Failure Mode and Effect Analysis
(FMEA)
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What is Failure Mode and
Effect Analysis (FMEA)?
FMEA is a quality audit procedure which
aims to anticipate failure in a product’s
functional design.
“Failure” may be the result of design,
manufacturing process, or use.
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FMEA
The aim of FMEA is to anticipate:
What might fail
What effect this failure would have
What might cause the failure
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FMEA
The significance of the failure is assessed
against:
The probability of failure
An assessment of the severity of the effect of
that failure
The probability of existing quality systems
spotting the failure before it occurs
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Where Does FMEA Occur?
Concept System-Level Detail Testing and Production
Planning Development Design Design Refinement Ramp-Up
Concept Design Process
FMEA FMEA FMEA
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Design Project FMEA
Design FMEA’s should cover:
all new components
carried over components in a new environment
any modified components
Mandatory on all control and load carrying
parts
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Design Project FMEA
“Failure” - a component or system not
meeting or not functioning to the design
intent
Design intent - may be stated in terms of
MTBF, load or deflection, coat thickness,
finish quality, etc.
“Failure” need not be readily detectable by a
customer
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FMEA Process
Identify a failure mode
Determine the possible effects or
consequences of the failure
Assess the potential severity of the effect
Identify the cause of failure (take action!)
Estimate the probability of occurrence
Assess the likelihood of detecting the failure
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Failure Mode
Failure mode - the manner in which a component
or system failure occurs (doesn’t meet design
intent)
Potential failure modes
Complete failure
Partial failure
Intermittent failure
Failure over time
Over-performance failure
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Failure Mode
Question to be asked: “How could the component
or system fail?”
Examples: Consider failure modes of a penlight’s
function defined as “Provide light at 3 0.5
candela.”
No light
Dim light
Erratic blinking light
Gradual dimming light
Too bright
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Failure Mode - Identification
List potential failure modes for the particular
part or function
assume the failure could occur, however unlikely
Sketch free-body diagrams (if applicable),
showing applied/reaction loads. Indicate
location of failure under this condition.
List conceivable potential causes of failure
for each failure mode
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Failure Mode – Effects
For each failure mode, identify the potential
downstream consequences of each failure mode
(the Effects)
Procedure for Potential Consequences
Beginning with a failure mode (FM-1) – list all its
potential consequences
Separate the consequences that can result when FM-1
occurs: “Effects of FM-1”
Write additional failure modes for remaining, depending
on circumstances
Add these to list of failure modes
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Failure Mode – Effects
Team brainstorms failure modes and effects
Example: Analyzing penlight bulb
Premature burnout – user could trip, fall, be
injured
While used in eye examination, bulb might
explode, resulting in injury
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Failure – Severity
To analyze risk, must first quantify the Severity of
the Effects
Assume that all Effects will result if the Failure
Mode occurs
Most serious Effect takes precedence when
evaluating risk potential
Design and process changes can reduce severity
ratings
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DFMEA Severity Table
Severity of Failure Rank
Hazardous – No warning: Unsafe 10
operation, without warning
Very high: Product inoperable; loss of 8, 9
primary function
High: Product operable, but at a 6, 7
reduced level
Low: Product operable; comfort or 4, 5
convenience items at reduced level
Minor: Fit/finish, squeak/rattle don’t 2, 3
conform; average customer notices
No effect 1
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Failure Mode – Causes
After Effects and Severity addressed, the Causes of
Failure Modes must be identified
In Design FMEA (DFMEA), design deficiencies
that result in a Failure Mode are Causes of failure
Assumes manufacturing and assembly specifications are
met
Process FMEA (PFMEA) has similar investigation
Causes are rated in terms of Occurrence
Likelihood that a given Cause will occur AND result in
the Failure Mode
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Failure Mode - Occurrence
Estimate the probability of occurrence on a
scale of 1 -10 (consider any fail-safe controls
intended to prevent cause of failure)
Consider the following two probabilities:
probability the potential cause of failure will
occur
probability that once the cause of failure occurs,
it will result in the indicated failure mode
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Failure Occurrence - Ranking
Occurrence Criteria Ranking
Remote: unreasonable to expect failure (1)
Low: similar designs have low failure rates (2,3)
Moderate: similar designs have occasional moderate
failure rates (4, 5, 6)
High: similar designs have failed in the past (7,8,9)
Very high: almost certain failure, in major way (10)
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Example DFMEA Occurrence Table
Probability of Failure Failure Rates Rank
Very High: Failure almost > 1 in 2 10
inevitable 1 in 3 9
High: Repeated failures 1 in 8 8
1 in 20 7
Moderate: Occasional 1 in 80 6
failures 1 in 400 5
1 in 2000 4
Low: Relatively few failures 1 in 15,000 3
1 in 150,000 2
Remote: Failure unlikely < 1 in 1,500,000 1
Current Controls
Design controls grouped according to
purpose
Type 1: Controls prevent Cause or Failure Mode
from occurring, or reduce rate of occurrence
Type 2: Controls detect Cause of Failure Mode
and lead to corrective action
Type 3: Controls detect Failure Mode before
product reaches “customer”
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Detection
Detection values are associated with Current
Controls
Detection is a measure of Type 2 Controls to
detect Causes of Failure, or ability of Type 3
Controls to detect subsequent Failure Modes
High values indicate a Lack of Detection
Value of 1 does not imply 100% detection
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DFMEA Detection Table
Detection Criteria: Likelihood of Detection Rank
Absolute Design Control does not detect, or 10
Uncertainty there is no Design Control
Very Remote Very remote chance Control will detect 9
Remote Remote chance Control will detect 8
Very Low Very low chance Control will detect 7
Low Low chance Control will detect 6
Moderate Moderate chance Control will detect 5
Moderately High Mod. High chance Control will detect 4
High High chance Control will detect 3
Very High Very high chance Control will detect 2
Almost Certain Control almost certain to detect 1
Reducing Risk
The fundamental purpose of the FMEA is to
recommend and take actions that reduce risk
Adding validation or verification can reduce
Detection scoring
Design revision may result in lower Severity and
Occurrence ratings
Revised ratings should be documented with
originals in Design History File
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Design Project FMEA - Results
Risk Priority Number (RPN)
RPN = Severity x Occurrence x Detection
Mathematical product of the seriousness of a
group of Effects (Severity), the likelihood that a
Cause will create the failure associated with the
Effects (Occurrence), and an ability to detect the
failure before it gets to the customer (Detection)
Note: S, O, and D are not equally weighted in
terms of risk, and individual scales are not linear
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Non-Intuitive Statistical
Properties of the RPN Scale
Incorrect Actual Statistical
Assumption Data
The average of all RPN The average RPN value
values is roughly 500 is 166
Roughly 50% of RPN 6% of all RPN values are
values are above 500 above 500
(median is near 500) (median is 105)
There are 1000 possible There are 120 unique
RPN values RPN values
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Criticality
Criticality = Severity x Occurrence
High Severity values, coupled with high
Occurrence values merit special attention
Although neither RPN nor Criticality are
perfect measures, they are widely used for
risk assessment
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Interpreting the RPN
No physical meaning to RPN
Used to “bucket problems”
Don’t spend a lot of time worrying about what a
measure of “42” means
Rank order according to RPN
Note that two failure modes may have the same
RPN for far different reasons:
S=10, O=1, D=2: RPN = 20
S=1, O=5, D=4: RPN = 20
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Actions
Actions taken are the important part of FMEA
Change design to reduce
Severity (redundancy?)
Occurrence (change in design, or processes)
Detection (improve ability to identify the problem before
it becomes critical)
Assign responsibility for action
Follow up and assess result with new RPN
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FMEA
Benefits Problems
Systematic way to Based on qualitative
manage risk assessment
Comprehensive Unwieldy
Prioritizes Hard to trace through
levels
Not always followed
up
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FMEA Levels
CFMEA – 1 (Concept) DFMEA – 2 (Design)
Failures in the concept Failures in current design
(inability to achieve (performance)
performance) Detection
Detection Highlighting failures during
Ability to find the failures the detail design phase
(i.e., use of historical data,
early models, etc.)
PFMEA – 3 (Process)
Failures in production
process
Detection
Finding the errors in the
production line
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Relationships (CFMEA, DFMEA,
PFMEA)
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FMEA
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FMEA
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END
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