Assignment 14.5 : Failure Mode and Effects Analysis ( FMEA )

Failure mode and effects analysis (FMEA) is a team-based methodology for identifying
potential problems with new or existing designs. It is one of the most frequently
used hazard-analysis tools. FMEA identifies the mode of failure of every component
in a system and determines the effect on the system of each potential failure. By failure
we mean inability to meet a customer’s requirements as opposed to actual catastrophic
material breakage or failure.
 

Thus, a failure mode is any way that a part could fail to perform its required
function. For example, a cable used to lift I-beams could fray from wear, kink from
misuse, or actually fracture from excessive load. Note that either fraying or kinking
could lead to fracture, but fracture might occur without these events if a design error
incorrectly estimated either the strength of the cable or the load it needed to support.

There are many variations in detailed FMEA methodology, but they are all aimed
at accomplishing three things: (1) predicting what failures could occur; (2) predicting
the effect of the failure on the functioning of the system; and (3) establishing
steps that might be taken to prevent the failure, or its effect on the function. FMEA
is useful in identifying critical areas of the design that need redundant components
and improved reliability. FMEA is a bottom-up process that starts with the required
functions, identifi es the components to provide the functions, and for each component,
lists all possible modes of failure.

Failure mode and effect analysis (FMEA) was one of the first systematic techniques for failure analysis. It was developed by reliability engineers in the 1950s to study problems that might arise from malfunctions of military systems. A FMEA is often the first step of a system reliability study. It involves reviewing as many components, assemblies, and subsystems as possible to identify failure modes, and their causes and effects. For each component, the failure modes and their resulting effects on the rest of the system are recorded in a specific FMEA worksheet. There are numerous variations of such worksheets. 

A FMEA is mainly a qualitative analysis. A few different types of FMEA analysis exist, like Functional, Design and Process FMEA. Sometimes the FMEA is called FMECA to indicate that Criticality analysis is performed also. An FMEA is an Inductive reasoning (forward logic) single point of failure analysis and is a core task in reliability engineering, safety engineering and quality engineering. Quality engineering is specially concerned with the "Process" (Manufacturing and Assembly) type of FMEA. A successful FMEA activity helps to identify potential failure modes based on experience with similar products and processes or based on common physics of failure logic. It is widely used in development and manufacturing industries in various phases of the product life cycle. Effects analysis refers to studying the consequences of those failures on different system levels.

Functional analyses are needed as an input to determine correct failure modes, both for functional FMEA or Piece-Part (hardware) FMEA. A FMEA is used to structure Mitigation for Risk reduction based on either failure (mode) effect severity reduction or based on lowering the probability of failure or both. The FMEA is in principle a full inductive (forward logic) analysis, however the failure probability can only be estimated or reduced by understanding the failure mechanism. Ideally this probability shall be lowered to "impossible to occur" by eliminating the (root) causes. It is therefore important to include in the FMEA an appropriate depth of information on the causes of failure (deductive analysis).

 Three factors are considered in developing a FMEA :

1) The severity of a failure. Many organizations require that potential failures with a 9 or 10  rating require immediate redesign.

2) The probability of occurrence of the failure.The probabilities given are very approximate and dependon the nature of the failure, the robustness of the design, and the level of quality developed in manufacturing.
 

3) The likelihood of detecting the failure in either design or manufacturing, before the product is used by the customer. Table 14.14 gives the scale for detection. Clearly, the rating for this factor depends on the quality review systems in place in the organization.