Page 28 - Mechanical Behavior of Materials

Section 1.3 Design and Materials Selection 29

Figure 1.11 Fuselage failure in a passenger jet that occurred in 1988. (Photo courtesy of
J. F. Wildey II, National Transportation Safety Board, Washington, DC; see [NTSB 89] for
more detail.)

process involves a wide range of activities and objectives. It is first necessary to assure that the
item is capable of performing its intended function. For example, an automobile should be capable
of the necessary speeds and maneuvers while carrying up to a certain number of passengers and
additional weight, and the refueling and maintenance requirements should be reasonable as to
frequency and cost.
However, any engineered item must meet additional requirements: The design must be such that
it is physically possible and economical to manufacture the item. Certain standards must be met as
to esthetics and convenience of use. Environmental pollution needs to be minimized, and, hopefully,
the materials and type of construction are chosen so that eventual recycling of the materials used is
possible. Finally, the item must be safe and durable.
Safety is affected not only by design features such as seat belts in automobiles, but also by
avoiding structural failure. For example, excessive deformation or fracture of an automobile axle or
steering component can cause a serious accident. Durability is the capacity of an item to survive
its intended use for a suitably long period of time, so that good durability minimizes the cost of
maintaining and replacing the item. For example, more durable automobiles cost less to drive than
otherwise similar ones that experience more repairs and shorter life due to such gradually occurring
processes as fatigue, creep, wear, and corrosion. In addition, durability is important to safety, as
poor durability can lead to a structural failure or malfunction that can cause an accident. Moreover,