Functional failure analysis

The Functional Failure Modes and Effects Analysis (Functional FMEA) or Functional Failure Analysis (as it is usually called in the USA) is typically performed to support safety analysis efforts early in the lifecycle and intended for iterative application. It tries to find all hypothetical failure modes to the defined functions of the considered system and assesses the operational effect thereof. That way functional failure modes that may be eliminated/mitigated by functional level design changes can be found, the confidence in the overall design concept is strengthened and areas requiring risk reduction can be identified. 

For this paper we treat hazard assessment as a combination of two interrelated concepts hazard identification, in which the possible hazardous events at the system boundary are discovered, and hazard analysis, in which the likelihood, consequences and severity of the events are determined. The hazard identification process is based on a model of the way in which parts of a system may deviate fi om their intended behaviour. Examples of such analysis include Hazard and Operability Studies (HAZOP, Kletz 1992), Fault Propagation and Transformation Calculus (Wallace 2005), Function Failure Analysis (SAE 1996) and Failure Modes and Effects Analysis (Villemeur 1992). Some analysis approaches start with possible deviations and determine likely undesired outcomes (so-called inductive approaches) while others start with a particular unwanted event and try to determine possible causes (so-called deductive approaches). The overall goal may be safety analysis, to assess the safety of a proposed system (a design, a model or an actual product) or accident analysis, to determine the likely causes of an incident that has occurred. 

Ensure consistency of results by integrating and relating all aspects of the safety assessment (e.g. hardware analysis with software analysis low-level probability studies with the high-level functional failure analysis) (Muari, 2000). 

For a criterion of failure, life tests should measure the time required for a material to deteriorate to a condition where it is no longer capable of performing its intended function. Careful analysis and testing are required to determine the most important condition or property at the time of failure or the point when a material becomes inadequate to its intended function. 

The acoustic microscopy s primary application to date has been for failure analysis in the multibillion-dollar microelectronics industry. The technique is especially sensitive to variations in the elastic properties of semiconductor materials, such as air gaps. SAM enables nondestructive internal inspection of plastic integrated-circuit (IC) packages, and, more recently, it has provided a tool for characterizing packaging processes such as die attachment and encapsulation. Even as ICs continue to shrink, their die size becomes larger because of added functionality in fact, devices measuring as much as 1 cm across are now common. And as die sizes increase, cracks and delaminations become more likely at the various interfaces. 

The mechanism of adhesion is also an important factor in failure analysis in composites [31]. Some adhesives work due to a physical entanglement of the resin into the wood structure whereas others require a free hydroxyl group on one of the cell wall polymers to participate in a chemical reaction with the resin. Substitution of hydroxyl groups was shown to decrease adhesion between chemically modified veneers due to the loss of hydroxyl functionality [32]. Resins that are water-soluble and depend on a hydrophilic substrate for penetration will be less efficient in chemically modified wood due to the decreased hydrophilic nature of the celt wall resulting from modification [33]. 

The output of the FMEA analysis is paramount to the determination of the business risk (consequence of function failure) presented by a system. This risk is used to determine the level of rigor applied to the validation, operational control, maintenance, and documentation/information needed to verify and maintain system performance as indicated by Figure 31.5. It follows that the documentation/information supporting system functions is as critical to the pharmaceutical organization as the system function itself. 

Process step definition Functional performance criteria Functional failure mode analysis Failure mode recovery requirements Informational requirements Information structures Legacy system interfaces Data entry range Data retention requirements Human/machine interface requirements Screen specifications Data entry modes Refresh rates Data migration... 

One of the consequences of corrosion is the failure of a machine or a system to function according to the specifications or the prescribed standards. The failure of a system to function according to specifications warrants a failure analysis of the system to identify the root causes of failure and suggestions to correct the situation. Corrosion damage, defects, and failures can have major and serious consequences on the operation of a system. Failure of a system with respect to corrosion is a final step or stage before the system undergoes corrosion damage and defects. 

Usually, these are not known. Instead the forces and moments are known and the strains are needed. This would be a typical situation for failure analysis. This means that Eqn (6.26) must be inverted to give the strains and curvatures as a function of the applied loads ... 

The assumption of independence between hazards and safety instrumented function failures seems very realistic. (NOTE If control functions and safety functions are performed by the same equipment, the assumption may not be valid Detailed analysis must be done to insure safety in such situations, and it is best to avoid such designs completely.) When hazards and equipment are independent, it is realized that a hazard may come at any time. Therefore, international standards have specified that PFDavg is an appropriate metric for measuring the effectiveness of a safety instrumented function. 

Chronic failures, which are very frequent events, and that when they are eliminated or the root causes controlled, restored functionality is achieved to its peak and the expected work level rises. These failures are difficult to control or eradicate, and this is only achieved by applying failure analysis, and they are often accepted as a normal part of the production... 

The functional approach stems from the fact that any system (or item) is merely the embodiment of a set of functions. A Functional Hazard Analysis (FHA) is a systematic, comprehensive top-down examination of each function of the system to consider the effects and probability of a functional failure, malfunction and/or normal response to unusual or abnormal external factors [AMC25.1309 para lOb(l)]. 

A system is a collection of components in a defined architecture with the sole purpose of accomplishing that system s function (refer to Fig. 3.1). The functional failure probability of that function is determined by the integrity of the constituent components as well as the logic of the systems architecture. The more complex the system, the more there is a need for an in-depth analysis technique to identify all possible combinations of failure that could result in loss of the system s integrity. The Fault Tree Analysis (FTA) is such a technique. A fault tree shows graphically, by means of a specified notation, the logical relationship between a particular system failure and all its contributing causes. 

In most civil aviation System Safety Assessments, this event originates from a Function Hazard Analysis (FHA, see Chapter 3), but it can also come from any other hazard identification technique (e.g. ZS A or PRA). An FTA is a deductive approach (i.e. top down) that determines how a given state (i.e. the undesired event) can occur. It does not identify all failures in a system in a way that inductive tproaches (such as an FMEA) would. 

RTC A/DO-254 defines Functional Failure Path (FFP) as the specific set of interdependent circuits that could cause a particular anomalous behaviour in the hardware that implements the function or in the hardware that is dependent upon the function. FFP Analysis (FFPA) is used to iteratively decompose the hardware functions into a hierarchy of subfunction to determine if it will be possible to fulfil completely the objectives of RTCA/DO-254 for each subfunction. If the assurance lifecycle data available or expected to be available is complete, correct and acceptable per the RTCA/DO-254 objectives and guidance, then no further decomposition is necessary. If it is not, then decomposition continues until such a stage as the FFP feasibly maps to one of the Development Assurance methods (and associated data set) as described in the previous section. For FFPs that are not Levels A or B, their interrelationships with the Level A or B FFPs should be evaluated using an F-FMEA, common mode analysis or dependency diagram to ensure that the Level A and B FFPs cannot be adversely impacted by the FFPs which are not Level A or B. 

This step involves the Safety Engineer highlighting to the Test Pilot and/or the HF Specialist all failure conditions identified via techniques such as the Functional Hazard Analysis (FHA) (Chapter 3), Failure Modes and Effects Analysis (EMEA) (Chapter 5), Common Mode Analysis (CMA) (Chapter 6), Particular Risk Analysis (PRA) (Chapter 7) and Zonal Safety Analysis (ZSA) (Chapter 8). 

Functional Hazard Analysis A Functional Hazard Analysis (FHA) is defined [SAE ARP4761 para 3.2] as a systematic, comprehensive examination of a system s functions to identify and classify potential Failure Conditions which the system can cause or contribute to, not only if it malfunctions or fails to function, but also in its normal response to unusual or abnormal external factors. 

The enterprise analyzes and prioritizes potential functional failure modes to define failure effects and identify the need for fault detection and recovery fimctions. Functional reliability models are established to support the analysis of system effectiveness for each operational scenario. Failures, which represent significant safety, performance, or environmental hazards, are modeled to completely understand system impacts. 

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