Failure Mode Analysis

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  • Topic: Reliability engineering, Failure mode and effects analysis, Failure
  • Pages : 13 (1502 words )
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  • Published : March 28, 2013
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FMEA & FTA
•FMEA/FMECA
•Fault Tree Analysis

Arnljot Hoyland, Marvin Rausand, System Reliability Theory,
John Wiley & Sons, Inc., 1994, ISBN 0-471-59397-4

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FMEA (Failure Mode and Effects
Analysis)
• Qualitative analysis
• Purpose: identify design areas where improvements are
needed to meet reliability requirements
• One of the first systematic techniques for failure analysis • Developed in the late 50s to study problems that might
arise from malfunctions of military systems
• Often used as the first step of a system reliability study • An FMEA becomes a failure mode, effects, and criticality analysis (FMECA) if criticalities or priorities are assigned • Information can be found in: MIL-STD-1629, IEC 812,

SAE ARP 926, IEEE std. 352
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FMEA Example Sheet
Syetem:
Ref. drawing no.:

Ref. No.

Description of unit
Function Operational
Mode

Performed by:
Date:

Page: of

Description of Failure
Effect of Failure
Ont the
Failure mode Failure
Detection of On
Mechanisms Failure
Components system
function
in the
subsystem

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Failure rate

Severity
ranking

Risk
reducing
measures

Comments

3

Bottom-up Approach
• Bottom-up (commonly used)
– Starting at the component level and
expanding upward
– The challenge: to decide which component
level the analysis should be conducted, since
the workload could be overwhelming
– A general rule: at a level where failure rate
estimates are available or can be obtained

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Top-down Approach
• The analysis is carried out in one or two stages
• The first stage: split the system into a number of
subsystems, and to identify possible failure
modes and failure effects of each subsystem
based on knowledge of the subsystem’s
required functions or on experience with similar
equipment
• If the subsystem has no failure modes that are
critical, then no further analysis of that
subsystem needs to be performed
• Drawback: It does not ensure that all failure
modes of a subsystem have been identified
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Basic Questions for conducting an
FMEA (according to IEEE std. 352)
• How can each part conceivably fail?
• What mechanisms might produce these mode of
failure?
• What could the effects be if the failures did
occur?
• Is the failure in the safe or unsafe direction?
• How is the failure detected?
• What inherent provisions are provided in the
design to compensate for the failure?
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Criticality Matrix
By combining the failure rate and the severity, we can obtain a ranking of the criticality of the different failure modes

Failure rate
Frequent
Probable
Occational
Remote
Very unlikely

Minor

Severity Group
Major
Critical

Catastrophic

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FTA (Fault Tree Analysis)
• Introduced in 1962 at Bell Lab, for a safety evaluation of the launching system for the intercontinental Minuteman
missile
• Top-down analysis: Start from a “top” event, which is the “root” of the fault tree
• The analysis can go to different levels of details (the leaf are called primal events or basic events)
• FTA may be qualitative, quantitative, or both
• FTA results can be

– A list of possible combinations of environmental factors, human errors, normal events and component failures that may result in a critical event
– The probability that the critical event will occur during a specified time interval
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http://www.relexsoftware.com/reliability/faulttree.asp
What is a Fault Tree Analysis?
A fault tree analysis (FTA) is a deductive, top-down method of analyzing system design and performance. It involves specifying a top event to analyze (such as a fire), followed by identifying all of the associated elements in the system that could cause that top event to occur. Fault trees provide a convenient symbolic representation of the combination of events resulting in the occurrence of the top...
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