Page 34 - Mechanical Behavior of Materials

Section 1.3 Design and Materials Selection 33

A prototype, or trial model, is often made and subjected to simulated service testing to
demonstrate whether or not a machine or vehicle functions properly. For example, a prototype
automobile is generally run on a test course that includes rough roads, bumps, quick turns, etc.
Loads may be measured during simulated service testing, and these are used to improve the initial
design, as the early estimate of loads may have been quite uncertain. A prototype may also be
subjected to simulated service testing until either a mechanical failure occurs, perhaps by fatigue,
creep, wear, or corrosion, or the design is proven to be reliable. This is called durability testing and
is commonly done for new models of automobiles, tractors, and other vehicles. A photograph of an
automobile set up for such a test is shown in Fig. 1.13.
For very large items, and especially for one-of-a-kind items, it may be impractical or
uneconomical to test a prototype of the entire item. A part of the item, that is, a component,may
then be tested. For example, wings, tail sections, and fuselages of large aircraft are separately tested
to destruction under repeated loads that cause fatigue cracking in a manner similar to actual service.
Individual joints and members of offshore oil well structures are similarly tested. Component testing
may also be done as a prelude to testing of a full prototype. An example of this is the testing of a new
design of an automobile axle prior to manufacture and the subsequent testing of the first prototype
of the entire automobile.
Various sources of loading and vibration in machines, vehicles, and structures can be simulated
by the use of digital computers, as can the resulting deformation and fracture of the material.

Figure 1.13 Road simulation test of an automobile, with loads applied at all four wheels and
the bumper mounts. (Photo courtesy of MTS Systems Corp., Eden Prairie, MN.)