Reliability is the ability of a system or component to function under stated conditions for a specified period of time. There are several reasons why failures occur, including design flaws, overstressing, wear and tear, vibration, incorrect specifications, misuse, and operating outside intended environments. The objectives of reliability engineering are to prevent or reduce failures, identify and correct causes of failures, determine ways to cope with failures, and estimate reliability of new designs. Reliability is defined as the probability of success and avoids downtime, repair costs, and warranty claims. Modes of failure include initial infant mortality failures, random stable failures, and wear-out failures over time depicted by a bathtub curve. Reliability influences system

1. Authored by: Ajay Verma

2. Reliability Engineering
TOE 02

3. What is reliability?
• Reliability is associated with unexpected failures
of products or services and understanding why
these failures occur is key to improving reliability.
The main reasons why failures occur include:
 The product is not fit for purpose or more specifically the design is
inherently incapable.
 The item may be overstressed in some way.
 Failures can be caused by wear-out
 Failures might be caused by vibration.

4. Wrong specifications may cause failures.
Misuse of the item may cause failure.
Items are designed for a specific operating
environment and if they are then used
outside this environment then failure can
occur.

5. It is clear that to ensure good reliability the
causes of failure need to be identified and
eliminated. Indeed the objectives of
reliability engineering are:
 To apply engineering knowledge to prevent or reduce the
likelihood or frequency of failures;
 To identify and correct the causes of failure that do occur;
 To determine ways of coping with failures that do occur;
 To apply methods of estimating the likely reliability of new
designs, and for analysing reliability data.

6. Definition of Reliability
• Reliability, describes the ability of a system or component
to function under stated conditions for a specified period
of time
• Reliability may also describe the ability to function at a
specified moment or interval of time (Availability).
• Reliability engineering represents a sub-discipline within
system engineering.
• Reliability is theoretically defined as the probability of
success (R=1-Pf ), as the frequency of failures; or in
terms of availability, as a probability derived from
reliability, testability and maintainability.

7. • Testability, Maintainability and Maintenance are
often defined as a part of "reliability engineering"
in Reliability Programs.
• Reliability plays a key role in the cost-
effectiveness of systems.
• Reliability engineering deals with the estimation,
prevention and management of high levels of
"lifetime“, engineering uncertainty and risk of
failure

8. • Reliability engineering relates closely to safety engineering and to
system safety, in that they use common methods for their analysis and
may require input from each other.
• Reliability engineering focuses on costs of failure caused by system
downtime, cost of spares, repair equipment, personnel, and cost of
warranty claims.
• Safety engineering normally emphasizes not cost, but preserving life
and nature, and therefore deals only with particular dangerous
system-failure modes. High reliability (safety factor) levels also result
from good engineering and from attention to detail, and almost never
from only reactive failure management.

9. Modes of Failure and Causes
• A failure is the partial or total loss or change
in the properties of a device in such a way
that its functioning is seriously affected or
totally stopped.
• The concept of failures and their details
help in the evaluation of quantitative
reliability of a device.

10. • In general, some components have well defined
failures; others do not.
• In the beginning, when the item or component is
installed, the item fails with high frequency, which
is known as initial failure or infant mortality.
• These are generally due to manufacturing defects.
They are very high at initial stages and gradually
decreases and stabilize over a longer period of
time.

11. • Stable or constant failures due to chance can
be observed on an item for a longer period.
• These types of failures are known as
random failures and characterized by
constant number of failures per unit of time.

12. • Due to wear and tear with the usage, the
item gradually deteriorates and frequency
of failures again increases.
• These types of failures are called as wear-
out failures. At this stage failure rate seems
to be very high due to deterioration.

13. • Therefore the whole pattern of failures
could be depicted by a bathtub curve

14. Reliability for System Effectiveness
• The term used to describe the overall
capability of a system to perform its
intended function is system effectiveness.
• The system effectiveness is defined as the
probability that the system can successfully
meet an operational demand within a given
time when operating under specified
condition.

15. Reliability for System
Effectiveness
• Effectiveness is influenced by the way the
system is designed, manufactured, used, and
maintained, and thus is a function of all life-
cycle activities.
• The effectiveness of the system is function
of several attributes, such as design
accuracy, performance measures, safety,
reliability, quality, and maintainability.

17. • Recently, the term assurance science, has
been used to increase the overall
effectiveness of any system.
• The assurance sciences are disciplines which
have the attainment of product integrity as
their common objective.