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Chapter 12
Failure Modes and Effects Analysis
Jaroslav Menčík
Additional information is available at the end of the chapter
http://dx.doi.org/10.5772/62364
Abstract
Failure Mode and Effect Analysis (FMEA) is a simple procedure for systematic revealing
of possible failures of structures or processes as early as in the design stage. The main
steps of this procedure are explained. Classification of severity, frequency and possibility
of early detection of the individual failure modes is shown, as well as the calculation of
the risk priority number, which serves for finding the most dangerous causes of failures.
The application of FMEA is shown on an example.
Keywords: Failure, failure mode, severity, frequency of occurrence, risk, FMEA
Until now, probabilistic methods were described. In this chapter, a nonprobabilistic method
will be explained, which can increase reliability in a very effective way.
Failure modes and effects analysis (FMEA) is a simple procedure for systematic revealing
possible failures of a structure or process as early as in the design or project stage and avoiding
or mitigating them. The basic idea is that the prevention of failures is better and cheaper than
their later detection and repairs. The term failure means here any loss of the ability of the object
to perform its functions properly.
FMEA was used for the first time in the Apollo project. Today, it is compulsory in the design
of aircrafts; very often it is used in the automotive industry and gradually spreads into other
branches. Its use is recommended by quality standards such as ISO 9000. In the past, good
designers and builders used a similar approach intuitively. The advantage of FMEA is the fact
that it is a systematic procedure guaranteeing that everything will be done to prevent expect‐
able failures of a component, structure, or process. A very important thing is that FMEA is not
a matter of one expert only, but uses the knowledge and experience of people from various
branches. Their cooperation can have synergic effects and bring further improvements into
the design.
© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons
Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,
and reproduction in any medium, provided the original work is properly cited.
90 Concise Reliability for Engineers
Failure modes and effects analysis can be done in 10 steps.
1. Formulation of the problem and establishing a FMEA team
FMEA can be done for a product (component or structure) or a process. A special team is
usually formed for the pertinent task. The team should consist of designers, technologists,
somebody responsible for the manufacture or building, and somebody representing the future
user. His practical experience with the operation and maintenance of similar objects is
invaluable.
Every FMEA team has its leader, either appointed by the management or selected by the team
itself. The role of the leader is to organize and facilitate the FMEA sessions, to ensure the
resources for the work, and to help the team to reach the consensus and to progress toward
the completion of FMEA.
Before starting the analysis, it is necessary to define well its scope, the relation of the team to
the management, and its competences and responsibility. It is also necessary to set the budget
for the analysis as well as the deadline. All this, including the names of the team leader and
members and the way of communication with the management should be written down in a
document.
2. Review of the construction or process
The purpose for a product FMEA is to reveal problems that could result in safety hazards,
product malfunctions, or a shortened life. The key question is “How can the product fail?”
The process FMEA should uncover the problems related to the manufacture, building, or
assembly of the product. It is helpful to consider the five elements of a process: people,
materials, equipment, methods, and environment. With these elements in mind, the key
question is ”How can the process failure affect the product, processing efficiency, or safety?“
During the first session, the members of the team should be sure that they understand all
necessary details of the construction or process and their interrelations. To ensure it, every
member should get in advance engineering drawings and documents of the product or a
detailed flowchart of the process or operation. It is helpful to have an expert on the construction
or process, who will be able to answer any questions the team might have.
3. Revealing of all potential failure modes
Once the team members understand the product (or process), they can begin thinking about
the potential failure modes that can affect the product quality, reliability and safety during its
Failure Modes and Effects Analysis 91
http://dx.doi.org/10.5772/62364
useful life. This should be done during one or more sessions organized according to the rules
of brainstorming; information on previous failures is also useful.
In such meetings, no idea or comments should be rejected. However, some people personally
involved in the design might feel offended by somebody’s finding the faults and mistakes. The
role of the team leader is to facilitate the process, enhance the people to bring ideas and
comments, and mitigate some negative psychological effects.
4. Listing of potential effects of each failure mode
Once the possible failure modes have been identified, they are written down into a special
form (Fig. 1). Then, the FMEA team reviews each failure mode and identifies the potential
effects of the failure should it occur. For every failure mode, there may be one or more effects.
Again, everything is written into the FMEA form. This is very important, as this information
is the base for assigning risk ratings to each failure mode. It is recommended to use the if-then
thinking: ”If this occurs, what are the consequences?” The form (Fig. 1) helps in taking
measures for the elimination of some failures or reduction of their severity.
Figure 1. Failure modes and effects analysis worksheet. (In real worksheets, both parts are printed together.)
Figure 26. Failure Modes and Effects Analysis (FMEA) worksheet.
(In real worksheets both parts are printed together.)
92 Concise Reliability for Engineers
5. Assigning severity, occurrence, and detection ratings for each effect
Each effect is assigned three numbers characterizing its severity, frequency, and probability
of early detection, and these numbers are written into the left part of the form (upper part of
Fig. 1). Often, each of the ratings is based on a 10-point scale, with 1 being the best case and 10
the worst case; for example,
Severity rating scale:
10 – consequences dangerously high (failure could injure or kill); 8 – consequences very serious
(failure renders the object unfit for use); 6 – moderate (failure results in partial malfunction);
4 – very low (there is minor performance loss); 3 – minor (the effects could be overcome without
performance loss); 1 – none (failure would not be noticeable).
Occurrence rating scale:
10 – very high probability of occurrence [failure is (almost) inevitable]; 8 – high probability
(repeated failures); 6 – moderate probability (occasional failures); 3 – low (relatively few
failures); 1 – negligible (failure is unlikely).
Detectability rating scale:
10 – probability of detection (POD) is zero (the object is not inspected or the defect is not
detectable); 8 – POD is low (the signs of failure are not easily detectable); 3 – POD is high (the
signs of failure are easily detectable, the objects are 100% controlled; 1 – detection of approach‐
ing failure is certain [the emerging defect is obvious or there is 100% automatic control (regular
inspections, if necessary)].
There are no fixed scales; the classification depends on the character of the object. However, it
is important to establish a clear description of the points on each scale so that all team members
have the same understanding and consensus of the ratings.
When assigning a severity rating, one must be aware that a single failure of a component can
have several effects, and each effect can have a different severity.
The best method for determining the occurrence rating is to use actual data from the same or
similar product or process. When actual failure data are not available, the team must estimate
how often the pertinent failure mode can occur.
The detection rating tells how likely a failure can be revealed before it happens. If there are no
controls, the probability of detection is low and the rating high (9 or 10).
6. Calculation of the Risk Priority Number (RPN) for each failure mode
Now, the RPN is calculated by multiplying the severity rating by the occurrence rating and
the detection rating for each item (see the special column in Fig. 1):
Failure Modes and Effects Analysis 93
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RPN = Severity × Occurrence × Detection . (1)
This number for a single item can be between 1 and 1000.
Then, the total RPN can be calculated by summing up the risk priority numbers for all failure
modes (Fig. 1, at the bottom of the table). This number alone is meaningless, because each
FMEA has a different number of failure modes and effects. However, it can serve for compar‐
ison with the revised total RPN once the improving measures have been proposed (see further).
7. Prioritizing the failure modes for action
The failure modes can now be ranked from the highest RPN to the lowest RPN. This can easily
be accomplished by common spreadsheet programs (e.g. Excel).
The team must now decide which failure modes will be worked on to reduce their RPN.
Usually, a limit value of RPN is chosen, and only those items are dealt with, whose RPN was
higher. However, special attention must also be paid to all cases with the highest severity
rating, such as 8 – 10.
8. Taking action for eliminating or reducing the high-risk failure modes
Each of the high-risk failure modes is discussed, and the team members propose measures to
reduce its RPN. This number is a product of three terms (severity, occurrence, and detectabil‐
ity), and the reduction of each of them will reduce the RPN. However, the best way is to
eliminate the reason for particular failure. For example, if a steel component can fail due to
corrosion, the use of a stainless steel can fully avoid this danger. If there is no failure, there is
no need to reduce its severity or frequency, nor improve its detectability.
Then, measures follow for the reduction of severity of a failure or their frequency. (Some of
the failure modes have similar reasons.) Improvement can be reached by new design, by using
other components or materials, by the improvement of input control for components or raw
materials, and discarding the unsuitable ones. The third way to reduce RPN strives at the
improvement of detection of failures in early stages (e.g. by building-in special elements or
sensors or by periodic inspections). However, this does not mean an actual improvement of
the structure.
9. Calculation of the resulting RPN as the failure modes are reduced
For each item corrected, new ratings are determined (severity, occurrence, and detectability)
as well as the risk priority number (see part 2 of Fig. 1). Then, the total RPN is calculated for
94 Concise Reliability for Engineers
the whole structure. This number can often be several tens of percent lower than the original
RPN, partly thanks to the elimination of reasons for some failures. The comparison of both
RPN shows how effective the FMEA was. It can also help in deciding what measures should
be taken in cases of several possible ways of improvement, with different RPNs.
10. Taking action for improvements
The recommended measures for improvement are written into the FMEA form, including their
ratings and RPN. However, the most important thing is to ensure that these measures will be
realized. Thus, it must also be proposed who will be responsible for the corrective action, the
date to which this action should be carried out, and the person who will check it (with respect
to the competences of the FMEA team). The final FMEA forms are then submitted to the
management.
Concluding remarks
Failure Mode and Effect Analysis, although it is very simple and does not work explicitly with
probabilities, can significantly reduce the number of mistakes happening during the design,
manufacture, and assembly or building of an object, as well as the number of failures occurring
during its life. Thus, FMEA reduces the total costs and increases the safety, reliability, lifetime,
and quality of the object. Very often, the design is improved.
Further details on FMEA can be found in the literature, e.g. [1 – 3]. FMEA has been incorporated
into reliability standards, such as IEC 60812, and also commercial computer programs for
FMEA are available, although the creation of own, purpose-tailored programs is easy.
A variant of FMEA exists, called FMECA (failure mode, effects, and criticality analysis), which
puts more emphasis on the assessment of consequences of possible failures [3]. The principle,
however, is the same as above.
Example 1
In a Failure Modes and Effects Analysis, done during the design of a home appliance, five
possible failure modes were revealed. Their severity (S), probability of occurrence (O), and
possibility of early detection (D) were classified as shown in the table below. Calculate the
RPN for each failure mode and the resultant RPN for the whole appliance.
Solution
The individual values of RPN (=S×O×D) and the resultant value (=∑RPNi) are written in italics.
Failure Modes and Effects Analysis 95
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Failure mode no. Severity Occurrence Detectability RPN
1 8 6 2 96
2 4.5 7 2 63
3 6 3 4 72
4 2.5 4 7 70
5 5 6 3 90
Total RPN 391
Author details
Jaroslav Menčík
Address all correspondence to: jaroslav.mencik@upce.cz
Department of Mechanics, Materials and Machine Parts, Jan Perner Transport Faculty,
University of Pardubice, Czech Republic
References
[1] McDermott R E, Mikulak R J, Beauregard M R. The Basics of FMEA. Portland: Re‐
source Engineering; 1996. 90 p.
[2] Menčík J. Failure mode and effect analysis - a tool for increasing the reliability and
quality of constructions. In: Proc. Int. Conf. „Quality and Reliability in Building In‐
dustry“, Levoča, 24. - 26. 10. 2001. Košice: Technical University of Košice; p. 346 – 351
[3] O´Connor Patrick D T. Practical Reliability Engineering. 4th ed. Chichester: John Wi‐
ley & Sons; 2002. 513 p.
