RELIABILITY
… INTRODUCTION
what is reliability?
reliability
reliability/
[ri-lahy-uh-bil-i-tee]
noun
is the probability that a system
has not failed in designated conditions
at a particular usage duration.
what is reliability?
reliability
what is failure ?
failure/
[feyl-yer]
noun
is the event of a system inherently
ceasing to perform a specified or
required function.
what is failure ?
… a system is said to have ‘failed’ once
failure has occurred …
… the state where it has ceased to
perform a specified or required function
… operational mission
… essential function
explicit failure/
… non-critical
[ik-splis-it feyl-yer]
… critical
noun
is the event of a system inherently
ceasing to perform aan explicitly
specified or required function.
… this allows ‘the customer’ to articulate to
the designers what failures are more important
than others …
… which allows specific requirements to be set
… why study
reliability?
the reliability function
the reliability function starts at 1
(when duration is zero) …
RELIABILITY - 𝑅
1
0.8
0.6
0.4
0.2
0
0
2
4
6
8
10
𝑡 – TIME TO FAILURE
… and decreases with as duration increases.
the reliability function
the reliability function starts at 1
(when duration is zero) …
… by studying this
(and other) functions,
we can learn about
how we can improve
reliability
… and decreases with as duration increases.
1
reliability - 𝑅
0.8
0.6
0.4
0.2
0
duration - 𝑡
the reliability function
supports the analysis of all failures,
regardless of when they happen
supports analysis
and optimization
of full life cycle
cost impact
1
0.8
RELIABILITY - 𝑅
technical support
and steering for
design teams
proper use of
components vs
increase quality
demands
use of published
standards
single point of
failure consideration
supplier consistency
0.6
supports tradeoffs
and alternatives are
considered by
management
0.4
0.2
0
0
promotes
engineering
development
testing
2
4
6
8
10
𝑡 – TIME TO FAILURE
assigns responsibility
for reliability to the
development team
shows how
reliability is
impacted by both
design and
manufacturing
processes
another way reliability
can be understood by
looking at the hazard rate
(which is sometimes
known as the failure rate)
hazard rate/
[haz-erd reyt]
another way reliability
noun
can
be
understood
by
is the rate at which a functioning
looking
at the
hazard
system
fails with
respect
to usage.rate
(which is sometimes
… perhaps the best known example of
known
therates
failure
rate)
aggregatingas
hazard
from different
processes is the bathtub curve.
hazard (or failure) rate
bathtub curve
Region I:
Wear-in or
quality
related
failures
Region II:
Constant
Hazard
Rate
usage
Region III:
Wear-out
hazard (or failure) rate
bathtub curve
… high likelihood of failure
during wear-in …
… and wear-out
usage
hazard (or failure) rate
bathtub curve
… this is only a ‘conceptual’
diagram …
… for hardware only …
usage
hazard (or failure) rate
bathtub curve
Region I:
Wear-in or
quality
related
failures
major
upgrade
/
software
more ‘wear-in’
usage
hazard (or failure) rate
bathtub curve
Region
I:
… but
there
is
another
major
Wear-in or
upgrade
quality
region
…
related
failures
… that occurs when asoftware
system is ‘dead on
more ‘wear-in’
arrival’ or DOA
/
usage
hazard (or failure) rate
bathtub curve
Region 0: Dead on Arrival
Region I:
Wear-in or
quality
related
failures
Region II:
Constant
Hazard
Rate
usage
Region III:
Wear-out
hazard (or failure) rate
bathtub curve
Region 0: Dead on Arrival
Region I:(DOA)/
Region II: Region III:
dead on arrival
Wear-in or Constant Wear-out
[ded on quality
uh-rahy-vuh
l]
Hazard
related
Rate
adjective failures
describes a system that is not functional
at the point of first usage.
usage
hazard (or failure) rate
bathtub curve
Region 0: Dead on Arrival
Region I:(DOA)
Region II:rate
Region
III:
/
dead on arrival
Wear-in or Constant Wear-out
[ded on quality
uh-rahy-vuh
l reyt]
Hazard
related
Rate
noun
failures
is the percentage of systems that can be
described as ‘dead on arrival.’
usage