric situations, SCLs are appropriate. SCPs are recommended for special cases, such as flat plate with penetrations, and are deemed appropriate where the geometry has a well-defined plane that can be directly related to the failure mode. 

A practical use of fitting a distribution to reliability data is to extrapolate to smaller failure rates or other environmental conditions. To simplify the equations, the expressions in the text refer to the mean life of the relevant portion of the assembly. If the constants that define the failure distribution are known, the time to reach a smaller proportion of failures may be readily calculated. For example, for failure modes that are described by a Weibull distribution, the time f to reach x% failures is given by ... 

The user must also define which failure modes are the feared events (EE). These events will be studied by the global analysis provided by the tool Safety Architect. A dysfunctional simulation of the system is then executed by propagating failures along the dataflow dependencies of components and until a feared event is reached. 

The failure module creates an instance of a separate common cause failure (CCF) module that stores all the CCFs. The hardware equipment instances are given a distinct ID number when they are instantiated. The ID numbers are used for handling common cause failures. A CCF affects many hardware components, so each common cause failure is modelled by defining a set of ID numbers that are affected by the CCF, and the failure mode related to that CCF. 

Failure mode defines all the failure modes of the item,... 

Failure cause defines possible causes of the failure modes,... 

Failure rate defines the failure rate of each failure mode,... 

Several PSFs may have a decisive contribution to the failure mode of a downstream PSF, in such a way that their omission could automatically cancel this failure mode. This situation is tackled by introducing another type of node to the network, namely the situational (S) node , whose frequency is the product of the multiplication of the frequencies of the converging nodes and importance is defined according to frequency measure. A case of a situational node in a network is presented schematically in Figure 5. 

The most known and diffused third generation maintenance management systems is the RehabU-ity-Centered Maintenance. The primary focus of RCM is to achieve a high level of understanding of the failure modes/causes, the likelihood of occurrence and the related effects, then to define a maintenance plan that prevents or proactively addresses the potential causes of failure in such a way that the overall cost of doing business is reduced. 

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