FMEA

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Methods for performing hazard analysis and risk assessment include safety review, checkhsts, Dow Fire and Explosion Index, what-if analysis, hazard and operabihty analysis (HAZOP), failure modes and effects analysis (FMEA), fault tree analysis, and event tree analysis. Other methods are also available, but those given are used most often. 

Failure Mode and Effects Analysis. The system design activity usually emphasizes the attainment of performance objectives in a timely and cost-efficient fashion. The failure mode and effects analysis (FMEA) procedure considers the system from a failure point of view to determine how the product might fail. The terms design failure mode and effects analysis (DFMEA) and failure mode effects and criticaUty analysis (EMECA) also are used. This EMEA technique is used to identify and eliminate potential failure modes early in the design cycle, and its success is well documented (3,4). 

Failure Mode and Ejfect Analysis (FMEA) This is a systematic study of the causes of failures and their effects. All causes or modes of failure are considered for each element of a system, and then all possible outcomes or effects are recorded. This method is usually used in combination with fault tree analysis, a quantitative technique. FMEA is a comphcated procedure, usually carried out by experienced risk analysts. 

The cost of performing the hazard identification step depends on the size of the problem and the specific techniques used. Techniques such as brainstorming, what-if analyses, or checklists tend to be less expensive than other more structured methods. Hazard and operability (HAZOP) analyses and failure modes and effects analyses (FMEAs) involve many people and tend to be more expensive. But, you can have greater confidence in the exhaustiveness of HAZOP and FMEA techniques—their rigorous approach helps ensure completeness. However, no technique can guarantee that all hazards or potential accidents have been identified. Figure 8 is an example of the hazards identified in a HAZOP study. Hazard identification can require from 10% to 25% of the total effort in a QRA study. 

Frequency Phase 1 Perform Qualitative Study, Typically Using HAZOP, FMEA, or What-if Analysis. To perform a qualitative study you should first (1) define the consequences of interest, (2) identify the initiating events and accident scenarios that could lead to the consequences of interest, and (3) identify the equipment failure modes and human errors that could contribute to the accident... 

For a eatastrophie failure in the aerospaee industry with a high probability of loss of life, whieh relates to an FMEA Severity Rating (S) = 10, a business eould quite possibly need insuranee eover well in exeess of 100 million. This will allow for eosts due to failure investigations, legal aetions, produet reeall and possible loss of... 

The role of FMEA in designing capable and reliable products... 

FMEA was first mentioned at the start of this ehapter. It is reeommended that the reader unfamiliar with FMEA refer to Appendix III and several other referenees provided to gain a firm understanding of its applieation in produet design. In general, an FMEA does the following (Leiteh, 1995) ... 

Teehniques sueh as FMEA, DFA and Quality Funetion Deployment (QFD) ean enhanee the sueeess of a produet, but alone they will not solve all produet development issues (Andersson, 1994 Jenkins et al., 1997a Kilt et al., 1993). They provide useful aids in the proeess of quality improvement, but they do not ensure produet quality (Andersson, 1994). There exists an important need for DFQ teehniques to aid design and support the produet development proeess (Andreasen and Olesen, 1990). In addition, it has been eited that in order to make further reduetions in produet development time requires new progress in these teehniques (Dertouzos et al., 1989). 

Design teehniques (for example, FMEA or Fault Tree Analysis (FTA))... 

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. 

Failure in the context here means that product performance does not meet requirements and is related back in the design FMEA to some component/character-istic being out of specified limits - a fault. The probability of occurrence of failure (O) caused by a fault can be expressed as ... 

While 30 ppm may be acceptable as a maximum probability of occurrence for a failure of low severity, it is not acceptable as severity increases. An example table of FMEA Severity Ratings was shown in Figure 2.20. In the definite return to manufacturer (a warranty return) or violation of statutory requirement region (S = 5 or S = 6), the designer would seek ways to enhance the process capability or else utilize some inspection or test process. Reducing d will reduce occurrence, as indicated by equation 2.11, but inspection or test is of limited efficiency. 

Figure 2.21 is a graph of Occurrence against Severity showing a boundary of the acceptable design based on these criteria for the case when d = 1 and Pof = 1. The graph is scaled in terms of FMEA ratings for Occurrence, equivalent probabilities and parts-per-million (ppm). 

The Conformability Map enables appropriate Cp values to be seleeted and through the link with the eomponent variability risks, and q, it is possible to determine if a produet design has eharaeteristies that are unaeeeptable, and if so what the eost eonsequenees are likely to be. The two modes of applieation are highlighted on Figure 2.23. Mode A shows that the quality loss assoeiated with a eharaeteristie at Cpk = 1 and FMEA Severity (S) = 6 eould potentially be 8% of the total produet... 

The Conformability Matrix (see later for an example) primarily drives assessment of the variability effeets. The Conformability Matrix requires the deelaration of FMEA Severity Ratings and deseriptions of the likely failure mode(s). It is helpful in this respeet to have the results from a design FMEA for the produet. 

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). 

E 7) r — S. i E Garnpcrcrij 3SS-c-nMv Failufe Descriptt n FMEA Severity Rating CS) Con-irner its jnciLcdlrig action Tor suppliers)... 

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. 

This case study concerns the initial design and redesign of a security cover assembly for a solenoid. The analysis only focuses on those critical aspects of the assembly of the product that must be addressed to meet the requirement that the electronics inside the unit are sealed from the outside environment. An FMEA Severity Rating (S) for the assembly was determined as S = 5, a warranty return if failure is experienced. 

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