Design failure mode effect analysis FMEA

8/10/2019 Design failure mode effect analysis FMEA

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Design failure mode effect

analysis

Dr. Ir. Muhammad Sabri


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Failure mode and effects analysis

Objective: To identify potential failure modes based on

past experience with similar products orprocesses.

To design those failures out of the systemwith the minimum of effort and resourceexpenditure.

Reducing development time and costs.

To estimate qualitative risk trough quantifynumbers of evaluation and show preventivemeasures of potential failure


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FMEA USAGE

To determine possible design and processfailure modes and sources of potentialvariation in manufacturing, assembly,

delivery, and all service processes To detect variations in customer usage;

potential causes of deterioration overuseful product life; and potential process

issues such as missed tags or steps,shipping concerns, and servicemisdiagnosis.


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DFMEA Steps


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FMEA Preparation stepsFormcolumn Work steps

Systems/Features Structure analysis

Potential failureFailure analysisPotential following failure

Cause of potential failure

Current conditionStructure analysis

Provided test measures

Risk assessment

Risk number 'occurrence'

Risk number 'concern'

Risk number 'detection'

Risk number precedence

Recommended correction

System improvementLiability

Improve conditionMeasures taken

Risk assessment

Risk number 'occurrence'

Risk number 'concern'

Risk number 'detection'

Risk number precedence


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FMEA

Eff S f ff d f d R


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Effect Severity of effect defined Rating

None No effect 1

Very minor Minor disruption to production line; a portion (100%) of the product may

have to be reworked on line but in station; fit/finish/squeak/rattle item does

not conform; defect noticed by discriminating customers

2

Minor Minor disruption to production line; a portion (100%) of the product may

have to be reworked on line but out of station; fit/finish/squeak/rattle item

does not conform; defect noticed by average customers

3

Very low Minor disruption to production line; product may have to be sorted and a

portion (100%) reworked; fit/finish/ squeak/rattle item does not conform;

defect noticed by most customers

4

Low Minor disruption to production line; 100% of product 5

may have to be reworked; vehicle/item operable, but some

comfort/convenience item(s) operable at reduced level of performance;

customer experiences some dissatisfactionModerate Minor disruption to production line; a portion (100%) may have to be

scrapped (no sorting); vehicle/item operable, but some

comfort/convenience item(s) inoperable; customers experience discomfort

6

High Minor disruption to production line; product may have to be sorted and a

portion (100%) scrapped; vehicle operable, but at a reduced level of

performance; customer dissatisfied

7

Very high Major disruption to production line; 100% of product may have to bescrapped; vehicle/item inoperable, loss of primary function; customer very

dissatisfied

8

Hazardous: with May endanger operator ; failure mode affects safe vehicle 9

warning operation and/or involves noncompliance with govern-ment regulation;

failure will occur with warning

Hazardous: May endanger operator ; failure mode affects safe vehicle 10

without warning operation and/or involves noncompliance with government regulation;

failure will occur without warning


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Probability of failure Occurrence RatingVery highpersistent

failures 100 per 1000 vehicles/items (10%) 10

50 per 1000 vehciles/items (5%) 9

Highfrequent failures 20 per 1000 vehicles/items (2%) 8

10 per 1000 vehicles/items (1%) 7

Moderateoccasional

failures

5 per 1000 vehicles/items (0.5%) 6



Lowrelatively few failures 0.5 per 1000 vehicles/items (0.05%) 3

0.1 per 1000 vehicles/items (0.01%) 2


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Detection Likelihood of detection Rating

Almost certain Design control will almost certainly detect a potential 1

cause/mechanism and subsequent failure mode

Very high Very high chance design control will detect a potential 2


High High chance design control will detect a potential 3


Moderately high Moderately high chance design control will detect a 4

potential cause/mechanism and subsequent failure mode

Moderate Moderate chance design control will detect a potential 5cause/mechanism and subsequent failure mode

Low Low chance design control will detect a potential cause/ 6

mechanism and subsequent failure mode

Very low Very low chance design control will detect a potential 7


Remote Remote chance design control will detect a potential 8


Very remote Very remote chance design control will detect a potential 9


Absolute Design control will not and/or cannot detect a potential 10

uncertainty cause/mechanism and subsequent failure mode; or

there is no design control


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The fundamentals of an FMEA

inputs


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FMEA FormCompany

Failure mode and effects analysisPart Name Part Number

FMEA Construction FMEA Process Model/System/Manufacture Technical

modification status

confirmation by concerneddepartments and/or supplierName/Dept./Supplier created by Date revised date

Systems

features

Failure

potential

Consequences

potential failure

D Cause of

potential

failure

current condition recommended

measurement

place

responsibility Improved condition

Intended

measurement

Oc

currence

Con

sequence

Detection

Riskpriorit

ynumbers(RPN)

Intended

measurement

Oc

currence

Con

sequence

Detection

Riskpriorit

ynumbers(RPN)


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PFMEA

is used to analyze manufacturing, assembly, or any otherprocesses such as those identified as transactional

focus is on process inputs. Software FMEA documents andaddresses failure modes associated with software functions.

Identifying potential manufacturing/assembly or production

process causes in order to place controls on eitherincreasing detection, reducing occurrence, or both

Prioritizing the list of corrective actions using strategies suchas mitigation, transferring, ignoring, or preventing the failuremodes

Documenting the results of their processes

Identifying the special potential process variables (PVs), froma failure standpoint, which need special controls


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DFMEA

Design Failure Mode Effects Analysis (DFMEA) is thedisciplined analysis of potential failures in the design

The DFMEA is a team effort usually conducted by afacilitator who collects the team's input and guidesthe processes.

the process will identify the key functional items,potential failure modes, their root causes and anycorrective action.

The process leads to a better design and can helpguide the testing and validation process. If used

correctly, it can provide context to the data thatphysical testing will produce so the behavior of thecompany can be influenced.


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DFMEA

is used to analyze designs before they arereleased to production

should always be completed well in

advance of a prototype build input to DFMEA is the array of functional

requirements

The outputs are (1) list of actions toprevent causes or to detect failure modesand (2) history of actions taken and futureactivities


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DFMEA

Function Estimating the effects on all customer segments

Assessing and selecting design alternatives

Developing an efficient validation phase withinthe DFSS algorithm

Prioritizing the list of corrective actions usingstrategies such as mitigation, transferring, ignoring,or preventing the failure modes

Identifying the potential special design parameters(DPs) in terms of failure

Documenting the findings for future reference


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Basic DFMEA parts:

Functional Item: The functional feature ordesign feature from the Bill of Material

(BOM).

Potential Failure Mode: Key word is potential.What failure modes could the feature

experience? Source for this is engineering

experience, warranty data and pureimagination.


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Basic DFMEA parts

Potential Effect(s) of Failure" What are the results ofthe failure on the function or behaviour of theproduct? Often the failure itself is not visible, butthe functional effect will be apparent. Forexample, a sealed bearing may have excessive

wear, but the failure is not visible externally. Theeffect of increased friction is a slowing of themotor or an increase in the power draw. Theeffects are key to designing instrumentation andoperational checks for tests to verify theexistence of failure modes.

Severity" How bad are the consequences of thefailure?


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Basic DFMEA parts

Criticality: How critical to the function of thedevice is the failure mode?

Potential Causes: The key word here is

mechanism. What can break the product?


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Basic DFMEA parts

Occurrence: What is the likelihood of failure?

Controls: What is the current design effort to

prevent the design from failing?

Detectability: How well can the failure modebe detected if it exists?


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Basic DFMEA parts

RPN: Risk Priority Number is the multiple ofall four ratings: Severity x Criticality x

Occurrence x Detectability.

Responsibility: Who will take responsibility forimplementing the recommended action?

Target Date: When will the recommended

action be completed?


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Basic DFMEA parts

Process of DFMEA should start at theconceptual design process

should be kept current throughout the

process and lead into the Process FailureMode Effects Analysis (PFMEA)

Use as a foundation for any follow on

development process


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Good practice of DFMEA

Initiation:A DFMEA should start as soon as the design

development process starts. At this stage, thedetails of specific design features may not beavailable and the potential failure modes willbe naturally broad. Going through thedisciplined process of capturing all potentialfailure modes at this stage will help to drivethe development more efficiently. TheDFMEA at this point can be used to developthegeneral outline of the validation plan.


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Design Iteration:

As the design is iterated, the DFMEA

should be kept current. This will include

adding details as design features aredeveloped and changing details as design

changes are made. The DFMEA at this

point should be used to begin planningthe details of the validation plan.


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Design Validation:.At this point the design should be nearly

complete The DFMEA should reflect all

the details of the design and thecorresponding potential failure modes.Most of the changes to the DFMEAshould now be reflecting the closing of

recommended actions. The validation planshould reflect checks on all the keyassumptions in the DFMEA.


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Production Validation:

The DFMEA should still reflect minor changes

implemented to improve or correct

production problems. Since manyDFMEA's are used as the basis for the

next project, this step is critical. Validation

should reflect checks on the changes.


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Production Run:

The DFMEA should still reflect minor changes

implemented to improve or correct

production problems. Since manyDFMEA's are used as the basis for the

next project, this step is critical. Validation

should reflect checks on the changes.


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Why is it important to have the DFMEAfit with the validation plan? Rememberthat the DFMEA is based on theindividual's assumptions of what thepotential failures are. The design will bedeveloped based on these assumptions.The purpose of validation is to validate

that the design will behave in the realworld as well as it does in the designer'smind


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Hypothesis and the DFMEA

If you consider the DFMEA from theviewpoint of the scientific method,

a couple of key points about the structureand use of the DFMEA becomes obvious.

The Failure Mode is actually the NullHypothesis. In other words, the potential failure mode is

what the design assumes will not happen.

What is not clear in the standard DFMEAformat is how the accuracy of theassumptions (Hypothesis/Null Hypothesis)are tested.


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The recommended actions may alsocontain other action items relevant to

design changes and other issues.

However, every assumption should haveobjective evidence clearly documented to

support the use of the design control or

the implementation of any contingency.


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Hypothesis and the DFMEA

Hypothesis: Toothbrush bristles will stay in toothbrush head.

Null Hypothesis: Bristles will fall out under some circumstances.

H = No bristle loss determined?

Ho = Bristle loss

If Ho then reject H

If Not (Ho) then accept H

Functional item

Failure mode

Where/how is this

determined

Where is this

Assumed outcomes


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Item

FunctionItem

Potentialoffailurem

ode

Potentialeffectoffa

ilure

Severity

Criticality

Potential

Causes(s)/Mechanism(s)of

Failure

Occurrences

Currentdesigncont

rol

Contingency

D

etectability

RPN

Recommendedaction

Responsibility

Targetdate

1

2

3

If Ho is true based on information Then ContingencyElse Design Control is accepted

H = Functional Item will not Fail because Current Design Controls work

Ho = Functional Item will fail due to Mechanism causing Failure Mode

Ho is accepted or rejected based on Recommended Action


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One logical failure mode to be consideredis that the insert area causes the handle

to split.

The effect of this could be the loss of therubber insert, separation of neck and

cutting or hurting the consumer's hand.

There are many potential causes: impact, thermal cycle, chemical attack/material

incompatibility, fatigue and Sharp radius.


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The critical part of this example is the detection methods.For each potential failure, there is an effect, a mechanismand a corresponding method of detection.

The method of detection should reflect the suspectedmechanism and take advantage of the potential effect todesign a test that will impose the mechanism and monitorfor the effect. For example, the loss of rubber due toimpact can be tested by imparting an impact andmonitoring for rubber loss.

DFMEA could lead to a very large number of discretetests. Just in a brief look at one failure in one design featureresults in four tests. Two of the tests are relatively quick(FEA model, load testing), and two of them could take a

significant amount of time (thermal cycle, chemicalexposure).

it will result in a very exhaustive list of discrete testing.


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To make the DFMEA more effective and tiedmore closely to the validation plan, add acolumn called Contingency next to the CurrentDesign Controls column.

This clearly shows that if the hypothesis iscorrect, the Current Design Controls willremain; if the null hypothesis is true, then theContingency will be tried.

Clearly declaring the contingency allows thedevelopment timeline to reflect the actualdecision based on the information.


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Item

Fu

nction

Item

Potential offailure mode

Potential

effect offailure

Se

verity

Criticality

Potential

Causes(s)/Mechanism(s) of Failure

Occurrences

Current design control

1 Handle split in grip

insert areaLoss of

rubber

3 impact 8 Impact resistant

plastic

FEA model

of impact

from 3

likely

direction

2 3 3 Thermal cycle 8 Thermal set plastic

with stable

material properties

from -30 deg c to

100 deg c

Thermal

cycle

testing

3 3 Chemical attack/material

incompatibility

8 Chemically inert

plastic to mild

alkalis and acids

Chemical

exposure

4 3 Sharp radius 5 All radius must be

greater than 1 mm

5 Cutting or

hurting

consumers

hand

8 8


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Exercise

Fill up all the functional item for the toothbush for the rest of the part

What conclusion can you grab from the

table analysis Is the RPN give you a good measurement

on intended design

Can you eliminated all failure causes usingthis technique?

Design failure mode effect analysis FMEA

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