Failure Mode and Effect Analysis (FMEA)

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.

FMEA The aim of FMEA is to anticipate: What might fail What effect this failure would have What might cause the failure

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

Where Does FMEA Occur? Concept FMEA Design FMEA Process FMEA Planning Development System-Level Design Detail Design Testing and Refinement Production Ramp-Up Concept FMEA Design FMEA Process FMEA

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

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

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

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

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

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

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

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

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

DFMEA Severity Table Severity of Failure Rank Hazardous – No warning: Unsafe operation, without warning 10 Very high: Product inoperable; loss of primary function 8, 9 High: Product operable, but at a reduced level 6, 7 Low: Product operable; comfort or convenience items at reduced level 4, 5 Minor: Fit/finish, squeak/rattle don’t conform; average customer notices 2, 3 No effect 1

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

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

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)

Example DFMEA Occurrence Table Probability of Failure Failure Rates Rank Very High: Failure almost inevitable > 1 in 2 10 1 in 3 9 High: Repeated failures 1 in 8 8 1 in 20 7 Moderate: Occasional failures 1 in 80 6 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”

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

Criteria: Likelihood of Detection DFMEA Detection Table Detection Criteria: Likelihood of Detection Rank Absolute Uncertainty Design Control does not detect, or there is no Design Control 10 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

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

Non-Intuitive Statistical Properties of the RPN Scale Incorrect Assumption Actual Statistical Data The average of all RPN values is roughly 500 The average RPN value is 166 Roughly 50% of RPN values are above 500 (median is near 500) 6% of all RPN values are above 500 (median is 105) There are 1000 possible RPN values There are 120 unique RPN values

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

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

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

FMEA Benefits Systematic way to manage risk Comprehensive Prioritizes Problems Based on qualitative assessment Unwieldy Hard to trace through levels Not always followed up

FMEA Levels CFMEA – 1 (Concept) DFMEA – 2 (Design) PFMEA – 3 (Process) 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 Finding the errors in the production line

Relationships (CFMEA, DFMEA, PFMEA)

FMEA

FMEA

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