Failure Modes and Effect Analysis (FMEA) is known to be a systematic procedure for the analysis of a system to identify the potential failure modes, their causes and effects on system performance (Cassanelli et al, 2006). FMEA is used to assist analysts to perform hazard analyses and it is regarded as a supplement rather than a replacement for hazard analyses. Safety analysts can use the FMEA to verify that all safety critical hardware has been addressed in the hazard analyses. The FMEA in hardware systems is an important technique for evaluating the design and documenting the review process.
All credible failure modes and their resultant effects at the component and system levels are identified and documented. Items that meet defined criteria are identified as critical items and are placed on the critical item list (CIL). Each entry of the CIL is then evaluated to see if design changes can be implemented so that the item can be deleted from the CIL. The analysis follows a well-defined sequence of steps that encompass (1) failure mode (2) failure effects (3) causes (4) detectability (5) corrective or preventive actions and (6) rationale for acceptance (Pillay & Wang, 2003).
FMEA is applicable at various levels of system decomposition from the highest level of block diagram down to the functions of discrete components. The FMEA is also an iterative process that is updated as the design develops. Design changes will require that relevant parts of the FMEA be reviewed and updated. Unfortunately, design faults may survive, in spite of all the checks and reviews, and eventually cause field failures. In this case the FMEA should be updated throughout the entire system’s life (Cassanelli et al, 2006).
Applying an FMEA to a production cycle means following a series of successive steps: analysis of the process or product in every single part, list of identified potential failures, evaluation of their frequency, severity (in terms of effects of the failure to the process and to the surroundings) and detection technique, global evaluation of the problem and identification of the corrective actions and control plans that could eliminate or reduce the chance of the potential failures. This task cannot be achieved on an individual basis because FMEA is a team function. The most important aspect of FMEA is the evaluation of the risk level of potential failures identified for every sub-system or component. The global value of the damages caused on the function or on the surroundings by every failure is indicated with the risk priority number (RPN). This number (from 1 to 1000) is an index obtained from the multiplication of three risk parameters, which are severity, occurrence and detection (Scipioni et al, 2002).
FMEA uses RPN to assess risk in three categories: Occurrence (O), Severity (S), and Detection (D). The rating is scaled from 1 to 10 for each category. The occurrence is related to the probability of the failure mode and cause. Occurrence ratings have been standardized by many electronics and automotive industries over the last few years. A ‘10’ on the occurrence table corresponds to a failure happening with every other part. A ‘1’ corresponds to one failure in a million parts (Rhee & Ishii, 2003). The severity index measures the seriousness of the effects of a failure mode. Thus, a severity index is assigned to the end effect of a failure. A ‘1’ on the severity index corresponds to a failure that does not affect anything, a ‘5’ corresponds to a performance loss, a ‘7’ corresponds to machine shut down, and a ‘10’ corresponds to a life-threatening failure. The detection index is generated on the basis of the likelihood of detection by relevant design reviews, testing, and quality control measures. A ‘1’ on the detection index corresponds to a failure mode that is almost certain to be detected and a ‘10’ corresponds to a failure that is almost impossible to detect. Taking the product of these three indices (occurrence, severity, and detection) generates the RPN. The RPN represents the risk associated to each failure mode (Rhee & Ishii, 2003).
For both street driving and racing, an efficient braking system is essential. Automotive braking systems normally employ conventional or ventilated brake discs and pads. In these systems, the brake discs are made of steel or gray cast iron, which are then paired with organic-composite brake pads. These types of friction materials are suitable for use in braking systems with moderate loads, where they exhibit a relatively high and stable friction coefficient, a low-wear rate and are quiet during operation. However, car manufacturers are tending to design more prestige and sports-class vehicles which require braking systems that provide more braking power than conventional braking systems. Accordingly, new materials are being introduced into braking systems (particularly in the case of disc materials), for example, carbon–ceramic C/C SiC composites, which can provide more friction and can operate at higher temperatures. It is clear that new counter-pad materials that can resist higher temperatures compared to conventional organic-based materials need to be used with these ceramic-based materials, for example, metal–matrix composites (MMC) (Kermc et al, 2005).
Development of a fibre reinforced, lightweight, ceramic brake disc for high performance vehicles, including Fl racing cars, is reported. C powder, C fibre and resin are pressed and ‘sintered’ at 1000oC to create a stable C structure which can easily be cut to size and shape. This is infiltrated with molten Si and Sic forms. The disc is finished on the working surfaces. The material is reported to have a service life of 300000 km, to withstand high temperatures and to allow high speed braking.1
The base metallic matrix consists of three phases: a copper-based phase (light grey), iron (dark grey) and steel with carbide precipitates (dark grey with white spots). Even when
1. Ceramic brake discs for high performance vehicles, literature review, Metal Power Report, Vl.56, issue 7-8, p.24
other additives (graphite, abrasives and friction modifiers) are added to the base matrix these three phases are present and the mixed laminar metallic area is formed on top. If abrasives are added the mixed metal area is not so well developed, probably because of the higher hardness of the pad material. In order to determine the average hardness of the lining, the micro-hardnesses of the different metallic phases are measured (Stadler et al, 2007).
The friction layer of a disk brake is a two-layer structure of iron and copper oxides. It is heterogeneous, uneven in its thickness, and with iron oxides as a lower layer and a copper oxides layer on top. This friction layer is formed on top of an approximately 30–40-?m-thick mixed metallic zone where the original metallic phases are plastically deformed due to the high temperature and the pressure during braking (Stadler et al, 2007).
The most commonly used rubbers in friction materials are SBR and NBR. Rubbers can be applied in two forms, i.e. rubber block and rubber powder. Generally speaking, application of rubber block in friction material production is complicated and less efficient, involving processes of mastication, phenolic resin compounding, compound pulverizing, and etc. By contrast, application of rubber powder is much simpler, where most of the industry use direct mixing process, the simplest one among resin/rubber compounding processes. In direct mixing process, all raw materials including powdered rubber, resin, fiber, filler, and etc. are directly mixed in a mixer and compounded into compounding material. Properties of rubber component can be better retained, and will have direct influence on the properties of cured phenolic resin binder and hence the properties of final products. Recent research has shown that nano powdered rubber has ideal application effect in various friction materials and is a kind of novel rubber modifier for friction materials (Liu et al, 2006).
Cassanelli, G., Mura, G., Fantini, F., Vanzi, M., & Plano, B. (2006) Failure Analysis-assisted FMEA, Microelectronics and Reliability, Vol. 46, issues 9-11, pp. 1795-1799
Kermc, K., Kalin, M., & Vizintin, J (2005) Development and use of an apparatus for tribological evaluation of ceramic-based brake materials, Wear, Vol.259, pp.1079-1087Liu, Y., Fan, Z., Ma, H., Tan, Y., & Quia, J. (2006) Application of nano powdered rubber in friction materials, Wear, Vol.261, issue 2, pp.225-229
Pillay, A., & Wang, J. (2003) Modified failure mode and effects analysis using approximate reasoning, Reliability Engineering & System Safety, Vol. 79, issue 1, pp. 69-85
Rhee, S.J., & Ishii, K. (2003) Using cost based FMEA to enhance reliability and serviceability, Advanced Engineering Informatics, Vol. 17, Issues 3-4, pp. 179-188
Scipioni, A., Saccarola, G., Centazzo, A., & Arena, F. (2002) FMEA methodology design, implementation and integration with HACCP system in a food company •, Food Control, Vol. 13, issue 8, pp. 495-501
Stadler, Z., Krnel, K., & Kosmac, T. (2007) Friction behavior of sintered metallic brake pads on a C/C–SiC composite brake disc, Journal of the European Ceramic Society, Vol. 27, issues 2-3, pp. 1411-1417
Teixeira, N., Cunha, G.,. Moreno, L., Pontes, M., Rosa, M., Jacob, K., Cardoso, I., Ferreiras, P., Ramalho, M.,. Carvoeiras, P et al. (2005) 263 Failure Modes and Effects Analysis (FMEA) applied to two modern radiotherapy centres, Radiotherapy and Oncology, Vol. 76, Supplement 2, p. S124