Reliability denitions
The reliability bathtub curve is often used to depict the expected failure rate of a family
of products over time. This model is comprised of three segments: infant mortality, useful
life and wearout, as illustrated in Figure 3.
Infant mortality is an initial period of failures usually resulting from manufacturing
defects or quality excursions, and has a decreasing failure rate over a relatively short
time frame. Product useful life is shown as the bottom portion of the bathtub curve. It is a
period of random failures with a nearly constant failure rate. The weakest component
in any system will determine the duration of this portion of the curve. At the end of
useful life, wearout failure modes, such as fatigue and material depletion, will cause
the failure rate to increase with time. This nal segment of the curve is called wearout.
Reliability during useful life is often a focus when considering products for a specic
application. In the case of GE outdoor LED systems, an exponential distribution may
be applied to model system reliability. Reliability values are often requested in the form
of an MTBF (Mean Time Between Failures) value. MTBF is often misunderstood since it
is expressed as a time value, but more accurately denes the failure rate during the
useful life of the product. The relationship is illustrated below, where λ is the failure
rate with units of hrs
-1
.
It is important to note that this failure rate is valid only during the useful life portion of the
bathtub curve where the failure rate is relatively constant. When the failure rate begins to
increase, a product has entered wearout and a dierent mathematical model is needed
to represent this behavior. For this reason, it is important to understand when wearout
failure modes begin. Both component-level and full-system testing are utilized by GE Lighting
engineers to understand and accurately model the reliability of outdoor LED systems.
Figure 3: Example of reliability bathtub curve
Time
Failure Rate
Product “useful life”
Approximate constant
failure rate
“infant mortality”
Decreasing failure rate
Product “wearout”
Increasing failure rate
(2 Seiten)
Manymanuals.com
Manymanuals.de
Manymanuals.fr
Manymanuals.it
Manymanuals.pl
Manymanuals.cz
Manymanuals.es
Manymanuals-pt.com
Kommentare zu diesen Handbüchern