FMEA Criticality analysis

Criticality Analysis Slide 1

http://www.fmea-fmeca-com

Criticality – Mil-Std-1629 Approach

CRITICALITY is a measure of the frequency of occurrence of an effect.

– May be based on qualitative judgement or

– May be based on failure rate data (most common)



Criticality Analysis

Qualitative analysis:

– Used when specific part or item failure rates are not available.

Quantitative analysis:

– Used when sufficient failure rate data is available to calculate criticality numbers.



Qualitative Approach

Because failure rate data is not available, failure mode ratios and failure mode probability are not used.

The probability of occurrence of each failure is grouped into discrete levels that establish the qualitative failure probability level for each entry based on the judgment of the analyst.

The failure mode probability levels of occurrence are:

– Level A - Frequent

– Level B - Reasonably Probable

– Level C - Occasional

– Level D - Remote

– Level E - Extremely Unlikely



Quantitative Approach

Failure Mode Criticality (CM) is the portion of the criticality number for an item, due to one of its failure modes, which results in a particular severity classification (e.g. results in an end effect with severity I, II, etc...).



Mil-Std-1629 Severity Levels

Category I - Catastrophic: A failure which may cause death or weapon system loss (i.e., aircraft, tank, missile, ship, etc...)

Category II - Critical: A failure which may cause severe injury, major property damage, or major system damage which will result in mission loss.

Category III - Marginal: A failure which may cause minor injury, minor property damage, or minor system damage which will result in delay or loss of availability or mission degradation.

Category IV - Minor: A failure not serious enough to cause injury, property damage or system damage, but which will result in unscheduled maintenance or repair.




The quantitative approach uses the following formula for Failure Mode Criticality:

Cm = βαλpt

Where Cm = Failure Mode Criticality

β = Conditional probability of occurrence of next higher failure effect

α = Failure mode ratio

λp = Part failure rate

T = Duration of applicable mission phase



Criticality Analysis Example

A resistor R6 with a failure rate of .01 failures per million hours is located on the Missile Interface Board of the XYZ Missile Launch System. If the resistor fails, it fails open 70 % of the time and short 30 % of the time. If it fails open, the system will be unable to launch a missile 30 % of the time, the missile explodes in the tube 20 % of the time, and there is no effect 50 % of the time. If it fails short, the performance of the missile is degraded 50 % of the time and the missile inadvertently launches 50 % of the time. Mission time is 1 hour.

λp = 0.01 in every case

α = 0.7 for open

β = 0.3 for unable to fire

β = 0.2 for missile explodes

β = 0.5 for no effect

α = 0.3 for short

β = 0.5 for missile performance degradation

β = 0.5 for inadvertent launch

Cm for R6 open resulting in being unable to fire is (.3)(.7)(.01)(1)=0.0021

Cm for R6 open resulting in a missile explosion is (.2)(.7)(.01)(1)=0.0014

Cm for R6 open resulting in no effect is (.5)(.7)(.01)(1)=0.0035

Cm for R6 short resulting in performance degradation is (.5)(.3)(.01)(1)=0.0015

Cm for R6 short resulting in inadvertent launch is (.5)(.3)(.01)(1)=0.0015




Item Criticality (Cr) is the criticality number associated with the item under analysis. For a mission phase, Cr is the sum of the item’s failure mode criticality numbers, Cm, which result in the same severity classification.




The quantitative approach uses the following formula for Item Criticality within a particular severity level:

Where Cr Item Criticality

n = The current failure mode of the item being analyzed

j = The number of failure modes for the item being analyzed.



Criticality Analysis Exercise

Criticality Analysis:

Determine failure mode criticality values and item criticality values for the R9 resistor, and create an item criticality matrix.




A resistor R9 with a failure rate of .04 failures per million hours is located on the Power Supply Board of the XYZ Missile Launch System. If the resistor fails, it fails open 70 % of the time and short 30 % of the time. If it fails open, the system will be unable to launch a missile 30 % of the time and there is no effect 70 % of the time. If it fails short, the performance of the missile is degraded 100 % of the time. Mission time is 1 hour.

λp = __ in every case α = __ for open β = __ for unable to fire β = __ for no effect α = __ for short β = __ for missile performance degradation Cm for R9 open resulting in being unable to fire is ___ Cm for R9 open resulting in no effect is ___ Cm for R9 short resulting in performance degradation is ___







Item

Cri

tica

lity

Severity Levels



Criticality Analysis - Answers

A resistor R9 with a failure rate of .04 failures per million hours is located on the Power Supply Board of the XYZ Missile Launch System. If the resistor fails, it fails open 70 % of the time and short 30 % of the time. If it fails open, the system will be unable to launch a missile 30 % of the time and there is no effect 70 % of the time. If it fails short, the performance of the missile is degraded 100 % of the time. Mission time is 1 hour.

λp = 0.04 in every case α = 0.70 for open β = 0.30 for unable to fire β = 0.70 for no effect α = 0.30 for short β = 1.00 for missile performance degradation Cm for R9 open resulting in being unable to fire is 0.0084 Cm for R9 open resulting in no effect is 0.0196 Cm for R9 short resulting in performance degradation is 0.012







R9(2)

R9(4)

R9(3)

Cri

tica

lity

Nu

mb

er x

10-6

Severity Levels

Date post:	20-Jan-2015
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FMEA Criticality analysis

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