Failure Mode and Effect Analysis

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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 products 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 Development
System-Level Design
Detail Design
Testing and Refinement
Production Ramp-Up
Planning
Concept FMEA
Design FMEA
Process FMEA
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Design Project FMEA

• Design FMEAs 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 (doesnt 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 penlights
  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
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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
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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
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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
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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
• Dont 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
• S10, O1, D2 RPN 20
• S1, O5, D4 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
• Systematic way to manage risk
• Comprehensive
• Prioritizes

• Problems
• Based on qualitative assessment
• Unwieldy
• Hard to trace through levels
• Not always followed up

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FMEA Levels

• CFMEA 1 (Concept)
• Failures in the concept (inability to achieve
  performance)
• Detection
• Ability to find the failures (i.e., use of
  historical data, early models, etc.)

• DFMEA 2 (Design)
• Failures in current design (performance)
• Detection
• Highlighting failures during the detail design
  phase
• 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