Page 241 - Shigley's Mechanical Engineering Design

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216 Mechanical Engineering Design
Figure 5–5

Valve-spring failure caused by
spring surge in an oversped
engine. The fractures exhibit
the classic 45° shear failure.

5–1 Static Strength
Ideally, in designing any machine element, the engineer should have available the results
of a great many strength tests of the particular material chosen. These tests should be
made on specimens having the same heat treatment, surface finish, and size as the element
the engineer proposes to design; and the tests should be made under exactly the same
loading conditions as the part will experience in service. This means that if the part is to
experience a bending load, it should be tested with a bending load. If it is to be subjected
to combined bending and torsion, it should be tested under combined bending and torsion.
If it is made of heat-treated AISI 1040 steel drawn at 500°C with a ground finish, the
specimens tested should be of the same material prepared in the same manner. Such tests
will provide very useful and precise information. Whenever such data are available for
design purposes, the engineer can be assured of doing the best possible job of engineering.
The cost of gathering such extensive data prior to design is justified if failure of the
part may endanger human life or if the part is manufactured in sufficiently large quan-
tities. Refrigerators and other appliances, for example, have very good reliabilities
because the parts are made in such large quantities that they can be thoroughly tested
in advance of manufacture. The cost of making these tests is very low when it is divided
by the total number of parts manufactured.
You can now appreciate the following four design categories:
1 Failure of the part would endanger human life, or the part is made in extremely
large quantities; consequently, an elaborate testing program is justified during
design.
2 The part is made in large enough quantities that a moderate series of tests is feasible.
3 The part is made in such small quantities that testing is not justified at all; or the
design must be completed so rapidly that there is not enough time for testing.
4 The part has already been designed, manufactured, and tested and found to be
unsatisfactory. Analysis is required to understand why the part is unsatisfactory
and what to do to improve it.