Page 264 - Handbook of Materials Failure Analysis

260 CHAPTER 11 The reliability design of mechanical system

Failure
rate, l
Bathtub curve with higher
failure rate

Bathtub curve
Straight line
with b
l G
T L t

FIGURE 11.1
Bathtub curve and straight line.

Reliability refers to the ability of system or component to perform a required
function under stated environmental and operational conditions for a specified period
of time [5]. Traditionally, the reliability over the product life can be illustrated by a
bathtub curve that has three regions: a decreasing failure rate, a constant failure rate,
and an increasing failure rate, as shown in Figure 11.1.
As the reliability of a product (or part) improves, failure rate of the product
declines in the field. The bathtub with higher failure rate may be changed into a
straight line with shape parameter β.
To find out the product reliability (or failure) in a straight line with β, product
lifetime L B and failure rate λ might be quantified in Equation 11.1:

FL B Þ ¼ 1 RL B Þ ¼ 1 e λL B ffi λL B (11.1)
ð
ð
where R is reliability function, λ is failure rate, and t is use time.
Today myriad innovations are constantly emerging through advanced technol-
ogy. Products based on these innovations improve our lives and widen their bound-
aries. But new products also experience frequent malfunctions that should have been
resolved before they were released into the global marketplace.
Recently, the Boeing 787 Dreamliner experienced some problems due to new
design elements—the fuselage of carbon-fiber-reinforced plastic (CFRP) and the
electrical system incorporating lithium-ion batteries, which resulted in grounding.
Such major issues are ultimately the responsibility of the Chief Executive Officer
of the business that launches the imperfect product. CEO must act in advance of
the release of new designs. But how can CEO understand concepts generally con-
signed to engineers? Let’s consider the concepts of product verification to find
out malfunctions in a new product, which might be the process CEO might ensure
before new product release.