Introduction to Engineering Reliability engineering Reliability Concepts
17022 Reliability
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Transcript of 17022 Reliability
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Monday, 22 April 2013 LOVELY PROFESSIONAL UNIVERSITY By- H imanshu Gupta
RELIABILITY
DEFINITION
Reliability is the ability of an item to perform its intended function
under stated operation conditions for a given period of time. The definition stresses on four significant elements probability,
intended function, time and operating conditions.
Probability
Consideration of variation makes reliability a probability. It ispossible to identify the frequency distribution of an item, whichpermits prediction of life of the item, e.g., the probability of an itemfunctioning is 0.85 for 60 hours indicates that only 85 times out of100, we would expect the item to be functioning for a period of 60hours.
Intended Function
For an item to be reliable, it must perform a certain functionssatisfactorily when called upon to do.
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RELIABILITY
Time
Time is the most important factor in the assessment of reliability,since it represents a measure of period during which one can expecta certain degree of performance from an item.
Stated Conditions
The applications and operating circumstances under which an item isput to use is an important component of reliability. As the operatingconditions change, the reliability of an item also changes. Operatingconditions such as temperature, humidity, torque, and corrosive
atmosphere all have a definite effect on performance.
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RELIABILITY
Failure
Failure of an item represents unreliability. Thus, to compute the
reliability of an item, it is necessary to understand the concept offailure. A deviation in the properties of an item from the prescribedconditions is considered as fault. A state of fault is denoted asFailure.
An item is considered to have failed under one of the followingconditions:
When it becomes completely inoperable.
When it is still operable, but no longer able to perform a requiredfunction.
When a serious deterioration makes the item unsafe for its continued
use.
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RELIABILITY
Causes of Failure
Some of the causes of failures are:
Deficiencies in design.
Improper selection of process and manufacturing technique.
Lack of knowledge and experience.
Error of assembly.
Improper service conditions.
Inadequate maintenance.
Variation in environmental and operating conditions.
Human errors.
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RELIABILITY
Nature/Modes of Failures
The different modes of failure are:
1) Catastrophic Failures: In this case, a normally operating itemsuddenly becomes inoperative. Example: Blowing of a fuse orelectric bulb.
2) Degradation (Creeping) Failures: These failures occur graduallybecause of change in some parameter with time. Example, change inresistance will affect the performance of a resistor.
3) Independent Failures: These are the failures, which occurindependently and does not depend on failure of the others.
4) Secondary Failures: A secondary failure occurs as a result of someprimary failure. For example tsunami occurred due to earthquake.
5) Failure due to improper handling and misuse: These are causedmainly due to certain factors like overloading, stressing beyond thecapacity.
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Bath Tub Curve (Phases of Failures)
Analysis of failure data has shown that failures in general can begrouped into different modes depending upon the nature of thefailure. When a large number of units are put into operation, it islikely that there is a large number of failures initially. Thesefailures are called Initial failures or Infant Mortality.
After the initial failures, for a long period of time of operationfewer failures are reported but it is difficult to determine theircauses. The failures during this period are often called random orcatastrophic failures. This is a period of normal operation.
As the time passes, the units get worn out due to wear and tearand begin to deteriorate. Here in this period, the failures are dueto wear and tear and due to ageing. This region is called the wearout region.
This curve is also called as Life Cycle Curve.
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Bath Tub Curve (Phases of Failures)
7
F
A
I
L
U
R
E
R
A
T
E
Time (hours, miles, cycles, etc)
EARLY LIFE
(burn-in or
break-in or
infant mortality
period)
(failure rate
decreases with time)
USEFUL LIFE
(or normal life)
(failure rate approx. const)
WEAROUT LIFE
(failure rate
increases with time)
Monday, 22 April 2013 LOVELY PROFESSIONAL UNIVERSITY By- H imanshu Gupta
Bath Tub Curve (Phases of Failures)
Early Failures
These failures occur at the beginning due to the probability ofdefective design, manufacturing or assembly and qualitycontrol techniques during manufacturing.
These are eliminated by debugging or burn in process. The weakand substandard products/components that fail during early hoursof system operation are replaced by good or tested components.Debugging is a method of accelerating the completion of earlyfailures by operating the system continuously for number ofhours, correcting them and then releasing the system for actualuse.
Debugging is done generally prior to dispatch to the user toensure the detection and elimination of early failures.
Warranty is based on the concept of early failures.
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Bath Tub Curve (Phases of Failures)
Catastrophic (Chance) Failures
These failures are predominant during actual working of the
system. They occur randomly and unexpectedly. The failure rate is fairly constant. These are caused due to sudden
stress accumulation beyond the design strength of the material.
This phase is called the useful life of the component. The failuresat this stage can be minimised by introducing redundancy in thesystem.
Wear Out Failures
The item is more likely to fail due to wear and tear and thenumber of failures will be high.
This is a typical ageing problem. Proper care and maintenancewill reduce the failures at this stage.
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Measures of Reliability/ Quantification of Reliability
1) Failure Rate
Failure rate is expressed in terms of failures per unit time i.e. asfailures per hour, or failures per 100 or 1000 hours. Failure rate isthe ratio of number of failures (f) during a specified test intervalto the total test time of items undergoing test.
= f/T
= Failure rate
f = Number of failures during the test interval
T= Total test time When the design is new, failure rate is high and when the design
is matured, the failure rate is fairly constant. Smaller the value offailure rate, higher is the reliability of the system.
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Measures of Reliability
2) Mean Time Between Failures (MTBF)
MTBF is referred to as the average time of satisfactory operation
of the system. Larger the MTBF, higher is the reliability of thesystem. It is applicable to repairable systems and is expressed inhours, e.g. If an item fails 8 times over a period of 40,000 hoursof operation, the MTBF would be 500 hours. During the operatingperiod, the failure rate is fairly constant.
MTBF is the reciprocal of the constant failure rate or the ratio oftest time to number of failures.
MTBF = 1/
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Measures of Reliability
3) Mean Time To Failures (MTTF)
This is applicable to non-repair systems. The mean time to failureis expressed as the average time an item is expected to functionbefore failure. If we have the life test information on n itemswith failure times t1, t2,.tn, then the mean time to failure isdefined as
MTTF = 1/nti, where i= 1 to n
The exponential formula for ReliabilityThe distribution of time between failures indicates the chance of a
failure-free operation for a specific time interval. When thefailure rate is constant, the probability of survival (reliability) isgiven by:
Ps = R = e-t
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System Reliability
System
A system is a collection of components, subsystem and/or assemblies
arranged to a specific design order to achieve desired functions with
acceptable performance and reliability.
Overall system results in the functioning of a product and a measure
of how well a system performs are based on the quality of design. The
basic steps for establishing system reliability are as follows:
The components and sub-systems constituting a given system and individual
reliability factors can be estimated, identified and computed.
A block diagram known as reliability block diagram represents the
configuration of a systems sub-components.
The condition for a successful operation of a system is then established todecide the functioning of components.
Combination rules of theory of probability are formulated to estimate a systems
reliability.
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System Reliability
Types of a System
Repairable System
It is a system that can be restored to an operating condition after a
failure by a repair or replacement of one or more components. The
following are the types of reparable systems:
Continuously Operating System: This type of system, when put in
operation to function, continues to operate till its failure or it stops for
planned maintenance schedules. For example, nuclear reactors or
satellites. On and off Operating System: Such a system can be operated at the
time when a consumer desires, and it can be re-operated when required,
for example, television, mobile phones.
Intermittently Operating System: This type of system is always ready
to perform but is operating intermittently, for example, cars and planes.
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System Reliability
Types of a System
Non Repairable System
It is also known as an instantaneous operating system with only one
cycle of performance, for example, fuses and flash bulbs.
Reliability Block Diagram
It is a diagram that represents how the components of a product,
represented by blocks, are arranged and related reliability-wise in a
larger system. It is often, but not necessarily, same as the way thatcomponents are physically related.
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System Reliability
System
A system is a collection of components, subsystem and/or assemblies
arranged to a specific design order to achieve desired functions with
acceptable performance and reliability.
Models of a System
Series Structure
For this system to work, both components 1 and 2 must work.
In this case, all n components must work in order for the whole
system to work.
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1 2
1 2 . . . n
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System Reliability
A Personal Computer
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Power
Supply
Motherboard ProcessorHard
Drive
A Simple Series system
Parallel Structures
1
2
1
2
.
.
.
n
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System Reliability
In a parallel system, the system will work as long as at least one
component works.
Combination of Series and Parallel Structures
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1 2
3
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System Reliability
Finding A Systems Reliability
A) A Series System
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1 2 n. . .
For a pure series system, the system reliability is equal to the
product of the reliabilities of its constituent components. Or:
ns
RRRR ...21
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System Reliability
Finding A Systems Reliability
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B) A Parallel System
1
2
n
..
.
For a pure parallel system, the overall
system reliability is equal to the product
of the component unreliability's.
Thus, the reliability of the parallel
system is given by:
Rs = 1 [(1 R1)(1-R2)(1-Rn)]
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System Reliability
Finding A Systems Reliability
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C) Combination of Series and Parallel Structures
Example:Consider a system with three components. Units 1 and 2 are connected
in series and Unit 3 is connected in parallel with the first two, as shown
in the figure below. Find the reliability of the system.
Solution:
R1
= 0.99 R2= 0.98
R3 = 0.97
Monday, 22 April 2013 LOVELY PROFESSIONAL UNIVERSITY By- H imanshu Gupta
System Reliability
Finding A Systems Reliability
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First, the reliability of the segment consisting of Units 1 and 2 is
calculated:
R1,2 = (R1)(R2)
= (0.99)(0.98)
= 0.97
The reliability of the overall system is then calculated by treating Units
1 and 2 as one with a reliability of 0.97 connected in parallel with Unit3.
Therefore: R1,2= 0.97
R3 = 0.97
Rs = 1 [(1 0.97)(1 0.97)]
= 0.9991
= 99.91%
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System Reliability
Finding A Systems Reliability
Bridge System
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A bridge system is a combination of all types of redundancies, and it is customized tomeet particular requirements. An inspection of this system reveals that any of the
following failures will cause the system to fail: failure of components 1 and 2, failures
of components 3 and 4, failure of components 1 and 5 and 4, and failure of components
2 and 5 and 3.
1 3
2
5
4
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System Reliability
Finding A Systems Reliability
Standby Redundancy
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In a standby redundancy, only one component is operating where one or
more components are in a standby mode to take over the operation, if the
already operating component fails without accumulating any time. In
case of parallel network, all the units in the configuration are active,
whereas in standby redundancy, they are not.
Reliability of a standby redundancy of n units in which one unit is
operating and n-1 units are standby, ready to take over, until operatingunit fails, is given by
Rst (t) = (t)re t/ r!
r= 1
n-1
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Reliability Improvement
A high degree of reliability is an absolute necessity for complexand modern systems to be used for industrial, military and other
scientific purposes. There are many ways by which reliability of acomponent or system can be enhanced. These are:
1) Design and Safety Factor: In order to design reliability intoproducts, reasons for product failures must be analyzedthoroughly. Generally, a product fails prematurely because ofinadequate design features, manufacturing and part defects,abnormal stresses induced, environmental condition and humanerror. Higher reliability could be achieved through mature design.
2) Parts and Material Selection: Designer has to choose betweenselecting standard parts and manufactured specialized parts with
higher reliability and greater tolerances. The trade off is usually incost but ease of parts availability, ease of repair, energyrequirements, weight and size may also be considered.
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Reliability Improvement
3) Redundancy: When it is not possible either to manufacture ahighly reliable component or the cost associated with suchmanufacturing is too high, the system reliability can be improvedby the techniques of redundancy.
Redundancy is the duplication of critical components or functionsof a system with the intention of increasing reliability of thesystem.
The various approaches of introducing redundancy in thesystem are:
a) To provide a duplicate or an additional path for the entire systemitself. This is known as system or unit redundancy.
b) To provide redundant path for each component individuallywhich is called component redundancy.
c) Use a combination of the above methods depending upon theconfiguration called mixed redundancy.
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Failure Analysis
Failure analysis techniques provide a methodical way to examine aproposed design for possible ways in which a failure can occur.There are two ways by which a failure analysis is done in reliabilityengineering:
1. Failure Mode and Effect Analysis (FMEA)/ Failure Mode andEffect Criticality Analysis (FMECA).
2. Fault Tree Analysis (FTA).
Fault Tree Analysis
The FTA is one of the many symbolic analytical logic techniquesfound in operations research and system reliability. Fault treediagrams are logic block diagrams that display state of system (top
event) in terms of the states of its components (basic events). FTDsprovide an alternative to RBDs. An FTD is built top-down in termsof events rather than blocks.
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System Safety and Fault Tree Analysis
Reliability and product safety are obviously related. Safety can bebroadly defined as the avoidance of conditions that can causeinjury, loss of life, or severe damage. Therefore the focus here ison failures that may create safety hazards.
The objective is to determine during design how these failures arelikely to occur, to estimate their probability of occurrence, and totake corrective action.
Fault Tree Analysis
A useful tool in performing a system safety analysis.
It is a graphical design technique that provides an alternative toreliability block diagrams.
It is a top-down, deductive analysis structured in terms of eventsrather than components. The perspective is on faults rather thanreliability.
All failures are faults, but not all faults may be consideredfailures.
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System Safety and Fault Tree Analysis
Fault Tree Analysis
There are four major steps to a fault tree analysis:
i. Define the system, its boundaries, and the top event.ii. Construct the fault tree, which symbolically represents the system
and its relevant events.
iii. Perform a qualitative evaluation by identifying thosecombinations of events that will cause the top event.
iv. Perform a quantitative evaluation by assigning failureprobabilities or unavailability's to the basic events and computingthe probability of the top event.
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System Safety and Fault Tree Analysis
Fault tree main symbols
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Commonly Used Symbols Occasionally Used Symbols
OR gate Incomplete event
AND gate An External Event
An Event / Fault gate
Basic Event
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System Safety and Fault Tree Analysis
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FTA Symbols Explained
Basic Event: A lower most event that can not be further developed.E.g. Relay failure, Switch failure etc.,
An Event / Fault: This can be a intermediate event (or) a top event. Theyare a result logical combination of lower level events.
E.g. Both transmitters fail, Run away reaction
OR Gate: Either one of the bottom event results in occurrence of the top event.
E.g. Either one of the root valve is closed, process signal to transmitter fails.
AND Gate: For the top event to occur all the bottom events shouldoccur.
E.g. Fuel, Oxygen and Ignition source has to be present for fire.
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System Safety and Fault Tree Analysis
32
FTA Symbols Explained
Incomplete Event: An event which has scope for further development butnot done usually because of insufficient data.
E.g. Software malfunction, Human Error etc.,
External Event: An event external to the system which can cause failure.E.g. Fire.
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System Safety and Fault Tree Analysis
33
Simple Examples
Example 1:
Example 2:
Transmitter Failed
OR
Transmitter 1
Failed
Transmitter 2
Failed
Valve Failed
Valve 1
Failed
Valve 2
Failed
AND
0.1 0.2
0.28
0.001 0.002
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System Safety and Fault Tree Analysis
34
Procedure
Steps to get the final Boolean equation:
1. Replace AND gates with the product of their inputs.
IE1 = A.B
IE2 = C.D
2. Replace OR gates with the sum of their inputs.
TOP = IE1+IE2
= A.B+C.D
3. Continue this replacement until all intermediate event gates
have been replaced and only the basic events remain in the
equation.
TOP = A.B+C.D
TOP
IE1 IE2
A B C D
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System Safety and Fault Tree Analysis
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Procedure
Boolean Algebra Reduction Example:TOP = IE1 + IE2
= (A.B) + (A + IE3)
= A.B + A + (C.D.IE4)
= A.B + A + (C.D.D.B)
= A + A.B + B.C.D.D (D.D = D)
= A + A.B + B.C.D (A + A.B = A)
= A + B.C.D A B A IE3
C DIE4
D B
TOP
IE1 IE2
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System Safety and Fault Tree Analysis
36
Faults can be classified as primary, secondary, and command.
A fault is primary if the component or part is functioning withinits design parameters when an inherent failure occurs.
A secondary failure occurs when an environmental stress or anexcessive operational stress causes the failure.
A command fault is one that occurs as a result of a correct actionbeing accomplished at a wrong time or place.
For example, a command fault may occur when turning power onprematurely or turning off a cooling subsystem before the systemhas been shut down.
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System Safety and Fault Tree Analysis
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System Safety and Fault Tree Analysis
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System Safety and Fault Tree Analysis
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Monday, 22 April 2013 LOVELY PROFESSIONAL UNIVERSITY By- H imanshu Gupta
System Safety and Fault Tree Analysis
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System Safety and Fault Tree Analysis
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Monday, 22 April 2013 LOVELY PROFESSIONAL UNIVERSITY By- H imanshu Gupta
System Safety and Fault Tree Analysis
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Failure Mode Effect and Criticality Analysis (FMECA)
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What Is A Failure Mode?
A Failure Mode is: The way in which the component, subassembly, product,
input, or process could fail to perform its intended
function.
Failure modes may be the result of upstream operations or
may cause downstream operations to fail
Things that could go wrong
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Failure Mode Effect and Criticality Analysis
(FMECA)/FMEA
44
FMEA
A structured approach to:
Identifying the ways in which a product or process can
fail
Estimating risk associated with specific causes
Prioritizing the actions that should be taken to reduce
risk
Evaluating design validation plan (design FMEA) orcurrent control plan (process FMEA)
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Failure Mode Effect and Criticality Analysis
(FMECA)/FMEA
45
The FMEA Form
Identify failure modes
and their effectsIdentify causes of the
failure modes
and controls
PrioritizeDetermine and assess
actions
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Failure Mode Effect and Criticality Analysis
(FMECA)/FMEA
46
Types of FMEAs
Design Analyzes product design before release to production,
with a focus on product function Analyzes systems and subsystems in early concept and
design stages
Process Used to analyze manufacturing and assembly processes
after they are implemented
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Failure Mode Effect and Criticality Analysis
(FMECA)/FMEA
47
FMEA Procedure
1. For each process input (start with high value inputs),determine the ways in which the input can go wrong
(failure mode)
2. For each failure mode, determine effects
Select a severity level for each effect
3. Identify potential causes of each failure mode
Select an occurrence level for each cause
4. List current controls for each cause
Select a detection level for each cause
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Failure Mode Effect and Criticality Analysis
(FMECA)/FMEA
48
FMEA Procedure
5. Calculate the Risk Priority Number (RPN)
6. Develop recommended actions, assign responsible persons,
and take actions
Give priority to high RPNs
MUST look at severities rated a 10
7. Assign the predicted severity, occurrence, and detectionlevels and compare RPNs
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Failure Mode Effect and Criticality Analysis
(FMECA)/FMEA
49
Severity, Occurrence, and Detection
SeverityImportance of the effect on customer requirements
Occurrence
Frequency with which a given cause occurs and
creates failure modes
Detection
The ability of the current control scheme to detect
or prevent a given cause
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Failure Mode Effect and Criticality Analysis
(FMECA)/FMEA
50
Rating Scales
Severity
1 = Not Severe, 10 = Very Severe
Occurrence
1 = Not Likely, 10 = Very Likely
Detection
1 = Easy to Detect, 10 = Not easy to Detect
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Failure Mode Effect and Criticality Analysis
(FMECA)/FMEA
51
Risk Priority Number (RPN)
RPN is the product of the severity, occurrence, and
detection scores.
Severity Occurrence Detection RPNX X =
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Acceptance Sampling Plan based on Life test
52
A common life testing plan involves choosing a sample of items
from the batch and observing their operation for a certain
predetermined time. If the number of failures exceeds a stipulated
acceptance number, the lot is rejected; if the number of failures is
less than or equal to the acceptance number, the lot is accepted.
Two options are possible. In the first option, an item that fails is
replaced immediately by an identical item. In the second, failed
items are not replaced.
In calculations for OC curve both Poisson and Binomial
distributions can be used. The OC curve shows the probability of
lot acceptance Pa, as a function of lot quality as indicated by the
mean life () or the mean time to failure of the item.
Operating Characteristic Curves
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Acceptance Sampling Plan based on Life test
53
The risk of rejecting a good lot (products with a satisfactory mean
life 0) is the producers risk.
The risk of accepting a poor lot (products with an unsatisfactory
mean life of1) is the consumers risk.
Operating Characteristic Curves
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Reliability and Life Testing Plans
54
Plans for reliability and life testing are usually destructive in nature.
They involve observing a sample of items until a certain number of
failures, observing over a certain period of time to record the number
of failures, or a combination of both. This testing is usually done at the
prototype stage, which can be expensive depending upon the unit cost
of the item.
Types of Tests
Three main types of tests
Failure Terminated Tests: In failure terminated plans, the tests are
terminated when a pre assigned number of failures occurs in the
chosen sample. Lot accepetance is based on the accumulated test time
of the items when the test is terminated.
Let predetermined number of failures be r, and the stipulated mean life
by C. From the test results, lets suppose the accumulated test time of
the items is T, from which the estimate of average life is,
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Failure Terminated Tests:
= T/r
If C lot is accepted.
Time Terminated Test:
A time terminated test is terminated when a preassigned time T is
reached. Acceptance of the lot is based on the observed number of
failures r` exceeds a preassigned value r, the lot is rejected, otherwise,
the lot is accepted.
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Reliability and Life Testing Plans
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Sequential Reliability Testing:
In sequential reliability testing, no prior decision is made as to the
number of failures or the time to conduct the test. Instead the
accumulated results of the test are used to decide whether to accept the
lot, reject the lot, or continue testing.
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Sequential Reliability Testing:
The cumulative number of failures based on the choice of the sampleis plotted versus, the accumulated test time of the items. Based on the
acceptable mean life 0, an associated producers risk , a minimum
mean life 1, and an associated consumer risk , equations for the
acceptance line and rejection line are found.
If the plot stays within the two lines, testing continues; if the plot falls
in acceptance region, the test is terminated and lot accepted; if the plot
falls in rejection region, the test is terminated and lot rejected.
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Reliability and Life Testing Plans
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Sequential Reliability Testing:
The cumulative number of failures based on the choice of the sample
is plotted versus, the accumulated test time of the items. Based on the
acceptable mean life 0, an associated producers risk , a minimum
mean life 1, and an associated consumer risk, equations for the
acceptance line and rejection line are found.
If the plot stays within the two lines, testing continues; if the plot falls
in acceptance region, the test is terminated and lot accepted; if the plot
falls in rejection region, the test is terminated and lot rejected.
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Availability
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An availability is a probability that a component/system is operational
at a given time, t (i.e. has not failed or been restored after failure).
The availability of an item is a probability that it is operatingsatisfactorily at any point of time, used in stated conditions including
an operating time, active repair time and logistic time.
If a vehicle has 99.9% availability, there is one time out of thousand
that someone needs to use the vehicle and finds out that the vehicle is
not operational because some part of the vehicle is either damaged or
in the process of being replaced.
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Maintainability
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Maintainability is defined as the probability of performing a successful
repair action within a given time.
It is a measure of ease and speed with which a system can be restored
to operational status after a failure occurs.
M(t) = 1- e-ut
Where u is the repair rate
MTTR (Mean Time To Repair) = 1/u