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TOOLS IN DESIGN FOR RELIABILITY

Preliminary Hazard Analysis (PHA)

Fault Tree Analysis

Failure Mode And Effect Analysis (FMEA)

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PRELIMINARY HAZARD ANALYSIS (PHA)

1. What is the subsystem or item under investigation?

2. What is the mode of operation?

3. What is the hazardous condition and why?

4. What event(s) triggers the hazardous condition?

5. What is the hazardous condition?

6. What event(s) triggers the potential accident?

7. What is the potential accident?

8. What is the possible effect(s) of the accident?

9. What is the classification of the severity of the hazard?

10. What measures are taken to contain or prevent occurences?

PHA is a broad study made in the early stages of design. The PHA consists of

breaking the engineering system down into subsystems or components, and for each

item answering the following questions:

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Classic Hazard Level Matrix

safeware-eng.com

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safeware-eng.com

Severity level classification from an aviation standard:

Class I: CatastrophicUn-survivable accident with hull loss.

Class II: Critical

Survivable accident with less than full hull loss; fatalities possibleClass III: Marginal

Equipment loss with possible injuries and no fatalities

Class IV: NegligibleSome loss of efficiency

Procedures able to compensate, but controller workload likely to be highuntil overall system demand reduced.

Reportable incident events such as operational errors, pilot deviations,surface vehicle deviation.

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PHA WORKSHEETS

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FAULT TREE ANALYSIS

Fault tree analysis is a technique that provides a systematic descriptionof possible occurrences in a system that can result in failure or severe

accident.

The four basic steps involved in developing a fault tree are as follows:

1. Develop the top undesired event of the system to be studied.

2. Develop a thorough understanding of the system under

consideration.

3. Determine the logical interrelationships of higher-level and lower-level

fault events.

4. Construct the fault tree using logical symbols.

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FAULT TREE ANALYSIS

Basic symbols used in fault trees:

Resultant fault event:

a rectangle denotes a fault event that results from a combination of

failure events through the input of a logic gate, such as an AND gate or

an OR gate

Basic fault event:

a circle denotes a basic fault event or the failure of an elementary

component. The values of the parameters, such as failure probability,

unavailability, failure rate, and repair rate, associated with the basic fault

event are obtained from empirical studies or other sources

AND gate:denotes that an output fault event occurs if all of the input fault events

occur

OR gate:

denotes that an output fault event occurs if one or more of the input fault

events occur

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FAULT TREE ANALYSIS

AND OR

Basic

fault events

AND gate OR gate

Gate

Resultant

fault event

All basic fault events must

occur in order for resultant

event to occur

At least one basic fault event

must occur in order for the

resultant event to occur

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The output fault occurrence probabilities forAND gate is:

1AND

mF F

ii

==

FAULT TREE ANALYSIS

F1

F2

FAND

Example:

F1 = 0.1

F2 = 0.05

FAND = 0.1 * 0.05 = 0.005INPUT EVENTS

OUTPUT EVENT

where

FAND

is the probability of occurrence of the AND gate output fault event

m is the number of independent input fault events

Fi

is the probability of occurrence of input fault event i, fori=1, 2, m

AND

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=m

ii

FFO

1R

The output fault occurrence probabilities forOR gate:

where

FOR is the probability of occurrence of the OR gate output fault event

m is the number of independent input fault events

Fiis the probability of occurrence of input fault event i, fori=1, 2, m

Note: For small (i.e. less than 10 percent) occurrence probabilities of

input fault events of the OR gate, the above equation reduces to:

1 (1 )1

ORmF F

ii

==

FAULT TREE ANALYSIS

F1

F2

FOR

Example:

F1 = 0.1

F2 = 0.05

FOR = 1 (1 - 0.1)*(1- 0.05) = 0.145INPUT EVENTS

OUTPUT EVENT

OR

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Example: Develop a fault tree for a system comprising of a windowless room with one switch

and three light bulbs. The switch can only fail to close, and the top undesirable event is the

room without light.

[Dhillon 1996]

FAULT TREE ANALYSIS

OR

OR

AND

FAULT EVENT

BASIC FAULT EVENT

TOP EVENT

BASIC FAULT EVENT

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Example: (cont'd) Assume that the probabilities of occurrence of basic fault events A ,B, C,D. E, and F are 0.1, 0.12,0.15, 0.15, 0.15, and 0.08, respectively.

Calculate the probability of occurrence of the top event (T) (i.e. the room without light).

Probability of having a room

without light is 27%

[Dhillon 1996]

FAULT TREE ANALYSIS

1 (1 )1

1 (1 0.1)(1 0.12) 0.208

O

mF F

ii

===

=

R 1

0.15*0.15*0.15 0.003375

AND

mF F

ii

===

=

OR

OR

AND

1 (1 )1

1 (1 0.208)(1 0.003375)(1 0.08)

0.27

O

mF F

ii

===

R

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Develop a fault tree for a climb. The top undesirable event is not making it to the summit byat least one member of your climbing your team

You climb in a team which can not split.

Team is lead by a guide.

Each team member on your team has 40% probability of failure

Guide has 10% probability of failure

Weather has 20% probability of turning bad

What is the probability of the climb failure?

FAULT TREE ANALYSIS

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OR

0.4 0.4 0.1

0.6761 (1- 0.4) (1- 0.4)(1- 0.1) = 0.676 0.2

OR

0.741 (1- 0.676) (1- 0.2) = 0.74

TWO CLIMBERS PLUS GUIDE

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OR

0.4 0.1

0.4601 (1- 0.4) (1- 0.1) = 0.460 0.2

OR

0.5681 (1- 0.46) (1- 0.2) = 0.568

TWO TEAMS OF SINGLE CLIMBER PLUS GUIDE

version 1

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OR

0.2 0.05

0.241 (1- 0.2) (1- 0.05) = 0.24 0.1

OR

0.3161 (1- 0.24) (1- 0.1) = 0.568

TWO TEAMS OF SINGLE CLIMBER PLUS GUIDE

version 2

Each team member on your teamhas 20% probability of failure

Guide has 50% probability of failure

Weather has 10% probability ofturning bad

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FAILURE MODES AND EFFECTS ANALYSIS (FMEA)

FMEAFailure modes

Failure effects

FMECAFailure modes

Failure effects

Failure criticality

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Failure Modes and Effects Analysis

is a detailed analysis of the malfunctions that can be produced in

the components of an engineering system. Similar to the QFD

approach, FMEA techniques involve charts that are developed,

amended and updated over time.

Primary Goal of Failure Modes and Effects Analysis

is to try to identify and list all possible ways in which product or a

process could fail to conform to its specified requirements.

[McMahon and Bowne 1993]

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FMEA poses the following questions:

What can fail/go wrong with each component of a product?

To what extend it might fail, and what are the potential

hazards produced by the failure?

What steps should be implemented to prevent the failure?

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Types of Failure Modes and Effects Analyses

There are several types of FMEAs, some are used much more often than others.

FMEAs should always be done whenever failures would mean potential harm or

injury to the user of the end item being designed.

The types of FMEA are:

System - focuses on global system functionsDesign - focuses on components and subsystems

Process - focuses on manufacturing and assembly processes

Service - focuses on service functions

http://www.npd-solutions.com/fmea.html

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If performed early in the design process, FMEA supports the product development in

reducing the risk of failure by:

Aiding in the objective evaluation of design requirements and design alternatives

Aiding in the initial DFM and DFA requirements

Increasing the probability that potential failure modes and their effects on system

operation have been considered in the design process

Providing additional information to aid in the planning of thorough and efficient

design improvements and development testing

Providing an open issue format for recommending and tracking risk reducing action

Providing future references to aid in analyzing filed concerns, evaluating design

changes and developing advanced designs

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Top-down, functional approach

This approach is used in early design, before parts have been identified.

The goal here is to look forlogic errors in the expected function and

operation of a product. One identifies a failure mode for the product as a

whole, then traces its causes "down" into subsystems or sub-functions.


Bottom-up, "hardware" approach

This approach is used when specific parts or at least major assemblies

have been designed. The goal here is to look forphysical errors in the

detailed design/manufacture of parts. One identifies a failure mode,

and then follows its effects "up" to the product as a whole in order to

predict how the product will respond to the failure

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Basis steps:

1. A complete list of the components and their function is prepared.

2. From an analysis of the operating and environmental conditions, the failuremechanisms that could affect each component are determined.

3. The failure modes of all components are identified.

4. Each failure mode is analyzed as to whether it has an effect on the entiresystem or product

5. The preventative measures or corrective actions that have been taken tocontrol or eliminate the hazard are listed.

6. The probability of failure of each component is listed, and the probabilitiesof failure of the subassemblies and complete system are caluclated fromreliability theory.


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Risk Priority Number (RPN) -is assigned to each failure mode based on occurrence, severity,

and detection of failure cause.

RPN = R occurrence x R severity x R detection

where 1 < RPN < 1000

RANKING PROCEDURE FOR FMEA

R occurrence

Identify every possible cause of each failure, and rank each cause

according to the likelihood of its occurrence on a scale of 1 to 10:

1 - cause will almost never arise ( ie. 1 in 106)

5 - occasional failure ( ie. 1 in 400)

10 regular occurrence

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Risk of occurrence

http://egweb.mines.edu/eggn491/lecture/FMEA/FMEA%20Homework.htm

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R severity Rate the severityof each possible failure on a scale of 1 to 10

1 - the customer would hardly notice the failure

5 - customer would be made uncomfortable or annoyed by the failure

10 - a major failure such as a significant safety hazard or non-compliance with a

government regulation






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Severity of effect of failure

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R detection

List current technologies being used to detect a failure cause and assign a likelihood of

detection prior to failure to each failure based on a scale of 1 to 10:

1 almost certain detection

10 practically undetectable mode






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Probability of detection

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Risk Priority Number


RPN = 1 failure is highly unlikely and unimportant

RPN = 30 it is OK

RPN = 100 failure will occur

RPN = 1000 hazardous and harmful failure will occur

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http://www.suppliermanager-online.com/training/corporation/fmea_training.pdf

STEPS IN FMEA AGAIN

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FMEA OF A CAR FRONT DOOR

Front door Corroded

interior

Deteriorate

dlifeleadin

gto

unsatisfa

ctoryappearance

andin

teriorhardware

malfun

ction

Upper edge of

wax too low 6 7 7 294

Wax layer

too thin

Durability

test T-118

4 7 7 196

Failure mode

(how part can fail?)

Failure effect on product/system

7 x 4 x 7=196

7 x 6 x 7=294

Causes of failure

Documents

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