Basics for FMEA and FMECA

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RELIABILITY ENGINEERING UNIT

ASST4403

Lecture 4: BASICS CONCEPTS FOR FMEA/FMECA

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Learning outcomes

terms to ones own situation

Demonstrate t e a ty to ana yse an asset, us ng t emetrics, to prepare for FMEA/FMECA

Articulate failures or situations that are less suitable to

use FMEA/FMECA for

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,effect analysis)?

A FMEA is a systemised group of activities intended

a) recognise and evaluate the potential failure of a,

b) identify actions which could eliminate or reducee c ance o a po en a a ure occurr ng, an

c) document the process.SAE J 1739 [3]

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BASIC CONCEPTS FOR FMEA/FMECA

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The purpose of FMEA/FMECA

(FMECA = failure modes, effect and criticality analysis )

Identify failures having unwanted effects on systemoperation

Satisfy contractual requirements of a customer

Allow improvements of system reliability

Allow im rovements of s stem maintainabilit

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Identification/evaluation of all unwanted effects within

Determination of criticality/priority foraddressing/mitigating each failure mode

Classification of the failure modes re ardin detection,

diagnostics, testability, , repair, maintenance, logistics

measures of the severity and probability

failure modes

Support the development of maintenance plan to reducethe likelihood of failure 6


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Failure mode

One of the possible states of a failed item, for a given

Example of a set of general failure modes

1 Failure during operation

2 Failure to operate at a prescribed time

3 Failure to cease operation at a prescribed time

4 Premature operation

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Class question:

What are the major failure modes of a car for you?

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Failure mode determination

The use of the system

The particular system element involved

The operational specification

The time constraints

e envronmen a s resses

The operational stresses

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Failure causes

Not always necessary to identify/describe depending

on t e a ure e ects an sever ty May be determined by analysing field failures or test

units. When design is new, expert opinion may be used

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Class question:

What are the causes to the major failure modes ofe car

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Failure effects

Defined as the consequence of a failure mode in terms,

May be caused by one or more failure modes of one or

Consequences of each failure mode should be identified,evaluated and recorded

Consider maintenance activities and s stem obectives

Evaluate also the effect on higher level at each level

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Example: a set of failure effects (for amotor vehicle starter

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Example: a failure effects probability

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The effects of the failure mode on the system item under

cons era on

Consequences of each possible failure on the output ofthe item should be considered

alternative provisions or devising recommendedcorrective actions

Example

If a battery gets flat, only its output will terminate, sothe local effect will be no supply of current/voltage.

u e a ery ea s, may ave ar more severe

and far-reaching effect. 15


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Relationship between failure modes andfailure effects in a s stem hierarch

,depend on the level of analysis and system failurecriteria

The failure effects identified at the lower level may

The failure modes at the lower level may become thea ure causes a e g er eve

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Class question:

modes?

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Detection methods

For each failure mode, one must determine the waye a ure s e ec e an e means y w c e

user/maintainer is made aware of the failure

It may be implemented by An automatic feature of the design (built-in test) Special checkout procedure before operating the

system

At start up of the system or continuously during

operation or at prescribed intervals

Failure detection and its annunciation must preventhazardous operating conditions21


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Failure mode detection evaluation criteria

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Class question:

How are the failure modes on your car detected?

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Failure compensating provisions

reduce the effect of the failure modes

Redundant items allowing continued operation i one ormore elements fail

Alternative means of operation

Means permitting effective operation or limiting damage

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compensating provisions

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Severity is an assessment of the significance of thea ure mo e s e ec on em opera on

The classification is dependent on the application

To do the assessment, consider

Nature of the system related to the failure effect on usersor environment

The functional performance of the system

Government or industry safety requirements

Requirements implied by warranty 26


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Example of severity classification for end

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Failure mode severity (SAE J1739)

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Risk/criticality matrix

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SINGLE POINT/COMBINATION OF

INDEPENDENT/DEPENDENT

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When FMEA/FMECA is less

su ta e Difficult and tedious for the case of complex systems that

have multiple functions involving different sets of system

components Presenting any relationships between individual or

groups of failure modes or causes of failure modes

since the main assumption of such analysis isindependency of failure modes

so ware ar ware uman n erac ons aninterdependency

involving complex failure logic and redundancy

ommon cause a ures

Cascading failures32


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Common cause failures (CCF)

A dependent failure in which two or more component,

interval, and are a direct results of a shared cause-

A CCF may be caused by

Common design or material deficiency Common installation error

Common harsh environment

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Cascading failures (propagating failure)

Multiple failures initiated by the failure of one component

effect

w - u w

fails

Components influence each other through internalenvironment e.g. one component failure may lead to

. .,increased pressure, temperature, humidity etc

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asca ng a ures

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Basics for FMEA and FMECA

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