
116 Decision Based Design
The second phase in a system’s lifetime is termed useful life. A (well-
designed) system spends most of its lifetime here. In this phase, failures occur
mostly because of chance events compounded by normal wear and tear. For
example, an automobile tire can fail because of a nail on the road. Failures
during the useful life are generally not attributable to design or manufactur-
ing aws; consequently, it is very hard to design for chance failures in the
useful life. Manufacturers therefore focus on reducing wear and tear from
normal use or prescribe strict operating conditions for the system. Hazard
function is more or ...