1. 1 Failures and fault classification Jørn Vatn NTNU jorn.vatn@ntnu.no

2. 2 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 A failure is then defined as the event that the possibility of performing the required function is terminated Fault A ‘fault’ or ’fault state’ denotes the state that the entity is not able to perform its required function Failure mode A failure mode is defined as the effect a failure has in the way it is observed on the entity that has failed

3. 3 Failure classification There are many principles to choose among for classifying failures. In this section we will consider the following dimensions Immediate  Gradual failure Hidden  Evident failure Physical  Functional failures

4. 4 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 performance

5. 5 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)

6. 6 The point of failure may be hard to define

7. 7 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 quantity

8. 8 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 automatically The term ‘evident’ relates to entities that are continuously demanded, and a failure will most likely be detected immediately Note 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)

9. 9 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 causes Some failures will become evident after a while Failure of a pump for maintaining pressure in an oil pressure container A 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 evident E.g., we need 3-out-of-4 bolts to keep a structure in place One loose bolt is not evident When the structure falls down, it is evident

10. 10 Exercise Give examples on Hidden failures Evident failures Evident by ”start-up” Evident after a while Evident only on system level

11. 11 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 etc A replacement with the component with a similar new one will not help For 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 location

12. 12 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 pump Root cause, for example bad maintenance, inadequate design etc

13. 13 Hierarchy of function, failure mode, failure cause and failure mechanism