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Failures and fault classification Failures and fault classification

Failures and fault classification - PowerPoint Presentation

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Failures and fault classification - PPT Presentation

Jørn Vatn NTNU jornvatnntnuno Definitions Failure In order to define the term failure we need first to introduce the term function A unit or system entity is designed for performing one or more functions ID: 277107

evident failure function failures failure evident failures function hidden term gradual system physical entity design pressure give relates level

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Slide1

Failures and fault classification

Jørn Vatn

NTNU

jorn.vatn@ntnu.noSlide2

Definitions

Failure

In order to define the term ‘failure’, we need first to introduce the term ‘function’

A unit or system (entity) is designed for performing one or more functionsA failure is then defined as the event that the possibility of performing the required function is terminatedFaultA ‘fault’ or ’fault state’ denotes the state that the entity is not able to perform its required functionFailure modeA failure mode is defined as the effect a failure has in the way it is observed on the entity that has failed Slide3

Failure classification

There are many principles to choose among for classifying failures. In this section we will consider the following dimensions

Immediate

 Gradual failureHidden  Evident failurePhysical  Functional failuresSlide4

Immediate  Gradual failure

We use the term ’immediate failure’ when the failure occurs spontaneously without any alert

This failure type is often related to situation where the entity is a binary function (only two states), e.g. a light bulb

A gradual failure is on the other side characterised by a observable gradual weakening of the performanceSlide5

Exercise

Give examples of immediate and gradual failures

Note that a component may have a gradual failure progression even though the function is binary, e.g., a crack development prior to a breakage (failure)Slide6

The point of failure may be hard to defineSlide7

Exercise

Give examples of situations where

The performance is weakened as a function of time

The loads which determines whether the situation is critical or not is a random quantitySlide8

Hidden  Evident failure

We often distinguish between hidden and evident failures

The term ’hidden’ often relates to entities that is not continuously demanded

For example the SIFA valve on a train (bleed of the air pressure by activation) is a hidden function, and a failure will not be detected automaticallyThe term ‘evident’ relates to entities that are continuously demanded, and a failure will most likely be detected immediatelyNote that the same SIFA-valve will also have a evident function (“not bleed of air pressure under normal operation) because an unintended activation immediately will be detected (breaks are activated)Slide9

More on Hidden  Evident failure

Some functions are not evident all the time, but may be verified e.g., at start-up (e.g., hand break in a car)

 Evident by ”start-up”

Some functions are hidden, but may be detected by ”self-test”, this is typical for detection systems”Partial stroke test” is a test that reveals some, but not all failure causesSome failures will become evident after a whileFailure of a pump for maintaining pressure in an oil pressure containerA filter without throughput will give system effects after a while

On a component level the function is hidden, but on a system level the function is evidentE.g., we need 3-out-of-4 bolts to keep a structure in placeOne loose bolt is not evidentWhen the structure falls down, it is evidentSlide10

Exercise

Give examples on

Hidden failures

Evident failuresEvident by ”start-up”Evident after a whileEvident only on system levelSlide11

Physical  Functional failures

Physical failures could be eliminated by a repair activity, or by replacing a unit with a new one

Typical causes behind physical failures could be

natural ageing (inside the design boundaries)external load (often outside the design boundaries)A functional failure relates to wrong design, wrong location, wrong usage etcA replacement with the component with a similar new one will not helpFor example if a smoke detector is mounted in an area where there will be no smoke in case of a fire, it will not cure the situation with a new detector at the same locationSlide12

Failure mechanisms and failure causes

Failure mechanisms relates to physical, chemical or other processes that deteriorates the entity, and leads to a failure

The term ‘failure cause’ is often used in two different ways

Failure on a lower level in the system hierarchy, e.g., a defect bearing in a pumpRoot cause, for example bad maintenance, inadequate design etc Slide13

Hierarchy of function, failure mode, failure cause and failure mechanism