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
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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