Design for reliability and safety Flashcards
Why is reliability important
people (customers) expect high reliability from products
Discuss the reliability of commercial aircraft
in 2017 there was no deaths from commercial air flight despite a huge number of flights occurring
The high reliability of aircraft is partly the result of reliability being designed into aircraft systems
Discuss the reliability of cars
A typical car engine lasts over 100,000 miles
Reliability has to be designed into the product
Give three examples of where reliability has been designed into a product
- aircraft
- cars
- washing machines
Defines single point failure
single failure leading to catastrophic failure
Define common mode failure
single effect that causes several sub systems to fail
Define cascade failure
one failure that leads to another etc
Define fault intolerant system
single failure causes the system to fail
Define fail safe
fail in a non-catastrophic way
Define safe life
period of safe operation
What does MTBF stand for
mean time between failure
How is mean time between failure (MTBF) calculated
MTBF = 1/failure rate (gamma)
What is the failure rate (gamma)
number of failures per time period t
At t = MTBF, the reliability = ____
and the failure = ____
reliability = 0.37 failure = 0.63
What are the three types of failure distribution (sketch them)
- Exponential
- normal
- weibull
What is component reliability R(t) defined as
R(t) = probability of survival as a fraction
R(t) = number of surviving parts after time t/(Total number)
For a constant failure rate what is the equation for component reliability
R(t) = e ^((-gamma)t)
What does R(t) stand for
probability of component surviving after time t
what does gamma stand for
failure rate (per hours)
How do you calculate gamma (failure rate)
gamma = -(lnR)/t
What does t stand for
time (hours)
for a constant failure rate: R(t) = e^((-gamma)t
When t=0 R=1 meaning..
no possibility of failure
for a constant failure rate: R(t) = e^((-gamma)t
When t=infinity R=0 meaning..
certainty of failure
What is the equation for component failure after time t (F(t))
F(t) = 1 - R(t)
Sketch the profiles of reliability and failure probabilities with respect to time
SKETCH
Draw a representation of a product that consists of a number of components that must all work for the product to function.
How is the reliability of this system calculated
SKETCH
Rtotal = R1 x R2 x R3 x R4
Draw a diagram for a fault intolerant system and describe how it can fail
failure of any component will cause failure of the system
SKETCH
Draw a representation of a product that has a ‘one in two’ component redundancy
How is the reliability of this system calculated
SKETCH
Rtotal = R1 x R4 x R2+3
where R2+3 = (R2+R3)-(R2xR3)
Draw a diagram for a fault tolerant system and describe how it can fail
there is some tolerance to failure of some components. The system will still function if only components 2 or 3 fail - same as ‘one in two’ redundancy
SKETCH
Give examples of two component redundancy systems
- Two computers on an aircraft
- Two tape players on a hi-fi
- Hospital generator for power supply
Draw a representation of a product that has a ‘one in three’ component redundancy
How is the reliability of this system calculated
SKETCH
Ra+b+c = 1-(1-Ra)(1-Rb)(1-Rc)
Is it better to have four small or two big engines on an aircraft
Four small engines are more reliable than two big engines
But it is often more expensive to have a larger number of smaller engines
What are the 6 steps for reliability modelling
- Identify components
- Calculate reliability of each component for time t from given failure rate (gamma)
- Formulate block diagram
- Calculate reliability
- Identify areas of low reliability
- Devise methods for improving reliability
Give examples of fail safe systems
- Overheating in an electrical product causes a fuse to fail and disconnects the electrical supply
- A ductile failure in a bicycle wheel rim prevents catastrophic loss of structural support
- most modern cars are fitted with twin hydraulic brake circuits, with two mast cylinders in tandem, in case one should fail
Give examples of design features that ensure fail safe
- Electrical fuses
- electrical circuit breakers
- kill cord on speed boats that switches acceleration off
- protect kevlar liners in aircraft fuel tanks to stop or reduce fuel leakage in the event of impact
- shields around aircraft engines to prevent/reduce debris flying out in the event of engine failure
Give the case study of the padstow speed boat disaster as an example of the importance of a fail safe system
A family were on a speed boat and no one was attached to the kill cord. The kill cord turns the accelerator off if the cord is pulled from the controls, where as a car requires constant force to keep the accelerator down, a boat often has a fixed accelerator due to the motion of the boat preventing an accelerator similar to a car
The boat did a sharp turn and everyone fell off board and the engine kept going resulting in serious injuries and death
Give the case study of the Concorde disaster
The concorde aircraft went over debris on the runway at take off which resulted in a large fire, stopping two engines from working and the aircraft crashing. Investigators concluded that failure (fuel leakage) would have been reduced if the aircraft had adequate shielding
Describe how the concorde disaster could be described as a cascade failure
A cascade failure involves one initial failure leading to another, then another and so on with these subsequent failures getting more and more out of control
- an engine wear strip had not been installed or manufactured properly which led to it falling of an aircraft on the runway
- the concorde ran over the engine strip and burst a tyre
- debris from the tyre hits the fuel tank
- pressure wave ruptures the weakest part of the tank
- fuel catches fire due to hot engines
- two engines stop worked
- plane turns and pilots reduce power on the two other engines
- plane stalls and crashes
What does FMECA stand for
failure modes effects and Criticality Analysis
What are the measures in a FMECA table
- Occurence
- Severity
- Detectability
What does RPN stand for and how is it calculated
Risk Priority Number
RPN = O x S x D
What would an FMECA table tell you
It tells you what would be the worst type of failure so that it can be mitigated
What does FTA stand for
Fault tree analysis
What is a fault tree analysis (FTA) and why is it used
The objective of a FTA analysis is to model or record how a failure or disaster can occur based on a series of events
Events can either be combined with an AND box or specified as independent events with an OR box.
FTA is used to help understand a past failure or predict how a future failure might occur
