1. Introduction
Principles of Reliability • No branch in engineering science, with the exception
of computer technology ( including communication
Engineering engineering) and environmental engineering, has
developed and advanced so phenomenally as Reliability
Engineering during last five decades.
• In today's technological world, everyone depends upon
a wide array of complex equipment, machinery,
systems and services to meet one’s everyday needs.
And their continued functioning is critical for the well-
being and quite often the survival of the humanity.
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Need for Product Performance Compulsions for Reliable
* Quest of Excellence (better and better Product
products) There are basically three main categories of
* Economic Implications ( lesser and lesser compulsions to go for reliable product:
costs)
* Dwindling Resources (for exploding 1. Economic
population) 2. Environmental
2 E i t l
* Environmental Consequences (Pollution) 3. Performance
-” No more than one or few decades remain before the chance to avert
the threats we confront, will be lost and the prospects for humanity
immeasurably diminished… A new ethics is required – a new These three are however interlinked.
attitude towards discharging responsibility for caring for ourselves
and for earth … this ethics must motivate a great movement,
convincing reluctant leaders, reluctant government and reluctant
people themselves to affect the needed changes.” -Warning to
humanity given by 102 Noble laureates at Rio Conference in 1992.
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2. Economic Compulsions Environmental Compulsions
• For products, the old concept of “use and • Development in the past century has rendered “free
throw” does not any more hold good, to resources ( like water, air and soil)” highly polluted.
keep the wheels of the industries turning as Further pollution may endanger the existence of
humanity on the Earth. Ozone layer depletion and
the cost of harnessing material and energy global warming are consequences.
goes up with time. • Effective utilizations of World resources is the need
• For success in business under the of the hour in the 21st century as exhaustible
resources become even more meagre and rare.
competitive market, a product must provide
• Demand of resources keeps increasing with the
trouble-free service and better customer population increasing at an exponential rate.
satisfaction. • Sustainable development is the solution to present
• Due to tough competition internationally, state of environmental health of Earth.
only a company that provides more reliable
products at cheaper cost will only survive.
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Performance Compulsions Reliability Definition
• Products are becoming more complex, designing for Reliability by IEC is defined as the
performance becomes even more challenging as
complexity and performance are contradictory capability of a product ( or system or
requirements. service) to perform its expected job
• Product longevity would imply less world pollution
and better utilization of remaining exhaustible under the specific conditions of use
resources. over an intended period of time.
• Hi h performance also results in l ss botheration
Higher p f m n ls sults less b th ti n
of after-sales service and thereby less :IEC
maintenance costs, reducing overall Life Cycle International Electrotechnical Commission
costs.
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3. Expected Job Obviously, now the IEC definition
The product or the unit functioning within raises questions, like …
the intended bounds of performance and is
operating satisfactorily.
– Can this capability be quantified ?
Conditions of Use (Prob. of Satisfactory Performance)
Conditions of use basically refer to the – Can this quantification be used with
environmental conditions under which the all kinds of products and system
p
product of the system has to operate.
y p without resorting to any redefinition
g y
from case to case ?
Intended (Mission) Time – Can this capability be used to
It is the length of time over which we don’t compare the performance different
want the product or system to have any engineering designs and technologies
failure at all and desired mission of the for a product or a system and assess
system should be carried out successfully their suitability?
without being impaired or aborted.
– Can this capability be engineered into
the products?
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Definition (contd.)
Definition of Reliability
• And, in reliability engineering, the goal is Reliability implies an aspect of engineering
to optimize, in probabilistic sense, the uncertainty, which is often reflected in its
performance of the system. probabilistic definition as:
It is the probability that a product will perform the
intended function without failure under the stated
conditions of use over the stated period of time.
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4. Objective of a Reliability Program
However one must remember ?
The prioritized objectives of a reliability programme are: • Instant of failure is unpredictable. Failure
– To apply engineering knowledge and specialized techniques to
can occur at any time and random.
prevent failures or to reduce the likelihood of the frequency of
failures.
Example: Failure of 0.9 and 0.6 reliable
systems)
– To identify and check the cause of failures those do occur,
despite the efforts to prevent them. • The importance of reliability lies in relative
– To determine ways of coping with the failures which may occur, if terms and not in absolute terms
terms.
their causes have not been checked.
• Failures are inevitable. No matter what you
– To apply methods for estimating the reliability of new designs,
and for analyzing reliability data with a view to improve future
do. One can only minimize but not eliminate
designs. altogether their occurrences.
• Hundred percent Reliability is impossible.
(LIMITATIONS OF RELIABILITY
13 ONCEPTS) 14
Failures do occur if: Causes of failures
• The design is inherently weak and consumes too much • Failures can be due to inherent weakness.
power, suffers resonance at the wrong frequency, etc.
The list of possible reasons are endless, every problem • Failures can be caused by wear out.
presents the potential for errors, omissions and • Failures can be caused by the time
commission. The more complex the design is, the
greater is the potential. dependent mechanisms.
• The actual strength of any population of components • Failures can also occur on account of
varies and there are bound to be some that are errors, such as incorrect specification,
, p f ,
relatively strong, others th t are relatively weak,
l ti l t th that l ti l k
however most will be of nearly average strength. Failure design or software coding, by faulty
will not occur so long as the applied load doesn’t exceed assembly or test, by inadequate or improper
the strength. maintenance, or simply by misuse.
• Sneaks can cause failures. A sneak is a
condition in which the system doesn't work
properly even though every part does.
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5. Aging can also lead to Failures
• Failures can occur at any point of time in a
product’s life. These failures always have a
reason.
• The failures would occur predominantly in early
life of a product if the manufacturer has been
t Such failures are called Q lit
complacent. S h f il
l ll d Quality
failures.
• Catastrophic or sudden failures can occur any
time in the life and are due to accidents.
• Wear out failures occur mainly towards the old
age. These occur due to deterioration
t2 > t1 17 18
Three Types of Failures Concern in Reliability Programme
• Our concern in Reliability is:
– How to extend the useful life of a product?
– How to minimize the chance failure of a product?
– How to reduce the initial hazard rate and to avoid
product failures during its period?
• The first two relate to the design and depend on material
selection and the choice of safety and derating factors,
with an emphasis being on prevention, reduction, or complete
elimination of chance f il
li i ti f h failures, which considerably affect th
hi h id bl ff t the
reliability in actual use.
• We are also concerned about the overhauling or preventive
replacement of the product during its design life.
• The third problem of concern is the initial period , which
may range from a few minutes to several hundred hours in
certain cases.
• To eliminate the possibility of failures immediately after
the delivery , manufacturers generally conduct test runs or
debugging tests before delivery to eliminate quality failures.
*Quality Failures *Sudden Failures *Wearout Failures
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6. Various Phases of Reliability
• Design phase
• Raw Materials, and Parts selection and Vendor
Control
• Manufacturing phase
• Packaging and Transhipment
• Storage
Reliability Function • Installation
• Usage and Consumer Education
• Maintenance phase
• Reliability Testing, Estimation & Demonstration
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Various Phases of Reliability (cont...)
Various Phases of Reliability (cont...)
• Design Phase • Raw Material, Parts Selection and Vendor
– If any phase in the entire life-cycle of a Control
product has maximum impact on its – Once the engineering design of a product is
reliability, it is the design phase. Analytical accomplished, the selection of raw material,
methods can help only to select an parts and assemblies is taken up as a next step
alternative design technology; thereafter where the purchase department has an
p p
the process is iterative and repetitive till
the specified performance goal is achieved. important role to play. A great majority of all
Design requires ingenuity and creativeness product failures results from the suppliers
and a designer ought to know what has been items. Suppliers/vendor selection, therefore
already been tried before and did not work. assumes a crucial role in reliability
New prototype is not build because the implementations programme since the quality of
designer knows that it will work better that materials, parts, assemblies and components
way, but because he has no reasons to supplied by vendors would invariably affect the
doubt why it will not work until it has been final quality or reliability of the manufactured
tried out.
product.
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7. Various Phases of Reliability (cont...) Various Phases of Reliability (cont…)
Vendor Control • Manufacturing phase
– The extent of supplies made by the vendors for – The human aspects affect the quality and workmanship in any
production process. Good workmanship depends considerably
manufacturing a product varies from the product upon the team spirit and the skill of a worker.
to product and from country to country. In an – The responsibility of quality control is to ensure that all in-
automotive industry in U.S.A., purchased plant operations and processes are performed in accordance
materials may account for about 50% of the with the specifications laid down. Also all incoming materials
and parts conform strictly to the specifications.
manufacturing cost, where as Japan, it is about – Reliance on an inspector causes worker to be less concerned
70%.
70% Thus a manufacturer of a final product about quality But recently emphasis has shifted on making
quality. recently,
depends very heavily for the quality and operators responsible for these attributes as is being done in
reliability of the supplies from the vendors. Japan. In Japan, QC is in fact design and process control-
oriented instead of inspection-oriented.
Vendor relation must involve a systematic – This has been achieved by organizing quality circles with a
exchange of technical, economic and managerial company. These circles are voluntary study groups.
information, and services. Some reputed
manufactures own joint research facilities with
their vendors.
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Various Phases of Reliability (cont…)
Various Phases of Reliability (cont...)
• Packaging and Transshipment
– It is an established fact that even with the use of • Storage
best packaging material, about 10 % of the – Raw materials, components and finished
shipment gets damaged in transit. Transportation
products are all stored for quite sometime
hazards may include temperature, humidity,
vibration, road conditions and shocks. before they are used or shipped. It is
therefore necessary to minimize deterioration
y
– In case of integral environment packaging one has
packaging,
to protect the product from the environment so as or degradation of a product on account of the
to preserve metals from rusting, keeping moisture storage. The measures to avoid degradation
out, shielding electronic products particularly include specifying the shelf-life of the
from electrostatic interferences. The design of product based on the laboratory tests and
such a packaging forms an integral part of the field data and by establishing standards
design of the original product and as such should thereof.
undergo all the design reviews just as is done in
case of the product itself.
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8. Various Phases of Reliability (cont...) Various Phases of Reliability (cont...)
• Installation • Usage and Consumer Education
– Before a product is put to use , it often – Manufactures usually prepare operating manuals
undergoes additional processing during to permit users to use the product properly and
distribution, assembly, installation and checkout effectively. For proper usage of a product,
etc. In situ installation may sometimes require consumer education is quite essential.
special facilities including a controlled – Consumer education is usually imparted by
environment,special tools and jigs, instruments
l l d providing product i f
idi d t information regarding th
ti di the
and special instructions. Preparation of user’s technical aspects of the product and its
site is sometimes required before the performance in the standard popular magazines,
installation of the product is carried out. The which are easily available to a user. A user must
necessarily know in advance the specifications and
instructions for use must accompany a product performance attributes of the product, he or she
to avoid its improper installation or use. has selected to purchase amongst the various
products of similar types available in the market.
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Various Phases of Reliability (cont...)
Maintenance Phase (cont...)
• Maintenance Phase – Preventive maintenance is advantageous for units whose
hazard rate increase with time. Therefore, the chance
– Besides one-shot equipments/devices, there are devices failures result into unscheduled maintenance whereas the
or products that can be maintained by carrying out scheduled maintenance reduces the occurrence of wear-
necessary preventive or corrective maintenance. out failures. The objective of schedule maintenance is to
– Preventive maintenance consists of taking preventive replace an item before it wears out.
measures at predetermined points of time during the
operation phase, to reduce the chances of failures of – To ensure high maintainability, a designer should ensure
the device whereas that the components with shorter lives would be easily
– corrective maintenance or repairs are carried out only accessible within the equipment and can be removed
without difficulty. This would help decreasing repair
after a failure has occurred. times and increasing availability of equipment.
– Sometimes maintenance or repair is carried out based on
the condition of the unit or based on the signature
analysis of certain parameters like vibration, noise etc.
Such an activity is known as predictive maintenance.
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9. Maintenance Phase (contd.)
Maintenance Phase (contd.)
– Maintenance has significant influence on the life of a
• Maintainability
product . – The maintainability is defined as the probability
– The supply function is concerned with providing of the of a unit being restored to a functional state
necessary personnel, material, parts and equipment to within a specified time interval known as down
support operation in the field. Collectively, maintenance time.
and supply efforts, facilities and manpower form the
logistics support. – Maintainability is related to the total down time,
– The logistics cost over the life time of a product often
g p which consists of administrative, logistic time
considerably exceeds the initial cost of the product. and the actual repair time.
d h l i i
– In fact, maintenance includes all actions necessary to keep – Actually the preparation time, fault location
the product in usable conditions through preventive time, part procurement time, actual repair time
measures which includes check-outs, testing, overhaul,
repairs, instructions, operational schedules, spares and and testing time after repairs, all add up to
last but not the least- personnel. increase the total down time.
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Maintenance Phase (cont...)
Reliability : A birth to death
• Availability
– It is defined as the probability that a
concern
product is working satisfactorily at any
given point of time when used under the
given conditions of use. Concept and
– Up time ( Available Time) is the time definition
during which product is working. Down time Design and
in which maintenance is done including
n wh ch ma ntenance s nclud ng Development
logistic and administrative time loss Manufacturing
– Free time is the time during which and Installation
operational use of a product is not Operation and
required. maintenance
– Storage time is the time during which the Maintainability
product is presumed to be in operable Quality
conditions but is being held as spare.
Reliability
35 Various phases in a product’s life 36
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10. Failure Data Broad Outline for Reliability Prediction
• Field failure data Define the system and its conditions.
– It constitutes a meaningful data source as they
represents experience from the real world. Define systems performance criteria (in terms of either
• Service Life data success or failure).
– Data on service life is necessary in assessing the Develop System models using either Reliability Logic
time characteristic of reliability Diagrams (RLD) or Fault Tree Methodology (FTA).
• Data from engineering tests Compile parts list for the system.
– V i
Various tests Assign failure rates and modify generic failure rates using
The problem of acquiring data is not an easy an appropriate procedure for the environment of use.
one. Although considerable failures data has
been generated and is available for the Select a prediction procedure and perform Parts Count or
electronic or electrical component and device, Parts stress method for the system.
Comparatively, not much published information is
available on the failure of mechanical Combine parts failure rates.
components. Estimate system reliability for the specified mission time.
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Journals
Reliability Education and Research
• IEEE Transactions on Reliability
• IIT Kharagpur, Reliability Engineering Center
(Est. in 1983) • American Society for Quality Control.
• Defence Establishments • Microelectronics Reliability.

• Nuclear Reliability and Safety Establishments • Reliability Engineering and Systems Safety.
• Space Programme • The International Journal of Quality and
• University of Maryland College park
Maryland, Reliability Management
R li bilit M m t
• The State University of New Jersey, Rutgers
• Quality and Reliability Engineering
• New Jersey Institute of Technology, Newark
• University of Arizona • International J of Performability Eng
• Int J of Uncertain System
• Int J of Quality Technology and
Quantitative management
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11. Thanks
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