Page 33 - Mechanical Behavior of Materials

34 Chapter 1 Introduction

This technology is now being used to reduce the need for prototype and component testing, thus
accelerating the design process. However, computer simulations are only as good as the simplifying
assumptions used in analysis, and the limitations on input data, which are always present. Thus,
some physical testing will continue to be frequently needed, at least as a final check on the design
process.

1.3.4 Service Experience

Design changes may also be made as a result of experience with a limited production run of a new
product. Purchasers of the product may use it in a way not anticipated by the designer, resulting in
failures that necessitate design changes. For example, early models of surgical implants, such as hip
joints and pin supports for broken bones, experienced failure problems that led to changes in both
geometry and material.
The design process often continues even after a product is established and widely distributed.
Long-term usage may uncover additional problems that need to be corrected in new items. If the
problem is severe—perhaps safety related—changes may be needed in items already in service.
Recalls of automobiles are an example of this, and a portion of these involve problems of
deformation or fracture.

1.4 TECHNOLOGICAL CHALLENGE

In recent history, technology has advanced and changed at a rapid rate to meet human needs. Some of
the advances from 1500 A.D. to the present are charted in the first column of Table 1.1. The second
column shows the improved materials, and the third the materials testing capabilities that were
necessary to support these advances. Representative technological failures involving deformation
or fracture are also shown. These and other types of failure further stimulated improvements in
materials, and in testing and analysis capability, by having a feedback effect. Such interactions
among technological advances, materials, testing, and failures are still under way today and will
continue into the foreseeable future.
As a particular example, consider improvements in engines. Steam engines, as used in the
mid-1800s for water and rail transportation, operated at little more than the boiling point of water,
◦
100 C, and employed simple materials, mainly cast iron. Around the turn of the century, the
internal combustion engine had been invented and was being improved for use in automobiles
and aircraft. Gas-turbine engines became practical for propulsion during World War II, when
they were used in the first jet aircraft. Higher operating temperatures in engines provide greater
efficiency, with temperatures increasing over the years. At present, materials in jet engines must
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withstand temperatures around 1800 C. To resist the higher temperatures, improved low-alloy
steels and then stainless steels were developed, followed by increasingly sophisticated metal alloys
based on nickel and cobalt. However, failures due to such causes as creep, fatigue, and corrosion
still occurred and had major influences on engine development. Further increases in operating
temperatures and efficiency are now being pursued through the use of advanced ceramic and
ceramic composite materials. These materials have superior temperature and corrosion resistance.
But their inherent brittleness must be managed by improving the materials as much as possible,