Reliability Introduction & Modes of Failure

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RELIABILITY

Reliability of a system (product) conveys the concept of dependability, successful operation or performance and the absence of failures. Reliability is the probability that a machine (product) can perform its intended function, without failure, for a specified interval of time when operating under standard conditions. It should be observed that the above stated definition stresses four significant factors; probability, intended function, time and operating conditions. These four elements play an important role in characterizing the reliability of an item. Increased reliability implies less failure of the machinery and consequently less downtime and loss of production. Product reliability is affected by usage mode, environment and maintenance actions. Quantitatively reliability may be considered as a measure of the success of an engineering system in meeting its stated objectives. The concept of reliability has been interpreted many different ways in numerous works out of which a few listed below:-
Reliability definitions:
The word reliable is commonly use in the same sense as dependable. Reliability is the probability of a device performing its purpose adequately for the period intended under the given operating conditions. 

This definition brings into focus four important factors, namely,
(a) The reliability of a device is expressed as a probability,
(b) The device is required to give adequate performance,
(c) The duration of adequate performance is specified, and

(d) The environmental and operating conditions are prescribed


MODES OF FAILURE

The different modes of failure are:
a) Catastrophic failures
This type of failure causes a normally operating system to suddenly become inoperative system. This type of failure occurs at random these failures are significant because they can result in injury, loss of life, or major damage.

b) Creeping failures
This type of failure occurs gradually because of change of some parameter with time. These failures can be detected and weeded out by proper inspection and maintenance. Preventive maintenance techniques are suitable for avoiding this kind of failure.

c) Independent failures
The failure which does not occur as a result of the effects generated by other failures is called independent failure

d) Secondary failures

A secondary failure occurs as a result of some primary failure. For example the spokes in a motor cycle traveling at a high speed, are broken as a result of Tyre burst.

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